PLUTONIUM ASSAY IN HEPA FILTERS AT THE 100 Bq/g THRESHOLD

The Atomic Weapons Establishment (AWE) at Aldermaston has investigated the performance of low and high resolution gamma-ray detectors for plutonium (Pu) assay in High Efficiency Particulate Air (HEPA) filters at the Drigg repository acceptance threshold (below 100 Bq/g total Pu alpha) for Low Level Waste (LLW). A technique is presented for rapidly sorting filters as LLW, that can be sent to Drigg, or as Intermediate Level Waste (ILW) which is above 100 Bq/g Pu alpha and must remain in storage at AWE. The strategy involved utilising the relatively high yield 60 keV emission from Am-241 ingrowth (Pu-241 daughter) and known isotopic ratios. This paper covers the determination of detector response factors for an Am-241 source positioned at various locations within a filter. These factors were weighted, according to the relative volumes that they represent, in order to derive a uniform response factor and quantify the systematic error for non-uniform distributions of contamination. Detection limits and random errors were also derived from the counting data. The high-resolution detector was shown to have the best detection limits and lowest systematic and random errors. Subsequent routine sentencing of 663 filters within 6 facilities resulted in 548 (83 %) LLW and 115 (17 %) ILW filters being identified.