Optimisation of Non Destructive Assay Equipment for Intermediate Level Waste in the B462.27 Vault Store at Harwell

The UKAEA owns and operates a suite of cells designed to process and repackage Remote Handleable Intermediate Level Waste (RH-ILW) at the Harwell site in the UK. Cans of RH-ILW are retrieved and assayed using Non Destructive Assay (NDA) prior to repackaging in Nirex approved drums for interim above — ground storage, and eventual disposal in a national repository. The NDA system comprises a combined passive/active neutron interrogator (NI) and also a gamma spectrometer (GS). The results are used in conjunction with an expert system to aid the reconciliation between the declared / historical records and measured fissile mass and β/γ radionuclide inventories. The assay system was originally designed to accommodate the unique diverse range of waste streams, which are stored within B462. To this end, a key feature of the design was the inclusion of state of the art features such as matrix compensation, fissile material location correction, and self-shielding compensation for the active cycle of the neutron interrogator. The systems were originally designed and calibrated in the laboratory during the early 1990’s. Commissioning proceeded through staged acceptance tests and commissioning trials for the equipment. Active commissioning using selected real waste drums has recently been completed, and the UKAEA is now operating the plant under a testing phase of full trial operations. The UKAEA is rapidly gaining experience in both the operation of the equipment, and its performance for various waste streams. This has led to a number of recommendations for significant improvements with respect to both the mode of operation, and the physics algorithms. By modifying the operating procedures, it is possible to improve the level of management control over the interpretation of the assay results, so that a greater degree of confidence can be demonstrated in the conclusions drawn. The approach has been to streamline the use of all the available information (historical records, assay and expert system data), coupled with extensive operator training, in order to ensure that operations staff are in control of the decision–making process. Sentencing decisions are now made using a proceduralised, open framework / methodology using the experience gained from the commissioning trials to provide extensive operator training, thereby eliminating the risks associated with previous “black box” treatment of expert systems. Technical changes include, for example, a simplification of the treatment of self-shielding in lumps of fissile material, leading to more robust data interpretation, and substantial throughput improvements. In establishing the new procedures and system enhancements, the UKAEA is following the new national Good Practice Guide for NDA measurements. In this paper, we describe the approach being followed to identify and implement these changes, as well as describing some of the important changes in some detail. We also describe UKAEA’s plans to undertake a comprehensive system refurbishment, in order to modernise the equipment for increased throughout and minimal plant down–time, using supportable high reliability components. This will also maximise the operational lifetime of the equipment.