A UK comparison for measurements of low levels of gamma-emitters in waste drums

Abstract

Much of the work of the UK nuclear industry is now concerned with decommissioning many of the existing power stations and other facilities. An important aspect of this work is the accurate measurement of low levels of radioactivity in waste forms such as building materials in order that these materials can be assigned to the correct waste streams. This has led to a call for suitable standards and reference materials, and the specific needs of UK users were identified at an NPL workshop in 2005. One of the highest priorities was for ‘soft waste’ spiked with γ-emitters in a 200 L drum format, with an activity concentration of just under 0.4 Bq g −1. In response, NPL prepared a single reference drum meeting this specification. The low density was achieved by loading the drum with plastic bottles, each partially loaded with ion-exchange resin. The resin in each bottle had been previously spiked with a mixture of 241Am, 137Cs and 60Co, all traceable to national standards. The drum would be used primarily as the basis of a comparison exercise, but feedback on its usefulness as a calibration standard would also be sought. The drum was measured by 17 radioassay groups at 15 UK sites. The monitors used were mostly commercial γ-spectrometry systems designed to accommodate waste drums. Some groups measured the drum on more than one monitor and some used more than one efficiency calibration. Many of the groups used mathematical modelling to derive their efficiencies. The results of the exercise were discussed at a second NPL workshop (2007), after which the participants were allowed to submit supplementary or replacement results (with reasons for any changes clearly stated). In total, 88 results were submitted. A total of 51 results were in agreement with the NPL values; of the remaining results, 24 were explained by the participants concerned (or were revised to provide supplementary values), but the other 13 results were either clearly discrepant or questionable. The exercise demonstrated differences between laboratories who had used the same modelling software for their efficiency calibrations, and indeed facilitated an exchange of models between these laboratories. The importance of an accurate knowledge of the form and structure of the sample in efficiency modelling was clearly demonstrated. Uncertainty budgets were of variable quality. Some participants quoted MDA values for one or more of the radionuclides. A follow-up comparison was requested and this is now being planned.