Improvement of Accuracy in Waste Characterisation by Applying Innovative Techniques (ASGS)

The safety consideration in the management of radioactive waste is largely related to its radionuclide inventory due to the harm of ionizing radiation. Depending on the origin and the purpose (free-release or disposal), different pre-conditions such as activity levels or waste container properties need to be applied. The most effective and accurate method to determine the nuclide inventory of a waste drum is Gamma-Spectrometry using High Purity Germanium Detectors (HPGe). These detectors allow the direct measurement of decay-radiation from the nuclear waste and, due to the high energy resolution, the determination of activity for each gamma-emitting nuclide. In the last decades, several measurement approaches based on HPGe detectors have been developed and all approaches are using the principle that the detector "views" the drum from the outside. The advantage is that the waste drum does not need to be opened, but since the waste matrix has some shielding effects for the gammaradiation, detailed knowledge about the drum content needs to be obtained in order to be able to calculate the nuclide activities. The less information that is available, the higher the uncertainties of the results are, which needs to be mitigated by adding some additional activity as a safety margin - so called virtual activity - to the best estimate of the activity obtained from the measurement. A large fraction of radioactive waste may deviate from calibration conditions applied in spectrometric radiation measurement, assuming a uniform activity distribution in their analysis model. This deviation results in a bias of the measurement result which needs to be accounted for as a so-called model-uncertainty of the measurement. Mirion Technologies (Canberra) developed and validated in cooperation with Aachen Institute for Nuclear Training (AiNT) an innovative measurement approach which allows more accurate results and therefore applying lower uncertainties. This new measurement approach extends the concept of a standard Segmented Gamma Scan (SGS) by performing sectorial measurements of single drum sectors in addition to segments, hereby gaining information on the spatial distribution of the activity within the drum. The goal is to perform gamma scans with high throughput and an accurate analysis based on the reconstruction of activities for each sector. The new approach has been developed, tested and validated with a measurement system and calibration drums. The comparison with SGS shows, that the novel method increases the measurement accuracy for non-uniform activity distribution hereby allowing the reduction of model uncertainties by a factor two to three, depending on the density of the waste matrix. (author)