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

摘要

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