Year
1992
Abstract
Accuracy of measuring plutonium content in waste via present-day nondestructive neutron techniques is - for some applications - inadequate and unreliable. Within an individual waste item that contains substantial neutron moderating material, plutonium sources can be detected with different efficiencies. Unknown distribution of plutonium content with respect to detection efficiencies (which might vary widely) is generally the largest source of error. Another error source, when plutonium content is moderate or high, is unknown multiplication. For much plutonium waste, the relation between total emitted neutrons and spontaneous fission neutrons is unknown, hence the measured passive singles rate (\"totals\") is often not directly relevant for assay. Reliance on the measured double correlation rate (\"reals\") alone can - because of large variation with detection efficiency - result in excessively large assay errors when detection efficiency for most of the contained plutonium is substantially lower than assumed. Utilization of both double and triple correlation measurements - when precision is adequate - could theoretically improve assay accuracy for waste that affords substantial moderation of emitted neutrons. Nevertheless, a potential for concealment of kilogram quantities of plutonium inside waste would persist, absent implementation of a reliable technique for detecting neutron shielding. Improved assay and shielding detection capabilities - for waste containing moderate or large quantities of plutonium - are important for enabling compliance with safeguards, transportation, and environmental regulations.