DELAYED GAMMA-RAY SPECTROSCOPY (2): EXPERIMENTAL STUDIES FOR DETERMINATION OF FISSILE MATERIALS RATIOS

摘要

In the field of nuclear safeguards, new and improved active-interrogation non-destructive assay (NDA) technologies are needed for the independent verification of the fissile composition in highly radioactive nuclear material (HRNM). For this reason, the Japan Atomic Energy Agency (JAEA) and the European Commission Joint Research Centre (JRC) are now collaborating to develop a neutron NDA system based on four different techniques. Among these, delayed gamma-ray spectroscopy (DGS) can be used to determine ratios of fissile nuclides present in the sample measuring the decay gamma rays from fission products (FP). Measurements of low-radioactivity nuclear material samples are underway using different facilities as the Pulsed Neutron Interrogation Test Assembly (PUNITA) in JRC Ispra, Italy and other facilities in Japan. To minimize the interference from the long-lived FP, it is important to use shielding and to consider those gamma rays above 3-MeV. Different compact neutron sources (14-MeV Deuterium-Tritium (D-T), ~2MeV-peak Cf-252) are available, but all of them need to be slowed down to the thermal energy region where the cross-sections of the fissile nuclides of interest are large. To achieve the required high-yield thermal flux that will produce sufficient amounts of gamma rays above 3-MeV, we are optimizing moderator and reflector materials surrounding the source and sample using Monte Carlo Code (MCNP). Optimization of the irradiation, transfer and measurement sequence is now underway to ensure statistically enough peak counts are obtained and to increase the probability to see the very short-lived FP. The experiments we are conducting using certified mono-elemental uranium and plutonium samples allow us to associate observed DG to the proportional isotopic compositions. In this paper we will present the current status of the optimization process and the experimental campaign for the detennination of the ratio of fissile materials of U-235, Pu-239, and Pu-241 in a sample.