Fissile mass quantification in radioactive waste packages using photofission delayed gamma rays

This paper reports a feasibility study, performed by numerical simulation with MCNPX, of fissile mass characterization in dense, large volume, long-lived and medium activity radioactive waste packages, using photofission delayed gamma rays. High-energy photon interrogation with a 15 MeV LINAC has been modelled for a 1.18 m
³
cemented waste package, coupled to high resolution spectroscopy with a HP Ge detector. The study was carried out by assessing the passive and active backgrounds respectively due to the waste package gamma emission, and to material activation during irradiation, in view to determine the detection limits for the main delayed gamma rays of interest. The obtained detection limits are lower than the expected uranium mass in the waste package. On the other hand, as the photofission signal results from both fissile
²³⁵
U and fertile
²³⁸
U isotopes, a method for uranium isotopes discrimination based on gamma-ray ratios has been evaluated, showing that photofission delayed gamma rays could be used to assess the fissile mass as well as the total uranium mass.