Combined HRGS/PNCC systems for the assay of plutonium contaminated waste for criticality control and identification of Low Level Waste candidates

摘要

Sellafield Ltd. has a large inventory of waste drums containing plutonium contaminated material (PCM) arising from its operations at the Sellafield site in Cumbria, UK over the past 60+ years. PCM is a contact handleable category of Intermediate Level Waste (ILW) and as such, there is currently no final disposal route in the UK for waste in this category; waste of this type must be conditioned and stored securely in suitable containment, usually on the site on which the waste was produced. Sellafield operates a number of engineered storage facilities for PCM drums which have been specifically designed for the safe and secure storage of waste of this type. However these are a limited resource and have a high operational cost. For this reason it is important to ensure that waste that meets the requirements for disposal as Low Level Waste (LLW) is not incorrectly consigned in the higher PCM category, thus causing the PCM drum stores to reach capacity prematurely. A new assay suite at the Sellafield PCM drum storage facility has recently been commissioned. The assay suite is a fully automated facility and incorporates a Radiometric Drum Measurement System (RDMS) intended to determine the fissile and other radionuclide content of drums of PCM prior to size reduction by supercompaction. The primary purpose of the RDMS is to ensure fissile and radionuclide limits are not exceeded when supercompacted drums are combined in their final disposal containers. However a secondary role of the RDMS is to identify any waste drums with activity limits below that for PCM, making them potential LLW candidates. This paper describes the main features of the RDMS and how it was designed, calibrated and tested to safely and reliably perform its required measurement functions. Results from the initial on-site performance testing and the first active commissioning measurements on real PCM drums using the new monitor are presented.