Implementation of a cross sections evaluation methodology for safety margin analysis: application to gadolinium odd isotopes

The second and third generation Light Water Reactors (LWRs) are often equipped with fuel with gadolinium-enriched fuel pins with the aim of extending the in-pile fuel life cycle without introducing any new safety issues at the Beginning Of Life (BOL). Gadolinium odd isotopes cross sections are crucial in assessing the neutronic performance, the economic performance, and the safety features of a reactor core. In detail, accurate evaluations of the neutron capture behavior of gadolinium burnable poisons are necessary for an exact evaluation of the economic gain due to the extension of fuel life, the residual reactivity penalty at EOL, and the reactivity peak for partially spent fuel for the criticality safety evaluation of Spent Fuel Pools. Nevertheless, present gadolinium odd isotopes neutron cross sections are somehow dated and poorly investigated in the high sensitivity thermal energy region for LWRs. Moreover, the thermal region gadolinium data are available with an uncertainty which is too high in comparison to the present day typical industrial standards and needs. This technical report presents in its first part a series of scientific motivations from which even the most recent gadolinium cross sections evaluations appear inadequate to provide accurate criticality calculations for a system with gadolinium fuel pins. For these reasons, sensitivity and uncertainty analyses (S/U) have been performed to investigate the effect of gadolinium odd isotopes nuclear cross sections data on the multiplication factor of some LWR fuel assemblies. The results have shown the importance of gadolinium odd isotopes in the criticality evaluation, and they confirmed the need to achieve an improvement of the gadolinium nuclear data accuracy by means of new experiments. This work has been the scientific justification to propose a re-evaluation of the neutron capture cross sections of 155Gd and 157Gd within the N_TOF project using new experimental measurements to be carried out at the European Council for Nuclear Research (CERN).In the second part of the report, a preliminary setup for EMPIRE calculations finalized to prepare an ENDF-formatted evaluation for gadolinium isotopes, once the experimental CERN data will be available, is presented.