Adjoint neutron spectrum calculation for heterogeneous cells

Abstract

Forward spectrum weighting has been used to generate broad-group cross sections. However, for some applications of perturbation theory, the adjoint spectrum is needed. Recently an approximate method has been proposed for the adjoint spectrum in a lattice cell, based on an equivalence principle. The approximations made can be avoided if the neutron energy dependence is treated in detail, and the neutron transport through the regions of a lattice cell is adequately represented. In this paper the neutron balance adjoint equations are derived for a heterogeneous unit cell based on the collision probabilities method, and the adjoint spectrum is calculated from this system of equations for different intervals of energy. Forward and adjoint spectra comparisons are done for a unit cell having in its inner region. In the numerical applications of the neutron balance adjoint equations the most important resonances of were considered. The results show significant variations between the forward and adjoint spectra, leading to remarkable differences in the effective resonance integrals.