Field Examples of Waste Assay Solutions for Curium-Contaminated Wastes

Passive neutron coincidence counting is a mature technique for the assay of Pu in nuclear material. It is widely deployed in safeguards and waste inventory verification applications. The presence of ²⁴²Cm and ²⁴⁴Cm in spent fuel wastes, often poses a severe challenge owing to the relatively short spontaneous fission half-life for these isotopes and the subsequent prolific spontaneous fission neutron emission. This is a well documented problem, compounded by the fact that for most waste assay applications, neutron assay techniques are not capable of distinguishing between these Cm isotopes and the even isotopes of Pu, which are normally of interest in waste assay applications. Therefore the presence of even small quantities of these isotopes can result in gross over-estimation of the Pu inventory, if an appropriate correction is not made. Previous theoretical studies carried out recently have illustrated the potential magnitude of the problem, with reference to the fundamental nuclear data and typical isotopic compositions of wastes. Neutron multiplicity counting can, in principle, differentiate between isotopes that undergo spontaneous fission, however in practice the uncertainties in waste assay are such that this is rarely beneficial. More practical “compensation” techniques use combinations of different assay techniques (for example passive and active neutron counting) and knowledge of the actinide ratios in the waste stream fingerprint. In this paper we describe various waste assay applications as case studies. For each example we describe the nature of the challenge and show how solutions have been developed for applications where the presence of curium has caused problems. We describe the technical solutions, showing the limitations and assumptions of each. We also emphasise the role of robust Quality Assurance procedures, to ensure that the techniques are implemented reliably and with predictable outcomes. Finally, we describe the benefits that have been realised for the plant operations teams, with regard to improved measurement accuracy, avoidance of false over-estimation of the Pu inventory and subsequent improvement in plant throughput.