Use of a Shielded Low Resolution Gamma Spectrometer for Segregation of Free Release and Low Level Waste

Abstract

In the UK, low level radioactive waste (LLW) is sent to the national Low Level Waste Repository (LLWR) at Drigg in Cumbria. Strict rules limit the specific activity of waste that is sent to the LLW Repository and waste producers and consignors have to demonstrate that the waste they send to the repository meets its conditions for acceptance. However, the limited capacity of the Low Level Waste Repository means that it is just as important for waste consigners to ensure that inactive ‘free release’ or ‘exempt’ waste is not inadvertently sent to the repository. Incorrect segregation of waste in a decommissioning activity can mean that large amounts of the waste produced is below the exemption limit and could therefore be disposed of in conventional landfill. Sellafield Ltd. is using a pair of Canberra WM2750 Clearance Monitors to assay 100 litre packages of soft waste produced in some of their decommissioning activities at Sellafield. The WM2750 uses low resolution gamma spectrometry (LRGS) to determine the radionuclide content of packages or drums of LLW up to a maximum of 140 litre capacity. It uses a lead shielded measurement chamber to reduce the local radiation background along with high efficiency sodium iodide (NaI) detectors in order to obtain the measurement sensitivity required to be able to distinguish between LLW and exempt waste in a measurement time of less than 1 minute per package. This paper describes the waste monitoring process and the design of the clearance monitor — in particular how it was calibrated and the performance testing that was carried out to ensure that waste items identified by the monitors as being exempt waste are suitable for disposal to a conventional landfill site.