FISSION PRODUCT POISON DEFINITION:
FISSION PRODUCT POISON PRODUCTION/REMOVAL: XENON (Xe) PRODUCTION:
1) Relatively high yield from fission (recall fission yield curve)
- There’s a lot of them!
- Xenon (Xe-135) and Samarium (Sm-149) mass numbers are towards
top of fission yield curve of larger fission product
2) High microscopic cross section for absorption
- Xe-135 - 2.6 x 106 Barns
- Sm-149 - 4.0 x 104 Barns
3) Does not fission
As Fission Product Poison concentrations increase, Thermal Utilization
Factor (f) drops, causing a negative reactivity affect.
- The Production is either directly from fission or decay of a
fission fragment
- The Removal is either from radioactive decay or neutron
absorption
- The Production Rate minus the Removal Rate equals the rate
of change of concentration
- The concentration is dependent on Power Level and various
decay characteristics
From I Decay: λINI (Decay Constant of Iodine times the amount of Iodine; half-life is 6.6 hrs)
Xenon (Xe-135):
XENON (Xe) REMOVAL:
Fuel
Σa
f=
Fuel
Σa +
Mod
Σa +
Poisons
Σa
It’s all about competition!
- Mass Number near peak of fission product yield curve
- About 0.3% of all fissions produce Xe-135 directly
- Xe-135 is also product of radioactive decay chain
(Te-135 to I-135 to Xe-135)
- Overall, produced in about 5.6% of all fissions
- Production of Xe-135 through decay is as follows:
βββ135
135
135
135
51 Sb
19 sec
I
53
6.6 hrs
54 Xe
XE CONCENTRATION DURING POWER CHANGES:
POWER DECREASE TO 50%
POWER
INCREASE
TO 100%
100% EQUILIBRIUM
XENON
XENON CONCENTRATION
EQUILIBRIUM XENON:
- Where production rate equals removal rate
- Takes about 40 - 50 hours to reach equilibrium
- Xenon concentration is directly related to flux
(reactor power)
- Equilibrium Xe levels at 50% power: -2100 pcm
- Equilibrium Xe levels at 100% power: -2700 pcm
- Reason it is NOT twice the 50% level is because the burnout
rate lessens the effect of the production rate at 100% power
- Xe-135 concentration numbers at 100% power are approximately:
- 5% production directly from fission
- 95% production from decay of Iodine
- 75% removal from neutron absorption
- 25% removal from xenon decay
1.7 sec
52 Te
POWER
INCREASE
TO 100%
50% EQUILIBRIUM
XENON
5 hrs
30-40 hrs
40-50 hrs
TIME (HOURS)
- Negligible amounts of Sm-149 are produced directly from fission
- 1.1% of all fissions result in production of Nd-149 or Pm-149
(decay chain of about 55 hours)
- Sm-149 has half-life of 1016 years and is considered stable
(no decay removal component)
- Once in the core, never Sm-149 free
SAMARIUM (Sm) PRODUCTION:
From Fission: γPmφΣ fFuel
From Promethium Decay: λPmNPm
Total Production: Two components above
SAMARIUM (Sm) REMOVAL:
From Absorption (Burnout): σ aSm NSmφ
From Decay: None (long half-life)
Total Removal Rate: Burnout term
EQUILIBRIUM SAMARIUM:
- Production of Promethium equals removal of SM-149 (burnout)
- The equilibrium value for Samarium concentration does not depend
upon flux, and therefore, does not depend upon power level
- Reactivity worth of equilibrium SM-149 is about -1000 pcm
Fuel
Total Production: γXeφ Σ f + λINI
(Microscopic cross section for absorption of Xenon times the amount of Xenon
From Absorption: σ a NXeφ times the flux; once Xe-135 absorbs a neutron, it is essentially gone because
Xe
Xe-136 has a very small microscopic cross section for absorption)
From Decay:
λXeNXe (Decay Constant of Xenon times the amount of Xenon; half-life is 9.1 hrs)
Total Removal Rate: σ a NXeφ + λXeNXe
Xe
XENON (Xe) CONCENTRATION AFTER SHUTDOWN/TRIP:
- When a reactor is shutdown, the neutron flux is reduced essentially to zero
- Xe-135 is no longer produced by fission and is no longer removed by burnup
- The only remaining production mechanism is the decay of the iodine-135
- The only removal mechanism for xenon-135 is decay
- Because the decay rate of iodine-135 is faster than the decay rate of xenon-135, the xenon concentration builds
to a peak (about -4700 pcm if tripped from 100% power)
- The time to “peak” is slightly < the square root of the power from the trip (7-9 hours at 100% power)
- Following the peak , the Xe-135 concentration will decrease at a rate controlled by the decay of I-135
- Approximately 24 hours from a trip from 100% power, Xe-135 concentration returns to the value prior to the trip
- It takes about 72 hours (3 days) for Xe-135 to all decay away (Xenon free)
XENON (Xe) CONCENTRATION DURING POWER TRANSIENT:
5-6 hrs 30-40 hrs
40-50 hrs
SAMARIUM (Sm-149):
Fuel
From Fission: γXeφ Σ f (Fission Yield of Xe, times the flux, times the macroscopic cross section of fuel for fission)
- On an uppower from 50% to 100%, initially Xe-135 concentration will drop because of the burnout effect from
the increase in flux. Eventually the increased concentration of I-135 will lead to an increase in Xe-135
- On a downpower from 100% to 50%, initially Xe-135 concentration goes up for the same reason as the trip
mentioned above, but will eventually drop due to the reduced production of I-135
XE-135 CHANGES OVER CORE LIFE:
- Xe-135 Concentration DECREASES slightly over core life (BOL to EOL):
- A couple of things we already know: Xe-135 is produced from the fission
of U-235, and it is flux dependent
- We also know that as fuel is burned out from BOL to EOL, we need more
neutrons passing a given area (flux) to cause the number of fissions to
maintain 100% power; therefore flux increases from BOL to EOL
- Based on the formula below, we can see that Xe-135 concentration
decreases from BOL to EOL (about 10% change in flux from BOL to EOL)
- Xe-135 Worth INCREASES slightly over core life (from BOL to EOL):
- Even though Xe-135 concentration decreases slightly over core life, the
worth of Xe-135 increases because there is less competition (boron
concentration decreases over core life to compensate for fuel burning out)
with Xe-135 to absorb neutrons
XENON OSCILLATIONS:
- Since Xe-135 is power dependent, Xe-135 concentrations affect power/flux changes
- If something happens to change flux (rod inserting, for example), a xenon oscillation is possible
- Rod insertion depresses the flux down in the core
- Since there is less flux in the top of the core, there is less Xe-135 burnout, causing flux to drop
even more
- For a given power level, flux will have to go up in the bottom of the core, which increases iodine
concentration
- After about 6.5 hours, I-135 starts decaying to produce Xe-135, increasing the concentration at the
bottom of the core
- Flux now starts to lower at the bottom
- At the top of the core, Xe-135 is decaying, leading to an increase in flux
- You can now see the oscillation of flux caused by xenon concentrations
- One of the concerns is the magnitude of the flux shifts will continue to increase and could cause
local linear heat rate limits to be exceeded
- At BOL, reactivity effects of moderator temperature tend to dampen oscillations and prevent
their growth
- Xenon oscillations are more prevalent at EOL because fuel is mostly burned out in axial center
of core, and there is less neutronic coupling between two core halves
- One of the methods that can be used to dampen xenon oscillations is to bump rods in when the
flux starts to shift to the top of the core, or bump them out when the flux shifts to the bottom
OPS Generic Fundamentals - PWR
192006 - Fission Product Poisons
Rev Date: May 17, 2017
For Training Use Only