Point flux nodal expansion scheme applied to the neutron noise analysis of the nuclear reactor core

Abstract

In this paper, a neutron noise simulator based on the point flux nodal expansion method (PFNEM) for nuclear reactor cores with rectangular geometry is developed. The point flux scheme is a kind of nodal expansion method (NEM) which is based on keeping balance of neutrons in every node and has been proved very efficient for LWR analysis. The method can be applied to mesh volumes as coarse as the reactor subassemblies to speed up calculations. In this work, the PFNEM is implemented for spatial discretization of neutron diffusion equation in the frequency domain in order to obtain the neutron noise, fluctuation of neutron flux around its mean value. The fluctuations of the absorption macroscopic cross sections in both fast and thermal energy groups are assumed as the neutron noise source. The adjoint noise analysis is also carried out to validate forward noise calculation results. Three test problems including 2-region slab reactor, 2D TWIGL seed-blanket reactor and 2D LRA BWR are assessed for evaluating the noise results. In addition, an adopted iterative scheme is also presented for the solution of neutron noise equations. Altogether, static and dynamic calculations results demonstrate the good accuracy of proposed approach.