An active neutron coincidence counter using a neutron generator as an interrogation source hasbeen suggested. Because of the high energy of the interrogation neutron source, 2.5 MeV, theinduced fission rate, which is the signature of the fissile contents in a sample, is strongly affectedby the moderator design. MCNP simulation has been performed to evaluate the effects by thesemoderators. A proper top-moderator between the interrogation source and the sample provides abetter 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 bottommoderatorsof polyethylene are significantly important to thermalize neutrons to induce fission.Based on the simulation results, the moderators are designed to be adapted to the experimentalsystem. Uranium oxide powder samples were prepared and tested as a preliminary performancetest. For the natural uranium sample of up to 3.5 kg (21.7g of 235U), preliminary experimentalresults showed 6.859 cps/g-235U coincidence events. The mean background error was 14.569, andthe resultant coincidence error was 25.4 cps in maximum. The effects by the counting efficiencyand asymmetric emission of interrogation neutrons from the generator are also considered. Theexperimental results show that the current status of an active counting using a neutron generatorhas some challenges to overcome. However, it is expected that the advantage of a controllableinterrogation source makes it more applicable for a variety of combinations with other nondestructivemethods like a passive coincidence counting especially under a harsh environmentsuch as a hot cell. More precise experimental setup and tests with higher enriched samples will befollowed to develop a system to apply it to an active measurement for the safeguards of a spentfuel treatment process.
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