Correction factors applied to isotopic concentrations in burnup credit implementation of PWR LEU applications with the recent JEFF3.1.1 / SHEM library - 7-14

The concept of taking credit for the reduction of the reactivity of nuclear spent fuel due to their burnup is referred to as 'Burnup Credit' (BUC). A rigorous methodology was developed by the CEA and AREVA-NC, carrying out the French BUC calculation route based on the code systems DARWIN and CRISTAL. It is proposed to take account of the biases of the spent fuel inventory calculation by the mean of Isotopic Correction Factors (ICF) applied to the BUC isotope concentrations in the criticality calculations. The individual reactivity worth, validated through integral experiments in the MINERVE reactors, can be taken into account by the use of k_eff penalty terms using reliable assessment methods for biases and errors determination due to nuclear data. This paper deals with the proposal of new ICFs to guarantee the conservativeness of the depleted fuel inventory in the criticality calculations, obtained with the latest versions of neutronic codes and libraries based on JEFF-3.1.1. Concentrations of most important isotopes towards criticality are now calculated within the target accuracy (±2%); JEFF-3.1.1 based predictions are particularly improved for minor actinides (²³⁶U, ²³⁷Np, ²³⁸Pu, ²⁴²Pu, ²⁴³Am, ²⁴¹Am) and important FPs (¹⁵¹Sm, ¹⁵²Sm, ¹⁵³Eu, ¹⁵⁵Gd). Therefore, ICF values are mainly driven by the experimental uncertainties, themselves mainly due to the uncertainty on the burnup of the various PWR rod cuts. The impact of the ICF values represents about 1200 pcm in a transport cask application for burnup of 30 GWd/tHM. This ICF penalty represents 5% of the full Burnup credit due to BUC nuclides