PNNL

Abstract

The direct measurement of plutonium in spent reactor fuel is an unmet challenge in international safeguards. In this simulation study, we investigate the use of the delayed gamma rays from fission product nuclei to determine the amount of fissile isotopes (Pu-239, Pu-241, and U-235) in irradiated light water reactor fuel assemblies. Fission is stimulated with an interrogating neutron source, and the radiation from the short lived fission products is measured. This measured gamma spectrum is then fit to a linear combination of spectra from pure Pu-239, Pu-241, and U-235 to determine the proportion of fissile isotopes present. In this paper, we describe the modelling and analysis methods used to represent the background of radioemissions from long-lived isotopes originally present in the spent fuel and the short time scale delayed gamma signal. Results are presented for simulations using a nominal instrument design on a library of fuel assemblies with burnups ranging from 0 to 60 GWd/MTU.