SLIDEREAD

REACTOR PHYSICS

COURSE INTRODUCTIONThis Training Manual assumes prior knowledge of Nuclear Theory. It extends this informationinto a discussion of Reactor Physics, particularly as it relates to CANDU reactors.The course begins with the general principles of reactor configuration required to maintain a selfsustaining chain reaction. It continues with reactor dynamics (in both the critical and subcriticalcore), reactivity feedback effects (temperature effects, fission product poisoning, and fuelburnup), and ends with operational considerations (at low and high power).The material covers four main areas, subdivided into eight major sectionsas follows: The Critical Reactor at Steady Power Output (Section 1) The Dynamic Reactor (Sections 2 and 3) Reactivity Feedback Effects (Sections 4, 5, and 6) Reactor Operations (Sections 7 and 8)

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training GroupiTABLE OF CONTENTSObjectives1The Critical Reactor at Steady Power Output91.0 INTRODUCTION91.1 Fission91.2 Harnessing Fission151.3 Movement of Neutrons Through the CANDU lattice Lattice181.4 The Finite Reactor26Response of The Critical Reactor to a Reactivity Change312.0 INTRODUCTION312.1 Exponential Power Rise312.2 Corrections to Exponential Reactor Response332.3 The Effect of Delayed Neutrons362.4 Prompt Criticality402.5 Power Rundown: The Prompt Drop42Responsiveness of The Subcritical Reactor453.0 INTRODUCTION453.1 Neutron Flux in a “Shut Down” Reactor453.2 Dynamics in the Subcritical Core503.3 Examples55Effects of Temperature and Voiding on Core Reactivity4.0INTRODUCTION61614.1 Feedback—An Introduction to Temperature Effects e614.2 The Physical Basis for Temperature Coefficients634.3 Temperature Coefficients of Reactivity724.4 Reactivity Variation with Temperature774.5 Void Reactivity79Effects of Fission Products on Core Reactivity855.0 INTRODUCTION855.1 Xenon and Iodine Buildup855.2 Transient Xenon Behaviour93

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training Groupii5.3 Xenon Oscillations1035.4 Samarium-149106Effects of Fuel Irradiation and On-Power Fuelling on Core Reactivity1116.0 INTRODUCTION1116.1 On-Power Fuelling1116.2 Fuel Burnup—General1126.3 Transient Reactivity Changes1156.4 Long-Term Reactivity Effects115Reactor Operations at Low Power1257.0 INTRODUCTION1257.1 Thermal Power, Neuron Power, AND Fission Power1257.2 Reactor Power Rundown1267.3 The Shutdown State1337.4 Approach to Critical1357.5 Low Power Operation Following Startup142Reactor Operations at High Power1458.0 INTRODUCTION1458.1 Flux Flattening1458.2 Flux Shape Details1508.3 High Power Protection161

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training GroupOBJECTIVESAt the end of training, the participants will be able to:The Critical Reactor at Steady Power OutputList the reaction products of the fission process and for each describe itsimportance for CANDU operation.1.0 Describe the characteristics of fission products with respect to yield,stability, radiation hazard, delayed neutron production, and capabilityto absorb neutrons.2.0 Identify the different energy contributions that make up theapproximately 200 MeV per fission deposited in the reactor.3.0 Define the terms:- unit cell multiplication factor,- effective neutron multiplication factor.4.0 Describe the neutron life cycle in terms of the following neutronprocesses:- fast fission of U-238,- resonance absorption in U-238,- absorption in non-fuel core materials,- thermal fission following absorption in fuel,- neutron leakage.5.0 Define reactivity and give its units of measurement.6.0 Define the terms:- reactivity worth,- excess core reactivity,- nominal core,- control reactivity.Response of the Critical Reactor to a Reactivity Change7.0 Define log rate and reactor period and state their relationship8.0 Describe the response of a CANDU reactor at low power to a smallstep insertion of positive reactivity9.0 Explain the effect of delayed neutrons on reactor control.10.0-Define the following and state how they arise:prompt jump,prompt drop.

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training Group11.0 Describe the change in reactor power following a positivereactivity insertion large enough to cause a prompt jump and state whypower first rises rapidly and then increases more slowly.12.0 Describe the change in reactor power following a large negativereactivity insertion and state why the power first drops rapidly andthen decreases more slowly.13.0 Define prompt criticality, explain how it arises, and state theapproximate reactivity insertion required to cause prompt criticality.Responsiveness of the Subcritical Reactor14.0Define the subcritical multiplication factor.15.0 Explain how subcritical multiplication of a neutron source in asubcritical core causes:- an observable, steady power level that is larger than the source,- a change in power level after a reactivity change that leaves thecore subcritical. Describe the rate of response to a reactivity change in a subcriticalcore. Indicate how and why the dynamic response changes in going froma deeply subcritical reactor to an almost critical reactor.Effects of Temperature and Voiding on Core Reactivity State the typical operating temperatures for the fuel, moderator,and coolant and the approximate range of temperatures encountered ingoing from cold shutdown to full power. Define temperature coefficients of reactivity for the:fuel,moderator, andheat transport coolant.Explain how thermal expansion of the moderator affects:neutron path lengths in the moderator,leakage from the core.Explain how increased molecular speeds caused by heating affectthe:- resonance Absorption in U-238 (Doppler broadening and selfshielding),- thermal neutron spectrum, and- thermal neutron path lengths.

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training Group Describe how changes in the thermal neutron temperatureaffect absorption in:- U-235,- Pu-239.16.0 Explain the effect on reactivity caused by a change in temperatureof the:- fuel,- moderator,- coolant. Given typical values of moderator, HT coolant and fuel reactivitycoefficients, calculate the reactivity change that would occur fortypical unit operations, including:- HT system warm up from cold to zero power hot,- increase in unit power from zero to full power, and- decrease in unit power from full power to zero.Define Power CoefficientCompare the sizes of the reactivity changes due to the moderator,coolant, and fuel for a given change in reactor power with reference to:- temperature coefficient,- temperature change,- time for the effect to show up.Describe the effect of a typical CANDU power coefficient on:- normal regulation,- a power transient following an upset. Define the term “void reactivity”. Explain how voiding of the coolant simultaneously increases fastfission and decreases resonance capture. Describe how the thermal neutron temperature changes on coolantvoiding and state the effects on fission rate. Explain how void reactivity leads to a lower limit on coolantisotopic. Explain the upper limit on heat transport isotopic.Effects of Fission Products on Core Reactivity Define the term, fission product poison.State the characteristics of xenon-135 that make it a significantfission product for reactor operations.

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training Group Describe how nuclear processes in the fuel:produce I-135 and Xe-135,remove I-135 and Xe-135.Define xenon load and iodine load. Explain the given curve shapes for:-Iodine load vs. time,Xenon load vs. time.for start up after a long shutdown.17.0Describe xenon buildup following a trip.18.0State the operational problem xenon causes when the reactor trips. Explain the following terms:poison out,poison prevent operation,poison override capability,poison override time,decision and action time.-19.0 Explain the following features of the reactivity change following atrip from full power:- initial rate of xenon buildup,- the peak,- eventual decrease in xenon concentration.20.0 State the approximate time for xenon to peak and the approximatereactivity worth of xenon at the peak for a reactor trip from full powerwith equilibrium fuel.21.0 Compare the sizes of the transient peaks following a reactor tripfrom equilibrium conditions for a trip from full power and a trip froma lower power level. Explain the power transient that follows a change in reactor powerin the high power range. Describe how the reactivity transient is counteracted after a returnto power:- before a poison out,- following a poison out. -Define the terms:xenon oscillation,flux tilt. State why large oscillations are unacceptable Describe how oscillations are controlled in CANDU reactors.

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training Group Explain how a small, local, reactivity change can cause a large fluxtilt in a reactor operating at high power without adequate spatialcontrol.Describe how a flux tilt changes with time if left to itself.Explain why an uncontrolled oscillation may continue indefinitely.State the characteristics of the samarium-149 that make it a fissionproduct poison. Describe how nuclear processes in the fuel:produce Pm-149 and Sm-149,remove Pm-149 and Sm-149.Compare the operational effects of samarium with those of xenonwith respect to:- initial build up,- transient following a trip or shutdown,- return to equilibrium following a restart,- transients on power changes.Effects of Fuel Irradiation and On-Power Fuelling on Core Reactivity List the chief characteristics associated with using on-powerfuelling for maintaining core reactivity. Define the terms:fresh fuel,fuel burnup, andequilibrium-fuelled reactor.State and explain the fuel burnup units22.0 Describe the transient change in Pu-239 following a shutdownfrom high power, and following return to high power after a shutdown. Define the terms “saturating fission product” and “non-saturatingfission product” and compare their long-term effects on the reactivityworth of a fuel bundle. Describe the changes that occur in the composition of a fuel bundleas it is exposed to neutron flux in the core.23.0 Given a graph showing the reactivity change of a fuel bundle withirradiation, explain the shape of the graph in terms of:- U-235 burnup and Pu-239 growth,- buildup of Pu-240 and Pu-241,- increasing fission products.

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training GroupReactor Operations at Low Power24.0 State and explain the reasons for non-linearity between changes inneutron power and changes in reactor thermal power.Explain the size and duration of the prompt drop given a curve of neutronflux decrease following a trip.Describe how prompt neutrons, delayed neutrons, photoneutrons, andspontaneous fission neutrons affect the shape of the rundown curve.25.0 Explain the following differences between the thermal power andneutron power rundown curves following a trip:- initial rate of drop,- duration,- cooling requirements.26.0 State the approximate value of decay heat at full power, and at 3minutes and 60 minutes after a trip from full power. Identify the reactivity changes that occur in a reactor after it isshutdown from extended operation at power and, for each change givethe:- sign,- approximate size,- time scale.27.0 Describe the variation in neutron flux while the reactor is in ashutdown condition for a long period of time.28.0 List the parameters specifically related to criticality that aremonitored and controlled during the approach to critical.29.0 List reactivity control mechanisms required during the approach tocriticality.30.0-Explain (for step reactivity increases) the observed changes in:stable count rate,flux detector time response,as the core is taken from deeply subcritical to almost critical. Predict the power level following a specified change of poisonconcentration, given the subcritical multiplication formula, themeasured power, and the poison concentration relative to criticality.31.0 Explain why start-up procedures require monitoring of neutronflux during start-up and do not depend solely on criticality predictions.32.0 List parameters that should be monitored and controlled onreaching criticality.

CNSCScience and Reactor Fundamentals – Reactor PhysicsTechnical Training Group33.0 Explain in general terms how a reactor, critical at low power, couldbecome subcritical:- if held in the low power “critical” state for a long time,- following power manoeuvres at low power.34.0 Describe the power response of a slightly supercritical reactor atlow power.35.0 Explain the change in power response as power approaches the lastdecade of reactor power. Describe how reactor power is increased to rated power fromhot shutdown.36.0 List the reactivity effects that occur as power rises and state howthey are expected to change as power increases.Reactor Operations at High Power37.0 Explain how flux-flattening permits increased reactor power outputwithout an unsafe increase in peak bundle or channel power.38.0to:-Describe and explain how flux flattening is achieved with respectthe overall flux shape,relative zone to zone flux shape,control of local flux peaks.39.0 Explain how a change of reactivity device configuration can causeflux peaks in the core and how this can affect bundle and channelpowers.40.0-Define the terms:reactivity device worth,differential reactivity device worth.41.0 Explain the variation in rate of reactivity insertion given a curve ofrod worth vs. position.42.0 Describe the response of the liquid zones while the adjusters aredriving from full in to full out.43.0 Describe the effect on flux shape of replacing h