Combined HRGS/PNCC Systems for the Assay of Plutonium Contaminated Waste Arising From the Drigg Retrieval Project

Between 1959 and 1967, Plutonium Contaminated Material (PCM) was stored in the magazines at the Low Level Waste Facility at Drigg. However, PCM is now classified as Intermediate Level Waste — rather than Low Level Waste — and so a programme to remove the PCM is underway. In support of this programme a suite of safety related non-destructive assay instrumentation has been supplied to the Drigg Site in order to characterise the PCM prior to transportation. Three identical assay systems employing High Resolution Gamma Spectrometry (HRGS) and Passive Neutron Coincidence Counting (PNCC) have been installed in the new waste retrieval facilities on three of the PCM magazines. These Retrieval Module Monitors allow the fissile content and radionuclide inventory of individual items of PCM to be determined before they are placed with other items into 200 litre drums and transported across the Drigg site to a temporary drum storage facility. In addition, two assay systems of a different design have also been installed in a new PCM Drum export facility. The purpose of these Drum Monitors is to determine the fissile content and radionuclide inventory of drums of PCM that have been retrieved from the magazines before they are transported to Sellafield. Like the Retrieval Module Monitors, the Drum Monitors also uses the HRGS and PNCC techniques to measure drums of up to 400 litre capacity but in this case a scanning detector is used instead of three fixed gamma detectors. This paper describes the characteristics of both the Retrieval and Drum Monitors, the particular requirements for these systems and the means by which they were designed, built and tested in order to ensure that they delivered their primary safety function, which was to not underestimate the fissile content of the items of PCM they measure.