Effect and uncertainties of H in Ice thermal scattering laws on the neutron multiplication factor for PWR fuel criticality applications

Abstract : In the context of the IAEA recommendation to ensure the transportation of fuel assemblies between 233 K and 311 K, thermal scattering laws of hydrogen in iced water have been produced with the LEAPR module of the NJOY code and included in the JEFF-3.3 nuclear data evaluation. Following this work, a benchmark was launched by the OECD/NEA Working Party on Nuclear Criticality-Safety subgroup-3 to evaluate the effect of the temperature on a PWR assembly criticality. This paper first focuses on the results obtained on this benchmark by CEA with the TRIPOLI-4 ® Monte-Carlo code. They show that, in terms of criticality-safety, computations made at 293 K are conservative and that the impact of density on the k eff is much stronger than the nature of the hydrogen bound or the adjustment of nuclear data to temperature. To go further, the uncertainties associated with the thermal scattering laws of hydrogen in iced water have been evaluated and propagated on one of the benchmark cases. The reference method to do so consists in a direct propagation of the LEAPR model parameters uncertainties. Another method, based on covariance matrix of the hydrogen in iced water scattering cross section, was also used in order to evaluate its relevance. The direct propagation leads to an uncertainty of 111 pcm. The uncertainty evaluated with the second method is lower by around 50 pcm. Whatever the method considered, those uncertainties remain acceptable in the criticality-safety context especially as the effect of the temperature on the k eff and the impact of the hydrogen bound nature are both low regarding density effects.