ANALYSIS OF TEMPERATURE EFFECTS ON REACTIVITY OF LIGHT WATER REACTOR FUEL ASSEMBLIES BY USING MVP-2 ADOPTING AN EXACT RESONACE ELASTIC SCATTERING MODEL

摘要

The Japan Atomic Energy Agency has installed a resonance elastic scattering model coupled with the thermal motion of target nucleuses (the exact model) to the continuous energy Monte Carlo code MVP-2 and also prepared the libraries of elastic scattering cross sections at 0 K for ~(235)U, ~(238)U, ~(238)Pu, ~(239)Pu, ~(240)Pu, ~(241)Pu, ~(242)Pu and ~(241)Am, which are necessary to apply the exact model. We applied the code and the libraries to analyze the neutron multiplication factors (k_(inf)'s) and Doppler reactivity of UO_2 and MOX fuel assemblies for light water reactors, and compared them with those obtained by using the conventional asymptotic slowing down model (the asymptotic model). The base condition of the assemblies was a hot operating condition with an in-channel void fraction of 40% and a fuel pellet temperature of 520 °C (793 K) for the BWR fuel assemblies and a hot zero power condition with that of 284 °C (557 K) for the PWR fuel assemblies. From the base condition only fuel pellet temperatures was raised to 1500 °C (1773 K). The calculated results showed that the difference in k_(inf)'s between the exact and the asymptotic models were -220 to -410 pcm (1 pcm=0.00001 Δk) at the 1500°C condition, and the exact model made the Doppler reactivity between the base and the 1500°C conditions more negative by 7 to 10%. The effect of the exact model for all the heavy isotopes other than ~(238)U was to make the Doppler reactivity less negative by a few %.