Development of active neutron NDA techniques for nuclear nonproliferation and nuclear security (6) Design study of neutron moderator and filter in a delayed gamma-ray measurement system using a 14 MeV D-T neutron source

For the fields of nuclear nonproliferation and security, the JAEA is developing delayed gamma-ray spectroscopy (DGS) using a compact pulsed D-T neutron generator (D-T generator) to determine the ratio of fissile nuclides contained in nuclear materials (NM). Since D-T neutrons are high energy (14 MeV), it is important to develop moderators suitable to decrease the energy of the neutrons into the thermal region for the purpose of increasing the possibility of fission reactions in the fissile nuclides of interest (e.g U-235, Pu-239, and Pu-241). In previous research, we demonstrated that combinations of tungsten, graphite, and polyethylene were appropriate as the moderator materials for 14-MeV neutrons by analyzing simple spherical models using MCNP6. In this research, MCNP6 was used to confirm the effectiveness of a more realistic model for the purpose of installation into an experimental environment. Results show the realistic model is able to increase ratio of thermal and fast + high energy neutrons but fast neutrons entering NM is not negligible even after moderation. Effects of fast- and high-energy neutrons can be suppressed by obtaining the difference between the measurement results of with and without neutron filter for the purpose of cutting off thermal neutrons. This work will present results from the realistic model study as well as details describing our method of fission yields contribution determinations.