An Advanced Nuclear Materials Assay System For Plutonium Contaminated Wastes

We describe the design philosophy and measured performance parameters of a state of the art nondestructive assay system for the monitoring of plutonium contaminated waste prior to long term disposal. The general requirement is to measure uranium or plutonium to specified disposal limits. Since alpha rates limit the disposal quantities, uranium may be present at higher mass values than plutonium. 239Pu at the 0.5 mg level, and dilute 235U at the 0.5 level. the system must measure the plutonium in the presence of uranium and the uranium in the presence of plutonium. The instrument is a combined passive/active system built around a highly efficient (45%) passive neutron coincidence counter designed to measure plutonium with a lower limit of detection (LLD) of 0.5 mg 239Pu equivalent (reactor grade) in a 200 l drum using 15 minute counting period. Simultaneous neutron coincidence counting gives a complementary measurement with a 1 mg LLD in the sam period. An integral 252 Cf shuffler provides active interrogation for the measurement of any uranium that may also be present. It will measure 235U with an LLD of 0.5 g using an 3 minute counting period. The performance has been successfully demonstrated using plutonium and uranium standards and a wide range of matrix types. In order to achieve the target detection limits in passive mode, the assay chamber is heavily shielded with concrete to suppress cosmic ray induced background events and the data is subjected to off-line statistical filtering. Matrix and location corrections are derived using matrix monitors, combined with the use of an indexing turntable and an analysis of the counts from the different neutron detector banks and different irradiation heights. This assay system brings the operator the advantages of rapid and reliable alpha assay of 200 l low level waste drums. It provides two complementary sub-milligram measures of plutonium (239Pu equivalent levels) and sub-gram measurement of 235U even when both may be present. This performance is well matched to measurements required to confirm compliance with disposal limits for low level wastes and allows efficient and economic optimisation of available disposal routes.