Year
2016
Abstract
Japan Atomic Energy Agency (JAEA) and Los Alamos National Laboratory (LANL) jointly developed the Epithermal Neutron Multiplicity Counter (ENMC). A measurement test using the standard samples was performed, and its results showed that the ENMC achieves high measurement accuracy (approx. 0.4%) for 240Pu effective mass under the optimum conditions. However, in the practical measurement for nuclear material accountancy or safeguards, bias was observed due to the variation of the sample properties. With this recognition, JAEA jointly with LANL conducted simulations for identifying the causes of this bias. The simulation results showed that the dominant cause of the bias was the variation in sample density and this bias could be mitigated by correcting neutron counting efficiency. JAEA and LANL evaluated the applicability of correction methods for the neutron counting efficiency by real measurement data and by simulation data. The results showed that the real measurement data is difficult to be applied to the correction because of its significant measurement error. For the simulation, we evaluated the neutron counting efficiencies for typical density of mixed-oxide (MOX) pellet and powder, and it was confirmed that the bias for 240Pu effective mass due to the variation of the sample properties was mitigated from 1.25% to 0.34%. By applying this improvement, total measurement uncertainty for plutonium mass quantification by using combination of the ENMC and nondestructive assay (NDA) for isotopic ratio of plutonium: High Resolution Gamma Spectrometry (HRGS) attains approx. 0.7% which is equivalent to the destructive assay level.