Using effective temperature as a measure of the thermal scattering law uncertainties to UOX fuel calculations from room temperature to 80 degrees C

The effective temperature T-eff is an important physical quantity in neutronic calculations. It can be introduced in a Free Gas Model to approximate crystal lattice effects in the Doppler broadening of the neutron cross sections. In the last decade, a few research works proposed analytical or Monte-Carlo perturbation schemes for estimating uncertainties in neutronic calculations due to thermal scattering laws. However, the relationship between the reported results with T-eff was not discussed. The present work aims to show how the effective temperature can measure the impact of the thermal scattering law uncertainties on neutronic calculations. The discussions are illustrated with Monte-Carlo calculations performed with the TRIPOLI-4 (R) code on the MISTRAL-1 benchmark carried out in the EOLE facility of CEA Cadarache (France) from room temperature to 354 K (80 degrees C). The uncertainty analysis is focused on the impact of the thermal scattering laws of H2O and UO2 on the neutron multiplication factor k(eff) for UOX fuel moderated by water. When using the H2O and UO2 candidate files for the JEFF-4 library, the variation range of T-eff leads to a k(eff) uncertainty of 2.3 pcm/K, on average. In the temperature range investigated in this work, T(eff )uncertainties of +/- 20K for H2O and +/- 10K for UO2 give uncertainties on the multiplication factor that remains close to +/- 50 pcm. Such a low uncertainty confirms the improved accuracy achieved on the modelisation of the latest thermal scattering laws of interest for light water reactors. In the future evaluated nuclear data libraries, uncertainty budget analysis associated with the low neutron energy scattering process will be a marginal contribution compared to the capture process.