REACTOR OPERATIONDOE-HDBK-1019/2-93Reactor Theory (Reactor Operations)StartupWhen a reactor is started up with unirradiated fuel, or on those occasions when the reactor isrestarted following a long shutdown period, the source neutron population will be very low. Insome reactors, the neutron population is frequently low enough that it cannot be detected by thenuclear instrumentation during the approach to criticality. Installed neutron sources, such asthose discussed in Module 2, are frequently used to provide a safe, easily monitored reactorstartup. The neutron source, together with the subcritical multiplication process, provides asufficiently large neutron population to allow monitoring by the nuclear instruments throughoutthe startup procedure. Without the installed source, it may be possible to withdraw the controlrods to the point of criticality, and then continue withdrawal without detecting criticality becausethe reactor goes critical below the indicating range. Continued withdrawal of control rods at thispoint could cause reactor power to rise at an uncontrollable rate before neutron level firstbecomes visible on the nuclear instruments.An alternative to using a startup source is to limit the rate of rod withdrawal, or require waitingperiods between rod withdrawal increments. By waiting between rod withdrawal increments,the neutron population is allowed to increase through subcritical multiplication. Subcriticalmultiplication is the process where source neutrons are used to sustain the chain reaction in areactor with a multiplication factor (keff) of less than one. The chain reaction is not"self-sustaining," but if the neutron source is of sufficient magnitude, it compensates for theneutrons lost through absorption and leakage. This process can result in a constant, orincreasing, neutron population even though keff is less than one.EstimatedCriticalPositionIn the first chapter of this module, 1/M plots were discussed. These plots were useful formonitoring the approach to criticality and predicting when criticality will occur based onindications received while the startup is actually in progress. Before the reactor startup isinitiated, the operator calculates an estimate of the amount of rod withdrawal that will benecessary to achieve criticality. This process provides an added margin of safety because a largediscrepancy between actual and estimated critical rod positions would indicate that the core wasnot performing as designed. Depending upon a reactor's design or age, the buildup of xenonwithin the first several hours following a reactor shutdown may introduce enough negativereactivity to cause the reactor to remain shutdown even with the control rods fully withdrawn.In this situation it is important to be able to predict whether criticality can be achieved, and ifcriticality cannot be achieved, the startup should not be attempted.For a given set of conditions (such as time since shutdown, temperature, pressure, fuel burnup,samarium and xenon poisoning) there is only one position of the control rods (and boronconcentrations for a reactor with chemical shim) that results in criticality, using the normal rodwithdrawal sequence. Identification of these conditions allows accurate calculation of controlrod position at criticality. The calculation of an estimated critical position (ECP) is simply amathematical procedure that takes into account all of the changes in factors that significantlyaffect reactivity that have occurred between the time of reactor shutdown and the time that thereactor is brought critical again.NP-04Rev. 0Page 24
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