LAUSR

Abstract The operator associated with the neutron transport equation is not self-adjoint. The solution to the adjoint transport equation is orthogonal to the solution of the neutron transport equation. For this reason, the former have a clear physical significance as the importance of neutrons within a particular system. The neutron importance depends on material cross sections as well as isotopic concentrations in the reactor core and during the operating cycle, it seems that the variation of this parameter is unavoidable. In this work, we investigated the variation of neutron importance during reactor operating cycles for a VVER-1000 reactor. A more realistic approach of calculation of the neutron importance during burnup cycle has been implemented by using the MCNIC method. The simulation for neutron importance quantification during reactor operating cycle influenced by burnup and operational condition has been carried out. The results show that the maximum absolute relative difference in the core average neutron importance from the beginning of cycle to the end of cycle is within 15%.