Revision 1
December 2014
Neutron Poisons
Student Guide
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ii
Table of Contents
INTRODUCTION ..................................................................................................................... 1
TLO 1 FUEL DEPLETION AND NEUTRON POISONS ................................................................ 2
Overview .......................................................................................................................... 2
ELO 1.1 Reactivity Effects of Fuel Depletion ................................................................. 3
ELO 1.2 Definition of Terms ........................................................................................... 5
ELO 1.3 Use of Burnable Neutron Poisons ..................................................................... 9
ELO 1.4 Burnable Neutron Poisons Design .................................................................. 11
ELO 1.5 Chemical Shim Advantages and Disadvantages ............................................. 16
ELO 1.6 Fixed Non-Burnable Neutron Poisons ............................................................ 17
ELO 1.7 Changes Over Core Life.................................................................................. 18
ELO 1.8 Excess Reactivity Over Core Life ................................................................... 21
ELO 1.9 Boron Concentration Changes During Natural Circulation ............................ 24
TLO 1 Summary ............................................................................................................ 25
TLO 2 XENON-135 ............................................................................................................. 28
Overview ........................................................................................................................ 28
ELO 2.1 Most Abundant Fission Product Poisons ........................................................ 29
ELO 2.2 Xenon and Samarium ...................................................................................... 31
ELO 2.3 Xenon Production and Removal ..................................................................... 32
ELO 2.4 Xenon Transient Terms ................................................................................... 34
ELO 2.5 Xenon Response During Reactor Operations ................................................. 40
ELO 2.6 Xenon Oscillations .......................................................................................... 47
ELO 2.7 Achieving a Xenon Free Condition................................................................. 48
ELO 2.8 Control Rod Motion Effects ............................................................................ 49
TLO 2 Summary ............................................................................................................ 52
TLO 3 SAMARIUM-149 ....................................................................................................... 56
Overview ........................................................................................................................ 56
ELO 3.1 Samarium Production and Removal................................................................ 56
ELO 3.2 Equilibrium Samarium .................................................................................... 58
ELO 3.3 Samarium Concentration Transients ............................................................... 60
ELO 3.4 Samarium-149 Effects Over Core Life ........................................................... 63
ELO 3.5 Xenon-135 to Samarium-149 Comparison ..................................................... 64
TLO 3 Summary ............................................................................................................ 67
NEUTRON POISONS SUMMARY ............................................................................................ 69
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Neutron Poisons
Revision History
Revision
Date
Version
Number
Purpose for Revision
Performed
By
10/31/2014
0
New Module
OGF Team
12/11/2014
1
Added signature of OGF
Working Group Chair
OGF Team
Introduction
With knowledge of the neutron life cycle and reactivity coefficients, it is
time to look deeper into core reactivities that have been mentioned but not
covered in depth. This lesson starts with a look at fuel depletion, its terms,
and methods used to lengthen the time between refueling outages. The
Rev 1
1
second terminal objective looks at fission product poisons starting with
xenon-135. As a major fission product poison, xenon-135 creates numerous
operational issues for operators and reactor engineers.
The last terminal objective looks into the second most significant fission
product poison, samarium-149. As with xenon-135, the operational issues
with samarium, including production and removal, are discussed, and a
comparison to xenon is presented. This module provides important
information related to the performance of reactor startups, shutdowns, and
power transients.
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of 80 percent
or higher on the following Terminal Learning Objectives (TLOs):
1. Describe how fuel depletion and neutron poison concentration affect
reactivity in a reactor core.
2. Describe the behavior of xenon-135 in a nuclear reactor and its effects
on reactor operation.
3. Describe the production, removal and effects of samarium-149 on the
operation of a nuclear reactor.
TLO 1 Fuel Depletion and Neutron Poisons
Overview
This section focuses on the reactivity effects from nuclear fuel depletion
and the methods used for increasing core life. The section includes
explanations for fuel depletion and neutron poison terms, as well as the
types of neutron poisons and methods used for kexcess control. Lastly, the
section presents the effects of core life and neutron poisons on moderator
and Doppler coefficients, thermal flux and control rod worth.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe fuel depletion for a nuclear reactor and how it impacts
reactivity over the life of the core.
2. Explain the following terms:
a. Fuel cycle
b. Fuel exposure
c. Conversion ratio
d. Burnable poison
e. Non-burnable poison
f. Chemical shim
3. Explain the concept and use of burnable neutron poisons in a reactor
core.
4. Describe the design of installed burnable poisons for a nuclear
reactor.
5. Explain the advantages and disadvantages of chemical shim over
fixed burnable poisons.
2
Rev 1
6. Describe fixed non-burnable poisons used in reactor cores; include an
example of material used as non-burnable poison.
7. Explain how the following nuclear reactor core parameters change
over core life due to fuel depletion and neutron poison concentration:
a. Moderator temperature coefficient
b. Doppler coefficient
c. Control rod worth
d. Core thermal flux
8. Explain the change in the value of excess reactivity over core life.
9. Describe the effect of changes in boron concentration on reactivity
during natural circulation conditions.
ELO 1.1 Reactivity Effects of Fuel Depletion
Introduction
Fission in the reactor fuel results in the burnup of fissionable nuclei and a
gradual depletion of fuel during power operation. The depletion of
fissionable nuclei in the fuel results in negative reactivity addition and a
slow decrease in kexcess. Reactor coolant system (RCS) dilution is used to
reduce boron concentration to offset this negative reactivity and maintain
keff = 1.0.
Control rods are fully withdrawn at 100 percent power; therefore, boron (B)
is the only variable positive reactivity source available to counter the
negative reactivity from fuel burnup throughout core life. Later sections
will include detailed discussion on this topic.
Reactivity Effects of Fuel Depletion
The excess fuel loaded in the core slowly burns out as fuel is depleted over
core life. The fuel mix of uranium, plutonium, and other fuel nuclei
changes in the burnup process. Different fuels affect the production of
fission products. These changing conditions affect the reactivity of the
core, the largest being the negative reactivity from fuel burnout.
Positive reactivity must be added to the reactor to maintain keff = 1.0. Fixed
poisons are used primarily during the first third of core life and soluble
boron is used throughout core life to maintain keff = 1.0.
Fuel Depletion Effects on keff
Fuel depletion will affect factors of the six-factor formula and keff as
follows:
Rev 1
3
Fast Fission Factor (ε)
There is a decrease in the number of fast fissions from uranium-235 with
depletion. This is a small impact for ε, about 1.04 for a new core to 1.03 for
a depleted core.
Resonance Escape Probability (ρ)
Some uranium-238 is converted to plutonium-240 as the core ages, by the
following reaction:
𝛽− 239
238
1
239 𝛽− 239
𝑈+ 𝑛→
𝑈→
𝑁𝑝 →
𝑃𝑢
92
0
92
93
94
239
1
240
𝑃𝑢 + 𝑛 →
𝑃𝑢
94
0
94
Uranium-238, with high resonance absorption cross-section peaks, depletes
and plutonium-240 builds in. Plutonium-240 resonance absorption crosssection peaks (approximately 30 times higher than uranium-238) resulting
in an increase in resonance capture or decrease in resonance escape
probability over core life.
Thermal Utilization (ƒ)
With the burnup of uranium-235, a decrease in the number of uranium-235
atoms causes a decrease in the thermal utilization factor. As the fuel
concentration decreases, the probability of neutron absorption by the
moderator increases due to an increase in moderator-to-fuel ratio. This
increase causes fewer neutrons to be available for absorption in the fuel and
a further decrease in the thermal utilization factor.
The combination of soluble boron, control rods, and burnable poisons in the
reactor will control kexcess from fuel loading. Control rods are normally
fully withdrawn, except during startup and shutdown, throughout the life of
the core. Reductions in boron concentration and burnable poison
concentrations compensate for fuel burnup and fission product poisons over
core life.
Lowering the concentration of boron decreases the probability that the
boron, a soluble in the moderator, will absorb thermal neutrons. This
results in an increase in the thermal utilization factor.
Additionally, plutonium-239 builds up over core life from the uranium-238
neutron capture. Since plutonium-239 is a fissionable fuel, its increase
causes an increase in the thermal utilization factor.
The overall effect over core life may be a slight increase in the thermal
utilization factor because of the changing boron concentration in the
coolant.
Reproduction Factor (η)
4
Rev 1
The reproduction factor is a function of plutonium-239 production from
neutron capture by uranium-238. The neutron yield per fission for
plutonium-239 is slightly higher than uranium-235, but production of
plutonium-239 lags depletion of uranium-235. Therefore, a slight decrease
in the reproduction factor occurs over life.
Non-Leakage Factors (Lth and Lf)
There are no significant changes over core life in non-leakage factors.
Overall Effects on keff
𝑘𝑒𝑓𝑓 = 𝜀(↘)𝐿𝑓 (→)𝜌(↓)𝐿𝑡ℎ (→)𝑓(↗)𝜂(↘)
The overall effect is that keff decreases without operator action.
Knowledge Check
Fuel depletion in a nuclear reactor causes thermal
utilization to decrease unless dilution of soluble boron
adds positive reactivity.
A.
True
B.
False
ELO 1.2 Fuel Depletion Common Terms
Introduction
This section introduces the terms related to fuel loads, fuel depletion, fuel
production, and control of kexcess. More detail on some of these terms is
provided later in this module.
Fuel Cycle
Pressurized water reactors (PWRs) are typically loaded with enough fuel to
operate at 100 percent power for an 18 or 24 month fuel cycle. The term
fuel cycle describes core operation between refueling events. A power
coastdown occasionally extends core life.
Utilities strive to maximize the fuel cycle in order to achieve the maximum
economic benefits from the fuel. These benefits include minimizing down
time (outages) during the fuel cycle as well as extending time between
refueling outages.
Fuel metallurgical limits and installed kexcess determine fuel cycle length and
the maximum amount of energy available from the fuel. Metallurgical
limits are needed to ensure integrity of the fuel and its cladding.
Maintaining integrity of the fuel and its cladding ensures fission products,
Rev 1
5
many of which are gases, are contained within the fuel. Reactivity limits
consider shutdown margin, temperature coefficients, and fuel thermal
limits. Fuel thermal limits restrict temperatures to maintain physical
integrity of the fuel.
Note
Many of the nuclides produced during the fission
process are gases, for example, iodine and xenon. These
fission product gases will exert pressure on the fuel
pellets and cladding as fuel temperature increases.
Undesirable distortion and loss of fuel cladding integrity
is possible.
Fuel Exposure
Glossary
Fuel exposure and fuel burnup are terms used to describe
the amount of energy released per unit weight of fuel.
Fuel exposure is measured in megawatt days per metric ton uranium
(Mwd/MTU).
Megawatts
Glossary
Megawatts (Mw) are units for the core thermal power
output.
For example, if a reactor produced 2,940 Megawatts (Mw) thermal for one
day, which would be 2,940 megawatt days (Mwd) of thermal output. If it
produced 1,470 Mw thermal constantly for two (2) days that would also be
2,940 Mwd of thermal output. The fuel exposure would be 2,940
Mwd/71.5 Metric Tons Uranium (MTU) = 41 Mwd/MTU if the same
reactor was loaded with 71.5 metric tons of uranium.
For an 18-month cycle, the average end-of-life (EOL) exposure (entire
cycle) is approximately 20,000 Mwd/MTU. Assemblies used for three (3)
cycles may reach 55,000 Mwd/MTU. Twice used fuel assemblies are
loaded into low-power regions of the reactor for their third burn to reduce
thermal stresses from their extended life.
Fuel exposure may be applied to an individual fuel element, the average
value of a section of the core, or to the entire reactor core. Ongoing efforts
exist to continuously improve thermal and nuclear design characteristics for
improved fuel lifetime and greater thermal output.
Conversion Ratio
One advantage of using low-enrichment fuel is that it breeds its own fuel.
When uranium-238 absorbs a neutron, it normally does not fission. Instead,
6
Rev 1
uranium-238 from neutron capture becomes uranium-239, which betaminus decays to neptunium-239, which beta-minus decays to plutonium239 as shown below:
𝛽− 239
238
1
239 𝛽− 239
𝑈+ 𝑛→
𝑈→
𝑁𝑝 →
𝑃𝑢
92
0
92
93
94
The microscopic thermal fission cross-section of plutonium-239 (747.4
barns) is larger than uranium-235 (584.4 barns), making plutonium-239 a
good fissionable fuel. When plutonium-239 absorbs a neutron and does not
fission, it forms plutonium-240, which becomes plutonium-241 from
neutron capture. Plutonium-241 has a larger microscopic thermal fission
cross-section that measures 1,012 barns, which is more than the
microscopic cross-sections of either uranium-235 or plutonium-239.
239
1
240
𝑃𝑢 + 𝑛 →
𝑃𝑢
94
0
94
Additional fuel is produced while uranium-235 is depleting because of these
neutron captures and beta decays. The ratio of this fuel production is called
the conversion ratio or the breeding ratio. Fast breeder reactors are capable
of sustaining a conversion ratio of one or greater.
Conversion ratios may also be expressed as a percent or a decimal.
Conversion ratios for PWRs are typically in range of 50 to 70 percent or 0.5
to 0.7. If the conversion ratio for a particular reactor was 50 percent, 50
fissionable plutonium-239/241 nuclei would be created per 100 uranium235 nuclei consumed.
At EOL the enrichment of uranium-235 is reduced considerably, and an
appreciable amount of plutonium has accumulated. For example, a 4.0
percent enriched uranium-235 fuel element with 40,000 Mwd/MTU of
burnup will have EOL enrichment of 0.8 percent and about 0.6 percent
plutonium-239 at EOL.
Burnable Poisons
Burnable poisons are utilized to control the large amounts of reactivity
associated with excess fuel. Good burnable poisons have a large
microscopic absorption cross-section for neutrons, do not fission, and create
products having low microscopic absorption cross-sections. For example,
Boron-10 has a thermal neutron microscopic absorption cross-section of
3,838 barns. It normally produces Helium-4 and Lithium-7 after absorbing
a neutron, both of which have low microscopic neutron absorption crosssections.
Self-shielding is a design feature that allows a burnable poison material to
burnout more evenly. Thick burnable poison materials benefit from the
effect of self-shielding. Initially, from self-shielding, absorptions taking
Rev 1
7
place in the outer layers reduce the number of neutrons available to
penetrate into the material.
Most neutrons are available to penetrate into the inner layers to continue
neutron absorption as outer layers convert to non-poison materials, from
neutron absorptions. This self-shielding process allows for an even
negative reactivity insertion over a longer period of time as the poison
depletes.
The negative reactivity from the burnable poisons decreases over core life
due to depletion of the burnable poisons. Ideally, this decrease in negative
reactivity should match the decrease in positive reactivity from burnup of
the fuel. As a result, kexcess would remain unchanged.
Compounds of boron or gadolinium comprise common burnable poisons,
and they are generally manufactured as rods contained within a fuel
element. These poisons are less disruptive to core power distribution since
they can be distributed more evenly than control rods.
Non-Burnable Poison
A non-burnable poison has a relatively large absorption cross-section for
neutrons, but does not fission, and creates products that have medium to
large absorption cross-sections. Non-burnable poisons maintain a constant
negative reactivity worth over the life of the core.
While not strictly non-burnable, certain materials are considered as nonburnable poisons. One example is hafnium. The neutron absorption of one
isotope of hafnium leads to the production of another neutron absorber, and
continues through a chain of five absorbers. This chain results in a longlived burnable poison, approximating non-burnable characteristics.
Consequently, the macroscopic cross-section of the hafnium remains large
for the entire fuel cycle.
Absorbers with low neutron absorption cross-sections can also be treated as
non-burnable. Normally, fixed non-burnable poisons are used for power
shaping (flux), or to prevent power peaking near high moderator regions of
the reactor.
Chemical Shim (Boration and Dilution)
Glossary
Chemical shim or boration and dilution are the terms
used to describe adjusting the concentration of boric acid
dissolved in the RCS.
The amount of soluble boron in the RCS influences the thermal utilization
factor, and enables an operator to control the large amount of kexcess from
required fuel loading. Soluble boron, primarily boron-10, allows much
finer control of reactivity than control rods. Boron-10 (20 percent of
naturally occurring boron) is the neutron absorbing poison and is 17.5
8
Rev 1
percent of the boric acid weight. An operator can adjust reactivity by
varying the concentration of boric acid (comprised of the isotope boron-10)
in the RCS through boration and dilution.

The boron-10 absorbs more neutrons, adding negative reactivity if the
boron concentration is increased.
 Positive reactivity is added if the boron concentration is diluted.
Changing boron concentration in a PWR is a slow process and used
primarily to compensate for fuel burnup, slower power transients, or fission
product poison buildup. Soluble boron produces a spatially uniform
neutron absorption method when dissolved in the moderator.
Adjusting boron concentration minimizes control rod use and allows for
their positioning at 100 percent withdrawn, creating a flatter flux profile for
even fuel burnout and reduced power peaking. The flatter flux profile is a
result of not having regions of depressed flux like those produced near
inserted control rods.
Knowledge Check
Adjusting the concentration of boric acid dissolved in the
reactor coolant system is called _____________.
A.
soluble hafnium
B.
chemical shim
C.
conversion ratio
D.
boron ratio
ELO 1.3 Use of Burnable Neutron Poisons
Introduction
Burnable poisons installed in the core work with soluble boron to control
the large amount of kexcess (excess fuel) needed for long duration fuel cycles.
This section discusses the concept and use of burnable poisons.
Design Objective
The design objective for burnable poisons is that they should burnup or
deplete at a rate that adds positive reactivity approximately equal to or faster
than the rate of negative reactivity addition from fuel depletion. Ideally, at
fuel cycle EOL, the burnable poisons should be fully exhausted.
Purpose of Using Burnable Neutron Poisons
Burnable poisons serve the following purposes in nuclear reactors:
Rev 1
9
1. Shape neutron flux to provide a more uniform power density.
2. Allow higher fuel enrichment at initial and refueling core load.
3. Reduce high concentrations of soluble boron at beginning-of-life
(BOL) that could cause a positive moderator temperature coefficient
of reactivity (MTC).
Commercial PWRs usually desire fuel cycles of 18 months or longer which
is accomplished by the use of higher enrichment fuel. Burnable poisons and
soluble poisons offset this additional kexcess.
Burnable poison depletion rates need to be balanced, such that the rate is
fast enough to keep up with the rate of depleted fuel, but not so fast that
kexcess could increase above its BOL value. In addition, an insufficient rate
would result in burnable poisons remaining in the core at EOL, effectively
limiting a full fuel cycle.
Ideally, installed burnable poison would keep the reactor exactly critical for
the entire cycle without the aid of control rods or soluble boron. However,
this is not possible. On the positive side, well-designed installed burnable
poisons minimize the need for soluble boron adjustments and control rod
repositioning with little negative reactivity remaining at EOL. Any
negative reactivity at EOL shortens core life.
Nuclear plant designers place burnable poisons in fixed, specific patterns
for shaping and controlling flux profiles in the reactor core. An operator
can decrease soluble boron concentrations at BOL to a point that a positive
moderator temperature coefficient of reactivity does not occur by using
burnable poisons for additional negative reactivity to counter kexcess.
Note
Recall from previous lessons that a positive MTC is
undesirable and limited by the plant technical
specifications. At BOL, specifications may allow a
small positive MTC, but MTC will generally become
negative by the time power increases to 100 percent.
Knowledge Check – NRC Bank
Instead of using a higher concentration of soluble boric
acid, burnable poisons are installed in a new nuclear
reactor core to _______________.
10
A.
prevent boron precipitation during normal operation
B.
establish a more negative moderator temperature
coefficient
C.
allow control rods to be inserted farther upon initial
criticality
Rev 1
D.
maintain reactor coolant pH above a minimum
acceptable value
ELO 1.4 Burnable Neutron Poisons Design
Introduction
Installed burnable poisons are effective, along with soluble boron, to
counter the kexcess needed for extended fuel cycles. This section discusses
the design, materials used, construction, and installation practices used for
installed burnable poisons.
Burnable Neutron Poisons Design
There are two general types of installed burnable poisons: burnable poison
rod assemblies (BPRAs) and poison integral to the fuel itself. The BPRAs
resemble control rods that are fixed within the fuel elements and therefore
cannot be moved during the cycle.
Westinghouse has a design in which the BPRA has an annular hole in the
middle of the rodlet. Westinghouse uses the term Wet Annular Burnable
Poison Assembly (WABPA). The advantage of this design is that the
poison is exposed to neutron flux from the inside out and from the outside
in.
The most popular installed burnable poison is a compound that contains
boron-10. Boron-10 has a thermal neutron cross-section of 3,838 barns.
This amount is greater than the absorption cross-sections of uranium-235,
plutonium-239, or plutonium-241.
Gadolinium is another popular burnable poison. Gadolinium-155 has a
cross-section of 61,000 barns and gadolinium-157 has a cross-section of
255,000 barns for thermal neutrons. When gadolinium is the burnable
poison, the fuel pellets include gadolinium distributed throughout the
pellets. As a result, the self-shielding effect slows the burnout of
gadolinium.
However, when the new core is first brought to power, the gadolinium-155
depletes faster than xenon-135 poison builds in, which means that more
positive reactivity is added from gadolinium than negative reactivity from
xenon. To offset this excess positive reactivity, boration via chemical shim
is required to add negative reactivity. Eventually, the depletion of
gadolinium results in dilution being required as the fuel depletes.
Westinghouse has designed a fuel pellet with a thin coating of a boron
compound on the surface of the pellet. This design results in a rapid
burnout of the neutron poison. This causes a similar effect to gadolinium;
however, the amount of boration needed is smaller and shorter. The rest of
the cycle utilizes dilution to counter kexcess after about 2,500 Mwd/MTU.
Rev 1
11
Burnable Poison Materials
The materials most commonly used as burnable poisons are boron and
gadolinium. The table below compares the cross-sections for absorption of
uranium-235 to the cross-sections of absorption for boron and gadolinium
isotopes.
Microscopic Cross-Section for Thermal Absorption
Isotope
Cross-Section
Uranium-235
683 barns
Boron-10
3,838 barns
Gadolinium-155
61,000 barns
Gadolinium-157
255,000 barns
Boron and gadolinium both have much higher cross-sections for absorption
than uranium-235, which makes these isotopes effective neutron poisons.
Boron-10 undergoes a neutron-alpha (n,  reaction, and both gadoliniumd155 and gadoloniumd-157 undergo neutron-gamma (n, γ reactions
(radiative capture). Neutron poison capture reactions generally result in
nuclei that have relatively small neutron absorption cross-sections and
stable daughter products. An exception to this is hafnium, which is
sometimes used in reactor control rods. Hafnium has a decay chain of five
successive isotopes with high thermal neutron cross-sections.
Note
Hafnium is not considered a burnable poison because of
its long decay chain of effective neutron absorptions. It
is therefore considered a non-burnable poison and is
discussed in more detail later in the course.
Burnable Poison Rods
For PWRs using boron in discrete burnable poison rods, these rods account
for approximately 6 to 8 percent Δk/k reactivity. The figure below shows
how burnable poisons are depleted during fuel burnup.
12
Rev 1
Figure: Burnable Poison Depletion Over Core Life
Burnable Poison Rod Construction
A burnable poison rod is shown in the figure below. A poison rodlet
consists of a borosilicate glass (borated silicate glass 12.5 wt. percent
without boron trioxide [B2O3]) tube contained within 304 stainless steel
cladding plugged and seal welded at both ends to encapsulate the glass.
The glass tube is supported along its inside diameter by a thin walled
tubular inner liner. Borosilicate glass is similar to Pyrex®. The
construction of these poison rods ensures that by-products of the neutron
absorption reactions are contained within the rods themselves.
Figure: Borosilicate Glass Burnable Poison Rod
Rev 1
13
Commercial PWRs that employ longer fuel cycles (usually 18 months) use
burnable poisons in all fuel cycles to offset the reactivity from the increased
fuel enrichment.
Burnable Poison Rod Positioning
Burnable poison rods are radially spaced within a reactor core in a
configuration that provides a flatter radial power distribution in addition to
reactivity control. The figure below shows the burnable poison rod
arrangement superimposed upon a fuel-loading pattern for a typical
commercial PWR.
Figure: Burnable Poison Rod Positioning in a PWR Core
The figure above shows that a greater number of burnable poison rods are
positioned near the center of the core. These poison rods help flatten the
radial neutron flux distribution by suppressing the neutron flux in the more
highly enriched center assemblies.
14
Rev 1
Note
Note that there are no burnable poison rods in region 1
fuel since these elements would be in a lower flux
(power) environment. Radial neutron flux distribution
in a PWR is principally controlled through fuel
enrichment and burnable poison rod positioning.
Borosilicate glass rods are effective at controlling the excess reactivity
associated with a new fuel load at the beginning of a core cycle. One
limitation in their use is that boron within the rods is not fully depleted over
core life resulting in a relatively high residual absorption rate at EOL. This
results in fuel that cannot be used to produce power because the residual
boron in these rods is still capable of suppressing neutron flux, prematurely
ending the fuel cycle.
Integrated Fuel Burnable Absorbers (IFBAs) are manufactured into
commercial PWR reactor fuel pellets to help minimize these consequences.
IFBA pellets are a type of fuel pellet coated with a thin film of zirconium
diboride (ZrB2). The boron in the coating reduces the thermal neutron flux
available for fission as compared to an uncoated pellet. The boron coating
on these pellets is normally depleted prior to the end of one fuel cycle.
Figure: IFBA Pellet
Neutron flux patterns within the fuel assemblies and the reactor can be
controlled by loading IFBA pellets in the middle of the fuel rod and
uncoated pellets in the ends of the rod. This loading allows increased fuel
enrichment without the associated higher power peaks, resulting in
improved uniform fuel burnup and lower peak centerline fuel temperatures.
Knowledge Check
The reactor engineering group has noticed that the
critical boron versus Mwd/MTU curve is indicating a
lower boron concentration than expected. Which one of
the following could be a cause for this? Note: the core is
halfway through its fuel cycle and power has been at 100
percent for two (2) months.
Rev 1
15
A.
Fuel enrichment is higher than design
B.
Installed fixed burnable poisons are depleting faster than
anticipated
C.
Dropped control rod is undetected
D.
Xenon has not reached equilibrium
ELO 1.5 Chemical Shim Advantages and Disadvantages
Introduction
This section compares the advantages and disadvantages of using soluble
poisons and fixed burnable poisons.
Chemical Shim Advantages
Advantages of chemical shim include the following:






Cost effective method of balancing kexcess to increase fuel loading
and increase fuel cycle time
Allows varying the reactivity contribution to kexcess over core life
Produces spatially uniform neutron absorption when dissolved in the
moderator coolant
Allows much finer control of reactivity than control rods
Minimizes control rod use and allows for their proper positioning for
control of flux profiles, improved fuel performance, lower fuel
temperatures, even fuel burnout and reduced power peaking
Allows for fewer control rods for kexcess control
Chemical Shim Disadvantages
Disadvantages of chemical shim include the following:

In high concentrations, can result in an undesirable positive
moderator coefficient
 Use of chemical shim involves additional costs for boric acid
handling, storage, processing, etc.
 Chemicals used result in metallurgical issues (corrosion)
 Changing boron concentration in a PWR is a slow process and cannot
be used to address faster transients
Fixed Burnable Poisons Advantages
Advantages of fixed burnable poisons include the following:

16
Fixed positions may be discretely loaded in specific locations in order
to shape or control flux profiles in the core
Rev 1

Allows soluble boron concentrations to be reduced, reducing potential
positive MTC, while still allowing for increased kexcess for improved
fuel cycle length
Fixed Burnable Poisons Disadvantages
Disadvantages of chemical shim include the following:

If not depleted at the end of a fuel cycle, the remaining fuel will not
be completely depleted (burned)
o Provides for an undesirable economic situation because fuel
may not be completely exhausted, resulting in shorter fuel
cycles
 Due to poisons being fixed, there is no ability to change poison
loading during the cycle
Knowledge Check
Adding fixed burnable poisons to the core allows soluble
boron concentrations to be reduced, reducing potential
positive moderator temperature coefficient.
A.
True
B.
False
ELO 1.6 Fixed Non-Burnable Neutron Poisons
Introduction
Fixed non-burnable poisons continue to maintain their reactivity worth
throughout the fuel cycle unlike fixed burnable poisons. They are useful for
shaping neutron flux levels within the core for an entire fuel cycle yet not
useful for negating kexcess.
Glossary
A non-burnable poison maintains a constant negative
reactivity worth over the life of the core. Certain
materials are considered as non-burnable poisons
because of their long life time as a poison but no neutron
poison is completely non-burnable. A good example of
this is hafnium.
Fixed non-burnable poisons can suppress neutron flux
levels in specific regions of the core for an entire cycle.
An example is suppressing power production at the edge
of a reactor core. This reduces fast neutron leakage to
the reactor vessel for moderating further neutron
embrittlement.
Rev 1
17
Fixed Non-Burnable Neutron Poisons Example
Hafnium has a long useful life as a neutron absorber because the absorption
of neutrons from one isotope of hafnium leads to the production of another
neutron absorber isotope continuing through a chain of five absorbers. This
results in a long-lived burnable poison with non-burnable characteristics.
It is possible to make the reactivity of a burnable poison material more
uniform over core life through self-shielding. As previously discussed, the
self-shielding effect occurs if the poison material is thick enough to allow
initially only outer layer exposure to the neutron flux. The inner layers
begin absorbing more neutrons as outer layers of poison absorb neutrons
and convert to non-poison materials. This results in uniform addition of
negative reactivity.
Fixed non-burnable poisons are used for power shaping, or preventing
excessive flux and power peaking near heavy moderator regions of the
reactor. Because these poisons do not burnout, they maintain their
reactivity worth throughout the fuel load, and therefore are not used to
balance kexcess of the fuel load.
Knowledge Check
Which of the following negative reactivities is not used
for controlling kexcess?
A.
Soluble boron
B.
Hafnium
C.
Control rods
D.
IFBA pellets
ELO 1.7 Changes Over Core Life
Introduction
Fuel burnup over core life causes a slow addition of negative reactivity and
indirectly affects other reactivity parameters.
o
o
o
o
o
o
18
Soluble boron reactivity coefficient
Moderator temperature reactivity coefficient
Fuel temperature reactivity coefficient
Control rod reactivity worth
Fission product poison concentration and reactivity worth
Neutron flux levels
Rev 1
Moderator Temperature Coefficient
Boron concentration in the coolant/moderator is a function of fuel burnup.
Boron concentration is reduced via dilution to compensate for the negative
reactivity due to fuel burnup as the reactor continues to operate.
The presence of boron results in the reduction of the thermal utilization
factor (ƒ) within the core since boron acts as a neutron absorber. In an
under moderated core, as designed in PWRs, with lower level boron
concentrations the change in the thermal utilization factor with respect to
moderator temperature change (Δf/ΔT) decreases. Moderator Temperature
Coefficient (MTC) becomes more negative over core life because f is the
positive factor in the MTC and its magnitude lowers. The resonance escape
probability factor is the negative influence on MTC and it becomes slightly
more negative over core life due to buildup of plutonium-240 (higher
resonance peaks).
Moderator temperature coefficient of reactivity becomes more negative over
core life.
Doppler Coefficient
The magnitude of the Doppler coefficient becomes larger with fuel burnup
as a result of the buildup of isotopes that have substantial resonance
absorption peaks, greater than uranium-238. The most important isotope is
plutonium-240. Plutonium-240 is produced from a neutron absorption by
uranium-238, two (2) beta-minus decays, and a radiative capture by
plutonium-239. Plutonium-240 has a large resonance peak at one (1) eV
(electron volt).
𝛽− 239
238
1
239 𝛽− 239
1
240
𝑈+ 𝑛→
𝑈→
𝑁𝑝 →
𝑃𝑢 + 𝑛 →
𝑃𝑢
92
0
92
93
94
0
94
This large resonance peak results in an increase in resonance capture or
decrease in resonance escape probability over core life. Therefore, the
Doppler coefficient becomes more negative over core life from the effects
of fission product buildup, shown below in the figure.
Figure: Fuel Temperature Coefficient Changes Over Core Life
Rev 1
19
However, there may be improved heat transfer from the fuel pellet to the
cladding due to pellet expansion (swelling) over core life. This results in
lower fuel temperatures at full power and a smaller coefficient, shown
below in the figure. The overall power defect may not change appreciably
because of lower fuel temperatures at high power while the coefficient (per
degree Fahrenheit) may become more negative from plutonium-240
buildup.
Figure: Average Fuel Temperature (°F)
The fuel temperature coefficient, also called the fuel Doppler reactivity
coefficient or Doppler broadening, is the change in reactivity per degree
change in fuel temperature as explained above. More information about
Doppler broadening is available in the Reactivity Coefficients module.
Control Rod Worth
Control rods are considered a non-burnable poison in the nuclear reactor
core. The combined effect of reducing burnable and soluble poison
concentrations and fuel depletion will increase neutron thermal diffusion
length. This results in the neutrons spending more time near the control
rods to thermalize and therefore a greater chance of absorption in the
control rods.
Thermal flux increases to maintain power, neutron flux levels near the
control rods increase, and control rod reactivity worth increases as the fuel
depletes. Therefore, control rod worth increases with increasing core life.
Core Thermal Flux
Core power output is proportional to the product of the neutron flux and the
fuel macroscopic cross-section for fission. To maintain 100 percent power,
the average thermal neutron flux must increase to compensate for the
overall decrease in fuel macroscopic fission cross-section due to fuel
depletion.
However, because of the creation of plutonium-239 and plutonium-241 fuel
isotopes, the increase in thermal flux level is smaller than without the fuel
20
Rev 1
conversion. Therefore, to operate at the same power over core life, thermal
flux must increase.
Knowledge Check
Which of the following statements accurately describes
the magnitude of nuclear reactor control rod worth over
core life?
A.
Control rod worth is constant over core life because the
effects of fuel depletion and boron dilution offset one
another.
B.
Control rod worth is constant over core life because
control rods are made of non-burnable neutron poisons.
C.
Control rod worth increases over core life because of fuel
depletion and burnable poison concentration reduction.
D.
Control rod worth decreases over core life because of
fuel depletion and burnable poison concentration
reduction.
ELO 1.8 Excess Reactivity Over Core Life
Introduction
The value of the excess multiplication factor (kexcess) varies over the life of a
nuclear reactor core. At the beginning of core life keff would be greater than
one (1) if there were no control rods, soluble boron, or burnable poisons
installed in the core. Excess multiplication factor (kexcess), from excess fuel,
is needed to make up for fuel depletion and fission product buildup over
core life.
Excess Reactivity Over Core Life
The shapes of the kexcess and critical boron curves are similar, but they are
not exactly the same. The next section covers the shape of these curves.
The starting conditions for the kexcess and critical boron concentration curves
are all rods out, 100 percent power, and xenon free. A graph of kexcess over
core life is shown in the below figure.
Rev 1
21
Figure: kexcess Over Core Life
The excess fuel loaded into the core at the beginning of a fuel cycle
provides a large amount of positive reactivity raising kexcess well above one
(1). Some negative reactivity source must compensate for this kexcess.
Normally, soluble boron (chemical shim) and installed burnable poisons act
to compensate for kexcess.
The graph above illustrates that the value of kexcess starts to decrease at the
beginning of core life due to the buildup of fission product poisons
including samarium, xenon, and others to equilibrium levels. This
represents a large amount of negative reactivity.
The buildup of xenon and samarium to an equilibrium value occurs over a
period of approximately 40 hours for xenon (after reaching a new steady
state power level) and 40 days for samarium. Over this time, kexcess steadily
declines because of the increase in fission product poisons.
After the buildup of fission product poisons to equilibrium values, the
positive reactivity added to the core by depletion of installed burnable
poisons overcomes the negative reactivity due to fuel depletion, causing
kexcess to increase. This increase in kexcess continues to about one-third core
age, but at a steadily slower rate. Eventually the reactivity loss due to fuel
burnout surpasses the reactivity addition produced by burnup of the
burnable poisons.
After about one-third of the fuel cycle or core age, the reactivity loss due to
fuel depletion becomes the dominant reactivity factor causing kexcess to
decrease steadily. At this point, the positive reactivity effect from burnable
poison depletion becomes negligible. Following the peak in kexcess,
additional fission product poison buildup from chemical elements other than
xenon and samarium as well as fuel depletion cause kexcess to continue to
decrease for the rest of the fuel cycle.
The above figure of kexcess tends to be more representative for a PWR using
gadolinium for fixed burnable poisons. The higher capture cross-section for
gadolinium causes it to burnup more rapidly during the first third of the fuel
cycle, resulting in an increase of kexcess.
22
Rev 1
However, many commercial PWRs tend to use boron instead of gadolinium.
In that instance, the below graph of kexcess over core life shows a shape more
like the following figure of critical boron concentration over core life.
Critical Boron Concentration Curve
Figure: Critical Boron Concentration Over Core Life
The shape of the curve above shows that the critical boron concentration
initially drops sharply due to the buildup of xenon and samarium in the
newly installed fuel. The shape will vary depending on poison
concentration, type, and fuel enrichment. This response is the same as the
one shown in the kexcess curve.
Following this initial drop, boron concentration and kexcess remain relatively
constant for a period due to the combined effects of negative reactivity
added by fuel burnup and positive reactivity added by the burnup of
burnable poison.
This curve differs from the previously described kexcess curve due to the
reactivity characteristics of the fixed burnable poison depletion rates. In
this case, the depletion rates of the burnable poisons and fuel are
approximately equal and opposite to provide a more constant kexcess and
critical boron concentration. A review of your plant's critical boron curve
will illustrate how your plant's burnable poisons deplete.
After the period with a relatively constant value, kexcess drops in an almost
linear fashion for the remainder of the fuel cycle due to the steady deletion
of the fuel. As a result, the amount of boron required to make up for the
excess reactivity goes down as well.
Rev 1
23
Knowledge Check
Excess reactivity (excess multiplication factor) decreases
early in core life due to ______________ and then
increases or levels out as the core approaches middle-oflife conditions due to _______________.
A.
fission product poison buildup; burnout of installed
poisons
B.
burnout of installed poisons; fission product poison
buildup
C.
boration; dilution
D.
fission product poison buildup; chemical shim
ELO 1.9 Boron Concentration Changes During Natural
Circulation
Introduction
This section contains a short discussion of boron concentration changes
during natural circulation conditions.
Boron Concentration Changes During Natural Circulation
Normally, boration and dilution are not performed during natural circulation
conditions. Without the Reactor Coolant Pumps (RCPs) running the RCS
flow rate will be much lower and the boron mixture distribution will be
uneven throughout the core. This could lead to a loss of shutdown margin.
To prevent a dilution accident, reducing boron concentration during natural
circulation conditions is not normally allowed. Any boron changes in
natural circulation will have the same reactivity effect as with forced
circulation, once completely mixed.
Prior to placing the Residual Heat Removal (RHR) System in service for
RCS cooldown, RHR boron concentration may be less than the boron
concentration in the RCS. To ensure boron concentrations are equal, RHR
loops are warmed-up, circulated, and sampled for boron concentration
before placing in service.
During a large break, loss of coolant accident boron can plate out on the
fuel cladding. For this reason, safety injection is transferred from cold-leg
to hot-leg injection and back again to flush boron back into solution.
24
Rev 1
Knowledge Check
A nuclear reactor has been shutdown for eight (8) hours
following a loss of offsite power. The reactor coolant
system (RCS) is in hot standby on single-phase natural
circulation. Compared to adding boric acid to the RCS
during forced circulation, adding boric acid during
natural circulation requires _________ time to achieve
complete mixing in the RCS; once completely mixed, a
one (1) part per million (ppm) increase in RCS boron
concentration during natural circulation will cause a/an
________ change in core reactivity.
A.
more; smaller
B.
more; equal
C.
less; smaller
D.
less; equal
TLO 1 Summary
1. Describe fuel depletion for a nuclear reactor and how it impacts
reactivity over the life of the core.
 Fission results in the destruction of fissionable nuclei in
reactor fuel, fuel depletion
 Over core life, this results in gradual insertion of negative
reactivity
2. Define the following terms:
 Fuel cycle - refers to the time between refueling
 Fuel exposure - describes fuel depletion or burnup, usually
expressed in (Mwd/T) or (Mwd/MTU)
 Conversion ratio - refers to the number of plutonium-239
nuclei produced per 100 uranium-235 nuclei consumed
 Burnable poison – used with soluble boron to control excess
fuel needed for fuel cycles
 Non-burnable poison - used in reactor cores to shape power
and to prevent excessive flux and power peaking near
moderator regions (ex: hafnium)
 Chemical shim - name for adjusting the concentration of
boric acid dissolved in the RCS
3. Explain the concept and use of burnable neutron poisons in a
reactor core.
 Installed during refueling, compensate for the excess
positive reactivity of the fuel
— Used to shape neutron flux for more uniform power
density, allow higher fuel enrichment, and allow for
Rev 1
25
lower concentrations of soluble boron to mitigate
positive MTC
4. Describe the design of installed burnable poisons for a nuclear
reactor.
 BPRAs and burnable poison integral to fuel pellet
 BRPA - one type of installed burnable poison is borosilicate
glass tube contained within type 304 stainless steel cladding
 Large number of burnable poison rods positioned near the
center of the core to flatten radial neutron flux distribution in
core by suppressing neutron flux in more highly enriched
fuel assemblies
 IFBA - a type of fuel pellet coated with thin film of
zirconium diboride (ZrB2)
— Normally burned out prior to end of one fuel cycle
— Loaded in middle of fuel rod assemblies to control
neutron flux patterns
— Allows increased fuel loading without higher power
peaks, yielding more uniform fuel burnup and lower
peak centerline temperatures
5. Explain the advantages and disadvantages of chemical shim over
fixed burnable poisons.
 Soluble neutron poison circulated in the reactor coolant
during normal operation, usually boron
— Cost effective method of balancing kexcess
— Reactivity can be varied during reactor operation
— Has a spatially uniform effect, flatter profile
— Finer control and allows for fewer control rods
 Disadvantages over fixed burnable poisons
— In high concentrations can result in positive MTC
— Additional costs in boric acid handling storage and
processing
— Creates metallurgical issue of corrosion
— Changing concentration/reactivity is slow process
 Fixed burnable poison advantages
— Can be loaded to specific locations
— Allows for soluble concentrations to be reduced
— Reduces potential for positive MTC
— Allows increased kexcess for longer fuel length
 Fixed burnable poison disadvantages
— If not completely depleted prior to EOL, reduces fuel
cycle
— Fixed poisons, can’t adjust during fuel cycle
6. Describe fixed non-burnable poisons used in reactor cores, include
an example of material used as non-burnable poison.
 Maintain reactivity worth constant throughout fuel cycle
 Hafnium is an example of a non-burnable poison, absorption
of a neutron by one isotope chains to multiple isotopes, also
with high absorption cross-sections
26
Rev 1
7. Explain how the following nuclear reactor core parameters change
over core life due to fuel depletion and neutron poison
concentration:
 MTC becomes more negative over core life largely due to
the significant drop in boron concentration as fuel depletes
 Doppler coefficient become larger as the core ages primarily
due to increase in Pu 240, which has larger resonance
capture
 Control rod worth increases over core from fuel burnup, less
boron, and a higher neutron flux over core life
 Core thermal flux increases over core life due to fuel burnup
8. Explain the change in the value of excess reactivity over core life.
 Initial drop in magnitude due to buildup of fission product
poisons
 kexcess increases from about day 25 to 1/3 fuel cycle due to
burnout of installed burnable poisons
 From 1/3 fuel cycle to EOL, reactivity from fuel burnout
dominates and kexcess decreases
9. Describe the effect of changes in boron concentration on reactivity
during natural circulation conditions.
 Takes longer when the RCS is operating in a natural
circulation mode versus forced circulation
Once the boron is completely mixed (or diluted), the same change in
reactivity in the core occurs during either forced or natural circulationNow
that you have completed this lesson, you should be able to do the following:
1. Describe fuel depletion for a nuclear reactor and explain the fuel
depletion effects on reactivity over the life of the core.
2. Explain the following terms:
a. Fuel cycle
b. Fuel exposure
c. Conversion ratio
d. Burnable poison
e. Non-burnable poison
f. Chemical shim
3. Explain the concept and use of burnable neutron poisons in a reactor
core.
4. Describe the design of installed burnable poisons for a nuclear
reactor.
5. Explain the advantages and disadvantages of chemical shim over
fixed burnable poisons.
6. Describe fixed non-burnable poisons used in reactor cores; include an
example of material used as non-burnable poison.
7. Explain how the following nuclear reactor core parameters change
over core life due to fuel depletion and neutron poison concentration:
a. Moderator temperature coefficient
b. Doppler coefficient
c. Control rod worth
d. Core thermal flux
Rev 1
27
8. Explain the change in the value of excess reactivity over core life.
9. Describe the effect of changes in boron concentration on reactivity
during natural circulation conditions.
TLO 2 Xenon-135
Overview
Xenon-135 is a significant negative reactivity component and has a large
impact on reactor operation. To accurately predict reactor response during
various operations, it is important for the operator to understand its
production and removal methods, and its response to various transients is
important in predicting reactor response. Xenon transients can limit reactor
operations, create unacceptably high power peaking in the fuel, and prevent
reactor startup.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe fission product poisons and how fission product poisons
affect the neutron life cycle.
2. List the most important fission product poisons to the operation of a
nuclear reactor.
3. Explain how xenon-135 is produced and removed in the core of a
nuclear reactor.
4. Explain the following terms:
a. Equilibrium iodine
b. Equilibrium xenon
c. Transient xenon
d. Peak xenon
e. Xenon free
f. Xenon precluded startup
g. Xenon dead time
5. Explain how xenon-135 concentration reacts during the following
nuclear reactor operations:
a. Xenon free initial reactor startup
b. Reactor shutdown
c. Decrease in reactor power
d. Increase in reactor power
e. Reactor startup with xenon present in the core
6. Describe the causes and effects of a xenon oscillation.
7. State the approximate time following a reactor shutdown at which the
reactor is considered xenon free.
8. Explain the effects of xenon concentration on a nuclear reactor core's
thermal flux profile for the following:
a. Control rod motion
b. Bore life
28
Rev 1
ELO 2.1 Most Abundant Fission Product Poisons
Introduction
Fission fragments resulting from fission events decay to produce a variety
of fission products. Fission product poisons are a major concern because
they absorb neutrons and remove them from the neutron life cycle.
There are dozens of long-lived and stable fission-product poisons that have
small to large neutron absorption cross-sections. These fission-product
poisons build up to equilibrium values over core life, because a decrease in
the thermal utilization factor (f), and add negative reactivity to the core.
In order to consider an isotope as a fission fragment poison it must meet
two basic criteria: first, it has a relatively large microscopic cross-section
for absorption and second, it must exist in sufficient quantities.
Most Abundant Fission Product Poisons
While there are several fission products that have significant neutron
absorption cross-sections, xenon-135 and samarium-149 have the most
substantial impact on reactor operation. The figure below shows the fission
yield curve for uranium-235. Other fuel isotopes have a similar fission
yield.
Figure: Fission Yield Curve for Uranium-235
Fission Product Poison Effect on Neutron Life Cycle
Xenon-135 and samarium-149 both have high absorption cross-sections: 2.6
x 106 barns for xenon-135 and 4.0 x 104 barns for samarium-149. They
have an impact on the thermal utilization factor (ƒ), reactivity, and keff
Rev 1
29
because xenon and samarium absorb neutrons that otherwise would be
absorbed in the fuel.
An increase in the macroscopic cross-section for absorption by any neutron
poison results in a decrease in the value of the thermal utilization factor (f),
based on the equation shown below:
𝑓=
∑𝑓𝑢𝑒𝑙
𝑎
𝑝𝑜𝑖𝑠𝑜𝑛
∑𝑓𝑢𝑒𝑙
+ ∑𝑚𝑜𝑑
+ ∑𝑜𝑡ℎ𝑒𝑟
+ ∑𝑎
𝑎
𝑎
𝑎
Therefore, as fission product poison concentration increases, the
denominator in the above equation increases, causing a decrease in the
value of the thermal utilization factor.
Fission Product Poison Concentration
The concentration of fission product poisons present in a nuclear reactor
core at any given time depends on the poisons production and removal
rates. Fission product poisons may be directly produced from fission or
produced from the decay (or decay chain) of certain fission products.
Removal of a fission product poison from the core occurs by radioactive
decay or neutron absorption. The removal generally results in an isotope
with a much lower neutron absorption cross-section.
Each isotope has a unique decay rate while the production and neutron
absorption (burnout) are dependent on the reactor's power level.
Fission Product Poison Equilibrium
Equilibrium is a term often associated with fission product poisons. At
equilibrium, the production rate of the poison equals the removal rate and
the concentration of the poison is constant. Equilibrium levels may be
power dependent, and the time to reach equilibrium is a function of the
change in power level, the rate of change of power, and the decay rate of the
particular isotope.
Other Fission Product Poisons
In addition to xenon and samarium, many other fission products have
appreciable cross-sections for neutron absorption. Neutron absorption will
not necessarily deplete the concentration of these poisons.
Due to their moderate cross-sections and continued production by fission,
these fission product poisons are termed permanent poisons. In a thermal
reactor, these poisons may accumulate at a rate of about 50 barns per
fission. This accumulation adds negative reactivity over core life.
30
Rev 1
Reactivity Effects of Fission Product Poisons
The reactivity effects of fission product poisons such as xenon and
samarium occur relatively slowly compared to other reactivity effects from
items including control rods, fuel, and moderator temperatures. Changes in
reactivity from fission product poisons occur over periods of hours to days
to years, rather than seconds or minutes with the exception of a rapid
burnout from peak xenon.
Knowledge Check
Fission product poisons contribute _______________
reactivity to a nuclear reactor as they buildup in the core
and _______________ reactivity to a nuclear reactor as
their concentration decreases in the core.
A.
negative; positive
B.
negative; negative
C.
positive; negative
D.
positive; positive
ELO 2.2 Xenon and Samarium
Introduction
Xenon-135 and samarium-149 are the most important fission product
poisons to consider with regard to reactor operations.
Xenon and Samarium
Xenon-135 and samarium-149 both have high cross-sections for neutron
absorption:


Xenon-135 — 2.6 x 106 barns
Samarium-149 — 4.0 x 104 barns
At equilibrium levels, they add considerable negative reactivity:


Xenon — approximately -3,000 per cent mille (pcm) (varies with
power level)
Samarium — approximately -700 pcm at BOL to -1,000 pcm at
EOL at 100 percent reactor power
On a reactor trip, Xenon peaks with a negative reactivity of almost 5,000
pcm and decays back to zero (0) pcm after about three (3) days. This is a
significant reactivity transient.
Rev 1
31
Unlike Xenon, samarium increases to a new equilibrium level after a reactor
trip, dependent on power history.
The details of xenon and samarium are discussed separately in the next
sections of this course.
Knowledge Check
Which of the following fission product poisons are of the
greatest concern to the operation of a nuclear reactor?
A.
Xenon and boron
B.
Zirconium and samarium
C.
Xenon and samarium
D.
Boron and gadolinium
ELO 2.3 Xenon Production and Removal
Introduction
Xenon-135 originates from two sources:
1. Directly from nuclear fission
2. By the beta-minus decay chain of Tellurium since iodine-131 is
usually considered the decay source because the half-life of tellurium135 is relatively short
Xenon-135 removal occurs by two methods:
1. Beta-minus decay to Cesium-135
2. Neutron capture reaction, which creates xenon-136 (burnout)
Xenon Production and Removal
Xenon-135 Production
As previously stated, xenon-135 comes directly from fission or the decay of
another fission product, iodine-135. Refer to the equation below:
𝛽 135 𝛽 135
𝛽 135
𝛽 135
135
𝑇𝑒 →
𝐼 →
𝑋𝑒 →
𝐶𝑠 →
𝐵𝑎 (𝑠𝑡𝑎𝑏𝑙𝑒)
52
53
54
55
56
19 𝑠𝑒𝑐
32
6.57 ℎ𝑟
9.10 ℎ𝑟
2.36 × 106 𝑦𝑒𝑎𝑟𝑠
Rev 1
Approximately 0.3 percent of all fissions yield xenon-135 as a fission
fragment, and approximately 6.0 percent of all fissions yield tellurium-135
directly, with beta-minus to iodine-135. Because of the short half-life of
tellurium-135 and longer half-life of iodine-135, tellurium-135 is ignored,
and iodine-135 is considered the source from fission. Consequently,
approximately 5 percent of all xenon-135 production is directly as a fission
fragment, and 95 percent of all xenon-135 production is from the betaminus decay of iodine-135.
Because 95 percent of xenon comes from the decay of iodine-135, which
has a half-life of six and a half (6.6) hours, there is a significant delay in the
production of xenon-135. This delay is important when considering
changing power levels, because production of xenon-135 depends on the
number of fissions occurring.
Xenon-135 Removal
Xenon-135, which has a half-life of 9.1 hours, is removed by beta-minus
decay to cesium-135 or neutron absorption to xenon-136 (burnout). Both
cesium-135 and xenon-136 have small neutron absorption cross-sections.
The ratio of xenon-135 removal due to burnout and decay varies with power
level. This is significant because it governs equilibrium values and
transient response.
Xenon-135 Production and Removal
The rate of change in xenon concentration equals the rate of production
minus the rate of removal. When these are in balance, xenon is in
equilibrium. The rate of change of xenon concentration is expressed by the
following equation:
𝑅𝑎𝑡𝑒 𝑜𝑓 𝑐ℎ𝑎𝑛𝑔𝑒
𝑥𝑒𝑛𝑜𝑛_ 135 𝑦𝑖𝑒𝑙𝑑 𝑖𝑜𝑑𝑖𝑛𝑒 _ 135 𝑥𝑒𝑛𝑜𝑛_ 135 𝑥𝑒𝑛𝑜𝑛_ 135
𝑜𝑓 𝑥𝑒𝑛𝑜𝑛_ 135 = 𝑓𝑟𝑜𝑚 𝑓𝑖𝑠𝑠𝑖𝑜𝑛 + 𝑑𝑒𝑐𝑎𝑦 − 𝑑𝑒𝑐𝑎𝑦 − 𝑏𝑢𝑟𝑛𝑢𝑝
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
∆𝑁𝑋𝑒
𝑓𝑢𝑒𝑙
𝑋𝑒
= 𝛾𝑋𝑒 ∑
Φ + 𝜆𝐼 𝑁𝐼 − 𝜆𝑋𝑒 𝑁𝑋𝑒 − 𝜎 𝑁𝑋𝑒 Φ
𝑓
𝑎
∆𝑡
Where:
NXe = xenon-135 concentration
∆𝑁𝑋𝑒 = change in xenon-135 concentration
γxe = fission yield of xenon-135
∑
𝑓𝑢𝑒𝑙
= macroscopic cross-section in fuel
𝑓
Φ = thermal neutron flux
NI = iodine-135 concentration
Rev 1
33
λI = decay constant for iodine-135
λXe = decay constant for xenon-135
𝜎
𝑋𝑒
= microscopic absorption cross-section for xenon-135
𝑎
∆𝑡 = change in time
The xenon-135 burnup term above, the last term in the equation, is the
neutron capture of xenon-135 to xenon-136.
1
136
135
𝑋𝑒 + 𝑛 →
𝑋𝑒 + 𝛾
0
54
54
Xenon-136 is not a significant neutron absorber; therefore, neutron
absorption by xenon-135 constitutes removal of poison from the reactor.
The burnup rate of xenon-135 depends on the neutron flux and the xenon135 concentration.
Xenon-135 decays, which has a nine and one-tenths (9.10) hour half-life,
(second-to-last-term in the equation) by beta emission to cesium-135.
Cesium-135, which is not considered a neutron poison, has a long half-life
(>106 years) and a small absorption cross-section for neutrons.
Knowledge Check
Which of the following is the greatest source of xenon135 production in an operating nuclear reactor core?
A.
The decay of tellurium-135 fission fragments
B.
Direct xenon-135 production from fission
C.
Neutron capture by samarium-134 fission fragments
D.
The decay of neodymium-149 fission fragments
ELO 2.4 Xenon Transient Terms
Introduction
An understanding of the various terms related to xenon transients will
provide a deeper understanding of xenon characteristics and limitations
during reactor operations.
34
Rev 1
Equilibrium Iodine (NI-eq)
Equilibrium iodine (NI-eq) is the constant iodine-135 concentration in the
reactor that eventually occurs after a power change. It is directly
proportional to reactor power, taking 20 to 25 hours after the power change
to reach equilibrium. Iodine-135 has to reach equilibrium before xenon-135
can reach equilibrium.
The rate of change in iodine concentration equals the rate of production
minus the rate of removal, expressed below in the equation:
𝑌𝑖𝑒𝑙𝑑 𝑓𝑟𝑜𝑚
𝑅𝑎𝑡𝑒 𝑜𝑓 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛
=
− 𝐷𝑒𝑐𝑎𝑦 𝑟𝑎𝑡𝑒 − 𝐵𝑢𝑟𝑛𝑢𝑝 𝑟𝑎𝑡𝑒
𝑓𝑖𝑠𝑠𝑖𝑜𝑛
𝑖𝑜𝑑𝑖𝑛𝑒 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
or
∆NI
𝑓𝑢𝑒𝑙
𝐼
= γI ∑
Φ − λI NI − 𝜎 NI Φ
𝑓
𝑎
∆t
Where:
NI = iodine-135 concentration
∆NI = change in iodine-135 concentration
ϒI = fission yield of iodine-135
∑
𝑓𝑢𝑒𝑙
= macroscopic cross-section in fuel
𝑓
Φ = thermal neutron flux
λI = decay constant for iodine-135
𝐼
𝜎 = microscopic absorption cross-section for iodine-135
𝑎
∆𝑡 = change in time
Since iodine-135 microscopic absorption cross-section (σIa) is small, the
burnup rate term may be ignored, which simplifies the below expression:
∆𝑁𝐼
𝑓𝑢𝑒𝑙
= 𝛾𝐼 ∑
Φ − 𝜆𝐼 𝑁𝐼
𝑓
∆𝑡
When the rate of iodine production equals the rate of removal, equilibrium
exists. The equation above can be solved for N (concentration) to obtain
equilibrium concentration.
𝛾𝐼 ∑
𝑁𝐼 (𝑒𝑞) =
Rev 1
𝑓𝑢𝑒𝑙
Φ
𝑓
𝜆𝐼
35
The equilibrium concentration equation above is proportional to reactor
power level since equilibrium iodine concentration is proportional to the
𝑓𝑢𝑒𝑙
fission reaction rate (γI ∑
Φ).
𝑓
Equilibrium Xenon (NXe [eq])
Equilibrium is established when the production and removal rates of xenon135 are equal. The equilibrium concentration of xenon-135 is designated
NXe (eq), and is shown below in the equation:
𝑓𝑢𝑒𝑙
Φ + ƛ𝐼 𝑁𝐼
𝑓
𝑋𝑒
𝜎 Φ + 𝜆𝑋𝑒
𝑎
𝛾𝑋𝑒 ∑
𝑁𝑋𝑒 (𝑒𝑞) =
Iodine-135 must also be in equilibrium for xenon-135 to be in equilibrium.
Substituting the expression for equilibrium iodine-135 concentration into
the equation for equilibrium xenon results in the below equation:
𝑓𝑢𝑒𝑙
(𝛾𝑋𝑒 + 𝛾𝐼 )
𝑓
𝑋𝑒
𝜎 Φ + 𝜆𝑋𝑒
𝑎
Φ∑
𝑁𝑋𝑒 (𝑒𝑞) =
From this equation, it can be seen that the equilibrium value for xenon-135
increases as power increases, because the numerator is proportional to the
fission reaction rate. Equilibrium xenon concentration increases with
higher reactor power, but is not directly proportional because thermal flux is
also in the denominator for xenon removal by burnout.
Equilibrium xenon at 25 percent power equals approximately 50 percent of
the 100 percent power equilibrium. Equilibrium xenon at 50 percent power
is between 70 percent and 80 percent of 100 percent power equilibrium.
These values are due to the magnitude of each of the removal terms in the
denominator of the above equation (flux levels for burnout).
Decay is the major removal term at low power and burnout is the major
removal term at high power. At 100 percent power and 100 percent
equilibrium, 70 percent of xenon-135 removal is by burnout and 30 percent
by decay. The two removal terms are approximately equal at 30 percent
36
Rev 1
power. The equilibrium iodine-135 and xenon-135 concentrations as a
function of neutron flux are illustrated below in the graphic.
Figure: Equilibrium Iodine-135 and Xenon-135 Concentrations Versus
Neutron Flux
Equilibrium xenon-135 concentration is reached quicker at higher reactor
power levels. This concentration is due to the faster production rate of
xenon at higher power levels. Equilibrium xenon concentration is typically
reached approximately 40 hours after a reactor startup to full power
operation. For a reactor startup to 50 percent power, approximately 44
hours are required to reach xenon equilibrium. This time extends to
approximately 48 hours at lower reactor power levels.
Transient Xenon
Xenon-135 undergoes a transient whenever reactor power is changed. It
takes 40 to 50 hours after the power change for xenon-135 to reach
equilibrium. Negative reactivity due to xenon-135 is directly proportional
to the xenon-135 concentration.
Following a power change, the operator will compensate for power defect
using boron and/or control rods. Because xenon-135 will continue to affect
reactivity for several hours after the power change, ongoing attention by the
operator will be required to maintain the appropriate reactivity balance.
Xenon-135 concentration will peak and then decrease to a new lower
equilibrium value or go to zero (0) (on a shutdown) any time the reactor
undergoes a rapid down power. A reactor trip is the most rapid down
power; therefore, results in the largest xenon peaks. Xenon peaks because
the thermal neutron flux lowers with power and reduces the burnout rate. In
addition, the xenon-135 production from iodine-135 decay remains almost
constant for a period of time with a six and a half (6.5) hour half-life, which
adds to the xenon peak.
Rev 1
37
Eventually production slows, decay becomes a closer match for the new
power level, and the xenon peaks. Following the peak, the removal rate
exceeds the production rate until reaching a new equilibrium or zero (0) if
the reactor is shut down.
Peak xenon occurs approximately 6 to 10 hours after a shutdown. The
following rule of thumb estimates the amount of time to reach peak xenon
in a shutdown reactor:

Time to peak xenon (in hours) is equal to the square root of the
percent (%) reactor power prior to the shutdown (trip).
Using this rule, a shutdown (trip) from 100 percent reactor power will result
in peak xenon concentration occurring about 10 hours later. Similarly, a
shutdown from 50 percent power will result in peak xenon concentration
approximately 7 hours later.
The greater the flux level (power) prior to shutdown, the greater the
concentration of iodine-135 at shutdown; therefore, the greater the peak in
xenon-135 concentration after shutdown. This can be seen in the following
figure, which illustrates the negative reactivity value of xenon-135
following shutdown from various neutron flux levels.
Figure: Xenon-135 Reactivity After Reactor Shutdown
The xenon-135 peak following a reactor shutdown can have an important
effect on plant operations. For example, if a reactor must be shut down
from full power operation for an extended period, the increasing
concentration of xenon-135 immediately following the reactor shutdown
increases the reactor shutdown margin by adding additional negative
reactivity to the core. After reaching a peak, the concentration of xenon135 slowly decreases until eventually reaching a xenon free condition.
38
Rev 1
The removal of xenon-135 by decay after the peak results in a decrease in
the shutdown margin (positive reactivity added). The shutdown margin in
the reactor will have returned to its initial value (at time of shutdown) after
approximately 20 to 24 hours. The reactor's shutdown margin continues to
decrease as the xenon-135 continues to decay.
In this case, shutdown margin requirements will be maintained by negative
reactivity inserted by the control rods (shutdown margin [SDM] assumes
most reactive rod fully stuck out, xenon free). However, any positive
reactivity added by a subsequent cooldown could reduce the shutdown
margin to the point of exceeding its limits. The operator would adjust
boron concentration as necessary to maintain the required shutdown margin.
Xenon Free
Xenon peaks then decays to essentially zero in approximately 70 to 80
hours after a reactor shut down.
Xenon Precluded Startup
The xenon peak following a reactor trip may provide sufficient negative
reactivity to prevent taking the reactor critical or maintaining criticality due
to insufficient positive reactivity available from control rods and/or dilution
to offset xenon.
This would likely happen late in core life when the reactor is shutdown
from a high power level, with excess reactivity level as low as one percent
Δk/k due to fuel depletion. The inability of the reactor to be started up due
to the effects of xenon is referred to as a xenon precluded startup.
The inability to restart the reactor due to xenon would persist for several
hours until the xenon-135 peak is decayed to the point where control rod
withdrawal and dilution can add sufficient positive reactivity to overcome
the negative reactivity attributed to the xenon.
Xenon Dead Time
Xenon dead time is the timeframe where the reactor is unable to override
the reactivity effects of xenon, such as during a xenon-precluded startup.
Knowledge Check
Two commercial pressurized water reactors are being
shut down from power range operation. Reactor A has
been operated for several days at 100% power. Reactor
B has been operated for several days at 75% power.
Which of the following correctly describes the behavior
of xenon-135 that can be expected for each reactor after
shutdown? Assume each reactor will be shut down for
several days.
Rev 1
39
A.
The peak xenon concentration for Reactor A will be
smaller in magnitude and will occur earlier than the peak
xenon concentration for Reactor B.
B.
The peak xenon concentration for Reactor A will be
smaller in magnitude and will occur later than the peak
xenon concentration for Reactor B.
C.
The peak xenon concentration for Reactor A will be
greater in magnitude and will occur later than the peak
xenon concentration for Reactor B.
D.
The peak xenon concentration for Reactor A will be
greater in magnitude and will occur earlier than the peak
xenon concentration for Reactor B.
ELO 2.5 Xenon Response During Reactor Operations
Introduction
Xenon-135 concentration changes occurring within the reactor affect the
amount of reactivity present in the core. Various reactor operations result in
significant changes in the concentration of xenon-135; and therefore,
significant changes in reactivity. To achieve or maintain a safe and desired
reactor power level, reactor operators must be able to recognize and adjust
for these effects.
Xenon Free Initial Reactor Startup
In a xenon free condition, the reactor core is considered free of xenon-135.
A xenon free condition exists at the beginning-of-core-life prior to reactor
operation, when no xenon has been produced within the core. It can also
occur anytime in core life when the reactor has been shut down long enough
to allow any xenon-135 to completely or almost completely decay away.
This condition occurs approximately 70 to 80 hours after reactor shutdown.
The figure below shows the time required to reach equilibrium xenon
concentration from a xenon free condition for three different power levels:
100 percent, 50 percent, and 25 percent reactor power.
40
Rev 1
Figure: Time to Reach Equilibrium Xenon for Various Power Levels
Reactor startup from a xenon free condition results in the immediate
production of xenon-135 directly from fission. Fission also produces a
large amount of Iodine-135. Over time, additional xenon-135 with a halflife of 6.5 hours is produced as a result of iodine-135 decay. These two
mechanisms lead to an increase in the concentration of xenon-135 in the
core.
Even as the concentration of xenon is building within the core, xenon is
being removed by burnout and decay. Eventually, production and removal
rates for xenon will balance and an equilibrium concentration of xenon-135
exists.
The reactor operator must add positive reactivity by diluting the
moderator/RCS boron concentration or withdrawing control rods without
changing the temperature of the reactor coolant during the buildup of xenon
to maintain a constant power level in the core (criticality).
If steam demand remains constant and the operator takes no action while
xenon concentration is increasing, the change in xenon concentration will
cause a change in the reactor coolant temperature to compensate for the
reactivity added by xenon. With a negative moderator temperature
coefficient, this will result in RCS temperature decreasing to add positive
reactivity.
Reactor Shutdown
When the reactor is shutdown, the total xenon in the core equals the xenon
present at the pre-shutdown equilibrium value and xenon produced by
iodine decay. Since the thermal neutron flux in the reactor is now
essentially zero, xenon removal will be from decay only. The sudden drop
in burnout will cause xenon to peak after shutdown due to production from
iodine decay being greater than removal by xenon decay.
Equilibrium iodine is directly proportional to the power level before
shutdown and like equilibrium xenon, increases as reactor power increases.
The xenon-135 concentration after shutdown is the sum of the xenon-135 at
shutdown, and the xenon-135 produced from iodine-135 decay.
Rev 1
41
Immediately after shutdown, iodine-135 decay will still be producing xenon
at a rate equal to the pre-shutdown power level. Xenon-135 removal will be
from decay only since burnout stops with the reactor shutdown. Therefore,
initially after shutdown, burnup decreases significantly and production from
iodine-135 decay remains almost constant. This causes large xenon peaking
(recall that 95 percent of xenon-135 production is from iodine-135 decay).
The largest peaks occur with reactor trips, not gradual shutdowns, because
there will be some xenon-135 burnout during the shutdown.
The largest magnitude xenon-135 peak after a trip occurs when the reactor
trips from 100 percent power and 100 percent equilibrium xenon and
iodine. The greater the flux level prior to shutdown, the greater the
concentration of iodine-135 at shutdown; therefore, the greater the peak in
xenon-135 concentration after shutdown.
The xenon-135 peak after trip has almost the same xenon concentration
throughout core life for a given power level trip. However, the reactivity of
the peaks gets more negative as the core ages, primarily due to the reduced
competition for thermal neutrons with boron-10 because the boron-10
reduces by dilution.
The equilibrium xenon-135 reactivity for 100 percent power is typically
between -2,500 pcm and -3,000 pcm. The peak xenon-135 reactivity for a
trip from 100 percent power is typically between -4,500 to -5,000 pcm.
For advanced cycle cores with lower thermal flux levels, the time to the
peak is 7.5 hours for a trip from 100 percent power, and the time to the peak
is 5 hours for a trip from 50 percent power. The figure below correlates
better (10 hours to peak from 100 percent) to lower enrichment cores with
higher thermal flux levels.
Figure: Xenon-135 Reactivity After Reactor Shutdown
42
Rev 1
Decrease in Reactor Power
Assume that a reactor is operating at 100 percent power with equilibrium
xenon.
There is an immediate decrease in xenon burnup when reactor power is
decreased from 100 percent to 50 percent power, resulting in an increase in
xenon-135 concentration. The decay rate of xenon-135 remains constant.
The iodine-135 concentration is still at the higher equilibrium level for 100
percent power and is still producing xenon-135 at the previously higher
rate.
The xenon-135 concentration continues to rise at a decreasing rate due to
the decay of iodine-135, until the rate of production of xenon becomes
equal to the rate of removal approximately roughly 8 to 10 hours after the
initial reduction in power level. During this time, the reactor operator will
withdraw control rods or dilute boron to add positive reactivity to adjust for
the negative reactivity added by xenon.
The Xenon-135 concentration then gradually decreases to the new
equilibrium level in about 50 to 60 hours. A greater power change requires
longer times for xenon to reach equilibrium. During this time, the reactor
operator will have to insert control rods or borate to add negative reactivity
to reverse the positive reactivity from the decreasing xenon until
equilibrium xenon is reached.
The magnitude of the decrease in burnout is less than for a reactor trip since
there is still some thermal neutron flux in the reactor on a power decrease.
This causes the magnitude of the peak to be lower and earlier. For example,
a load rejection from 100 percent to 50 percent power will have a xenon135 peak that is lower in magnitude and earlier than the peak following a
trip from 50 percent power.
The more gradual the down power, the lower the xenon-135 peak, and the
earlier the peak will occur because the thermal neutron flux remains higher
for a longer time. There will be no xenon-135 peak if the power gradually
decreases.
Rev 1
43
Figure: Xenon-135 Variations During Power Changes
Increase in Reactor Power
Xenon concentration initially decreases because the burnup of xenon is
increased at the new higher power level when the reactor power is increased
(up power transient). The decay of xenon remains constant.
An immediate increase in the direct production of xenon-135 from fission
also occurs. The burnout term dominates and the concentration of xenon in
the core initially decreases since the direct production of xenon from fission
only accounts for approximately 5 percent of the xenon in the core.
Because 95 percent of the xenon production in the core is from iodine-135
decay, which has a six to seven hour half-life, the production of xenon
remains constant for several hours. After roughly four to six hours,
depending on power levels, the rate of production of xenon from iodine and
fission equals the rate of removal of xenon by burnup and decay. At this
point, the xenon concentration reaches a minimum and begins building to
equilibrium. Equilibrium occurs in 20 to 30 hours.
Note
Note
44
The magnitude and the rate of change of xenon
concentration during the initial four (4) to six (6) hours
following the power change depends on the initial power
level and amount and rate of change in power level. The
most rapid burnout of xenon occurs when a reactor is
startedup and operated at full power while a maximum
peak xenon condition exists. Gradual power increases
prevent the xenon dip.
Rev 1
Increase in Reactor Power
The reactor operator must add negative reactivity by inserting control rods
or borating to maintain constant reactor power levels and stable reactor
coolant temperature during periods of xenon burnout. When xenon
concentration turns and begins increasing, the reactor operator will respond
by adding positive reactivity.
Figure: Xenon-135 Variations During Power Changes
Reactor Startup With Xenon Present
Commercial reactor operations may require a reactor to be started up after a
short-term shutdown and prior to the time where all of the xenon-135 has
decayed. The figure below shows a graph of xenon concentration versus
time after startup with xenon present.
Figure: Xenon Behavior During Reactor Startup With Xenon Present in the
Core
Rev 1
45
The first portion of figure illustrates xenon concentration approaching peak
xenon following a reactor shutdown. At time zero, a reactor startup
commences. The xenon-135 concentration starts to decrease due to burnout
with neutron flux levels increasing once the reactor attains a significant
power level (5 to 10 percent).
This decrease is much faster than normal xenon decay or xenon rate
changes and the operator must monitor conditions carefully, as it may be
necessary to add large amounts of negative reactivity to offset the decrease
in xenon concentration.
This accelerated decrease in xenon concentration during the startup is a
result of two factors:


The burnout rate is very high and is higher than the rate normally
experienced in an up power transient due to the high concentration
of xenon-135.
Iodine-135 and xenon-135 are now produced directly from fission,
but it takes several hours to re-establish equilibrium conditions.
The burnout mechanism for xenon would not alter the xenon removal rate if
the reactor were to be taken critical, but the power level was at about one to
two percent or lower. In this case, the xenon decay curve follows the
normal shutdown decay curve and control of the reactor would be more
stable.
The lag in recovery of xenon-135 concentration during this startup
condition is attributed to the half-life of iodine-135. Although the iodine
concentration starts to recover immediately, the production of xenon from
iodine decay lags behind due to the 6.5-hour half-life of iodine-135.
Knowledge Check
Equilibrium iodine-135 concentration in an operating
nuclear reactor is directly proportional to which of the
following?
46
A.
The rate of decay of xenon-135
B.
The decay constant for iodine-135
C.
The fission reaction rate
D.
The rate of neutron capture by tellurium-135
Rev 1
ELO 2.6 Xenon Oscillations
Introduction
Axial xenon oscillations are common in PWR operations. The main
concern with xenon oscillations is the resultant local fuel power peaking.
Xenon Oscillations
Large thermal reactors with little flux coupling between regions may
experience spatial power oscillations because of the non-uniform presence
of xenon-135. The following four steps describe the mechanism:
1. An initial lack of symmetry in the core power distribution (for
example, individual control rod movement or misalignment)
causes an imbalance in fission rates within the reactor core, and
therefore, in iodine-135 buildup and xenon-135 absorption.
2. In the high-flux region, (higher burnout) xenon-135 burnout
allows the flux to increase further, while in the low-flux region,
the increase in xenon-135 (lower burnout) causes a further
reduction in flux. The iodine concentration increases where the
flux is high and decreases where the flux is low.
3. As soon as the iodine-135 levels build up sufficiently, iodine135 undergoes beta-minus decay to xenon, reversing the initial
situation. Flux decreases in this higher power area will cause
power to decrease, and the former low-flux region increases in
power.
4. Repetition of these patterns leads to xenon oscillations in the
core with periods of about 15 hours.
Xenon oscillations can either converge or diverge. Converging oscillations
die out by themselves. Diverging oscillations continue to grow, unless
corrected by operator action.
Xenon oscillations can be radial (side to side) or axial (top to bottom).
Radial oscillations are uncommon. Any rapid change in the axial power
distribution can cause an axial xenon oscillation. A change in axial power
distribution is identified by the Axial Flux Difference (AFD) indicators and
is also called or known as Delta Flux or Delta I.
The core is not as strongly coupled axially as it is radially; this is the reason
axial flux tilts and xenon oscillations are more likely. Xenon oscillations
can cause AFD to approach and exceed technical specification limits. This
can require a reduction in reactor power to less than 50 percent or in
extreme cases, cause a reactor trip.
Knowledge Check
Which of the following could result in a xenon
oscillation in the core of a nuclear reactor?
Rev 1
47
A.
A change in reactor power level due to a change in steam
demand
B.
Individual control rod insertion
C.
A reactor trip from a high power level
D.
Reactor coolant system flow oscillations
ELO 2.7 Achieving a Xenon-Free Condition
Introduction
This section reviews xenon response following a shutdown or trip.
Following a peak in xenon-135 concentration 7 to 10 hours after shutdown,
the concentration decreases at a rate controlled by the decay of iodine-135,
which has a half-life of 6.5 hours, to xenon-135 and the decay rate of
xenon-135, with a half-life of 9.1 hours.
The xenon-135 concentration will be about equal to the full power
equilibrium xenon concentration (level at the time of trip) approximately 20
to 24 hours after shutdown from full power.
The xenon-135 concentration will have decreased to a small percentage of
its pre-shutdown level, and assuming the reactor is xenon free, introduces
only insignificant error into reactivity calculations about 3 days (70 to 80
hours) after shutdown.
Note that the higher the reactor power level at the start of a shutdown, the
longer it takes to reach a xenon free condition.
Knowledge Check
A commercial pressurized water reactor that has been
operated at 100 percent for several weeks is being shut
down. How much time must elapse prior to a subsequent
startup attempt to ensure that reactivity due xenon-135 is
no longer present in the core?
48
A.
80 hours
B.
40 hours
C.
20 hours
D.
10 hours
Rev 1
ELO 2.8 Control Rod Motion Effects
Introduction
This section discusses the effect xenon has on neutron flux levels with
changes in core reactivity and includes review of the previously covered
topic on xenon oscillations.
Control Rod Motion Effects
When the control rods are inserted a short distance (while maintaining a
constant reactor power), the thermal flux in the top half of the core
decreases while the thermal flux in the bottom half of the core increases.
AFD will go strongly negative. The rate of xenon-135 burnup in the lower
portion of the core increases immediately. This increase exaggerates the
power peak in the bottom of the core over the next several hours, and the
xenon-135 peak (less flux) in the top of the core will further suppress the
power in the top. AFD goes more negative at the same time.
The power peak in the bottom of the core creates a larger iodine-135
concentration in the bottom. About 6.5 hours after power peaks in the
bottom, the xeon-135 from the iodine-135 will produce so much xenon-135
that the power will decrease in the bottom of the core and increase in the
top. At the top, the xenon concentration is low because of the reduced
iodine-135 production at the top. The axial thermal flux and xenon
concentration profiles shown in the next figure (D) will exist in the core
after about 19.5 hours.
The iodine-135 concentration increases and eventually produces a xenon135 peak in the top of the reactor after the power increases in the top of the
reactor, once again suppressing the power in the top to repeat the cycle.
Power oscillates back and forth top to bottom as mentioned previously. The
period of the cycles (peak in top to peak in top, or peak in bottom to peak in
bottom) is 24 to 28 hours.
Rev 1
49
Figure: Thermal Flux Versus Xenon Concentration After Control Rod
Insertion
Rapid down powers can cause axial xenon oscillations. Power distribution
will peak in the top of the core relative to the core average power if rods are
100 percent withdrawn and only boration keeps RCS Tavg on program. This
peak is because RCS Thot decreases far more than RCS Tcold. This adds
more positive reactivity to the top of the core than to the bottom since MTC
is negative and delta temperature change is larger in the top.
This rapid change in power distribution can trigger a xenon oscillation, in
the same way as discussed with control rod insertion. However, the down
power xenon oscillation is opposite in phase to the rod insertion xenon
oscillation. AFD can be kept in a tight band to prevent the xenon oscillation
from starting by inserting the control rods judiciously while the core goes
down in power. Many utilities have reactivity control programs that advise
the operator on how much rod insertion and how much boration to use
during a down power so that AFD is kept in a tight enough band to prevent
a xenon oscillation from starting.
During xenon oscillations, operators must avoid inserting control rods just
as the oscillation causes power to peak at the top of the core. This would
result in the subsequent power peak in the bottom of the core to be greater
in magnitude and occur 12 to 14 hours later. Additionally the power peak
50
Rev 1
in the top of the core that occurs 24 to 28 hours later will also be greater in
magnitude. The correct time to insert control rods is just after power
peaked in the bottom of the core. This dampens the oscillation if performed
correctly.
Core Age Effects
Xenon oscillations converge when neutrons from the high power region are
shifted to the region of high xenon concentration. The neutrons reduce the
size of the xenon-135 peaks by burnout. This in turn dampens the
oscillations. No neutron actually makes it from the top to the bottom of the
reactor or from bottom to top. The shifted neutrons cause fissions in the
center of the reactor, and those fissions release neutrons that cause more
fission. Eventually, the neutrons reach the region of the high xenon
concentration and reduce the size of the peak by burnout.
At beginning of core life, the reactivity effects due to moderator
temperature tend to dampen xenon oscillations and prevent their growth.
Xenon oscillations are more prevalent at the end of core life because the
fuel is mostly depleted in the axial center of the core, and there is less
neutronic coupling or neutron sharing between the upper and lower halves
of the core. The magnitude of these flux shifts could continue to increase
and could result in violations of reactor thermal limits if left unchecked.
Westinghouse created a fuel design that reduces the probability of creating
diverging xenon oscillations. The fuel pellets near the top and bottom of
the fuel element have annular holes. This reduces the disparity in the fuel
concentration at EOL between the edges and the center. It also creates more
space for the helium and fission product gas expansion when clad creep and
fuel pellet swell closes the gap between the fuel pellets and the cladding.
Effects of Xenon On Core Profiles
The production of xenon-135 and its precursor, iodine-135, depends on
thermal neutron flux. Therefore, the production rate of these two isotopes
at any local point in a reactor core depends on the thermal neutron flux level
felt by the fuel at that location in the core. The production and removal
rates and isotopic concentrations of xenon-135 and iodine-135 will not be
uniform since neutron flux is not uniform over the entire volume of the
core. As a result, xenon-135 can produce a local reactivity effect, which
tends to change the thermal neutron flux profile across the core.
The thermal neutron flux will be depressed in areas of the core where local
xenon concentration is relatively high, and will tend to be greater in areas of
the core where local xenon concentration is relatively low. The effect on
the magnitude of thermal flux in these local areas of the core is due to the
effects of xenon-135 on the neutron life cycle. Areas of high xenon-135
concentration absorb a greater number of thermal neutrons, removing them
from the neutron life cycle, as compared to areas of lower xenon-135
concentration.
Rev 1
51
An example of this localized effect of xenon-135 concentration on the
thermal neutron flux profile in a nuclear reactor occurs during a xenon
oscillation.
MTC causes the equilibrium thermal neutron flux to shift to the bottom of
the core during core operation. This flux shift is due to having the colder,
more-dense water near the bottom of the core. The increased moderator
density at the core bottom leads to an increase in the thermalization of
neutrons near the bottom of the core compared to the top of the core. This
causes thermal flux at the bottom of the core to be greater than the thermal
flux at the top.
Equilibrium iodine and xenon concentrations are directly proportional to the
local thermal neutron flux level. The concentration of iodine and xenon is
also greater near the bottom of the core since the thermal flux is greater in
the bottom half of the core from MTC.
Knowledge Check
A nuclear reactor is experiencing a xenon oscillation. At
what time in core life would this xenon oscillation be of
greatest concern from a reactor operational standpoint?
A.
Just after initial core startup.
B.
Early in core life.
C.
Toward the middle of core life.
D.
Late in core life.
TLO 2 Summary
1. Describe fission product poisons and how fission product poisons
affect the neutron life cycle.
 Act as parasitic absorbers of neutrons, removing neutrons from
the neutron life cycle
 An increase in the macroscopic cross-section for absorption by
any neutron poison will result in a decrease in the value of ƒ:
𝑓=

52
∑𝑓𝑢𝑒𝑙
𝑎
𝑝𝑜𝑖𝑠𝑜𝑛
∑𝐹𝑢𝑒𝑙
+ ∑𝑚𝑜𝑑
+ ∑𝑜𝑡ℎ𝑒𝑟
+ ∑𝑎
𝑎
𝑎
𝑎
At equilibrium, production rate of the poison equals the removal
rate of the poison
Rev 1

Depending on the poison, equilibrium levels are power
dependent and time to reach equilibrium is related to both power
and decay rates
2. List the most important fission product poisons to the operation of a
nuclear reactor
 Xenon-135 – 2.6 x 106 barns
 Samarium-149 – 4.0 x 104 barns
 At equilibrium levels, they add considerable negative reactivity:
— Xenon – approximately -3,000 pcm (varies with power)
— Samarium – approximately -700 - 1,000 pcm (100%
BOL/EOL)
3. Explain how xenon-135 is produced and removed in the core of a
nuclear reactor.
 Produced from two sources:
— Directly as a fission fragment
— Beta-minus tellurium-135 decay chain
 Removed by:
— Beta-minus decay to Cesium-135 (Cs-135)
— Neutron capture reaction which creates Xe-136
𝛽 135 𝛽 135
𝛽 135
𝛽 135
135
𝑇𝑒 →
𝐼→
𝑋𝑒 →
𝐶𝑠 →
𝐵𝑎 (𝑠𝑡𝑎𝑏𝑙𝑒)
52
53
54
55
56
19 𝑠𝑒𝑐
6.57 ℎ𝑟
9.10 ℎ𝑟
2.36 × 106 𝑦𝑒𝑎𝑟𝑠

Of the total xenon-135 production:
— Approximately 5% is directly as a fission fragment
— 95% is from the beta-minus decay of I-135
 With 95% coming from I-135, and I-135 having a half-life of 6.6
hours, a significant delay in the production occurs
 Rate of change of the xenon concentration is equal to the rate of
production minus the rate of removal (when these are in balance,
xenon is said to be in equilibrium)
4. Explain the following terms: equilibrium iodine, equilibrium xenon,
transient xenon, peak xenon, xenon free, xenon precluded startup,
xenon dead time.
 Equilibrium iodine is the constant I-135 concentration in the
reactor that occurs after a power change
— Directly proportional to reactor power
— Takes 20 to 25 hours after the power change to reach
equilibrium
 Equilibrium xenon
𝑓𝑢𝑒𝑙
Φ∑
(𝛾𝑋𝑒 + 𝛾𝐼 )
𝑓
𝑁𝑋𝑒 (𝑒𝑞) =
𝑋𝑒
𝜎 Φ + 𝜆𝑋𝑒
𝑎
 Xenon concentration is not liner with power level; at 25%
power, approximately equal to 50% of its 100% power level
— At low powers, decay is the major removal term
— At high powers, burnout is the major removal term
Rev 1
53

Transient xenon
— When a power change in the reactor occurs, Xe-135
undergoes a transient
— Takes 40 to 50 hours after the power change for Xe-135 to
reach equilibrium
— Negative reactivity due to Xe-135 is directly proportional
to the Xe-135 concentration.
 Peak xenon
— Anytime the reactor undergoes a rapid down power, Xe135 concentration will peak and then decrease to a new
lower equilibrium value or go to zero (on a shutdown)
— A reactor trip is the most rapid down power, and therefore
results in the largest xenon peaks
— Xenon peaks because the thermal neutron flux goes down
with power, reducing the burnout rate
— Xe-135 production from I-135 decay remains almost the
same for a period of time (6.5 hour half-life) adding to the
xenon peak
— Peak xenon is reached in approximately 6 to 10 hours after
a shutdown
 Xenon precluded S/U and dead time
— Following a reactor trip, xenon peaks
— The period of time where the reactor is unable to override
the effects of xenon is called xenon dead time
5. Explain how xenon-135 concentration reacts during the following
nuclear reactor operations:
 Xenon-free initial reactor startup
— The greater the flux level prior to shutdown, the greater the
concentration of iodine-135 at shutdown; therefore, the
greater the peak in xenon-135 concentration after
shutdown
 Reactor S/U with xenon present
— When reactor attains a 5-10% power, xenon-135 starts to
decrease due to burnout with increasing flux levels
— This decrease is much faster than normal xenon decay or
xenon rate changes and must be monitored very carefully
as very large reactivity addition rates can be created
6. Describe the causes and effects of a xenon oscillation.
 May be caused by a rapid change in the core power distribution
— Can change local power levels in the core by a factor of
three or more
— More prevalent at the end of core life because the fuel is
mostly depleted in the axial center of the core, and there is
less neutronic coupling between the upper and lower
halves of the core
7. State the approximate time following a reactor shutdown at which the
reactor can be considered "xenon free."
 3 days (70 to 80 hours) after shutdown, xenon-135 concentration
has decreased to a small percentage of its pre-shutdown level
54
Rev 1

The higher the reactor power level at the start of a shutdown, the
longer the time required to reach a xenon-free condition
8. Explain the effects of xenon concentration on a nuclear reactor core's
thermal flux profile for the following:
 Control rod motion
— When inserted a small amount (while maintaining a
constant reactor power), the thermal flux in the top half of
the core decreases, while the thermal flux in bottom half of
the core increases (AFD goes strongly negative)
— Control rod movement can result in the concentration of
xenon-135 oscillating between the top and bottom portions
of the core, resulting in a change in the axial thermal flux
profile within the core
 Core life
— At BOL, the reactivity effects due to MTC tend to dampen
xenon oscillations
— Xenon oscillations are more prevalent at EOL because fuel
is mostly depleted in the axial center of the core
— Less neutronic coupling (neutron sharing) between the
upper and lower halves of the core
— In areas of the core where local xenon concentration is
relatively high, the thermal neutron flux will be depressed
— In areas of the core where local xenon concentration is
relatively low, the thermal neutron flux will tend to be
greater
Objectives
Now that you have completed this lesson, you should be able to do the
following:
1. Describe fission product poisons and how fission product poisons
affect the neutron life cycle.
2. List the most important fission product poisons to the operation of a
nuclear reactor.
3. Explain how xenon-135 is produced and removed in the core of a
nuclear reactor.
4. Explain the following terms:
a. Equilibrium iodine
b. Equilibrium xenon
c. Transient xenon
d. Peak xenon
e. Xenon free
f. Xenon precluded startup
g. Xenon dead time
5. Explain how xenon-135 concentration reacts during the following
nuclear reactor operations:
a. Xenon free initial reactor startup
b. Reactor shutdown
c. Decrease in reactor power
Rev 1
55
d. Increase in reactor power
e. Reactor startup with xenon present in the core
6. Describe the causes and effects of a xenon oscillation.
7. State the approximate time following a reactor shutdown at which the
reactor can be considered xenon free.
8. Explain the effects of xenon concentration on a nuclear reactor core's
thermal flux profile for the following:
a. Control rod motion
b. Core life
TLO 3 Samarium-149
Overview
This section includes discussion and comparison of the reactivity effects
from samarium-149 (samarium-135 and xenon-135. Samarium-149 effects
on the neutron life cycle are much less significant than xenon-135 although
it is the second largest fission product poison. Samarium is not as visible to
the operator as xenon; however, it is important for the operator to
understand and predict its response to reactor transients. This chapter
discusses the samarium-149 effects in a reactor.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Explain how samarium-149 is produced and removed from the reactor
core during reactor operation.
2. Describe equilibrium samarium-149 concentration.
3. Explain how equilibrium samarium-149 concentration varies with the
following reactor operations:
a. Initial reactor startup
b. Reactor shutdown
c. Reactor startup after shutdown
4. Describe the effects of samarium-149 concentration on reactor
operation over core life.
5. Compare the effects of samarium-149 to the effects of xenon-135 on
reactor operation.
ELO 3.1 Samarium Production and Removal
Introduction
Xenon-135 and samarium-149 are the most significant fission product
poisons. This section explains where samarium comes from and how it is
removed.
Samarium Production
Negligible amounts of samarium-149 are produced directly from fission.
However, 1.1 percent of all fissions result in the production of either
56
Rev 1
neodymium-14 or promethium-149. Samarium-149 is the end product of a
decay chain containing these isotopes. The decay chain is shown below:
𝛽− 149
𝛽− 149
149
𝑁𝑑 →
𝑃𝑚 →
𝑆𝑚
60
61
62
1.73 ℎ𝑜𝑢𝑟𝑠 53 ℎ𝑜𝑢𝑟𝑠
Both neodymium and promethium decay by beta-minus; however, the halflife of promethium-149 is much longer than neodymium-149 and all other
149 precursors (other fission products that decay to neodymium-149).
Consequently, the time dependence behavior of samarium-149 can be
explained by assuming that the only samarium-149 precursor is
promethium-149 with a yield of 1.1 percent. This is similar to tellurium135 in the xenon-135 chain.
Samarium-149 Removal
Samarium-149 has half-life of 1016 years and is considered stable.
Therefore, the only removal mechanism for samarium-149 is by neutron
capture (burnout). Neutron capture results in the conversion of samarium149 to samarium-150 as shown below:
149
1
150
𝑆𝑚 + 𝑛 →
𝑆𝑚 + 𝛾
62
0
62
The microscopic cross-section for absorption for samarium-149 is 4.1 x 104
barns. Samarium-150 is also stable, but has a low absorption cross-section
for neutrons of approximately 103 barns.
Knowledge Check
Samarium-149 is removed from a reactor by which one
of the following processes?
Rev 1
A.
It is stable, so it is only removed when the reactor is
defueled
B.
Neutron capture reactions producing samarium-150
C.
Beta-minus decay to promethium-149
D.
Beta-minus decay to neodymium-149
57
ELO 3.2 Equilibrium Samarium
Introduction
This chapter discusses equilibrium concentrations of promethium-149 and
samarium-149 and shows how equilibrium Samarium-149 is independent of
power level.
Equilibrium Samarium
The equilibrium samarium-149 concentration is independent of power level;
time to attain equilibrium is not. The 1.73-hour half-life of neodynium-149
is significantly shorter than the 53.1-hour value for promethium-149.
Therefore, promethium-149 may be considered a direct fission product.
This assumption, neglecting a small amount of promethium burnup, is
shown as:
Rate of change of promethium-149 = yield from fission – decay of
promethium-149
𝑑𝑁𝑃𝑚
𝑓𝑢𝑒𝑙
= 𝛾𝑃𝑚 ∑
Φ − 𝛷𝑃𝑚 𝑁𝑃𝑚
𝑓
𝑑𝑡
Where:
NPm = promethium-149 concentration
γPm = promethium-149 fission yield
λPm = decay constant for promethium-149
Solving for the equilibrium value of promethium-149 gives the following:
𝛾𝑃𝑚 ∑
𝑁𝑃𝑚 (𝑒𝑞) =
𝑓𝑢𝑒𝑙
𝛷
𝑓
𝜆𝑃𝑚
The rate of samarium-149 formation is:
Samarium-149 rate of change = yield from fission + promethium-149 decay
- samarium-149 burnup.
𝑑𝑁𝑆𝑚
𝑓𝑢𝑒𝑙
𝑆𝑚
= 𝛾𝑆𝑚 ∑
𝛷 + 𝜆𝑃𝑚 𝑁𝑃𝑚 − 𝑁𝑆𝑚 𝜎 𝛷
𝑓
𝑎
𝑑𝑡
Where:
58
Rev 1
NSm = samarium-149 concentration
γSm = samarium-149 fission yield
σaSm = microscopic absorption cross-section of samarium-149
Since the fission yield of samarium-149 is nearly zero (0), the equation
shortens:
𝑑𝑁𝑆𝑚
𝑆𝑚
= 𝜆𝑃𝑚 𝑁𝑃𝑚 − 𝑁𝑆𝑚 𝜎 𝛷
𝑎
𝑑𝑡
By substituting the equilibrium concentration of promethium-149 (shown
below),
𝛾𝑃𝑚 ∑
𝑁𝑃𝑚 (𝑒𝑞) =
𝑓𝑢𝑒𝑙
𝛷
𝑓
𝜆𝑃𝑚
into the formula, and then solving for the equilibrium concentration of
samarium-149:
𝑓𝑢𝑒𝑙
𝛷
𝑓
𝑆𝑚
𝜎
𝑎
𝛾𝑃𝑚 ∑
𝑁𝑆𝑚 =
The formula above shows that equilibrium samarium-149 concentration is
independent of neutron flux and power level. Therefore, the samarium
concentration undergoes a transient following a power level change, but it
returns to its original value.
The decay of promethium-149 to samarium-149 has a half-life of 53 hours.
The decay of neptunium-239 to plutonium-239 has a half-life of 56.4 hours
(fuel conversion). Conversion to plutonium-239 adds positive reactivity,
while samarium-149 production adds negative reactivity at close to the
same decay rate. These two reactivities are close in magnitude and offset
each other. Because of this offset, we ignore the reactivity effects of
building samarium-149 to equilibrium.
However, the samarium-149 and plutonium-239 conditions in a reactor
shutdown are different. There is a relationship between samarium-149
Rev 1
59
peaks during a reactor shutdown and plutonium-239; however, these
reactivities do not offset the same way. A later chapter covers this in detail.
Knowledge Check
Similar to the fact that iodine-135 must be in equilibrium
for xenon-135 to reach equilibrium, promethium-149
must be in equilibrium for samarium-149 to reach
equilibrium.
Which one of the following is correct concerning
equilibrium samarium-149 concentration?
A.
Equilibrium samarium-149 concentration at 10 percent
power is much less than at 100 percent power.
B.
Equilibrium samarium-149 concentration at 50 percent
power is approximately half of the equilibrium
samarium-149 concentration at 100 percent power.
C.
Promethium-149 concentration has to reach equilibrium
before samarium-149 can reach equilibrium.
D.
Since samarium-149 is stable, it takes years of reactor
operation before it reaches equilibrium.
ELO 3.3 Samarium Concentration Transients
Introduction
This section on samarium concentration transients discusses the samarium149 and promethium-149 response for startup and shutdown scenarios.
Initial Reactor Startup
Power level affects the amount of time required to reach equilibrium
samarium although equilibrium is independent of power level.
Promethium-149 production begins with no samarium-149 present when
taking a new core critical and increasing power for the first time. As
promethium-149 builds, it decays to samarium-149. The increase in
samarium-149 is shown below in the figure.
60
Rev 1
Figure: Samarium-149 Buildup to Equilibrium
Samarium-149 concentration reaches equilibrium in about 20 to 25 days if
the reactor is operated at significant power levels (time to equilibrium is
power dependent). It takes longer to reach equilibrium samarium-149 at
lower power levels. The concentration remains essentially constant during
reactor operation and the reactor is never again samarium free since
samarium-149 is considered stable due to its extremely long half-life of
10-16 years. This means that reused fuel contains samarium-149.
Neptunium-239 is decaying to plutonium-239 during the decay of
promethium-149 to samarium-149. During this period, the operator may
not need to make reactivity adjustments for the increasing samarium-149
since these two reactivities approximately offset each other. However, the
operator will need to make adjustments for the xenon-135 buildup. It would
be hard to differentiate reactivity effects from samarium-149. Actual
reactivity from equilibrium samarium-149 is about -700 to -1,000 pcm
(BOL to EOL).
Reactor Shutdown
Samarium-149 peaks after shutdown due to the decay of equilibrium
promethium-149 (no samarium decay and burnout stops). After shutdown,
the size of the peak depends on promethium-149 concentration: the greater
the concentration of promethium-149 in the core, the higher the peak.
Therefore, the samarium-149 peak depends on the power level prior to
shutdown (equilibrium promethium-149).
Over time, the promethium-149 completely decays to samarium-149, but
the samarium-149 peak levels off at a constant higher value since it does not
decay (power is zero (0) percent, so no burnout). It takes about 20 days
after shutdown for samarium-149 to peak and level. The figure below
shows the samarium response over time.
Rev 1
61
Figure: Behavior of Samarium-149 in a Pressurized Water Reactor
The rate of samarium-149 production at shutdown is:
𝑑𝑁𝑆𝑚
= 𝜆𝑃𝑚 𝑁𝑃𝑚
𝑑𝑡
plutonium-239 also peaks after shutdown from the decay of neptunium-239.
An operator may sum these two reactivity effects, along with other minor
effects, into one curve of reactivity added during shutdown to define overall
effects. The operator uses this combined curve in shutdown margin
calculations and subsequent estimated critical position calculations.
With these two reactivities combined, the net reactivity change can be
positive or negative. Typical values range from + 60 pcm to -250 pcm.
Usually, the largest effect is from samarium-149. Both samarium-149 and
plutonium-239 are stable and remain in the used fuel after shutdown.
Actual reactivity added from samarium-149 peaking is approximately -860
pcm to -1,300 pcm (100 percent power trip). As previously stated, this is
power dependent, the higher the power history at shutdown, the higher the
peak.
Reactor Startup after Shutdown
The samarium-149 peak from shutdown will immediately start to burn
down to the equilibrium samarium-149 concentration after startup and
increasing thermal flux levels. There is a delay in the significant formation
of additional samarium-149 due to the low inventory and long half-life of
promethium-149.
The figure below shows the samarium response over time.
62
Rev 1
Figure: Behavior of Samarium-149 in a Pressurized Water Reactor
Samarium-149 burnout continues for several days, and for a short time,
samarium actually decreases below its equilibrium level until promethium149 production and decay rates can fully restore samarium-149 to
equilibrium.
The plutonium-239 peak also burns down, offsetting most of the samarium149 reactivity effect. An operator is unlikely to see the samarium-149
reactivity effect with xenon-135 reactivity changes occurring
simultaneously.
Knowledge Check
Every samarium-149 transient is partially offset in terms
of reactivity by a transient of which of the following?
A.
Plutonium-239 from neptunium-239 decay
B.
Plutonium-241 from neptunium-241 decay
C.
Plutonium-242 from neptunium-242 decay
D.
Uranium-233 from protactinium-233 decay
ELO 3.4 Samarium-149 Effects Over Core Life
Introduction
This session provides information about samarium-149 effects over core
life.
Rev 1
63
Samarium-149 Effects Over Core Life
The greatest change in samarium-149 concentration occurs immediately
following the initial startup of a new reactor core that contains no reused
fuel. However, samarium-149 and other unspecified poisons continue to
increase in concentration throughout core life and remain in the core. This
continued increase provides a constant source of negative reactivity.
Reactivity values due to equilibrium and peak samarium-149 become
increasingly more negative as the core ages. There is less thermal neutron
competition from the decreasing concentration of soluble boron (boron-10)
and from the depleting burnable poisons, as well as increasing thermal
neutron flux levels over core life. While samarium-149 reactivity should
not prevent a restart, its value is greatest at EOL.
The negative reactivity added to the core by samarium-149 combined with
the lack of positive reactivity due to fuel burnout late in core life could
prevent an operator from taking the reactor critical.
Knowledge Check
At what point in core life will samarium-149
concentration in the core of a nuclear reactor have the
greatest impact on the operator’s ability to start up the
reactor?
A.
Toward the end of core life
B.
Toward the middle of core life
C.
Toward the beginning of core life
D.
Samarium-149 is never a concern during a reactor startup
ELO 3.5 Xenon-135 to Samarium-149 Comparison
Introduction
This section gives a brief review and overview of the differences between
xenon and samarium neutron poisons.
Xenon-135 to Samarium-149 Comparison
Xenon-135 has a significant effect on the net reactivity of the core. Starting
up, or shutting down, the operator must consider and compensate for xenon135 reactivity whether the reactor is operating at power. The estimated
critical position must estimate xenon concentration at the time of expected
criticality if xenon concentration is changing during a reactor startup.
64
Rev 1
Shutdown margin calculations must also consider xenon. Power transients,
especially up power, can add a considerable amount of reactivity from
xenon burnout. The reactor operator must always be aware of the reactivity
magnitude and change rate of xenon.
Samarium neutron effects are minor when compared to xenon. Plutonium239 production from neptunium-239 decay offsets the samarium reactivity
and the samarium reactivity is smaller in magnitude and changes at a much
slower rate. The samarium effect on reactor operations, estimated critical
position calculations, and shutdown margin are small as a result.
Comparison Between Xenon-135 and Samarium-149
The following table provides a comparison between xenon-135 and
samarium-149.
Effect
Xenon-135
Samarium-149
2.6 x 106 Barns
Microscopic CrossSection for
Absorption (σa)
4.1.x 104 Barns
Time to Peak
Concentration
Square Root of Power
Prior to Shutdown
(S/D) or Trip
≈ 20 Days
Time to Equilibrium
Concentration
40 to 48 Hours
25 to 35 Days
Reactivity Worth
(These Are
Approximate
Reactivity Values —
Change Over Core
Life)
-2.7 Percent Δk/k at
100 Percent Power
Equilibrium
-0.7 Percent Δk/k at
Power Equilibrium
-4.7 Percent Δk/k at
Peak
-1.1 Percent Δk/k at
Peak
Removal by Decay
Yes
No
Equilibrium
Dependent on Power
Yes
No
Distribution Problem
(Oscillations)
Yes
No
Rev 1
65
As seen in the chart above, samarium-149 presents a much smaller
operational problem for a reactor than xenon-135. Compared to xenon, the
negative reactivity added by samarium is much smaller, and changes related
to the concentration and reactivity of samarium occurs over a period of days
rather than hours.
Additionally, the negative reactivity value attributable to xenon-135 at
equilibrium concentration is nearly four times that of samarium-149 at 100
percent reactor power.
The negative reactivity value of xenon-135 at its peak is comparable to the
amount of positive reactivity required for a reactor to operate approximately
one year ( 5 percent Δk/k).
Samarium reaches a peak after a reactor shutdown and remains at that peak,
since it does not decay. Xenon reaches a peak after a reactor shutdown and
then slowly decays.
A reactor operator is seldom aware that samarium exists, as compared to
xenon. Xenon can present significant operational problems, especially if an
operator attempts reactor startup when xenon concentration is near its peak
value.
Other Fission Product Poisons
Numerous other fission products have a neutron poisoning effect on reactor
operations because of their concentrations and thermal neutron absorption
cross-sections. Individually, these effects are small, but when combined,
they have a more significant impact. These may accumulate at an average
rate of 50 barns per fission event in the reactor.
Knowledge Check
A commercial pressurized water reactor experienced a
reactor trip from 100 percent reactor power. Which of
the following would contribute the greatest magnitude of
negative reactivity to the reactor core, assuming the
reactor remains shutdown for an extended period of
time?
66
A.
Samarium-149 concentration approximately 12.5 days
after the trip
B.
Smamarium-149 concentration 20-25 days after the trip
C.
Xenon-135 concentration approximately 10 hours after
the trip
D.
Xenon-135 concentration approximately 70 hours after
Rev 1
the trip
TLO 3 Summary
1. Samarium-149 is produced directly from fission and from the decay of
promethium-149 during reactor operation.
𝛽− 149
𝛽− 149
149
𝑁𝑑 →
𝑃𝑚 →
𝑆𝑚
60
61
62
1.73 ℎ𝑜𝑢𝑟𝑠
53 ℎ𝑜𝑢𝑟𝑠

Samarium-149 is removed from the core by neutron absorption;
it does not decay
149
1
150
𝑆𝑚 + 𝑛 →
𝑆𝑚 + 𝛾
62
0
62

2. The equation for equilibrium samarium-149 concentration is:
3.𝑁𝑆𝑚 =
𝑓𝑢𝑒𝑙
𝑓
𝑆𝑚
𝜎
𝑎
𝛾𝑃𝑚 ∑

Independent of power level (time to achieve equilibrium is not)
3. During initial reactor startup, production of promethium-149 begins (to
produce samarium), promethium-149 decays to samarium-149.
 Samarium equilibrium concentration is reached in about 25 to 35
days if the reactor is operated at significant power levels
 After a reactor shutdown, the samarium-149 concentration
increases due to the decay of the promethium-149 inventory in
the core and the loss of the burnup factor
 Samarium-149 concentration decreases as samarium is burned
up and the delay in promethium-149 producing samarium-149
catches up to return samarium to equilibrium if the reactor is
restarted following a shutdown
4. Samarium-149 remains in core unless burned out, presenting a constant
source of negative reactivity.
 At end of core life, available positive reactivity from fuel, etc.
may be insufficient to override added negative reactivity from
samarium-149 (and xenon-135)
5. Effects of xenon versus samarium
Effect
Xenon-135
Samarium-149
Microscopic CrossSection for
Absorption (σa)
2.6 x 106 Barns
4.1.x 104 Barns
Rev 1
67
Effect
Xenon-135
Samarium-149
Time to Peak
Concentration
Square Root of Power
Prior to Shutdown
(S/D) or Trip
≈ 20 Days
Time to Equilibrium
Concentration
40 to 48 Hours
25 to 35 days
Reactivity Worth
-2.7 Percent Δk/k at
100 Percent Power
Equilibrium
-0.7 Percent Δk/k at
Power Equilibrium
-4.7 Percent Δk/k at
Peak
-1.1 Percent Δk/k at
Peak
Removal by Decay
Yes
No
Equilibrium
Dependent on Power
Yes
No
Distribution Problem
(Oscillations)
Yes
No
Now that you have completed this lesson, you should be able to do the
following:
1. Explain how samarium-149 is produced and removed from the reactor
core during reactor operation.
2. Describe equilibrium samarium-149 concentration.
3. Explain how equilibrium samarium-149 concentration varies with the
following reactor operations:
a. Initial reactor startup
b. Reactor shutdown
c. Reactor startup after shutdown
4. Describe the effects of samarium-149 concentration on reactor
operation over core life.
5. Compare the effects of samarium-149 to the effects of xenon-135 on
reactor operation.
68
Rev 1
Neutron Poisons Summary
Objectives
Now that you have completed this module, you should be able to
demonstrate mastery of this topic by passing a written exam with a grade of
80 percent or higher on the following TLOs:
1. Describe fuel depletion for a nuclear reactor and explain the fuel
depletion effects on reactivity over the life of the core.
2. Describe fission product poisons and how fission product poisons
affect the neutron life cycle.
3. Explain how samarium-149 is produced and removed from the reactor
core during reactor operation.
Rev 1
69