Reactor Theory (Reactor Operations)DOE-HDBK-1019/2-93REACTOR OPERATIONThere is also another effect that is a consideration only on reactors that use dissolved boron inthe moderator (chemical shim). As the fuel is burned up, the dissolved boron in the moderatoris slowly removed (concentration diluted) to compensate for the negative reactivity effects of fuelburnup. This action results in a larger (more negative) moderator temperature coefficient ofreactivity in a reactor using chemical shim. This is due to the fact that when water density isdecreased by rising moderator temperature in a reactor with a negative temperature coefficient,it results in a negative reactivityaddition because some moderator is forced out of the core.With a coolant containing dissolved poison, this density decrease also results in some poisonbeing forced out of the core, which is a positive reactivity addition, thereby reducing themagnitude of the negative reactivity added by the temperature increase. Because as fuel burnupincreases the concentration of boron is slowly lowered, the positive reactivity added by the abovepoison removal process is lessened, and this results in a larger negative temperature coefficientof reactivity.The following effect of fuel burnup is most predominant in a reactor with a large concentrationof uranium-238. As the fission process occurs in a thermal reactor with low or mediumenrichment, there is some conversion of uranium-238 into plutonium-239. Near the end of corelife in certain reactors, the power contribution from the fission of plutonium-239 may becomparable to that from the fission of uranium-235. The value of the delayed neutron fraction(b) for uranium-235 is 0.0064 and for plutonium-239 is 0.0021. Consequently, as core burnupprogresses, the effective delayed neutron fraction for the fuel decreases appreciably. It followsthen that the amount of reactivity insertion needed to produce a given reactor period decreaseswith burnup of the fuel.ShutdownA reactor is considered to be shut down when it is subcritical and sufficient shutdown reactivityexists so there is no immediate probability of regaining criticality. Shutdown is normallyaccomplished 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 decays immediately following shutdown is similar forall reactors provided a large amount of negative reactivity is inserted. After a large negativereactivity addition the neutron level undergoes a rapid decrease of about two decades (promptdrop) until it is at the level of production of delayed neutrons. Then the neutron level slowlydrops off as the delayed neutron precursors decay, and in a short while only the longest-livedprecursor remains in any significant amount. This precursor determines the final rate ofdecrease in reactor power until the neutron flux reaches the steady state level corresponding tothe subcritical multiplication of the neutron source.Rev. 0NP-04Page 31
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