# Reactor Kinetics and Operation

Reactor Kinetics and Operation

Reactor Kinetics and Operation

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Lectures on Applied <strong>Reactor</strong> Technology <strong>and</strong> Nuclear Power SafetyLecture No 5Title:<strong>Reactor</strong> <strong>Kinetics</strong> <strong>and</strong> <strong>Reactor</strong> <strong>Operation</strong>Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy TechnologyKTHSpring 2005Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 1

Outline of the Lecture (1)• <strong>Reactor</strong> <strong>Kinetics</strong>– <strong>Reactor</strong> Period– Effective Delayed Neutron Fraction– Effective Delayed Neutron Precursor Decay Constant– Prompt Criticality– Stable Period Equation– <strong>Reactor</strong> Startup Rate (SUR)– Doubling TimeApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 2

Outline of the Lecture (2)• <strong>Reactor</strong> <strong>Operation</strong>– Startup– Estimated Critical Position– Core Power Distribution– Power Tilt– Shutdown Margin– <strong>Operation</strong>– Temperature– Pressure– Power Level– Flow– Core Burnup– Shutdown– Decay heatApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 3

<strong>Reactor</strong> Period (2)• The smaller the value of τ, the more rapid the change inreactor power• If the reactor period is positive, reactor power isincreasing• If the reactor period is negative, reactor power isdecreasingApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 5

<strong>Reactor</strong> Period (3)• There are numerous equations used to express reactorperiod, but Equation shown below, or portions of it, willbe useful in most situationsτ =l *+ρβλeffeff− ρρ + & ρlβλ*effρeff& ρ=====prompt generation lifetimeeffective delayed neutron fractionreactivityeffective deleyed neutron precursor decay constantrate of change of reactivity• The first term in Equation is the prompt term <strong>and</strong> thesecond term is the delayed termApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 6

Effective Delayed Neutron Fraction (1)• Recall that , the delayed neutron fraction, is the fractionof all fission neutrons that are born as delayed neutrons• The value of β depends upon the actual nuclear fuelused• the delayed neutron precursors for a given type of fuelare grouped on the basis of half-life• The table on the next slide lists the fractional neutronyields for each delayed neutron group of three commontypes of fuelApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 7

Effective Delayed Neutron Fraction (2)Group Half-Life Uranium-235 Uranium-238 Plutonium-239(sec)1 55.6 0.00021 0.0002 0.000212 22.7 0.00141 0.0022 0.001823 6.22 0.00127 0.0025 0.001294 2.30 0.00255 0.0061 0.001995 0.61 0.00074 0.0035 0.000526 0.23 0.00027 0.0012 0.00027TOTAL - 0.00650 0.0157 0.00200Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 8

Effective Delayed Neutron Fraction (3)β• The term (beta-bar) is the average delayed neutron fractionβ• The value of is the weighted average of the total delayed neutronfractions of the individual types of fuel• Each total delayed neutron fraction value for each type of fuel is weightedby the percent of total neutrons that the fuel contributes through fission• If the percentage of fissions occurring in the different types of fuel in areactor changes over the life of the core, the average delayed neutronfraction will also change• For a light water reactor using low enriched fuel, the average delayedneutron fraction can change from 0.0070 to 0.0055 as uranium-235 isburned out <strong>and</strong> plutonium-239 is produced from uranium-238Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 9

Effective Delayed Neutron Fraction (4)• Delayed neutrons do not have the same properties as prompt neutronsreleased directly from fission• The average energy of prompt neutrons is about 2 MeV. This is muchgreater than the average energy of delayed neutrons (about 0.5 MeV)• The fact that delayed neutrons are born at lower energies has twosignificant impacts on the way they proceed through the neutron life cycle• First, delayed neutrons have a much lower probability of causing fastfissions than prompt neutrons because their average energy is less than theminimum required for fast fission to occur• Second, delayed neutrons have a lower probability of leaking out of the corewhile they are at fast energies, because they are born at lower energies <strong>and</strong>subsequently travel a shorter distance as fast neutronsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 10

Effective Delayed Neutron Fraction (5)• These two considerations (lower fast fission factor <strong>and</strong>higher fast non-leakage probability for delayed neutrons)are taken into account by a term called the importancefactor (I)• The importance factor relates the average delayedneutron fraction to the effective delayed neutron fraction• The effective delayed neutron fraction β effis defined asthe fraction of neutrons at thermal energies which wereborn delayedβeff= β ⋅IApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 11

Effective Delayed Neutron Fraction (6)• In a small reactor with highly enriched fuel, the increasein fast non-leakage probability will dominate thedecrease in the fast fission factor, <strong>and</strong> the importancefactor will be greater than one• In a large reactor with low enriched fuel, the decrease inthe fast fission factor will dominate the increase in thefast non-leakage probability <strong>and</strong> the importance factorwill be less than one (about 0.97 for a commercial PWR).Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 12

Effective Delayed Neutron Precursor DecayConstant (1)• Another new term has been introduced in the reactor period (τ )equation• That term is λ eff (pronounced lambda effective), the effectivedelayed neutron precursor decay constant• The decay rate for a given delayed neutron precursor can beexpressed as the product of precursor concentration <strong>and</strong> the decayconstant ( λ) of that precursor• The decay constant of a precursor is simply the fraction of an initialnumber of the precursor atoms that decays in a given unit timeApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 13

Effective Delayed Neutron Precursor DecayConstant (2)• A decay constant of 0.1 sec -1 , for example, implies thatone-tenth, or ten percent, of a sample of precursoratoms decays within one second• The value for the effective delayed neutron precursordecay constant, λ eff , varies depending upon the balanceexisting between the concentrations of the precursorgroups <strong>and</strong> the nuclide(s) being used as the fuelApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 14

Effective Delayed Neutron Precursor DecayConstant (3)• If the reactor is operating at a constant power, all the precursorgroups reach an equilibrium value• During an up-power transient, however, the shorter-lived precursorsdecaying at any given instant were born at a higher power level (orflux level) than the longer-lived precursors decaying at the sameinstant• There is, therefore, proportionately more of the shorter-lived <strong>and</strong>fewer of the longer-lived precursors decaying at that given instantthan there are at constant power• The value of λ eff is closer to that of the shorter-lived precursorsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 15

Effective Delayed Neutron Precursor DecayConstant (4)• During a down-power transient the longer-livedprecursors become more significant• The longer-lived precursors decaying at a given instantwere born at a higher power level (or flux level) than theshorter-lived precursors decaying at that instant• Therefore, proportionately more of the longer-livedprecursors are decaying at that instant, <strong>and</strong> the value ofλ eff approaches the values of the longer-lived precursorsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 16

Effective Delayed Neutron Precursor DecayConstant (5)• Approximate values for λ eff are 0.08 sec -1 for steadystateoperation, 0.1 sec -1 for a power increase, <strong>and</strong> 0.05sec -1 for a power decrease• The exact values will depend upon the materials used forfuel <strong>and</strong> the value of the reactivity of the reactor coreApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 17

Effective Delayed Neutron Precursor DecayConstant (6)• Let us recall the equation for reactor period:*lτ = +ρprompttermβλeffeff− ρρ + & ρdelayedterm• If the positive reactivity added is less than the value of , theemission of prompt fission neutrons alone is not sufficient toovercome losses to non-fission absorption <strong>and</strong> leakage• If delayed neutrons were not being produced, the neutron populationwould decrease as long as the reactivity of the core has a value lessthan the effective delayed neutron fractionβ effApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 18

Effective Delayed Neutron Precursor DecayConstant (7)• The positive reactivity insertion is followed immediatelyby a small immediate power increase called the promptjump• This power increase occurs because the rate ofproduction of prompt neutrons changes abruptly as thereactivity is added• Recall that the generation time for prompt neutrons is onthe order of 10 -13 seconds• The effect can be seen in Figure on the next slideApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 19

Effective Delayed Neutron Precursor DecayConstant (8)Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 20

Effective Delayed Neutron Precursor DecayConstant (9)• After the prompt jump, the rate of change of power cannot increaseany more rapidly than the built-in time delay the precursor half-livesallow• Therefore, the power rise is controllable, <strong>and</strong> the reactor can beoperated safely• Conversely, in the case where negative reactivity is added to thecore there will be a prompt drop in reactor power• The prompt drop is the small immediate decrease in reactor powercaused by the negative reactivity addition. The prompt drop isillustrated in Figure on the next slideApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 21

Effective Delayed Neutron Precursor DecayConstant (10)Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 22

Prompt Criticality (1)• It can be readily seen from Equation for the reactorperiod that if the amount of positive reactivity addedequals the value of β eff , the reactor period equationbecomes the followingτ =*lρ+βλeffeff− ρρ + & ρβ⇒eff= ρτ =*lρApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 23

Prompt Criticality (2)• In this case, the production of prompt neutrons alone isenough to balance neutron losses <strong>and</strong> increase theneutron population• The condition where the reactor is critical on promptneutrons, <strong>and</strong> the neutron population increases asrapidly as the prompt neutron generation lifetime allowsis known as prompt critical• The prompt critical condition does not signal a dramaticchange in neutron behaviorApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 24

Prompt Criticality (3)• The reactor period changes in a regular manner between reactivitiesabove <strong>and</strong> below this reference• Prompt critical is, however, a convenient condition for marking thetransition from delayed neutron to prompt neutron time scales• A reactor whose reactivity even approaches prompt critical is likelyto suffer damage due to the rapid rise in power to a very high level• For example, a reactor which has gone prompt critical couldexperience a several thous<strong>and</strong> percent power increase in less thanone secondApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 25

Prompt Criticality (4)• Because the prompt critical condition is so important, a specific unitof reactivity has been defined that relates to it• The unit of reactivity is the dollar ($), where one dollar of reactivity isequivalent to the effective delayed neutron fraction• A reactivity unit related to the dollar is the cent, where one cent isone-hundredth of a dollar• If the reactivity of the core is one dollar, the reactor is prompt critical• Because the effective delayed neutron fraction is dependent uponthe nuclides used as fuel, the value of the dollar is also dependenton the nuclides used as fuelβ effApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 26

Stable Period Equation (1)• For normal reactor operating conditions, the value of positivereactivity in the reactor is never permitted to approach the effectivedelayed neutron fraction, <strong>and</strong> the reactor period equation is normallywritten as followsτ =βλeffeff− ρρ + & ρ• The above Equation is referred to as the transient period equationsince it incorporates the ρ& term to account for the changing amountof reactivity in the core*• The l ρ term (prompt period) is normally negligible with respect tothe remainder of the equation <strong>and</strong> is often not includedApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 27

Stable Period Equation (2)• For conditions when the amount of reactivity in the coreis constant ( ρ& = 0), <strong>and</strong> the reactor period is unchanging,the reactor period Equation can be simplified further toEquation shown below which is known as the stableperiod equationτ =βeff−λeffρρApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 28

Doubling Time (1)• Sometimes it is useful to discuss the rate of change of reactor powerin terms similar to those used in radioactive decay calculations• Doubling or halving time are terms that relate to the amount oftime it takes reactor power to double or be reduced to one-half theinitial power level• If the stable reactor period is known, doubling time can bedetermined as follows:• Doubling time DT = τ ln 2, where τ is stable reactor periodApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 31

<strong>Reactor</strong> <strong>Operation</strong> - Startup (1)• When a reactor is started up with unirradiated fuel, or onthose occasions when the reactor is restarted following along shutdown period, the source neutron population willbe very low• In some reactors, the neutron population is frequentlylow enough that it cannot be detected by the nuclearinstrumentation during the approach to criticality• Installed neutron sources, such as those discussed inearlier parts of this course, are frequently used toprovide a safe, easily monitored reactor startupApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 32

<strong>Reactor</strong> <strong>Operation</strong> - Startup (2)• The neutron source, together with the subcritical multiplicationprocess, provides a sufficiently large neutron population to allowmonitoring by the nuclear instruments throughout the startupprocedure• Without the installed source, it may be possible to withdraw thecontrol rods to the point of criticality, <strong>and</strong> then continue withdrawalwithout detecting criticality because the reactor goes critical belowthe indicating range• Continued withdrawal of control rods at this point could causereactor power to rise at an uncontrollable rate before neutron levelfirst becomes visible on the nuclear instrumentsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 33

<strong>Reactor</strong> <strong>Operation</strong> - Startup (3)• An alternative to using a startup source is to limit the rateof rod withdrawal, or require waiting periods between rodwithdrawal increments• By waiting between rod withdrawal increments, theneutron population is allowed to increase throughsubcritical multiplication• Subcritical multiplication is the process where sourceneutrons are used to sustain the chain reaction in areactor with a multiplication factor (k eff ) of less than oneApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 34

<strong>Reactor</strong> <strong>Operation</strong> - Startup (4)• The chain reaction is not "self-sustaining," but if theneutron source is of sufficient magnitude, it compensatesfor the neutrons lost through absorption <strong>and</strong> leakage• This process can result in a constant, or increasing,neutron population even though k eff is less than oneApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 35

Estimated Critical Position (1)• In the previous lecture, 1/M plots were discussed• These plots were useful for monitoring the approach tocriticality <strong>and</strong> predicting when criticality will occur basedon indications received while the startup is actually inprogress• Before the reactor startup is initiated, the operatorcalculates an estimate of the amount of rod withdrawalthat will be necessary to achieve criticalityApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 36

Estimated Critical Position (2)• This process provides an added margin of safety because a largediscrepancy between actual <strong>and</strong> estimated critical rod positionswould indicate that the core was not performing as designed• Depending upon a reactor's design or age, the buildup of xenonwithin the first several hours following a reactor shutdown mayintroduce enough negative reactivity to cause the reactor to remainshutdown even with the control rods fully withdrawn• In this situation it is important to be able to predict whether criticalitycan be achieved, <strong>and</strong> if criticality cannot be achieved, the startupshould not be attemptedApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 37

Estimated Critical Position (3)• For a given set of conditions (such as time since shutdown,temperature, pressure, fuel burnup, samarium <strong>and</strong> xenon poisoning)there is only one position of the control rods (<strong>and</strong> boronconcentrations for a reactor with chemical shim) that results incriticality, using the normal rod withdrawal sequence• Identification of these conditions allows accurate calculation ofcontrol rod position at criticality• The calculation of an estimated critical position (ECP) is simply amathematical procedure that takes into account all of the changes infactors that significantly affect reactivity that have occurred betweenthe time of reactor shutdown <strong>and</strong> the time that the reactor is broughtcritical againApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 38

Estimated Critical Position (4)• For most reactor designs, the only factors that changesignificantly after the reactor is shut down are theaverage reactor temperature <strong>and</strong> the concentration offission product poisons• The reactivities normally considered when calculating anECP include the following– Basic reactivity of the core– Direct xenon reactivity– Indirect xenon reactivity– Temperature reactivityApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 39

Estimated Critical Position (5)• Basic Reactivity of the Core:– The reactivity associated with the critical control rod position fora xenon-free core at normal operating temperature. Thisreactivity varies with the age of the core (amount of fuel burnup)• Direct Xenon Reactivity:– The reactivity related to the xenon that was actually present inthe core at the time it was shutdown. This reactivity is correctedto allow for xenon decayApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 40

Estimated Critical Position (6)• Indirect Xenon Reactivity:– The reactivity related to the xenon produced by the decay ofiodine that was present in the core at the time of shutdown• Temperature Reactivity:– The reactivity related to the difference between the actual reactortemperature during startup <strong>and</strong> the normal operatingtemperatureApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 41

Estimated Critical Position (7)• To arrive at an ECP of the control rods, the basicreactivity, direct <strong>and</strong> indirect xenon reactivity, <strong>and</strong>temperature reactivity are combined algebraically todetermine the amount of positive control rod reactivitythat must be added by withdrawing control rods to attaincriticality• A graph of control rod worth versus rod position is usedto determine the estimated critical positionApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 42

Core Power Distribution (1)• In order to ensure predictable temperatures <strong>and</strong> uniform depletion ofthe fuel installed in a reactor, numerous measures are taken toprovide an even distribution of flux throughout the power producingsection of the reactor• This shaping, or flattening, of the neutron flux is normally achievedthrough the use of reflectors that affect the flux profile across thecore, or by the installation of poisons to suppress the neutron fluxwhere desired• The last method, although effective at shaping the flux, is the leastdesirable since it reduces neutron economy by absorbing theneutronsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 43

Core Power Distribution (2)• A reactor core is frequently surrounded by a "reflecting"material to reduce the ratio of peak flux to the flux at theedge of the core fuel area• Reflector materials are normally not fissionable, have ahigh scattering cross section, <strong>and</strong> have a low absorptioncross section• Essentially, for thermal reactors a good moderator is agood reflectorApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 44

Core Power Distribution (3)• Water, heavy water, beryllium, zirconium, or graphite arecommonly used as reflectors• In fast reactor systems, reflectors are not composed ofmoderating materials because it is desired to keepneutron energy high• The reflector functions by scattering some of theneutrons, which would have leaked from a bare(unreflected) core, back into the fuel to produceadditional fissionsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 45

Core Power Distribution (4)• Figure on the next slide shows the general effect ofreflection in the thermal reactor system where corepower is proportional to the thermal flux• Notice that a reflector can raise the power density of thecore periphery <strong>and</strong> thus increase the core averagepower level without changing the peak power• As illustrated in the Figure, the thermal flux in thereflector may actually be higher than that in theoutermost fuel since there are very few absorptions inthe reflectorApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 46

Core Power Distribution (5)Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 47

Core Power Distribution (6)• Varying the fuel enrichment or fuel concentrations in the coreradially, axially, or both, can readily be used to control powerdistribution• The simplified example illustrated in Figure on the next slide showsthe effect of using a higher enrichment in the outer regions of thecore• Varying fuel concentrations or poison loading for flux shaping isfrequently referred to as zoning• In the example illustrated the large central peak is reduced, but theaverage power level remains the sameApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 48

Core Power Distribution (7)Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 49

Core Power Distribution (8)• The previous examples discuss changes in radial power distribution• Large variations also exist in axial power distribution• Figure (A) on the next slide illustrates the power distribution thatmay exist for a reactor with a cylindrical geometry• The control rods in this reactor are inserted from the top, <strong>and</strong> theeffect of inserting control rods further is shown in Figure (B)• The thermal flux is largely suppressed in the vicinity of the controlrods, <strong>and</strong> the majority of the power is generated low in the core• This flux profile can be flattened by the use of axial fuel <strong>and</strong>/orpoison zoningApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 50

Core Power Distribution (9)Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 51

Power Tilt (1)• A power tilt, or flux tilt, is a specific type of core power distributionproblem• It is a non-symmetrical variation of core power in one quadrant ofthe core relative to the others• The power in one portion might be suppressed by over-insertion ofcontrol rods in that portion of the core, which, for a constant overallpower level, results in a relatively higher flux in the remainder of thecore• This situation can lead to xenon oscillations, which were previouslydiscussedApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 52

Shutdown Margin (1)• Shutdown margin is the instantaneous amount ofreactivity by which a reactor is subcritical or would besubcritical from its present condition assuming all controlrods are fully inserted except for the single rod with thehighest integral worth, which is assumed to be fullywithdrawn• Shutdown margin is required to exist at all times, evenwhen the reactor is critical• It is important that there be enough negative reactivitycapable of being inserted by the control rods to ensurecomplete shutdown at all times during the core lifetimeApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 53

Shutdown Margin (2)• A shutdown margin in the range of one to five percentreactivity is typically required• The stuck rod criterion refers to the fact that theshutdown margin does not take credit for the insertion ofthe highest worth control rod• The application of the stuck rod criterion ensures that thefailure of a single control rod will not prevent the controlrod system from shutting down the reactorApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 54

<strong>Reactor</strong> <strong>Operation</strong> (1)• During reactor operation, numerous parameters such astemperature, pressure, power level, <strong>and</strong> flow arecontinuously monitored <strong>and</strong> controlled to ensure safe<strong>and</strong> stable operation of the reactor• The specific effects of variations in these parametersvary greatly depending upon reactor design, butgenerally the effects for thermal reactors are asdiscussed in the followingApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 55

Temperature Effects (1)• The most significant effect of a variation in temperatureupon reactor operation is the addition of positive ornegative reactivity• As previously discussed, reactors are generally designedwith negative temperature coefficients of reactivity(moderator <strong>and</strong> fuel temperature coefficients) as a selflimitingsafety feature• A rise in reactor temperature results in the addition ofnegative reactivityApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 56

Temperature Effects (2)• If the rise in temperature is caused by an increase inreactor power, the negative reactivity addition slows, <strong>and</strong>eventually turns the increase in reactor power• This is a highly desirable effect because it provides anegative feedback in the event of an undesired powerexcursionApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 57

Temperature Effects (3)• Negative temperature coefficients can also be utilized in watercooled <strong>and</strong> moderated power reactors to allow reactor power toautomatically follow energy dem<strong>and</strong>s that are placed upon thesystem• For example, consider a reactor operating at a stable power levelwith the heat produced being transferred to a heat exchanger foruse in an external closed cycle system• If the energy dem<strong>and</strong> in the external system increases, more energyis removed from reactor system causing the temperature of thereactor coolant to decrease• As the reactor temperature decreases, positive reactivity is added<strong>and</strong> a corresponding increase in reactor power level resultsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 58

Temperature Effects (4)• As reactor power increases to a level above the level of the newenergy dem<strong>and</strong>, the temperature of the moderator <strong>and</strong> fuelincreases, adding negative reactivity <strong>and</strong> decreasing reactor powerlevel to near the new level required to maintain system temperature• Some slight oscillations above <strong>and</strong> below the new power level occurbefore steady state conditions are achieved• The final result is that the average temperature of the reactor systemis essentially the same as the initial temperature, <strong>and</strong> the reactor isoperating at the new higher required power level• The same inherent stability can be observed as the energy dem<strong>and</strong>on the system is decreasedApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 59

Temperature Effects (5)• If the secondary system providing cooling to the reactorheat exchanger is operated as an open system withonce-through cooling, the above discussion is notapplicable• In these reactors, the temperature of the reactor isproportional to the power level, <strong>and</strong> it is impossible forthe reactor to be at a higher power level <strong>and</strong> the sametemperatureApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 60

Pressure Effects (1)• The pressure applied to the reactor system can alsoaffect reactor operation by causing changes in reactivity• The reactivity changes result from changes in the densityof the moderator in response to the pressure changes• For example, as the system pressure rises, themoderator density increases <strong>and</strong> results in greatermoderation, less neutron leakage, <strong>and</strong> therefore theinsertion of positive reactivityApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 61

Pressure Effects (2)• A reduction in system pressure results in the addition ofnegative reactivity• Typically, in pressurized water reactors (PWR), themagnitude of this effect is considerably less than that ofa change in temperature• In two-phase systems such as boiling water reactors(BWR), however, the effects of pressure changes aremore noticeable because there is a greater change inmoderator density for a given change in system pressureApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 62

Power Level Effects (1)• A change in reactor power level can result in a change inreactivity if the power level change results in a change insystem temperature• The power level at which the reactor is producingenough energy to make up for the energy lost to ambientis commonly referred to as the point of adding heatApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 63

Power Level Effects (2)• If a reactor is operating well below the point of addingheat, then variations in power level produce nomeasurable variations in temperature• At power levels above the point of adding heat,temperature varies with power level, <strong>and</strong> the reactivitychanges will follow the convention previously describedfor temperature variationsApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 64

Power Level Effects (3)• The inherent stability <strong>and</strong> power turning ability of anegative temperature coefficient are ineffective belowthe point of adding heat• If a power excursion is initiated from a very low powerlevel, power will continue to rise unchecked until thepoint of adding heat is reached, <strong>and</strong> the subsequenttemperature rise adds negative reactivity to slow, <strong>and</strong>turn, the rise of reactor power• In this region, reactor safety is provided by automaticreactor shutdown systems <strong>and</strong> operator actionApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 65

Flow Effects (1)• At low reactor power levels, changing the flow rate of thecoolant through the reactor does not result in ameasurable reactivity change because fuel <strong>and</strong>moderator temperatures <strong>and</strong> the fraction of steam voidsoccurring in the core are not changed appreciably• When the flow rate is varied, however, the change intemperature that occurs across the core (outlet versusinlet temperature) will vary inversely with the flow rate• At higher power levels, on liquid cooled systems,increasing flow will lower fuel <strong>and</strong> coolant temperaturesslightly, resulting in a small positive reactivity insertionApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 66

Flow Effects (2)• A positive reactivity addition also occurs when flow is increased in atwo-phase (steam-water) cooled system• Increasing the flow rate decreases the fraction of steam voids in thecoolant <strong>and</strong> results in a positive reactivity addition• This property of the moderator in a two-phase system is usedextensively in commercial BWRs• Normal power variations required to follow load changes on BWRsare achieved by varying the coolant/moderator flow rateApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 67

Core Burnup (1)• As a reactor is operated, atoms of fuel are constantlyconsumed, resulting in the slow depletion of the fuelfrequently referred to as core burnup• There are several major effects of this fuel depletion• The first, <strong>and</strong> most obvious, effect of the fuel burnup isthat the control rods must be withdrawn or chemical shimconcentration reduced to compensate for the negativereactivity effect of this burnupApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 68

Core Burnup (2)• Some reactor designs incorporate the use ofsupplemental burnable poisons in addition to the controlrods to compensate for the reactivity associated withexcess fuel in a new core• These fixed burnable poisons burn out at a rate thatapproximates the burnout of the fuel <strong>and</strong> they reduce theamount of control rod movement necessary tocompensate for fuel depletion early in core lifeApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 69

Core Burnup (3)• As control rods are withdrawn to compensate for fueldepletion, the effective size of the reactor is increased• By increasing the effective size of the reactor, theprobability that a neutron slows down <strong>and</strong> is absorbedwhile it is still in the reactor is also increased• Therefore, neutron leakage decreases as the effectivereactor size is increasedApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 70

Core Burnup (4)• The magnitude of the moderator negative temperaturecoefficient is determined in part by the change in neutronleakage that occurs as the result of a change inmoderator temperature• Since the fraction of neutrons leaking out is less with thelarger core, a given temperature change will have less ofan effect on the leakage• Therefore, the magnitude of the moderator negativetemperature coefficient decreases with fuel burnupApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 71

Core Burnup (5)• There is also another effect that is a consideration onlyon reactors that use dissolved boron in the moderator(chemical shim)• As the fuel is burned up, the dissolved boron in themoderator is slowly removed (concentration diluted) tocompensate for the negative reactivity effects of fuelburnup• This action results in a larger (more negative) moderatortemperature coefficient of reactivity in a reactor usingchemical shimApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 72

Core Burnup (6)• This is due to the fact that when water density is decreased by risingmoderator temperature in a reactor with a negative temperaturecoefficient, it results in a negative reactivity addition because somemoderator is forced out of the core• With a coolant containing dissolved poison, this density decreasealso results in some poison being forced out of the core, which is apositive reactivity addition, thereby reducing the magnitude of thenegative reactivity added by the temperature increase• Because as fuel burnup increases the concentration of boron isslowly lowered, the positive reactivity added by the above poisonremoval process is lessened, <strong>and</strong> this results in a larger negativetemperature coefficient of reactivityApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 73

Core Burnup (7)• The following effect of fuel burnup is most predominantin a reactor with a large concentration of uranium-238• As the fission process occurs in a thermal reactor withlow or medium enrichment, there is some conversion ofuranium-238 into plutonium-239• Near the end of core life in certain reactors, the powercontribution from the fission of plutonium-239 may becomparable to that from the fission of uranium-235Applied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 74

Core Burnup (8)• The value of the delayed neutron fraction (b) foruranium-235 is 0.0064 <strong>and</strong> for plutonium-239 is 0.0021• Consequently, as core burnup progresses, the effectivedelayed neutron fraction for the fuel decreasesappreciably• It follows then that the amount of reactivity insertionneeded to produce a given reactor period decreases withburnup of the fuelApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 75

<strong>Reactor</strong> Shutdown (1)• A reactor is considered to be shut down when it issubcritical <strong>and</strong> sufficient shutdown reactivity exists sothere is no immediate probability of regaining criticality• Shutdown is normally accomplished by insertion of some(or all) of the control rods, or by introduction of solubleneutron poison into the reactor coolant• The rate at which the reactor fission rate decaysimmediately following shutdown is similar for all reactorsprovided a large amount of negative reactivity is insertedApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 76

<strong>Reactor</strong> Shutdown (2)• After a large negative reactivity addition the neutron levelundergoes a rapid decrease of about two decades(prompt drop) until it is at the level of production ofdelayed neutrons• Then the neutron level slowly drops off as the delayedneutron precursors decay, <strong>and</strong> in a short while only thelongest-lived precursor remains in any significant amount• This precursor determines the final rate of decrease inreactor power until the neutron flux reaches the steadystate level corresponding to the subcritical multiplicationof the neutron sourceApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 77

<strong>Reactor</strong> Shutdown (3)• The half-life of the longest lived delayed neutronprecursor results in a reactor period of around -80seconds or a startup rate of -1/3 DPM for most reactorsafter a reactor shutdown• One noticeable exception to this is a heavy water reactor• In a heavy water reactor, the photo-neutron source isextremely large after shutdown due to the amount ofdeuterium in the moderator <strong>and</strong> the large number of highenergy gammas from short-lived fission product decayApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 78

<strong>Reactor</strong> Shutdown (4)• The photo-neutron source is large enough to have asignificant impact on neutron population immediatelyafter shutdown• The photo-neutron source has the result of flux levelsdecreasing more slowly so that a heavy water reactorwill have a significantly larger negative reactor periodafter a shutdownApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 79

<strong>Reactor</strong> Shutdown (5)• Throughout the process of reactor shutdown the nuclearinstrumentation is closely monitored to observe thatreactor neutron population is decreasing as expected,<strong>and</strong> that the instrumentation is functioning properly toprovide continuous indication of neutron population• Instrumentation is observed for proper overlap betweenranges, comparable indication between multipleinstrument channels, <strong>and</strong> proper decay rate of neutronpopulationApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 80

<strong>Reactor</strong> Shutdown (6)• A distinction should be made between indicated reactorpower level after shutdown <strong>and</strong> the actual thermal powerlevel• The indicated reactor power level is the power produceddirectly from fission in the reactor core, but the actualthermal power drops more slowly due to decay heatproduction as previously discussed• Decay heat, although approximately 5 to 6% of thesteady state reactor power prior to shutdown, diminishesto less than 1% of the pre-shutdown power level afterabout one hourApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 81

<strong>Reactor</strong> Shutdown (7)• After a reactor is shutdown, provisions are provided for the removalof decay heat• If the reactor is to be shut down for only a short time, operatingtemperature is normally maintained• If the shutdown period will be lengthy or involves functions requiringcooldown of the reactor, the reactor temperature can be lowered bya number of methods• The methods for actually conducting cooldown of the reactor varydepending on plant design, but in all cases limitations are imposedon the maximum rate at which the reactor systems may be cooledApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 82

<strong>Reactor</strong> Shutdown (8)• These limits are provided to reduce the stress applied to systemmaterials, thereby reducing the possibility of stress induced failure• Although a reactor is shut down, it must be continuously monitoredto ensure the safety of the reactor• Automatic monitoring systems are employed to continuously collect<strong>and</strong> assess the data provided by remote sensors• It is ultimately the operator who must ensure the safety of thereactorApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 83

Decay Heat (1)• About 7% of the 200 MeV produced by an averagefission is released at some time after the instant offission• This energy comes from the decay of the fissionproducts• When a reactor is shut down, fission essentially ceases,but decay energy is still being produced• The energy produced after shutdown is referred to asdecay heatApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 84

Decay Heat (2)• The amount of decay heat production after shutdown isdirectly influenced by the power history of the reactorprior to shutdown• A reactor operated at full power for 3 to 4 days prior toshutdown has much higher decay heat generation than areactor operated at low power for the same period• The decay heat produced by a reactor shutdown fromfull power is initially equivalent to about 5 to 6% of thethermal rating of the reactorApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 85

Decay Heat (3)• This decay heat generation rate diminishes to less than1% approximately one hour after shutdown• However, even at these low levels, the amount of heatgenerated requires the continued removal of heat for anappreciable time after shutdown• Decay heat is a long-term consideration <strong>and</strong> impactsspent fuel h<strong>and</strong>ling, reprocessing, waste management,<strong>and</strong> reactor safetyApplied <strong>Reactor</strong> Technology <strong>and</strong>Nuclear Power Safety - Lecture 5Henryk AnglartNuclear <strong>Reactor</strong> Technology DivisionDepartment of Energy Technology, KTHSlide No 86