Program VAREX. A tool for variational analysis of reactivity effects with XSDRNPM

A description of the program VAREX is given, which can be used to calculate changes of reactivity due to changes in the microscopic cross sections or due to changes in the atmic densities of separate isotopes. To this end, it uses first-order perturbation theory, which is also shortly reviewed. The main purpose of the program is to calculate the contribution of isotopes to the Doppler reactivity coefficient or to the moderator density coefficient. The program reads cross sections from AMPX working libraries, atomic densities from XSDRNPM output files, and fine-mesh fluxes from XSDRNPM flux files. A few examples are given, which show the applicability and limitations of the program. (orig.)