Non-destructive assay methods for the free release of dismantling wastes
The determination of isotope specific activities, and above all, the identification of radioisotopes is an important issue in the waste management of nuclear wastes whatever their origin. Systems based on the detection of the gamma rays emitted by the radionuclides allow the non-destructive assay (NDA) of relatively large volumes of waste. Different NDA systems are commercially available for this purpose. NDA of waste items is subject to the influence of many different physico-chemical characteristics of the sample, most of which are generally not known. In many cases, however, NDA is the only approach that allows measuring these large samples. The unconditional free release of wastes originating from nuclear activities for a great deal relies on NDA systems and procedures for decision-making on free release. The assay of waste of very low activity and even the decision if a radionuclide has been detected or not, is very often difficult. The poor measurement precision, caused by the bad counting statistics, combined with a generally unknown bias of the measurement complicate the interpretation of the results, and accurate measurements are seldom obtained in these cases. The aim of the present paper is to give an overview of the efforts made at SCK-CEN to come to a consistent system that allows performing free release measurements, involving a high throughput of large amount of materials, as expected from a dismantling project.