Determining axial fuel-rod power-density profiles from in-core neutron flux measurements

Abstract

A method is proposed for determining power-density profiles in nuclear reactor fuel rods from neutron flux measurements obtained by multiple in-core micropocket fission detectors (MPFD). A general formulation is presented that relates the power-density profiles in a reactor's fuel rods to the flux density at detector locations. Key to this formulation is the construction of an appropriate response function that relates the flux at any position in the core to the rate at which neutrons are born at different depths in the fuel rods. In this preliminary study simple, although not unrealistic, response functions for thermal neutrons and for fast neutrons are derived and used to illustrate the analysis methods. To find the unknown power-density profiles in the fuel rods, given the measured fluxes at various locations through the core, requires inversion of an ill-posed Fredholm integral equation. With numerical quadrature, this integral equation can be converted into a (generally, underdetermined) set of algebraic equations for the power densities at various axial positions in the fuel rods. To solve these underdetermined equations, the linear regularization method is employed. Results are presented for simulated data for a single fuel rod and for multiple rods in various configurations. This preliminary study indicates that in-core flux measurements can indeed be used to infer power-density profiles in reactor fuel rods.