On the Reactivity Void Response of an Advanced Pressurized Water Reactor

The results of the effect of various parameters on the reactivity void response of an advanced pressurized water reactor (APWR) containing mixed-oxide (plutonium and ²³⁸U) fuel are presented. The parameters studied include the moderator-to-fuel ratio; the presence of ²³⁸U and ²⁴⁰Pu in the fuel; the presence of parasitic (thermal) absorbers; the variation of η of ²³⁹Pu as a function of energy; the assumption that the water in the reactor core upon voiding remains uniform in density; the ratio of ²³⁹Pu/²⁴¹Pu atoms; and the treatment of neutrons in the resonance energy range. It is shown that using the WIMS-D code to determine neutron group constants at low levels of water loss (<40%) from the core is usually adequate for APWR studies and is conservative over the entire range studied. In borderline situations, the use of a methodology that provides a more rigorous treatment of neutron interaction in the resonance energy range is required. This is shown by use of the RABBLE code. Data on the effect of various parameters on the initial conversion ratio are given. In general, effects that harden the neutron spectrum for a fixed k_eff tend to increase the conversion ratio. In addition, the conversion ratio tends to increase as the fraction of ²⁴¹Pu in the fissile fuel increases.