© Copyright 2014Operator Generic Fundamentals Operator Generic Fundamentals Reactor Theory – Reactor Operational Physics.

© Copyright 2014Operator Generic Fundamentals 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 INTRO 2

© Copyright 2014Operator Generic Fundamentals 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. INTRO 3 Terminal Learning Objectives

© Copyright 2014Operator Generic Fundamentals 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 TLO 1 TLO 1 – Explain the use of the estimated critical position (ECP) calculation and nuclear instrumentation during reactor start-up. 4 Reactor Critical Conditions

© Copyright 2014Operator Generic Fundamentals 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 TLO 1 5

© Copyright 2014Operator Generic Fundamentals 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. TLO 1 6

© Copyright 2014Operator Generic Fundamentals Estimated Critical Conditions Calculations 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 ELO 1.1 ELO 1.1 – Describe the reactivity variables involved in an estimated critical position calculation and how they are used to predict criticality. 7

© Copyright 2014Operator Generic Fundamentals 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 ELO 1.1 8 Estimated Critical Conditions Calculations

© Copyright 2014Operator Generic Fundamentals 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 ELO 1.1 9 Estimated Critical Conditions Calculations

© Copyright 2014Operator Generic Fundamentals 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 ELO 1.1 10 Estimated Critical Conditions Calculations

© Copyright 2014Operator Generic Fundamentals 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 ELO 1.1 11 Estimated Critical Conditions Calculations

© Copyright 2014Operator Generic Fundamentals ELO 1.1 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. 12 Estimated Critical Conditions Calculations

© Copyright 2014Operator Generic Fundamentals ELO 1.1 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. 13 Estimated Critical Conditions Calculations

© Copyright 2014Operator Generic Fundamentals Reactor Startup Nuclear Instrumentation 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 ELO 1.2 ELO 1.2 – Describe the nuclear instrumentation response during a reactor startup to criticality. 15

© Copyright 2014Operator Generic Fundamentals 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 k eff 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 ELO 1.2 16 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals As k eff 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) ELO 1.2 18 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals 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 ELO 1.2 19 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals 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 ELO 1.2 20 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals 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. ELO 1.2 21 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals 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 5- second 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. ELO 1.2 22 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals 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. ELO 1.2 23 Reactor Startup Nuclear Instrumentation

© Copyright 2014Operator Generic Fundamentals 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 TLO 1 24 TLO 1 Summary

© Copyright 2014Operator Generic Fundamentals –As k eff 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 TLO 1 25 TLO 1 Summary

© Copyright 2014Operator Generic Fundamentals 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 TLO 2 TLO 2 – Describe the operation of a nuclear reactor during startup. 26 Nuclear Reactor Startup Operations

© Copyright 2014Operator Generic Fundamentals 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. TLO 2 27 Enabling Learning Objectives for TLO 2

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 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. 28 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.1 29 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 30 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 1/M value (ICRR) plotted following each reactivity addition from control rod pulls As k eff 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 ELO 2.1 31 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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) ELO 2.1 33 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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 x- axis and 1.0 line on y-axis IMPORTANT CONCEPT: Always allow indicated source range count rate to stabilize for data points ELO 2.1 34 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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 k eff makes it too early for an accurate prediction Process repeated ELO 2.1 36 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 38 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 39 B&W Plants

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 41 Combustion Engineering Plant

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 Figure: Conservative and Non-conservative 1/M Plots 43 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 1.Perform a 1/M plot utilizing your plant’s 1/M (ICRR) procedures –Your instructor will provide data to use 2.Compare critical rod predictions and discuss ELO 2.1 44 Group Project

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 Figure: Reactor Startup – Time (Rod Pulls) vs. Count Rate Increase 45

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.1 46 1/M and Subcritical Multiplication Indication of Approach to Criticality

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.1 47 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.1 48 Predicting Criticality Using a 1/M Plot

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.2 ELO 2.2 – Describe why shutdown control rod assemblies are withdrawn prior to starting up the reactor. 49 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.2 50 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.2 51 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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 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. ELO 2.3 52 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals A critical reactor is defined by k eff = 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 ELO 2.3 53 Identifying Criticality

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.3 54 Criticality

© Copyright 2014Operator Generic Fundamentals 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! ELO 2.3 55 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals Difference between critical and subcritical multiplication: 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. ELO 2.3 Figure: Neutron Count Rate Increases during a Reactor Startup (Log Scale) 56 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.3 57 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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 k eff 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 K eff gets closer to 1.0, a small prompt jump may be observed ELO 2.3 58 Monitoring the Approach to Criticality

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.3 59 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.3 60 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.3 61 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.3 62 Reactivity Control Mechanisms for Reactor Startup

© Copyright 2014Operator Generic Fundamentals 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? ELO 2.4 ELO 2.4 – Describe reactor response and operator responsibilities when operating a reactor in the intermediate range, both above and below the POAH. 63 Critical Reactor Above and Below POAH

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.4 64 Reactor Operation – Intermediate Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.4 65 Reactor Operation – Intermediate Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.4 66 Reactor Operation – Intermediate Range

© Copyright 2014Operator Generic Fundamentals 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) ELO 2.4 67 Point of Adding Heat

© Copyright 2014Operator Generic Fundamentals 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 68 Critical Reactor Above and Below POAH ELO 2.4

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.4 69 Reactivity Effects Above POAH

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.4 70 Critical Reactor Above and Below POAH

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.4 71 Critical Reactor Above and Below POAH

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.4 72 Critical Reactor Above and Below POAH

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.4 73 Critical Reactor Above and Below POAH

© Copyright 2014Operator Generic Fundamentals 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 T Avg at setpoint ELO 2.5 ELO 2.5 – Describe the basic startup sequence for the secondary (steam) plant, including how reactor power is affected as steam flow is increased. 74 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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 –T Sat of steam generators ELO 2.5 75 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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/T Avg ELO 2.5 76 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.5 77 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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 T Avg 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 ELO 2.5 78 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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 (T Avg ) in accordance with a ramped "T Avg versus power program" ELO 2.5 Figure: Typical Westinghouse T Avg versus Power Program Graph 79 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals Operators at B&W stations tend to adjust T Avg differently with respect to reactor power Between about 20 and 100 percent reactor power, B&W plant operators strive to hold T Avg constant At a B&W plant, integrated control system (ICS) keeps T Avg 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 ELO 2.5 Figure: Typical B&W T avg versus Power Program Graph 80 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals As steam flow increased, reactor must produce more heat, therefore fuel temperature must increase Before fuel temperature can increase, T Avg 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 ELO 2.5 81 Moderator & Fuel Temperature Coefficients

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.5 82 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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 ELO 2.5 IMPORTANT CONCEPT: Above POAH, steam demand controls reactor power, control rods and boron concentration control RCS temperature 83 Raising Reactor Power to 100%

© Copyright 2014Operator Generic Fundamentals 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 T avg. ELO 2.5 84 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.5 85 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 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. ELO 2.5 86 Steam Plant Startup

© Copyright 2014Operator Generic Fundamentals 1.As k eff 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 TLO 2 87 TLO 2 Summary

© Copyright 2014Operator Generic Fundamentals –Given count rates vs. control rod position, predict when reactor will become critical through use of a 1/M plot TLO 2 88 TLO 2 Summary 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 2014Operator Generic Fundamentals 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 TLO 2 89 TLO 2 Summary

© Copyright 2014Operator Generic Fundamentals 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 TLO 2 90 TLO 2 Summary

© Copyright 2014Operator Generic Fundamentals 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 TLO 2 91 TLO 2 Summary

© Copyright 2014Operator Generic Fundamentals –Power level increase: o As steam flow increased, reactor must produce more heat, therefore fuel temperature must increase o Before fuel temperature can increase, T Avg 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 TLO 2 92 TLO 2 Summary

© Copyright 2014Operator Generic Fundamentals Nuclear Reactor 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 TLO 3 TLO 3 – Describe the operation of a nuclear reactor during power range operation. 93

© Copyright 2014Operator Generic Fundamentals 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. TLO 3 94 Enabling Learning Objectives for TLO 3

© Copyright 2014Operator Generic Fundamentals Reactor Power Range Operation Rolling the turbine and placing it on line requires precise balancing of steam flow and RCS average temperature (T Avg ) during power operations Steam flow changes affect T Avg and require continuous attention by operator ELO 3.1 – Describe the monitoring and control of reactor coolant system temperature and power during power range operations. ELO 3.1 95

© Copyright 2014Operator Generic Fundamentals Steam dumps to condenser used to improve control of T Avg and reactor power while rolling main turbine and placing it on line –Operators raise power by withdrawing control rods causing T Avg 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 ELO 3.1 96 Reactor Power Range Operation

© Copyright 2014Operator Generic Fundamentals 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 T Avg 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 ELO 3.1 97 Reactor Power Range Operation

© Copyright 2014Operator Generic Fundamentals CE and Westinghouse plants have a programmed T Avg function based on a referenced T Avg (called T Ref in Westinghouse plants) T Ref programmed as a function of power level –Operator’s responsibility to match T Avg to T Ref Figure is for a typical Westinghouse plant ELO 3.1 98 Reactor Power Range Operation Figure: Typical Westinghouse T Avg versus Power Program Graph

© Copyright 2014Operator Generic Fundamentals Note that T Avg will rise (or slide) as a function of reactor power (T Ref ) Reason for sliding T Avg upward (or holding T Cold constant) is to enable steam pressure to remain higher at 100 percent turbine load With a constant T Avg, steam pressure would be too low for existing turbine design and too expensive to build a turbine that would work ELO 3.1 99 Reactor Power Range Operation

© Copyright 2014Operator Generic Fundamentals B&W plants adjust T Avg differently with respect to reactor power Between 0 and about 20 percent reactor power, value of T Avg ramps up rapidly, then levels out between about 20 and 100 percent reactor power ELO 3.1 Figure: Typical B&W T Avg versus Power Program Graph 100 Reactor Power Range Operation

© Copyright 2014Operator Generic Fundamentals B&W plants have an ICS that automatically maintains T Avg 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 high- pressure turbine –Eliminates need for moisture separators ELO 3.1 101 Reactor Power Range Operation

© Copyright 2014Operator Generic Fundamentals ELO 3.1 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. 102 Reactor Power Range Operation

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.2 – Describe the process of raising reactor power to rated core power. ELO 3.2 103

© Copyright 2014Operator Generic Fundamentals 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 T Avg to T Ref –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 ELO 3.2 104 Raising Reactor Power to Rated Core Power

© Copyright 2014Operator Generic Fundamentals Match T Avg to T Ref –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 ELO 3.2 105 Raising Reactor Power to Rated Core Power

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.2 106 Raising Reactor Power to Rated Core Power

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.2 107 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.2 108 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.2 109 Compensation for the Power Defect

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.2 110

© Copyright 2014Operator Generic Fundamentals Raising Reactor Power to Rated Core Power ELO 3.2 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. 111

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 ELO 3.3 – Describe the effects of control rod motion and boration/dilution on reactor operation in the power range. 112 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 113 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 114 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 115 Axial Flux Oscillations

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 116 Boron Concentration Changes

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 117 Control Rod Movement

© Copyright 2014Operator Generic Fundamentals 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 ELO 3.3 118 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals High boron concentrations cause thermal utilization factor, only major positive contributor to MTC to become even more positive This stronger positive effect to k eff overrides negative effects of non- leakage 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 ELO 3.3 119 Positive Moderator Temperature Coefficient

© Copyright 2014Operator Generic Fundamentals 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. ELO 3.3 120 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 TLO 3 ELO 3.4 – Explain how boron concentration affects core life. 121 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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 TLO 3 122 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 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. ELO 3.4 123 Reactivity Control in the Power Range

© Copyright 2014Operator Generic Fundamentals 1.At power: o Steam flow controls power o Operators control T Avg using control rods and boron –CE and Westinghouse plants have a programmed T Avg function based on a referenced T Avg (called T Ref in Westinghouse plants) –B&W plants - between 0 \ and about 20 percent reactor power, value of T Avg ramps up rapidly, then levels out between about 20 and 100 percent –B&W plants have an integrated control system (ICS) that automatically controls T Avg –B&W plants utilize a vertical once through steam generator, unlike U-tube design of Westinghouse and CE TLO 3 124 TLO 3 Summary

© Copyright 2014Operator Generic Fundamentals 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 T Avg to T Ref –Matching T Avg to T Ref o T Avg 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 TLO 3 125 TLO 3 Summary

© Copyright 2014Operator Generic Fundamentals –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 TLO 3 126 TLO 3 Summary

© Copyright 2014Operator Generic Fundamentals –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 TLO 3 127 TLO 3 Summary

© Copyright 2014Operator Generic Fundamentals –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” TLO 3 128 TLO 3 Summary

© Copyright 2014Operator Generic Fundamentals 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 TLO 3 129 TLO 3 Summary

© Copyright 2014Operator Generic Fundamentals 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 TLO 4 TLO 4 – Describe the characteristics of and process used when performing a reactor shutdown. 130 Reactor Shutdown and Decay Heat

© Copyright 2014Operator Generic Fundamentals 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. TLO 4 131 Enabling Learning Objectives for TLO 4

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.1 – Explain how reactor power decays following a reactor trip. ELO 4.1 132 Reactor Shutdown and Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.1 133 Reactor Shutdown and Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.1 Figure: Reactor Trip Power Decay Response 135 Reactor Shutdown and Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.1 136 Reactor Trip Power Decay

© Copyright 2014Operator Generic Fundamentals 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) ELO 4.1 Decay heat produced in reactor immediately following a trip is approximately 7 percent of rated thermal power. 137 Reactor Trip Power Decay

© Copyright 2014Operator Generic Fundamentals 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. ELO 4.1 138 Reactor Trip Power Decay

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.2 – Explain the basic reactor shutdown process for a commercial nuclear reactor. ELO 4.2 139 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals Plant shutdown consist of: –Turbine power reduction –Maintain T Avg equal to T Ref 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 T Avg controlled with control rods and boron, allows for: –Maintaining axial flux in its acceptable band –Complying with rod insertion limits to maintain shutdown margin ELO 4.2 140 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.2 141 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.2 142 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.2 143 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.2 144 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.2 145 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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. ELO 4.2 146 Reactor Shutdown Procedure

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.3 – Describe the relationship between decay heat generation, power history, and time after reactor shutdown. ELO 4.3 147 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.3 148 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.3 149 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals Decay heat first hour following reactor shutdown After an hour, still greater than 1 percent of full power ELO 4.3 Figure: Decay Heat Production for the First Hour Following a Reactor Trip 150 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals Figure plots decay heat for a uranium-fueled reactor vs. a 32- year time period Infinite operating time assumed ELO 4.3 Figure: Decay Heat Production for 32 Years Following a Reactor Trip 151 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 ELO 4.3 152 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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. ELO 4.3 153 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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. ELO 4.3 154 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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. ELO 4.3 155 Reactor Decay Heat

© Copyright 2014Operator Generic Fundamentals 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 TLO 4 156 TLO 4 Summary

© Copyright 2014Operator Generic Fundamentals –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 TLO 4 157 TLO 4 Summary

© Copyright 2014Operator Generic Fundamentals 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 TLO 4 158 TLO 4 Summary

© Copyright 2014Operator Generic Fundamentals 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. TLOs 160 Reactor Operational Physics Summary

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© Copyright 2014Operator Generic Fundamentals Operator Generic Fundamentals Reactor Theory – Reactor Operational Physics.

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