IMPROVEMENTS IN THE ACCURACY OF THE U235 MEASUREMENT IN WASTE DRUMS AT BNFL'S DRIGG SITE

Accurate determination of the U235 content of Plutonium Contaminated Material (PCM) wastes generated during retrieval operations at BNFL's Drigg Site is complicated by the potential presence of self-shielding lumps of uranium. The routine assay functions of the Drigg Drum Monitors are unable to determine whether uranium lumps are present, and hence pessimistic assumptions regarding the degree of self-shielding in any uranium present have been applied in the calibration, to meet criticality safety requirements. A significant number of drums have been measured as having U235 content greater than the limits defined in the criticality clearance certificates, however it is likely that the U235 content of these drums has been grossly overestimated because of the assumed self-shielding (which equates to approximately a factor of eight compared to infinitely dilute uranium). These high U235 content drums present significant operational problems. To address this issue, BNFL Instruments have developed a technique for identifying whether lumps of uranium are present. If the uranium present can be proven to be dilute, then the U235 result is reduced by removal of the assumed level of self-shielding from the calibration. To support the introduction of this technique and to determine whether lumps of uranium are genuinely present in the waste, twelve of the highest U235 content drums were selected for physical repacking. These drums were opened and individual waste packets removed and physically inspected to look for evidence of lumpy material. The packets were then placed within new drums and re-assayed to determine their fissile content. Additional measurements were then performed of the highest U235 content packets from each original drum, to obtain high quality gamma spectra for the analysis. The analysis results indicated that two of the twelve drums contained significant lumps of uranium, with the remaining ten being dilute. The waste measurement results presented in this paper demonstrate the applicability of this technique, which will now be applied to future high U235 content waste drums to improve the accuracy of the U235 measurement and to avoid unnecessary drum repacking.