Effect of Reactivity Feedback on Reactivity Accidents in Light Water Power Reactor

Reactivity feedback effects by moderator temperature coefficient and void coefficient have hitherto been believed to have less importance in controlling a fast power excursion, such as a reactivity accident in a light water power reactor, since they are mainly caused by heat transfer from fuel to coolant occurring after considerable time delay. Recent SPERT-III-E-core transient tests, however, have revealed that moderator feedback effects compensate significantly the magnitude of reactivity, under reactorconditions of high temperature or high power level, and that this moderator feeaback isgenerated by prompt neutron slowing down energy.In the present work, the SPERT experiments have been analyzed anewly by the multi-channel nuclear thermal hydrodynamic kinetics code, EUREKA, with the view to examining the moderator feedback effect. As a result, the SPERT conclusion mentioned above has been confirmed.The report also analyses the influence of differences in reactor system temperature, initial power level, coolant velocity, reactor period and moderator temperature coefficient on the moderator feedback reactivity effect through comparisons with the SPERT experiments, and presents the conciusion that safety analyses on reactivity accidents in light water power reactor would involve a significant amount of error, if moderator feedback effect is ignored.