Statistical Modelling Applied to the Contents of Waste Drums

Gamma spectrometry is widely used to determine the radioactive content of waste drums. However, the results of such surveys often result in large numbers of limit-of-detection (LOD) results. In this paper we will show how simple statistical methods can be used to obtain useful information on the average drum activities, even in these unfavourable circumstances. Results from measurements on ⁶⁰Co, ¹⁵²Eu, ¹⁵⁴Eu activities in drums of waste from decommissioning of the GLEEP reactor suggest that these activities are lognormally distributed with geometric standard deviations (GSD) ranging from 3–4. This statistical model can be used to extract information from ²³⁵U, ^234mPa and ²³⁴Th datasets which show only LOD activity results. In a repository of N drums, each with activity <L, the total activity is clearly <NL. However, we can use the lognormal model to make a much stronger statement about the total waste activity. This model is developed quantitatively in the paper.