Alternative Weighting Functions for Group Collapsing of Nuclear Data in Lead Fast Reactors Time-dependent Analyses

The generation of accurate few-group constants for time-dependent analyses is a challenging problem in the physics of fission reactors, especially the ones featured by a fast spectrum. Re- cently, many efforts have been devoted to optimising the selection of the group structure for the constants calculation, for instance using genetic algorithms. Another approach to improve the quality of few-group constants consists in the adoption of alternative weighting functions with respect to the one usually employed, i.e. the k-mode fundamental flux distribution. In this paper, the performances of the most common eigenvalue formulations of the neutron transport equation (e.g., the time or collision eigenproblems) are compared when employed in the collapsing process. The effectiveness of these alternative weighting spectra is studied by performing some full-core transient calculations on a Lead Fast Reactor study case with the FRENETIC nodal diffusion code. The paper shows that, depending on the type of the transient and reactivity insertion, the few-group constants obtained using these alternative weighting spectra yield integral quantities (e.g., the total power) that are more accurate than the ones obtained with the k-mode collapsed data.