Low Temperature Effects on PWR Fuel Assembly Criticality Calculations

One of the parameters affecting the neutron multiplication factor k_eff of a system containing fissile material is the system temperature. Therefore, the effect of temperature on criticality safety analyses is an area of international interest. In this context, the Working Party on Nuclear Criticality Safety (WPNCS) of the OECD Nuclear Energy Agency (NEA) formed a subgroup to define and execute a code-to-code comparison benchmark to investigate the effect of temperature on k_eff for PWR fuel assemblies. Two configurations of a generic water-moderated PWR fuel assembly were analysed at different temperatures between 233 K and 588 K, and with different assembly burnups. Based on this benchmark, GRS performed an additional study to investigate the impact of the moderator densities, the neutron reaction cross sections and the thermal scattering data on k_eff separately. The benchmark results show the expected decrease of k_eff with temperature and a considerable jump in k_eff at the phase transition of the moderator. The additional investigation demonstrates that the jump in k_eff is mainly caused by the change of the moderator density due to the phase transition. The change of the thermal scattering data due to the phase transitions leads to a similar but smaller jump in k_eff. Furthermore, the actual impact of the different parameters on k_eff depend strongly on the considered fuel assembly configuration.