ACTIVE NEUTRON COINCIDENCE COUNTING USING A NEUTRON GENERATOR FOR A FISSILE MATERIAL ACCOUNTING

An active neutron coincidence counter using a neutron generator as an interrogation source has been suggested. Because of the high energy of the interrogation neutron source, 2.5 MeV, the induced fission rate, which is the signature of the fissile contents in a sample, is strongly affected by the moderator design. MCNP simulation has been performed to evaluate the effects by these moderators. A proper top-moderator between the interrogation source and the sample provides a better performance in the case that a side moderator is not applicable in the counting system. However, with a side moderator, a top-moderator should not be used. The side- and bottom-moderators of polyethylene are significantly important to thermalize neutrons to induce fission. Based on the simulation results, the moderators are designed to be adapted to the experimental system. Uranium oxide powder samples were prepared and tested as a preliminary performance test. For the natural uranium sample of up to 3.5 kg (21.7g of 235U), preliminary experimental results showed 6.859 cps/g-235U coincidence events. The mean background error was 14.569, and the resultant coincidence error was 25.4 cps in maximum. The effects by the counting efficiency and asymmetric emission of interrogation neutrons from the generator are also considered. The experimental results show that the current status of an active counting using a neutron generator has some challenges to overcome. However, it is expected that the advantage of a controllable interrogation source makes it more applicable for a variety of combinations with other non-destructive methods like a passive coincidence counting especially under a harsh environment such as a hot cell. More precise experimental setup and tests with higher enriched samples will be followed to develop a system to apply it to an active measurement for the safeguards of a spent fuel treatment process.