REACTIVITY COEFFICIENTSDOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters)SummaryThe important information in this chapter is summarized below.Reactivity Coefficients SummaryThe temperature coefficient of reactivity is the change in reactivity per degreechange in temperature.A reactor is under moderated when a decrease in the moderator-to-fuel ratiodecreases keff due to the increased resonance absorption. A reactor is overmoderated when an increase in the moderator-to-fuel ratio decreases keff due tothe decrease in the thermal utilization factor.Reactors are usually designed to operate in an under moderated condition so thatthe moderator temperature coefficient of reactivity is negative.Increasing the moderator temperature will decrease the moderator-to-fuel ratio.Decreasing the moderator temperature will increase the moderator-to-fuel ratio.A negative temperature coefficient of reactivity is desirable because it makes thereactor more self-regulating. An increase in power, resulting in an increase intemperature, results in negative reactivity addition due to the temperaturecoefficient. The negative reactivity addition due to the temperature increase willslow or stop the power increase.The fuel temperature coefficient is more effective than the moderator temperaturecoefficient in terminating a rapid power rise because the fuel temperatureimmediately increases following a power increase, while the moderatortemperature does not increase for several seconds.The Doppler broadening of resonance peaks occurs because the nuclei may bemoving either toward or away from the neutron at the time of interaction.Therefore, the neutron may actually have either slightly more or slightly less thanthe resonant energy, but still appear to be at resonant energy relative to thenucleus.Uranium-238 and plutonium-240 are two nuclides present in some reactor fuelsthat have large resonance absorption peaks.NP-03Rev. 0Page 28
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