Cocomputer simulation study and validation to select a gamma spectrometer for the determination of fissile uranium in solid radioactive waste

SCK CEN and Institut des Radioéléments (IRE) are collaborating on the Recovery of Uranium from Mo-99 production -RECUMO project to build a new facility in Europe with the aim to process highly enriched uranium radioactive residues (HEUR) from the production of radioisotopes until 2038. The facility will produce High-Level Solid Waste (HLSW) during the hot cell processes containing a small amount of fissile U-235[1]. It is important to determine the mass of U-235 in the waste for processing and for fulfilling safeguards declarations to the International Atomic Energy Agency (IAEA) and Euratom. Previous studies to estimate waste properties and determine the most suitable technology concluded that gamma spectrometry, specifically In Situ Object Counting System (ISOCS) measurement, was the most suitable analytical technique for this purpose[2]. Following this previous result, the aim of the master thesis is to select a suitable gamma spectrometer for the project among several detector candidates with different crystal sizes and geometries. This selection is based on three main criteria: the Minimum Detectable Activity (MDA) and the capacity of withstand high dose rate determine by computer simulation (Nucleonica and Visiplan) and experimental validation; as well as the detector price. The major result of this work is the choice of a coaxial gamma detector with a medium to large crystal size because it has a good balance between the selection criteria: a MDA that falls in the low mg range, a higher dose rate resistance compare to the other detector types and a lower price. Our work also shows that gamma spectrometry is in fact appropriate for the determination of U-235 in RECUMO waste.