Conceptual Design of Hybrid Nuclear Material Accounting Instrument for U/TRU Ingot of Pyroprocessing

One of the final products from the pyroprocessing with spent nuclear fuels is a U/TRU ingot consisting of rare earth elements, uranium, and transuranic elements including plutonium and curium. The amount of nuclear materials in the U/TRU ingot should be measured as precisely as possible for securing the safeguardability of a pyroprocessing facility because the ingot contains the most amount of Pu element in spent nuclear fuel. In the present study, we, KAERI and STS, propose a novel nuclear material accounting method to measure the Pu mass in the U/TRU ingot. This is a hybrid system combining two measurement techniques based on (1) fast- and (2) thermal-neutron induced fission events. For technique #1, the changes in the average neutron energy will be measured using a so-called ring ratio method for which there are two detector rings close and far from the sample to be measured. The average neutron energy increases as the Pu content in the ingot increases due to increased fast-neutron induced fission events. We call this technique, fast-neutron energy multiplication (FNEM). For technique #2, the changes in neutron population by the Cd liner in the cavity wall will be measured. This is the so-called Cd ratio of the PNAR (Passive Neutron Albedo Reactivity) technique, which is based on thermal- neutron induced fission events. The signature for the Pu mass will be significantly enhanced using hybrid techniques, i.e., FNEM×PNAR. In the present study, performance of the measurement system was investigated with different detector parameters in order to determine the optimum geometry by Monte Carlo simulations with MCNP6.