Reactor Theory (Reactor Operations)DOE-HDBK-1019/2-93REACTOR OPERATIONFor most reactor designs, the only factors that change significantly after the reactor is shut downare the average reactor temperature and the concentration of fission product poisons. Thereactivities normally considered when calculating an ECP include the following.Basic Reactivity of the Core-The reactivity associated with the critical control rodposition for a xenon-free core at normal operatingtemperature. This reactivity varies with the age of the core(amount of fuel burnup).Direct Xenon Reactivity -The reactivity related to the xenon that was actually presentin the core at the time it was shutdown. This reactivity iscorrected to allow for xenon decay.Indirect Xenon Reactivity -The reactivity related to the xenon produced by the decayof iodine that was present in the core at the time ofshutdown.Temperature Reactivity -The reactivity related to the difference between the actualreactor temperature during startup and the normal operatingtemperature.To arrive at an ECP of the control rods, the basic reactivity, direct and indirect xenon reactivity,and temperature reactivity are combined algebraically to determine the amount of positive controlrod reactivity that must be added by withdrawing control rods to attain criticality. A graph ofcontrol rod worth versus rod position is used to determine the estimated critical position.CorePowerDistributionIn order to ensure predictable temperatures and uniform depletion of the fuel installed in areactor, numerous measures are taken to provide an even distribution of flux throughout thepower producing section of the reactor. This shaping, or flattening, of the neutron flux isnormally achieved through the use of reflectors that affect the flux profile across the core, orby the installation of poisons to suppress the neutron flux where desired. The last method,although effective at shaping the flux, is the least desirable since it reduces neutron economy byabsorbing the neutrons.A reactor core is frequently surrounded by a "reflecting" material to reduce the ratio of peakflux to the flux at the edge of the core fuel area. Reflector materials are normally notfissionable, have a high scattering cross section, and have a low absorption cross section.Essentially, for thermal reactors a good moderator is a good reflector. Water, heavy water,beryllium, zirconium, or graphite are commonly used as reflectors. In fast reactor systems,reflectors are not composed of moderating materials because it is desired to keep neutron energyhigh. The reflector functions by scattering some of the neutrons, which would have leaked froma bare (unreflected) core, back into the fuel to produce additional fissions. Rev. 0NP-04Page 25
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