Operator Generic Fundamentals
Reactor Theory – Reactor Operational Physics
© Copyright 2014
Operator Generic Fundamentals
2
Reactor Operational Physics
• This module introduces the student to actual PWR reactor
operations. Among the topics covered are:
– Estimating critical conditions
– 1/M plots
– Identifying criticality
– Response to steam demand
– Use of control rods
– Use of boron
– Reactor trip response
– Decay heat
© Copyright 2014
INTRO
Operator Generic Fundamentals
3
Terminal Learning Objectives
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 on the following TLOs:
1. Explain the use of the estimated critical position calculation and
nuclear instrumentation during reactor start-up.
2. Describe the operation of a nuclear reactor during startup.
3. Describe the operation of a nuclear reactor during power range
operation.
4. Describe the characteristics of and process used when performing
a reactor shutdown.
© Copyright 2014
INTRO
Operator Generic Fundamentals
4
Reactor Critical Conditions
TLO 1 – Explain the use of the estimated critical position (ECP)
calculation and nuclear instrumentation during reactor start-up.
• It is important for the reactor operator to have a clear understanding
of the theory and practices used for performing an estimate of critical
conditions (ECC) and reactor startup.
• This chapter covers the following topics related to reactor startups:
– Determining target values for control rod height and boron
concentration when the reactor will be critical on a start-up
– Nuclear instrumentation response
© Copyright 2014
TLO 1
Operator Generic Fundamentals
5
Estimated Critical Conditions (ECC/ECP)
• Estimating critical conditions in a reactor is essential for safe and
controlled startup
• Important for:
– Reactor safety
– Verification of core design
– Knowledge of when to expect criticality on a reactor startup
• Included is an explanation of the nuclear instrument response on a
reactor startup
© Copyright 2014
TLO 1
Operator Generic Fundamentals
6
Enabling Learning Objectives for TLO 1
1. Describe the reactivity variables involved in an estimated critical
position calculation and how they are used to predict criticality.
2. Describe the nuclear instrumentation response during a reactor
startup to criticality.
© Copyright 2014
TLO 1
Operator Generic Fundamentals
7
Estimated Critical Conditions Calculations
ELO 1.1 – Describe the reactivity variables involved in an estimated
critical position calculation and how they are used to predict criticality.
• Performing a safe reactor startup requires consideration of all core
reactivities:
– Desired rod height and boron concentration
– Plant parameters to ensure criticality is possible
– Expectations of when the reactor should "go critical”
(Anticipate criticality at any time during a S/U!)
• 1/M plot during actual startup validates the ECC
• Called an ECC, estimate of critical conditions, or ECP, estimated
critical position
– Same concept, different names
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
8
Estimated Critical Conditions Calculations
• Provide an added margin of safety for two main reasons:
1. Identification of critical boron concentration and control rod
position
2. Check that reactor core is performing as designed
• An ECP/ECC is simply a mathematical calculation accounting for
changes in reactivities associated with:
– Time since shutdown
– Temperature
– Core life
– Samarium and xenon poisoning
– Boron concentration
– Rod position
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
9
Estimated Critical Conditions Calculations
• Many plants may have desired rod positions for criticality with
boron concentration adjusted for positions or choose to minimize
boron concentrations by having an acceptable range of rod
positions
• On longer shutdowns:
– Xenon will have decayed off to almost zero
– Large amount of positive reactivity added to ECC/ECP
calculation
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
10
Estimated Critical Conditions Calculations
• Reactivities considered when calculating an ECP include:
– Critical control rod position
– Boron concentration
– Time in core life
– Power defect (if critical data was at power)
– Fission product poisons, xenon and samarium reactivity
• PWRs have a minimum temperature for criticality and normal band
for RCS pressure and temperature (NOPT)
– These reactivities not normally considered
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
11
Estimated Critical Conditions Calculations
• Plant curve book and computer data used to obtain values for
reactivities
• Other procedures utilized to determine amount of water or boric
acid needed to adjust the RCS boron concentration
• Actual critical conditions during startup must be within a certain
reactivity amount of the ECC
• Reactor engineering tracks actual to design data
• 1/M predictions must track within established band
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
12
Estimated Critical Conditions Calculations
Knowledge Check
Near the end of core life, critical rod position has been calculated for a
nuclear reactor startup 4 hours after a trip from 100 percent power
equilibrium conditions. The actual critical rod position will be lower
than the predicted critical rod position if...
A. the RCS temperature is maintaining 3 degrees higher than its
normal percent power temperature.
B. actual boron concentration is 10 ppm lower than the assumed
boron concentration.
C. one control rod remains fully inserted during the approach to
criticality.
D. the startup is delayed until 8 hours after the trip.
Correct answer is B.
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
13
Estimated Critical Conditions Calculations
Knowledge Check
To predict critical control rod position prior to commencing a nuclear
reactor startup, which of the following would have the greatest
negative reactivity effect 2 weeks after a trip from 100 percent power
(reactor has been maintained in hot shutdown conditions):
A. RCS boron concentration
B. Samarium
C. Power defect
D. Xenon
Correct answer is B.
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
14
Group Project
• Perform an ECC for your plant using your plant’s procedure and
data supplied by your instructor
• Compare and discuss outcome
© Copyright 2014
ELO 1.1
Operator Generic Fundamentals
15
Reactor Startup Nuclear Instrumentation
ELO 1.2 – Describe the nuclear instrumentation response during a
reactor startup to criticality.
• This section describes the response of the nuclear instrumentation
during a reactor startup.
• Important to the reactor operator:
– Identifying proper from improper response
– Correctly identifying criticality
– Preventing an uncontrolled reactivity addition
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
16
Reactor Startup Nuclear Instrumentation
• During control rod withdrawal, source range nuclear instruments
respond to increased neutron flux levels
• Insertion of positive reactivity
– Slight positive startup rate (SUR) and count rate will increase
• When rod motion stopped in a subcritical reactor:
– Source range count rate achieves new equilibrium level
– SUR will decay to zero
• As keff approaches 1.0, a longer period of time required to reach an
equilibrium neutron level
– When reactor close to criticality, this will be several minutes
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
17
Reactor Startup Nuclear Instrumentation
Figure: Reactor Startup – Time (Rod Pulls) vs. Count Rate Increase
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
18
Reactor Startup Nuclear Instrumentation
• As keff comes closer to 1.0, count rate increases more per rod pull
and time to reach equilibrium increases
• A reactor that is exactly critical has a stable neutron count rate after
control rod motion has ceased
• Difficult to determine between reactor exactly critical or effect of
subcritical multiplication
– More apparent when reactor is supercritical
– Common to call reactor critical when reactor slightly supercritical
• Reactor is supercritical when:
– Constant positive startup rate
– Steadily increasing neutron population
– No control rod motion (or positive reactivity addition)
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
19
Reactor Startup Nuclear Instrumentation
• Source range count rate will have doubled five to seven times
during startup
• As neutron level increases, intermediate range nuclear instruments
indicate increasing power level
• Power is raised to 10-8 amps on the intermediate range
– Less than point of adding heat
– High enough to negate effects of gammas affecting neutron
population indication
• Critical data recorded
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
20
Reactor Startup Nuclear Instrumentation
• Information used for startup records and for referencing to future
ECC/ECPs and validating core design data
• Typically recorded:
– Time and date of criticality
– Power level
– Rod position
– RCS temperature
– Boron concentration
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
21
Reactor Startup Nuclear Instrumentation
Knowledge Check – NRC Bank
A nuclear reactor startup is in progress and the reactor is slightly
subcritical. Assuming the reactor remains subcritical, a short control
rod withdrawal will cause the reactor startup rate indication to
increase rapidly in the positive direction, and then...
A. stabilize until the point of adding heat (POAH) is reached,
then decrease to zero.
B. rapidly decrease and stabilize at a negative 1/3 dpm.
C. gradually decrease and stabilize at zero.
D. continue a rapid increase until the POAH is reached, then
decrease to zero.
Correct answer is C.
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
22
Reactor Startup Nuclear Instrumentation
Knowledge Check – NRC Bank
A nuclear reactor startup is in progress with a stable source range
count rate and the reactor is near criticality. Which one of the following
statements describes count rate characteristics during and after a 5second control rod withdrawal? Assume reactor remains subcritical.
A. There will be no change in count rate until criticality is
achieved.
B. The count rate will rapidly increase (prompt jump) to a stable
higher value.
C. The count rate will rapidly increase (prompt jump) then
gradually increase and stabilize at a higher value.
D. The count rate will rapidly increase (prompt jump) then
gradually decrease and stabilize at the previous value.
Correct answer is C.
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
23
Reactor Startup Nuclear Instrumentation
Knowledge Check – NRC Bank
During a nuclear reactor startup, if the startup rate is constant and
positive without any further reactivity addition, then the reactor is...
A. subcritical.
B. prompt critical.
C. supercritical.
D. exactly critical.
Correct answer is A.
© Copyright 2014
ELO 1.2
Operator Generic Fundamentals
24
TLO 1 Summary
1. Reactivities normally considered when calculating an ECP include:
– Control rod position
– Boron concentration
– Time in core life
– Power defect (if critical data was at power)
– Fission product poisons, xenon and samarium reactivity, from
critical data time to time of startup
2. During control rod withdrawal, source range nuclear instruments
indicate increased neutron flux levels
– With each insertion of positive reactivity, an indication of a slight
positive SUR and count rate increase
– When rod motion is stopped in a subcritical reactor:
o Source range count rate stabilizes at new higher equilibrium
level
o Indicated SUR will eventually decay to zero
© Copyright 2014
TLO 1
Operator Generic Fundamentals
25
TLO 1 Summary
– As keff comes closer to 1.0:
o Count rate increases more per rod pull
o Time to reach equilibrium increases
– Reactor is supercritical with a constant positive SUR and a
steadily increasing neutron population with no control rod motion
– Normally, source range count rate will have doubled five to
seven times before criticality is achieved
© Copyright 2014
TLO 1
Operator Generic Fundamentals
26
Nuclear Reactor Startup Operations
TLO 2 – Describe the operation of a nuclear reactor during startup.
• This chapter discusses methods for:
– Performing reactor startups to
criticality
– Increasing power into power
range
– Starting up steam plant
• Included are:
– Reactor response during power
increase
– Instrumentation monitored
– Definition of criticality
© Copyright 2014
TLO 2
Operator Generic Fundamentals
27
Enabling Learning Objectives for TLO 2
1. Explain the use of inverse multiplication (1/M) plots during the
approach to criticality and how they predict when criticality will
occur.
2. Describe why shutdown control rod assemblies are withdrawn
prior to starting up the reactor.
3. Describe how reactor response changes when criticality is
reached and the parameters that must be monitored and
controlled as a nuclear reactor approaches criticality.
4. Describe reactor response and operator responsibilities when
operating a reactor in the intermediate range, both above and
below the POAH.
5. Describe the basic startup sequence for the secondary (steam)
plant, including how reactor power is affected as steam flow is
increased.
© Copyright 2014
TLO 2
Operator Generic Fundamentals
28
Predicting Criticality Using a 1/M Plot
ELO 2.1 – Explain the use of inverse multiplication (1/M) plots during the
approach to criticality and how they predict when criticality will occur.
• 1/M plot used to predict when control rods have inserted sufficient
reactivity to bring reactor critical
• 1/M plots also used if reactor is being taken critical by dilution of
boron and for loading fuel
• In case of loading fuel, 1/M plot not used to predict criticality, used
to ensure that reactor will not go critical as each fuel element loaded
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
29
Predicting Criticality Using a 1/M Plot
1.
Start with 1/M =1. 0 with control rods at 0. (1/M is also
referred to as ICRR (inverse count rate ratio). Plot the 1.0
data point.
2.
Follow plant procedures or incremental control rod
withdrawals, stopping at each interval, allowing time to
reach count rate equilibrium, then calculating the 1/M value.
3.
On the 1/M form, plot the 1/M data point at each
incremental rod position.
4.
Using the plotted points on the 1/M form and a straight
edge predict the critical (1/M=0) rod position.
5.
Repeat steps 2 through 4 to extrapolate successive and
more accurate predictions of critical control rod position.
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
30
Predicting Criticality Using a 1/M Plot
• To plot effects of subcritical multiplication , 1/M plot or ICRR used for
prediction of criticality
– Prediction of control rod position
– Prediction of boron concentration
• In PWRs, normally criticality is achieved using control rods
– Our discussions illustrate performance of 1/M plots to predict
criticality using control rods
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
31
Predicting Criticality Using a 1/M Plot
• 1/M value (ICRR) plotted following each reactivity addition from
control rod pulls
• As keff approaches one, 1/M value approaches zero
• After several points have been plotted on a 1/M plot, a conservative
estimate of critical rod position can be obtained by:
– Extrapolating slope between last data points (or baseline and
last data point) on plot and reading rod position off horizontal
axis
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
32
Predicting Criticality Using a 1/M Plot
Figure: 1/M Plot for a Reactor Startup
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
33
Predicting Criticality Using a 1/M Plot
• Specific use of 1/M plots during reactor startups and other
evolutions depends on reactor design and vendor requirements
– Westinghouse
– Babcock & Wilcox (B&W)
– Combustion Engineering (CE)
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
34
Predicting Criticality Using a 1/M Plot
• Baseline 1/M value typically established after fully withdrawing
shutdown banks (Westinghouse)
– Safety groups (B&W)
– First two of six CEA groups (CE)
• Baseline is plotted at intersection of rod position fully inserted on xaxis and 1.0 line on y-axis
IMPORTANT CONCEPT:
Always allow indicated source range
count rate to stabilize for data points
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
35
Predicting Criticality Using a 1/M Plot
Figure: Typical ICRR Plot for Westinghouse Plant
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
36
Predicting Criticality Using a 1/M Plot
• In this example, control rod bank A initially pulled to 50 steps
• After allowing count rate to stabilize, count rate is recorded and 1/M
is calculated
– 1/M = 9.0/9.4 = 0.958
• 1/M then plotted against current rod position (A bank at 50 steps)
• Extrapolating a line from baseline data point to next data point yields
approximate critical rod height
– Compared to estimated critical conditions (ECC) calculations
– In this case, small change in keff makes it too early for an
accurate prediction
• Process repeated
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
37
Predicting Criticality Using a 1/M Plot
Figure: Typical ICRR Plot for Westinghouse Plant with Criticality Prediction
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
38
Predicting Criticality Using a 1/M Plot
• Useful prediction of critical rod position does not occur until control
rods are at B bank at 172 steps and not highly accurate until C
bank is at 119 steps
• Bank C at 119 steps predicts critical control rod position early,
which is ideal for early anticipation of criticality
• If 1/M plot does not predict rod position within limits of ECP:
– Startup must be terminated, cause must be identified
– Errors more commonly associated with reactivity balance
calculation of ECP
• Reactor engineering staff notified
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
39
B&W Plants
• Baseline ICRR typically established after first four control rod
assembly (CRA) groups withdrawn
– Referred to as safety groups
• 1/M plots at B&W plant may be used:
– While shutdown and diluting boron concentration
– Plant heatup to 525F
– Approach to criticality
– Plant cooldown
– Plot 1/M against safety rod positions
– During refueling operations
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
40
B&W Example
Figure: 1/M Plot for B&W Plant Startup
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
41
Combustion Engineering Plant
• Baseline typically established after first two of six CEA groups
withdrawn
– 1/M plot normally established for CEA groups 3, 4, 5, and 6
withdrawal
• Other uses of 1/M plots at CE plants include:
– While shutdown and diluting boron concentration
– During refueling operations
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
42
Combustion Engineering Example
Figure: ICRR Plot at a CE Plant
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
43
Predicting Criticality Using a 1/M Plot
• Preferably, 1/M plot to be shaped like line A to provide most
conservative estimate of criticality
• Least conservative approach to criticality represented by line C
Figure: Conservative and Non-conservative 1/M Plots
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
44
Group Project
1.
2.
Perform a 1/M plot utilizing your plant’s 1/M (ICRR) procedures
– Your instructor will provide data to use
Compare critical rod predictions and discuss
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
45
1/M and Subcritical Multiplication
Indication of Approach to Criticality
• As reactor comes
closer to criticality,
count rate makes a
much larger change
and time to reach
equilibrium increases
considerably
Figure: Reactor Startup – Time (Rod Pulls) vs. Count Rate Increase
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
46
1/M and Subcritical Multiplication
Indication of Approach to Criticality
• Following items used to identify that criticality may occur during next
rod pull:
– Count rate change becomes much larger (a prompt jump also
becomes more evident)
– Longer time for subcritical multiplication to equalize neutron count
rate
– 1/M plot data points occur with much less rod pull and become
more accurate in predicting criticality
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
47
Predicting Criticality Using a 1/M Plot
Knowledge Check
When is a 1/M plot the most accurate in predicting critical rod position?
A. As the 1/M approaches 0
B. The same throughout the plot
C. As the 1/M approaches 1.0
D. About halfway through the plot
Correct answer is A.
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
48
Predicting Criticality Using a 1/M Plot
Knowledge Check
As the reactor approaches criticality, the neutron generation time
_________, the time to reach equilibrium counts ____________, and
length of rod pulls __________.
A. stays the same; increases; shortens
B. increases; increases; shortens
C. shortens; decreases; stays the same
D. stays the same; decreases; shortens
Correct answer is A.
© Copyright 2014
ELO 2.1
Operator Generic Fundamentals
49
Reactivity Control Mechanisms for
Reactor Startup
ELO 2.2 – Describe why shutdown control rod assemblies are withdrawn
prior to starting up the reactor.
• Should an event occur that could cause an unanticipated criticality
accident or the probability of one is increased during a reactor
startup, then:
– Reactivity mechanisms exist to ensure the reactor can be
immediately and safely shutdown
• Shutdown rod banks (other names for CE and B&W) are pulled prior
to actually starting 1/M plot
– Provides a reserve of negative reactivity that can be quickly
inserted
© Copyright 2014
ELO 2.2
Operator Generic Fundamentals
50
Reactivity Control Mechanisms for
Reactor Startup
• Shutdown rods can be inserted or tripped manually
– If conditions exist calling for a reactor trip, this would happen
automatically (if not completed first by operators)
• If reactor becomes critical below rod T.S. insertion limits, insertion of
shutdown rods immediately adds sufficient negative reactivity to shut
down reactor
– In this case, minimum shutdown margin may not be met and
addition of boron would be required
• Reactor startups on PWRs performed at normal operating pressures
and temperatures (NOPT)
• Minimum temperature for criticality specified in plant technical
specifications
© Copyright 2014
ELO 2.2
Operator Generic Fundamentals
51
Reactivity Control Mechanisms for
Reactor Startup
Knowledge Check
The control rod shutdown rod banks are withdrawn at the beginning of
a reactor start-up to ensure that …
A. the reactor will remain shutdown as the xenon decays during
reactor recovery.
B. the reactor can be shutdown during the positive reactor
insertion caused by reactor cooldown.
C. reactivity mechanisms exist to ensure the reactor can be
immediately and safely shutdown.
D. none of the above.
Correct answer is C.
© Copyright 2014
ELO 2.2
Operator Generic Fundamentals
52
Reactivity Control Mechanisms for
Reactor Startup
ELO 2.3 – Describe how reactor response changes when criticality is
reached and the parameters that must be monitored and controlled as a
nuclear reactor approaches criticality.
• Important for a operator to recognize and understand criticality
• This section adds to building blocks of information about 1/M plots
and subcritical multiplication covered in previous sections
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
53
Identifying Criticality
• A critical reactor is defined by keff = 1.0 or reactivity = 0
• An exactly critical reactor has a stable neutron count rate after
control rod motion has ceased
• Subcritical multiplication also results in a stable neutron count rate
and a startup rate equal to zero in a subcritical reactor
– Could make it difficult to identify criticality
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
54
Criticality
• So how is a critical reactor clearly identified?
ANSWER:
When the reactor is supercritical!
Normally when the reactor operator
identifies the reactor as critical, it will
actually be slightly supercritical.
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
55
Reactivity Control Mechanisms for
Reactor Startup
• A supercritical reactor is identified by:
– Constant positive startup rate and steadily increasing neutron
population with no control rod motion
– Equilibrium neutron counts is not reached as in subcritical
multiplication
• Normally, source range count rate will have doubled five to seven
times to criticality during a startup
– This is only a rule of thumb!
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
56
Reactivity Control Mechanisms for
Reactor Startup
• Difference between critical and subcritical multiplication:
Figure: Neutron Count Rate Increases during a Reactor Startup (Log Scale)
• Note that neutron level is plotted on a log scale, therefore a straight
line on this plot results in an exponential rise in neutron flux.
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
57
Reactivity Control Mechanisms for
Reactor Startup
• As neutron count rate increases, intermediate range nuclear
instruments come on scale
– Possible to go critical in source range or intermediate range
• Nuclear instrumentation response verified by checking for proper
overlap between source and intermediate
• When intermediate range operation verified:
– Source range blocked and de-energized to prevent burnout of
its detectors from high neutron flux
– Source range no longer needed as it will be “pegged” upscale
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
58
Monitoring the Approach to Criticality
• During rod withdrawal, source range NI responds to increasing
neutron flux levels
– Level increase
– SUR increase
• SUR meter first indication of reactivity addition
• When rod motion is stopped in a subcritical reactor, source range
count rate achieves new equilibrium level
• As keff approaches 1.0, a longer period of time required for count
rate to reach equilibrium
– Close to criticality, this time period is several minutes
– As Keff gets closer to 1.0, a small prompt jump may be observed
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
59
Reactivity Control Mechanisms for
Reactor Startup
• Reactor operator is monitoring:
– Neutron level and startup rate
– Source range and intermediate range until SR blocked
– Control rod position –rod step counters verified by individual rod
position indicators
o Ensure any stuck/dropped rods are identified
– Boron concentration, chemistry provides samples as needed
– Moderator temperature, ensure minimum temperatures are met
and steam dumps are maintaining temperature
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
60
Reactivity Control Mechanisms for
Reactor Startup
Knowledge Check
In a nuclear reactor with source neutrons, a constant neutron flux
over a few minutes is indicative of criticality or...
A. the point of adding heat.
B. supercriticality.
C. subcriticality.
D. equilibrium subcritical count rate.
Correct answer is D.
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
61
Reactivity Control Mechanisms for
Reactor Startup
Knowledge Check
Which one of the following parameters should not be closely
monitored and controlled during the approach to criticality?
A. Axial flux difference (axial shape index)
B. Reactor startup rate
C. Source range (neutron) count rate
D. Rod position
Correct answer is A.
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
62
Reactivity Control Mechanisms for
Reactor Startup
Knowledge Check – NRC Bank
A reactor startup is in progress and the reactor is slightly subcritical.
Assuming the reactor remains subcritical, a short control rod withdrawal
will cause the reactor startup rate indication to increase rapidly in the
positive direction, then...
A. gradually decrease and stabilize at zero.
B. stabilize until the point of adding heat (POAH) is reached and
decrease to zero.
C. continue a rapid increase until the POAH is reached and
decrease to zero.
D. rapidly decrease and stabilize at a negative 1/3 dpm.
Correct answer is A.
© Copyright 2014
ELO 2.3
Operator Generic Fundamentals
63
Critical Reactor Above and Below POAH
ELO 2.4 – Describe reactor response and operator responsibilities when
operating a reactor in the intermediate range, both above and below the
POAH.
• PWR inherent safety feature is moderator and fuel temperature
coefficients
• If moderator or fuel increases in temperature, effects to neutron life
cycle (six-factor formula) results in:
– Negative reactivity added to the reactor
o Results in reactor power decrease to stop temperature
increase
• How do these coefficients respond above and below POAH?
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
64
Reactor Operation – Intermediate Range
• After criticality, control rods are withdrawn to increase reactor
power to 1 x 10-8 amps in intermediate range
• Power increase is stopped and allowed to stabilize
• Once stabilized data is taken:
– Time and date
– Control rod positions
– RCS boron concentration
– RCS loop temperatures
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
65
Reactor Operation – Intermediate Range
• 10-8 data is typically reviewed by plant’s nuclear engineering group
and is used to:
– Update ECC/ECP correction factors
– Update reactivity curves
– Validate core design data
• After critical data, control rods are withdrawn to increase power to
POAH
– 5 x 10-6 to 10-5 amps
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
66
Reactor Operation – Intermediate Range
• Until POAH is reached, reactor fission process is not contributing
sufficiently to heat RCS
– Negates any effects from negative moderator and fuel
temperature coefficients
• When POAH reached, moderator and fuel temperature will
increase, causing power to be turned from negative moderator and
fuel temperature (Doppler) coefficients
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
67
Point of Adding Heat
• Up to 1 percent reactor power decay heat and RCPs are major
heat producers to RCS
– Their heat input more than enough to overcome primary heat
losses and raise RCS temperatures
• At about 1 percent reactor power, reactor’s heat production from
fission causes clear measurable increase to RCS temperatures
– Defined as point of adding heat (POAH)
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
68
Critical Reactor Above and Below POAH
• Control of reactor is more difficult when critical and below POAH
– Negative moderator temperature coefficient and fuel
temperature coefficient do not provide feedback
• To turn a reactor power increase without insertion of negative
reactivity, POAH must be reached to raise RCS temperature
• Below POAH requires additional attention by reactor operator to
maintain reactor power
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ELO 2.4
Operator Generic Fundamentals
69
Reactivity Effects Above POAH
• Above POAH, as positive reactivity is added, reactor power
increases cause RCS to heat up
– Negative moderator temperature coefficient and fuel
temperature coefficient provide feedback
IMPORTANT CONCEPT:
With reactor power above the POAH reactivity feedback
from the fuel (Doppler) and moderator temperature
coefficient is available to limit reactor power increases.
Below the POAH this feedback does not exist.
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
70
Critical Reactor Above and Below POAH
• Procedurally, power raised 2 percent to 4 percent with steam dumps
maintaining RCS temperature and power
– Operator positions control rods and boron as needed to
compensate for xenon to keep reactor at desired power level
• Control of reactor is more stable with moderator/ Doppler reactivity
feedback
• Either above or below the POAH, if reactor is allowed to go
subcritical (i.e. xenon building in) this feedback will not work in
opposite direction
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
71
Critical Reactor Above and Below POAH
Knowledge Check – NRC Bank
A nuclear reactor is critical below the point of adding heat (POAH).
The operator adds enough reactivity to attain a startup rate of 0.5
decades per minute. Which one of the following will decrease first
when the reactor reaches the POAH?
A. Startup rate
B. Pressurizer level
C. Reactor coolant temperature
D. Reactor power
Correct answer is A.
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
72
Critical Reactor Above and Below POAH
Knowledge Check
The point of adding heat is defined as that power level where the
nuclear reactor is producing enough heat...
A. to support main turbine operations.
B. to cause a measurable temperature increase in the fuel and
coolant.
C. for void coefficient to produce a negative reactivity feedback.
D. for Doppler coefficient to produce a positive reactivity
feedback.
Correct answer is B.
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
73
Critical Reactor Above and Below POAH
Knowledge Check
Given a critical nuclear reactor operating below the point of adding
heat (POAH), what reactivity effects are associated with reaching the
POAH?
A. There are no reactivity effects because the reactor is critical.
B. The decrease in fuel temperature will begin to create a
negative reactivity effect.
C. The increase in fuel temperature will begin to create a
negative reactivity effect.
D. The increase in fuel temperature will begin to create a positive
reactivity effect.
Correct answer is C.
© Copyright 2014
ELO 2.4
Operator Generic Fundamentals
74
Steam Plant Startup
ELO 2.5 – Describe the basic startup sequence for the secondary (steam)
plant, including how reactor power is affected as steam flow is increased.
• After reactor power has been raised
above 1 percent:
– Steam plant heat up can be started
– Turbine rolled and placed on line
• Moderator and fuel temperature
coefficients allow reactor power to follow
steam demand (turbine power)
• Operator actions via control rods and
boron concentration maintain TAvg at
setpoint
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
75
Steam Plant Startup
• Sequence as follows:
– Main steam lines warmed
– Steam pressures equalized
– Main steam valves opened
– Condenser vacuum established
• Steam dumps to condenser may now be utilized and main turbine
rolled and placed on line
• Reactor power and temperature controlled using control rods or
dilution and steam dumps
• Steam dumps control SG pressure at setpoint to control RCS
temperature at no load value
– TSat of steam generators
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ELO 2.5
Operator Generic Fundamentals
76
Steam Plant Startup
• Steam flow increases from various steam loads at startup
– RCS temperatures controlled by operator with rods, boration, or
dilution
• More steam load requires more reactor power
– RCS temperature will drop from greater steam flow
• Operator responds by withdrawal of rods or dilution to restore RCS
temperature, in affect raising power
• Feedback process to control RCS temperature and reactor power:
– Reactor operator using control rods or adjusting boron
concentration
– Automatic operation of steam dumps controlling steam
pressure/TAvg
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ELO 2.5
Operator Generic Fundamentals
77
Steam Plant Startup
• During turbine generator startup, steam dump system bleeds steam
directly to condenser in order to maintain steam pressure
• As turbine generator draws more steam, steam dump system
automatically reduces amount of steam bled to condenser
• Operators maintain RCS temperature and reactor power at
approximately constant levels
• When turbine generator’s steam load has increased to match reactor
power, steam dumps will fully close
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ELO 2.5
Operator Generic Fundamentals
78
Steam Plant Startup
• Steam dumps may be used in manual to raise reactor power by
increasing steam demand
1. Steam dumps open
2. RCS temperature drops
3. Reactor operator withdraws control rods to restore TAvg
4. Reactor power at new higher power level to match increased
steam demand
• In affect, more reactivity added to balance reactivities for new
higher power level
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ELO 2.5
Operator Generic Fundamentals
79
Steam Plant Startup
• When reactor power increases above 15 percent, operator may
select automatic control for control rods
• Control rods will then step out automatically to raise average reactor
coolant temperature (TAvg) in accordance with a ramped "TAvg versus
power program"
Figure: Typical Westinghouse TAvg versus Power Program Graph
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
80
Steam Plant Startup
• Operators at B&W stations tend to
adjust TAvg differently with respect to
reactor power
• Between about 20 and 100 percent
reactor power, B&W plant operators
strive to hold TAvg constant
• At a B&W plant, integrated control
system (ICS) keeps TAvg constant
• In doing so, ICS also endeavors to
maintain steam pressure
approximately constant
• A B&W station operates with
approximately 35°F to 50°F of
superheat on steam system
© Copyright 2014
ELO 2.5
Figure: Typical B&W Tavg versus
Power Program Graph
Operator Generic Fundamentals
81
Moderator & Fuel Temperature Coefficients
• As steam flow increased, reactor must produce more heat,
therefore fuel temperature must increase
• Before fuel temperature can increase, TAvg starts to decrease from
reactor producing less power than demanded by steam flow
• Moderator temperature decrease adds positive reactivity, causing
reactor power to increase (no operator action)
• Reactor power increase heats fuel, adding negative reactivity to
core because of fuel temp coefficient
• Without operator action, these two reactivities balance each other,
resulting in:
– New higher power level at a reduced RCS temperature
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Operator Generic Fundamentals
82
Steam Plant Startup
• Magnitude of temperature drop a function of value of these
coefficients
– They change over core life, especially moderator temperature
coefficient (MTC)
• This is why the reactor operator responds, on steam flow increases,
by withdrawing control rods (or dilution) to add enough positive
reactivity to maintain temperature
• Steam flow reductions - insert control rods or borate
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
83
Raising Reactor Power to 100%
• Operators raise reactor power to 100 percent through combination of
control rod withdrawal and boron dilution
• If using control rod position alone to raise power, reactor operator will
pull control rods until control bank fully withdrawn
– Further automatic rod withdrawal impossible
• Continued plant power increases (increased steam flow rates) will
cause RCS temperature to decrease, adding positive reactivity to
reactor (via MTC) to compensate for power defect
• Decreasing RCS temperatures results in lower steam pressures in
secondary plant and lower plant efficiency
• To prevent a decrease in RCS temperature, operator must start boron
dilution to compensate for plant power increase
IMPORTANT CONCEPT:
Above POAH, steam demand controls reactor power, control
rods and boron concentration control RCS temperature
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
84
Steam Plant Startup
IMPORTANT CONCEPT
Because of this response of reactor power to steam
demand in a PWR, it is said that reactor power follows
steam demand in power range, and control rods and
boron control Tavg.
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
85
Steam Plant Startup
Knowledge Check
A nuclear power plant has been operating at 80 percent of rated
power for several weeks. A partial steam line break occurs and 2
percent total steam flow is escaping. Turbine load and control rod
position remain the same. Assuming no operator or automatic
actions, when the plant stabilizes, reactor power will be
____________ and average reactor coolant temperature will be
____________.
A. higher; higher
B. unchanged; higher
C. higher; lower
D. unchanged; lower
Correct answer is C.
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
86
Steam Plant Startup
Knowledge Check – NRC Bank
A nuclear reactor is operating just above the point of adding heat. To
raise reactor power to a higher stable power level, the operator must
increase...
A. steam generator levels.
B. steam demand.
C. average reactor coolant temperature.
D. reactor coolant system boron concentration.
Correct answer is B.
© Copyright 2014
ELO 2.5
Operator Generic Fundamentals
87
TLO 2 Summary
1. As keff approaches one, 1/M plot approaches zero
– An estimate of critical rod position can be obtained by
extrapolating the slope between data points on plot and reading
rod position off horizontal axis
– More accurate after a few data points have been taken and
plotted
– Reactor operator knows that criticality very close criticality when:
o Count rate change becomes much larger
o Longer time for subcritical multiplication to stabilize neutron
count rate
o 1/M plot data points occur with much less rod pull and become
more accurate in predicting criticality
© Copyright 2014
TLO 2
Operator Generic Fundamentals
88
TLO 2 Summary
– Given count rates vs. control rod position, predict when reactor
will become critical through use of a 1/M plot
1.
2.
3.
4.
5.
© Copyright 2014
Start with 1/M =1. 0 with control rods at 0. (1/M is also
referred to as ICRR (inverse count rate ratio). Plot the 1.0
data point.
Follow plant procedures or incremental control rod
withdrawals, stopping at each interval, allowing time to reach
count rate equilibrium, then calculating the 1/M value.
On the 1/M form, plot the 1/M data point at each incremental
rod position.
Using the plotted points on the 1/M form and a straight edge
predict the critical (1/M=0) rod position.
Repeat steps 2 through 4 to extrapolate successive and more
accurate predictions of critical control rod position.
TLO 2
Operator Generic Fundamentals
89
TLO 2 Summary
2. Provides for a reserve of negative reactivity that can be quickly
inserted should need arise to ensure adequate shutdown margin
or compensate from some unforeseen positive reactivity addition
3. An exactly critical reactor will have a stable neutron count rate
after control rod motion has ceased
– Supercritical reactor has a constant positive startup rate and
steadily increasing neutron population with no control rod motion
o When reactor is supercritical, identifying criticality more
accurate (subcritical multiplication effect)
– Reactor operator monitors:
o Source range or other low range neutron flux instrumentation
o Control rod position
o Boron concentration
o Moderator temperature
© Copyright 2014
TLO 2
Operator Generic Fundamentals
90
TLO 2 Summary
4. Point of adding heat
– When reactor’s heat production from fission causes a clear
measurable increase to RCS temperatures
– Above POAH, reactivity feedback from fuel and moderator
temperature coefficient available to limit power increase
– Above or below POAH, if reactor allowed to go subcritical,
moderator and fuel coefficient feedback will not work in opposite
direction
o Temperature controlled by steam dumps and RCPs at no load
temperatures
o No MTC available to add positive reactivity to stop power
decrease
o Power level continues to decrease (reactor will go subcritical)
until positive reactivity added by operator to stop decrease
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TLO 2
Operator Generic Fundamentals
91
TLO 2 Summary
5. Steam plant startup performed by:
– Warming main steam lines
– Equalizing steam pressures
– Opening main steam valves
– Establishing condenser vacuum
– Reactor power being controlled by control rods or dilution of
boron and steam dumps
– Once condenser vacuum is established, steam dumps to
condensers used for steam generator pressure and RCS
temperature control
– Reactor power raised using steam dumps and rods/boron
© Copyright 2014
TLO 2
Operator Generic Fundamentals
92
TLO 2 Summary
– Power level increase:
o As steam flow increased, reactor must produce more heat,
therefore fuel temperature must increase
o Before fuel temperature can increase, TAvg starts to decrease
from reactor, producing less power than demanded by steam
flow
o Moderator temperature decrease adds positive reactivity
causing reactor power to increase (no operator action)
o Without operator action, a new higher power level at a
reduced RCS temperature
o Reactor operator responds, on steam flow increases, by
withdrawing control rods (or dilution) to add enough positive
reactivity to maintain temperature
© Copyright 2014
TLO 2
Operator Generic Fundamentals
93
Nuclear Reactor Power Range Operation
TLO 3 – Describe the operation of a nuclear reactor during power range
operation.
This chapter discusses:
– Monitoring and control of reactor during power operations
– Use of control rods and boration/dilution for reactor control
– Effects of boron concentration on core life
© Copyright 2014
TLO 3
Operator Generic Fundamentals
94
Enabling Learning Objectives for TLO 3
1. Describe the monitoring and control of reactor coolant system
temperature and power during power range operations.
2. Describe the process of raising reactor power to rated core power.
3. Describe the effects of control rod motion and boration/dilution on
reactor operation in the power range.
4. Explain how boron concentration affects core life.
© Copyright 2014
TLO 3
Operator Generic Fundamentals
95
Reactor Power Range Operation
ELO 3.1 – Describe the monitoring and control of reactor coolant system
temperature and power during power range operations.
• Rolling the turbine and placing it on line requires precise balancing
of steam flow and RCS average temperature (TAvg) during power
operations
• Steam flow changes affect TAvg and require continuous attention by
operator
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
96
Reactor Power Range Operation
• Steam dumps to condenser used to improve control of TAvg and
reactor power while rolling main turbine and placing it on line
– Operators raise power by withdrawing control rods causing TAvg
to increase and steam dumps to open for steam pressure control
• Rods are withdrawn until power is 7 to 15 percent, exact power level
dependent on plant
• Turbine generator rolled, observing heatup limits, to synchronous
speed and placed on line
• As turbine load increases, turbine steam flow increases and steam
dumps close in response to decreasing steam pressure
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Operator Generic Fundamentals
97
Reactor Power Range Operation
• When turbine generator steam load has increased to match reactor
power, steam dumps will be fully closed
– Now reactor power increases with turbine load
• Provides for a smooth transition for placing turbine on line and
keeping power and TAvg steady and controlled
• Not all plants use this method
– Some plants raise power to 3 or 4 percent on steam dumps to
atmosphere, roll turbine, and place it on line
o This method may not offer as smooth a transition
• Turbine load further increased using turbine control system in
manual or automatic at rate prescribed by plant procedures
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
98
Reactor Power Range Operation
• CE and Westinghouse plants have a programmed TAvg function
based on a referenced TAvg (called TRef in Westinghouse plants)
• TRef programmed as a function of power level
– Operator’s responsibility to match TAvg to TRef
• Figure is for a typical Westinghouse plant
Figure: Typical Westinghouse TAvg versus Power Program Graph
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
99
Reactor Power Range Operation
• Note that TAvg will rise (or slide) as a function of reactor power (TRef)
• Reason for sliding TAvg upward (or holding TCold constant) is to
enable steam pressure to remain higher at 100 percent turbine load
• With a constant TAvg, steam pressure would be too low for existing
turbine design and too expensive to build a turbine that would work
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
100
Reactor Power Range Operation
• B&W plants adjust TAvg
differently with respect to
reactor power
• Between 0 and about 20
percent reactor power, value
of TAvg ramps up rapidly, then
levels out between about 20
and 100 percent reactor
power
Figure: Typical B&W TAvg versus Power Program Graph
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
101
Reactor Power Range Operation
•
•
•
•
B&W plants have an ICS that automatically maintains TAvg
B&W plants utilize a vertical once through steam generator
Design allows for approximately 35-50F of superheated steam
B&W plants have advantage of higher quality steam entering highpressure turbine
– Eliminates need for moisture separators
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
102
Reactor Power Range Operation
Knowledge Check – NRC Bank
A nuclear power plant is operating at 100 percent power near the end
of a fuel cycle with all control systems in manual. The reactor
operator inadvertently adds 100 gallons of boric acid to the reactor
coolant system (RCS). Which one of the following will occur as an
immediate result of the boric acid addition? Assume a constant main
generator output.
A. Pressurizer level will decrease and stabilize at a lower value.
B. Reactor power will decrease and stabilize at a lower value.
C. RCS pressure will increase and stabilize at a higher value.
D. Average RCS temperature will increase and stabilize at a
higher value.
Correct answer is A.
© Copyright 2014
ELO 3.1
Operator Generic Fundamentals
103
Raising Reactor Power to Rated Core
Power
ELO 3.2 – Describe the process of raising reactor power to rated core
power.
• Numerous considerations must be observed during power increase
to full rated load
• Technical specifications place certain operating limits on core
• Other reactivity inputs besides those coming directly from power
increase must also be observed and compensated for by operators
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
104
Raising Reactor Power to Rated Core
Power
• Reactor power increased to 100 percent load by process of
increasing turbine load either manually or automatically using the
plant’s installed turbine load control system
• As load (steam flow) increased, reactor operator will use a
combination of control rods and boron dilution to:
– Match TAvg to TRef
– Maintain axial flux distribution within power distribution limits
– Maintain control rod position above minimum level to ensure
required shutdown margin
– Compensate for xenon build up (negative reactivity)
– Compensate for samarium burnout (positive reactivity)
– Compensate for power coefficient
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
105
Raising Reactor Power to Rated Core
Power
• Match TAvg to TRef
– Required to ensure proper steam pressures to turbine and that
temperatures are in boundary of any safety analysis studies
• Maintain axial flux distribution within power distribution limits
– Vertical positioning of controlling bank of control rods affects
axial flux distribution
– Important for safety at higher power levels to ensure fuel power
limits not exceeded
– By the time power is increased to 100 percent, control bank
rods will be fully or close to fully withdrawn
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
106
Raising Reactor Power to Rated Core
Power
• Maintain control rod position above minimum level for ensuring
required shutdown margin
• Shutdown margin required for all reactor operating modes: full
power to cold shutdown/refueling
• When reactor is critical, called available shutdown margin
• If reactor trips, available shutdown margin from control rods
compensates for positive reactivity added from power defect
– Only happens if control rods sufficiently withdrawn to ensure
necessary negative reactivity
• Rod insertion limits higher as power increased because power
(Doppler) defect is larger
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
107
Reactivity Control in the Power Range
• Compensation for xenon build up (negative reactivity)
– When power increased, xenon must be compensated for
– Depending on previous power history, xenon may be:
o “Burning out” thereby adding positive reactivity
o “Building in” to add negative reactivity
– Either way, operator must compensate
– Maneuvering data usually provided by reactor engineering to
guide operator on power changes
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
108
Reactivity Control in the Power Range
• Compensation for samarium burnout (positive reactivity)
– Samarium is a reactivity poison similar to xenon with its
reactivity effect related to power history
– Chances are, on a return to power, samarium will be burning
out from its peak, adding positive reactivity
– Much smaller reactivity affect compared to xenon
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
109
Compensation for the Power Defect
• Consists primarily of reactivity from fuel (Doppler - FTC) and
moderator temperature changes (MTC)
• Responsible for 1,000 to 1,500 PCM negative reactivity on power
increase to 100 percent
• Reactivity value changes over core life, primarily due to a larger
MTC at end of life
• Control rod and boron dilution necessary to overcome this large
negative reactivity input
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
110
Other Reactivity Issues
• Insertion of reactivity simultaneously via two methods normally not
allowed
– i.e. control rods and boron dilution at the same time
• Reactor response to boron dilution has a lag time for it to mix in
large volume of RCS
– Operators must plan ahead when using boron concentration
changes for reactivity control
• Reactivity effects from control rods is instantaneous and useful if
reactivity adjustment needed quickly
– Reactivity changes must be made deliberately
• During a normal power increase from off line to 100 percent,
significant dilution required
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
111
Raising Reactor Power to Rated Core
Power
Knowledge Check – NRC Bank
A nuclear power plant has been operating at 75 percent power for
several weeks when a partial main steam line break occurs that
releases 3 percent of rated steam flow. Assuming no operator or
automatic actions occur, reactor power will stabilize __________ 75
percent and average reactor coolant temperature will stabilize at a
__________ temperature.
A. greater than; lower
B. at; higher
C. greater than; higher
D. at; lower
Correct answer is A.
© Copyright 2014
ELO 3.2
Operator Generic Fundamentals
112
Reactivity Control in the Power Range
ELO 3.3 – Describe the effects of control rod motion and boration/dilution
on reactor operation in the power range.
• Control rod movement at power results in changes to core axial flux
distribution
• Core flux distribution limits specified by plant technical
specifications
• Axial flux distribution limits used to ensure:
– Even power production (kW/ft) top to bottom
– Uniform depletion of fuel
– Acceptable axial xenon distributions
– Operation within core peaking factors
– Assurance of assumptions made in plant safety analysis
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
113
Reactivity Control in the Power Range
• Axial flux differences sensitive to many core-related parameters:
– Control bank position
– Core power level
– Axial burn up
– Axial xenon distribution and, to a lesser extent,
– Reactor coolant temperature and boron concentration
• Quadrant power tilts, caused by a mis-positioned or a dropped
control rod, also of concern for similar reasons as axial flux tilts
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
114
Reactivity Control in the Power Range
• Axial flux outside limits can also initiate axial xenon oscillations
that will magnify axial flux “tilt”
• A dropped peripheral control rod can initiate radial xenon
oscillations as flux is suppressed in vicinity of control rod
• Both of these conditions produce undesirable and possibly limiting
effects on core power distribution
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
115
Axial Flux Oscillations
• In large nuclear reactor, slow axial xenon oscillations induced by
differences in neutron flux top to bottom
• In high flux portion of core, initially xenon concentration decreases,
reducing neutron capture to increase neutron flux (power increase)
• Continues until delayed production of xenon from iodine decay
causes increase of xenon to decrease neutron flux (power)
• Opposite effect occurs in other portion of core with lower flux
• Result: continuous variation of core xenon concentration top to
bottom with resulting flux changes
– As flux increases in one portion, it decreases in other
• Period of these oscillations is about 24 hours
• Xenon oscillations self-dampening or converging as long as
operator response does not aggravate it by improper control rod
movement
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
116
Boron Concentration Changes
• Most nuclear units “base-loaded”
• Load changes made by changes to RCS boron concentration and
minimizing rod movement
• Control rods used to maintain axial flux distribution within unit limits
• Operation in this manner minimizes potential of operation outside
allowable axial flux band that could result in unacceptable xenon
oscillations
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
117
Control Rod Movement
• Control rod movement for reactivity control, when base loaded,
undesirable because it distorts natural axial neutron flux distribution
• Effort made to maintain controlling rod bank fully withdrawn
• In general, minimizing control rod movement over core life and
handling reactivity changes with boron/dilution serves to minimize
axial flux shifts
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
118
Reactivity Control in the Power Range
• As reactor operated, fuel atoms constantly being depleted
– Core burn up is a negative reactivity addition
– Positive reactivity must be added to compensate
• Control rods not available since they are full out, leaving boron
concentration dilution to compensate
• Some reactor designs incorporate use of fixed burnable poisons
installed in fuel assemblies
– Add positive reactivity while “burning out”
– Increase core life while keeping boron concentration low
enough at BOL to minimize positive temperature coefficient
– Slows down decrease in boron concentration at BOL
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
119
Positive Moderator Temperature
Coefficient
• High boron concentrations cause thermal utilization factor, only
major positive contributor to MTC to become even more positive
• This stronger positive effect to keff overrides negative effects of nonleakage factors (fast and thermal) from moderator density change
– Result is positive MTC with high born concentrations
– Limited by technical specifications
• As fuel burns out, boron concentration quickly reduces to point
where MTC becomes negative
• Importantly, plant technical specification has limits on MTC, which
are factors for various accident analysis studies
• Reactor core designs limited with regards to positive MTC
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
120
Reactivity Control in the Power Range
Knowledge Check
The major reason boron is used in a nuclear reactor is to permit...
A. a reduction in the shutdown margin.
B. an increase in the amount of control rods installed.
C. an increase in core life.
D. a reduction in the effect of resonance capture.
Correct answer is C.
© Copyright 2014
ELO 3.3
Operator Generic Fundamentals
121
Reactivity Control in the Power Range
ELO 3.4 – Explain how boron concentration affects core life.
• Constant depletion of fuel during reactor operation termed core
burn-up
– Negative reactivity addition
• Positive reactivity must be added to compensate
• During full load conditions with control rods fully withdrawn, no
positive reactivity available from control rods
• Diluting boron concentration will add necessary positive reactivity to
compensate for core burnup
• Fixed burnable poison rods also used to lengthen core life
© Copyright 2014
TLO 3
Operator Generic Fundamentals
122
Reactivity Control in the Power Range
• Some reactor designs incorporate fixed burnable poisons installed
in fuel assemblies to compensate for reactivity associated with
excess fuel in new core
– If boron concentration high, positive moderator temperature
coefficient can result - not good!
– Plant technical specification limits for positive MTCs are
restrictive; core designs consider these limitations
• As core ages and fuel burns out, boron concentration quickly
reduces to point where MTC becomes negative
© Copyright 2014
TLO 3
Operator Generic Fundamentals
123
Reactivity Control in the Power Range
Knowledge Check – NRC Bank
A high boron concentration is necessary at the beginning of a fuel
cycle to...
A. compensate for excess reactivity in the fuel.
B. produce a negative moderator temperature coefficient.
C. flatten the axial and radial neutron flux distributions.
D. maximize control rod worth until fission product poisons
accumulate.
Correct answer is A.
© Copyright 2014
ELO 3.4
Operator Generic Fundamentals
124
TLO 3 Summary
1. At power:
o Steam flow controls power
o Operators control TAvg using control rods and boron
– CE and Westinghouse plants have a programmed TAvg function
based on a referenced TAvg (called TRef in Westinghouse plants)
– B&W plants - between 0 \ and about 20 percent reactor power,
value of TAvg ramps up rapidly, then levels out between about 20
and 100 percent
– B&W plants have an integrated control system (ICS) that
automatically controls TAvg
– B&W plants utilize a vertical once through steam generator,
unlike U-tube design of Westinghouse and CE
© Copyright 2014
TLO 3
Operator Generic Fundamentals
125
TLO 3 Summary
2. Reactor power increased to rated 100 percent load by a process
of increasing turbine load
– As load increased, reactor operator will use a combination of
control rods and boron dilution to match TAvg to TRef
– Matching TAvg to TRef
o TAvg programmed and controlled to ensure proper steam
pressures to turbine and that temperatures are in boundary
of any safety analysis studies
– Maintain axial flux distribution within power distribution limits
o Ensures fuel power limits not exceeded and safety analysis
design basis met
© Copyright 2014
TLO 3
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TLO 3 Summary
– Maintain control rod position above minimum level for ensuring
required shutdown margin
o When reactor operating, called available shutdown margin
o If reactor trips, available shutdown margin from control rods
provides necessary negative reactivity to offset reactivity
added from power defect
o Rod insertion limits higher as power increased because power
(Doppler) defect is larger
– Compensate for xenon build up (negative reactivity)
o When power raised, xenon must be compensated for
– Compensate for samarium burnout (positive reactivity)
o Samarium is a reactivity poison similar to xenon with its
reactivity effect related to power history
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TLO 3 Summary
– Compensate for power defect
o Consists primarily of reactivity from fuel and moderator
temperature increase
o Together, responsible for 1,000 to 1,500 PCM negative
reactivity on power increase to 100 percent
o Value affected over core life primarily to a larger MTC at EOL
o Control rods and boron dilution necessary to overcome this
large negative reactivity input
3. Flux distribution limits used:
– To ensure even power production (kW/ft) top to bottom
– For uniform depletion of fuel
– Acceptable axial xenon distributions
– Operation within core peaking factors
– Assurance of assumptions made in plant safety analysis
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TLO 3 Summary
– Axial flux differences sensitive to many core related parameters:
o Control bank position
o Core power level
o Axial burnup
o Axial xenon distribution and, to a lesser extent,
o Reactor coolant temperature and boron concentration
– Axial flux outside their limits, besides affecting fuel power
distribution, can also initiate axial xenon oscillations
o Magnifies axial flux “tilt”
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TLO 3 Summary
4. Most nuclear units operated “base-loaded”
– When load changes made, they are made by changes to RCS
boron concentration and minimizing rod movement
– Control rods used to maintain axial flux distribution
o Minimize potential of operation outside allowable axial flux
band that could result in unacceptable xenon oscillations
– Control rod movement
o For reactivity, control undesirable because it distorts natural
axial neutron flux distribution
o Minimizing control rod movement over core life and handling
reactivity changes with boron/dilution serves to minimize axial
flux shifts
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Reactor Shutdown and Decay Heat
TLO 4 – Describe the characteristics of and process used when
performing a reactor shutdown.
This chapter will consider:
– Reactor power decrease on a reactor trip
– Basic shutdown process for a commercial nuclear reactor
– Decay heat source and heat output
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Enabling Learning Objectives for TLO 4
1. Explain how reactor power decays following a reactor trip.
2. Explain the basic reactor shutdown process for a commercial
nuclear reactor.
3. Describe the relationship between decay heat generation, power
history and time after reactor shutdown.
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Reactor Shutdown and Decay Heat
ELO 4.1 – Explain how reactor power decays following a reactor trip.
• This section discusses reactor power response on a reactor trip
– Also mentioned:
o Prompt drop
o Decay of delayed neutron precursors
• This section provides knowledge for operator to recognize correct
response of a reactor trip as indicated on available instrumentation
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Reactor Shutdown and Decay Heat
• A reactor trip is a rapid, usually unplanned, automatic or manual
shutdown of a reactor by a rapid insertion of control rods
– Usually caused by equipment failure, uncontrolled transient, or
reactor protective function
• Emergency procedures used to mitigate effects
• Cause of reactor trip may be immediately determined or could be
difficult to determine
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Reactor Shutdown and Decay Heat
Figure: Reactor Trip Power Decay Response
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Reactor Shutdown and Decay Heat
• Fission rate immediately
drops two decades, indicated
power drops to bottom of
power range (prompt drop)
• Rapid drop occurs because
prompt neutrons make up
about 99.4 percent of total
neutron population
• Prompt neutron generation
time is about 10-5 sec
• Prompt drop takes place
very quickly
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Figure: Reactor Trip Power Decay Response
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Reactor Trip Power Decay
• With prompt neutrons gone, neutron level drops at rate equal to
production of delayed neutrons, short and long-lived
• Short-lived precursors decay off quickly
• When short-lived precursors are decayed, neutron population
controlled by delayed neutrons from long-lived precursors
• Half-life of longest-lived delayed neutron precursors (bromine-87,
56 sec.) results in reactor period of -80 seconds or -1/3 DPM SUR
after a reactor trip
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Reactor Trip Power Decay
• Operators should expect source range nuclear instruments to come
on-line in approximately 15 to 18 minutes after a trip from 100
percent equilibrium power
• Reactor power “decay” following a reactor trip and actual thermal
“decay heat” power not the same
– Indicated (NIS) reactor power level is power produced directly
from fissions
– Decay heat is thermal power as a result of heat produced from
fission product decay – no direct control room indication
– Decay heat is considerable source of heat that must be removed
form fuel to prevent damage (part of ECCS acceptance criteria)
Decay heat produced in reactor immediately following a trip
is approximately 7 percent of rated thermal power.
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Reactor Trip Power Decay
Knowledge Check
On a reactor trip, approximately how long will it take for the neutron
level to drop to the bottom of the intermediate range (Westinghouse
intermediate range scale is 10-3 to 10-11 amps where the POAH is
equal to approximately 10-5 amps).
A. 15 minutes
B. 18 minutes
C. 21 minutes
D. 24 minutes
Correct answer is B.
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Reactor Shutdown Procedure
ELO 4.2 – Explain the basic reactor shutdown process for a
commercial nuclear reactor.
• Similar to a reactor startup, the plant startup process will be a
carefully scripted procedure and followed precisely
• This section gives an overview of a unit shutdown and reactor
shutdown
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Reactor Shutdown Procedure
• Plant shutdown consist of:
– Turbine power reduction
– Maintain TAvg equal to TRef
• Secondary plant equipment aligned for lower secondary power
conditions
– Reducing number of operating feedwater, condensate, and
heater drain pumps, moisture separators out of service, etc.
• Reactor power reduced in this manner, with TAvg controlled with
control rods and boron, allows for:
– Maintaining axial flux in its acceptable band
– Complying with rod insertion limits to maintain shutdown margin
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Reactor Shutdown Procedure
• With reactor power reduced to less than 10 percent, the turbine can
be tripped without the reactor tripping
• After turbine tripped, control rods and steam dumps used to
maintain reactor power at 1 to 2 percent
• Reactor is shutdown, either tripped or shutdown by manual
insertion of control rods
– Control rods driven into core in reverse order of their bank
overlapping sequence to maintain a relatively constant
differential control rod worth
• Reactor is considered shutdown when subcritical and sufficient
shutdown reactivity (shutdown margin) exists
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Reactor Shutdown Procedure
• Nuclear instrumentation monitored to observe that reactor neutron
population decreasing as expected
• If reactor is tripped, neutron level SUR is verified decreasing at -1/3
DPM from decay of long-lived delayed neutron precursors
• Verify source range energizes and confirm proper overlap
• To maintain hot shutdown conditions, decay heat removed using
steam dumps and feedwater
• If cooling to cold shutdown, steam generators used to cooldown
RCS until residual heat removal system in service
– Residual heat removal provides long-term decay heat removal
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Reactor Shutdown Procedure
• During cooldown, maximum cooldown rates apply to minimize
thermal stresses to reactor and pressurizer vessels
• Before starting RCS cooldown, boron concentration increased to
meet xenon-free shutdown margin required by plant technical
specifications for RCS temperature below 200°F
• Important to have this cold shutdown boron concentration to ensure
sufficient negative reactivity exists to negate large amount of
positive reactivity added by cooldown
• Once this boration is completed and RCS boron concentration has
been verified by chemical analysis, reactor cooldown is initiated
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Reactor Shutdown Procedure
• Although a reactor is shut down, it must be continuously monitored
• Instrumentation besides the nuclear instrumentation is available
• Ultimately, operators must ensure safety of reactor
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Reactor Shutdown Procedure
• Shutdown margin is instantaneous amount of reactivity by which a
reactor is subcritical or would be subcritical from its present
condition, assuming all control rods fully inserted except for single
rod with highest integral worth (assumed to be fully withdrawn)
• Stuck rod criterion ensures failure of a single control rod will not
prevent control rod system from shutting down reactor
• Shutdown margin required to exist at all times, even when reactor
is critical
• Shutdown margin must ensure complete shutdown at all times
during core lifetime
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Reactor Shutdown Procedure
Knowledge Check
Which one of the following describes the process for inserting control
rods during a normal reactor shutdown?
A. Control rods are inserted in reverse order one bank at a time
to maintain a rapid shutdown capability from the remainder of
the control rods.
B. Control rods are inserted in reverse order in a bank
overlapping sequence to limit the amount of positive reactivity
added during a rod ejection accident.
C. Control rods are inserted in reverse order one bank at a time
to maintain acceptable power distribution.
D. Control rods are inserted in reverse order in a bank
overlapping sequence to maintain a relatively constant
differential control rod worth.
Correct answer is D.
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Reactor Decay Heat
ELO 4.3 – Describe the relationship between decay heat generation,
power history, and time after reactor shutdown.
• Decay heat is heat the reactor continues to release for a very long
time following shutdown
– Must be removed following reactor shutdown
• This section addresses decay heat source and how it is affected by
reactor's power history and time after shutdown
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Reactor Decay Heat
• Approximately 200 MeV of energy released per fission event
during power operation
– In form of kinetic energy of fission fragments and fission
neutrons
– Appears instantaneously after fission event
• 6 to 7 percent of this energy released some time later
– Decay of fission products created from millions of fission
events at power
– Known as decay heat
– During shutdowns, method for removal must be provided
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Reactor Decay Heat
• Majority of decay heat in a reactor core from gamma and beta
decay of fission products
• Even with reactor shut down, these fission fragments continue to
decay and release heat into reactor core
• Number of fission fragments undergoing decay a function of:
– Power history (# of fission)
– Time since they occurred
IMPORTANT CONCEPT:
Amount of decay heat being released in core depends
upon core power history and time since shutdown
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Reactor Decay Heat
• Decay heat first hour following reactor shutdown
• After an hour, still greater than 1 percent of full power
Figure: Decay Heat Production for the First Hour Following a Reactor Trip
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Reactor Decay Heat
• Figure plots decay heat for a uranium-fueled reactor vs. a 32year time period
• Infinite operating time assumed
Figure: Decay Heat Production for 32 Years Following a Reactor Trip
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Reactor Decay Heat
• Fission products are considered in equilibrium following operation
at full power for one month
• Reactor decay heat is approximately 7 percent of RTP and
decreases to about 1 percent RTP in about 2 ¾ hours
• For a 3,000 MW power reactor, heat generation in core is still at
45-50 MW one hour after shutdown
• Good rule of thumb is decay heat drops to approximately 1
percent of initial reactor power level in 4-6 hours following
shutdown
This amount of decay heat production is more than
sufficient to damage reactor’s fuel if cooling is lost
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Reactor Decay Heat
Knowledge Check
A 1,000 MW nuclear reactor was shut down one month ago following
several months of operation at 100 percent power. Reactor coolant
is being maintained at 547°F and all 4 reactor coolant pumps are
operating. The principle source of heat input to the reactor coolant is
from...
A. subcritical thermal fission of U-235 and Pu-239.
B. fission product decay.
C. subcritical fast fission of U-238.
D. reactor coolant pumps.
Correct answer is D.
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Reactor Decay Heat
Knowledge Check – NRC Bank
The magnitude of decay heat generation is determined primarily by...
A. core burnup.
B. power history.
C. final power at shutdown.
D. control rod worth at shutdown.
Correct answer is B.
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Operator Generic Fundamentals
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Reactor Decay Heat
Knowledge Check
A nuclear power plant has been operating at rated power for six months
when a reactor trip occurs. Which one of the following describes the
source(s) of core heat generation 30 minutes after the reactor trip?
A. Fission product decay and delayed neutron-induced fission are
both significant sources and produce approximately equal rates
of core heat generation.
B. Fission product decay is the only significant source of core heat
generation.
C. Delayed neutron-induced fission is the only significant source of
core heat generation.
D. Fission product decay and delayed neutron-induced fission are
both insignificant sources and generate core heat at rates that
are less than the rate of ambient heat loss from the core.
Correct answer is B.
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TLO 4 Summary
1. Following a reactor trip, fission rate immediately drops almost two
decades
– With prompt neutrons gone, neutron level drops at a rate equal
to production of delayed neutrons – short-lived and long-lived
– Short-lived precursors decay off quickly
– When short-lived precursors gone, neutron population controlled
by delayed neutrons from longest-lived precursors
o Results in reactor period of -80 seconds and startup rate of 1/3 DPM
2. Turbine power reduction performed by decreasing external
electrical load on turbine generator
– Use of boron and rods for maintaining axial flux and complying
with rod insertion limits to maintain shutdown margin
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TLO 4 Summary
•
•
•
•
– Reactor power reduced to less than 10 percent (Westinghouse
value) turbine tripped without reactor tripping
– After turbine tripped, control rods and steam dumps to maintain
1 to 2 percent reactor power
– Reactor tripped or shutdown by manual insertion of control rods
in reverse order of their bank overlapping sequence
If reactor tripped,1/3 DPM confirmed
If reactor plant maintained at hot shutdown, decay heat removed
using steam dumps and feedwater
If RCS to cold shutdown conditions, steam generators first used
then residual heat removal (RHR) system
– RHR removes decay heat long term
Before starting RCS cooldown, boron concentration increased to
meet xenon-free shutdown margin for RCS temperature < 200°F
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TLO 4 Summary
3. 200 MeV of energy per fission event during power operation
– Kinetic energy of fission fragments and fission neutrons
– Appears instantaneously after fission event
– 6 to 7 percent of this energy released some time later from
decay of fission products created from millions of fission events
at power
– Known as decay heat
– Majority from gamma and beta decay of fission products
– After reactor shutdown for 1 hour, greater than 1 percent full
power
– Amount of decay after shutdown dependent on power history
and time after shutdown
– Fission products considered to be in equilibrium following
operation at full power for one month
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Crossword Puzzle
• It’s crossword puzzle time!
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Reactor Operational Physics Summary
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 on the following TLO's:
1. Explain the use of the estimated critical position calculation and
nuclear instrumentation during reactor start-up.
2. Describe the operation of a nuclear reactor during startup.
3. Describe the operation of a nuclear reactor during power range
operation.
4. Describe the characteristics of and process used when
performing a reactor during shutdown.
© Copyright 2014
TLOs
Operator Generic Fundamentals