High Precision Nondestructive Assay to Complement DA

Large scale spent fuel reprocessing plants and fuel fabrication facilities require large numbers of grab samples taken for destructive analysis (DA) for nuclear material accountancy and safeguards. The resulting cost and radioactive waste generation is large and significant time is lost between the sample taking and the DA results. This paper presents an approach to combine nondestructive assay (NDA) with DA calibration and normalization to reduce the number of chemical analysis and shorten the time to obtain the results. For plutonium bearing samples, the preferred NDA system is a modified version of the high efficiency epithermal neutron counter. These are well-type neutron counters with efficiencies in the range of 60-70 % and short neutron die-away times. The counting precisions of the new counters are an order of magnitude better than for prior thermal-neutron counters for multiplicity analysis. The accuracy goal for the NDA is in the range of 0.2-0.5 % for the Pu-240 effective mass. The stability and statistical precision of the neutron counters are better than 0.1%, and the total measurement error will be dominated by how well the standards from the DA laboratory match the samples measured with the NDA. The analysis of the neutron data can be performed in the simple coincidence (doubles) mode, or the “known-alpha ” mode, or the “multiplicity ” mode. Each of these modes has advantages and disadvantages depending on the experimental conditions. This paper will evaluate the analysis methods to obtain to obtain the best accuracy. Also, the preliminary design of a neutron counter that is optimized for the measurement of small samples such as powder and pellets will be presented