Reactivity Coefficients Summary

REACTIVITY COEFFICIENTS DOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters) Summary The important information in this chapter is summarized below. Reactivity Coefficients Summary The temperature coefficient of reactivity is the change in reactivity per degree change in temperature. A reactor is under moderated when a decrease in the moderator-to-fuel ratio decreases k_eff due to the increased resonance absorption. A reactor is over moderated when an increase in the moderator-to-fuel ratio decreases k_eff due to the decrease in the thermal utilization factor. Reactors are usually designed to operate in an under moderated condition so that the 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 the reactor more self-regulating. An increase in power, resulting in an increase in temperature, results in negative reactivity addition due to the temperature coefficient. The negative reactivity addition due to the temperature increase will slow or stop the power increase. The fuel temperature coefficient is more effective than the moderator temperature coefficient in terminating a rapid power rise because the fuel temperature immediately increases following a power increase, while the moderator temperature does not increase for several seconds. The Doppler broadening of resonance peaks occurs because the nuclei may be moving either toward or away from the neutron at the time of interaction. Therefore, the neutron may actually have either slightly more or slightly less than the resonant energy, but still appear to be at resonant energy relative to the nucleus. Uranium-238 and plutonium-240 are two nuclides present in some reactor fuels that have large resonance absorption peaks. NP-03 Rev. 0 Page 28