A formulation of a reactivity variation for taking into account spatial and energy collapsing

This paper presents an original method to evaluate the reactivity worth induced by space or energy condensation procedures commonly used by reactor physicists. Exact and approximate formulas are derived, which allow assigning a reactivity worth to each computation region in order to determine its importance. Some examples are shown aiming at highlighting that results obtained using the proposed formulation have a physical meaning and could be used to define an optimised spatial mesh and to analyse in a new way the usual energy collapsing process. This formulation can be considered as complement to the classical perturbation theory definition of a reactivity variation.