R&D in support of the development of a Low-Enriched Uranium based radioisotope production target (PhD, Postdoc)

Under the pressure of international treaties in the frame of non-proliferation, the global civil use of high-enriched uranium is gradually reduced.  Two main areas in which high-enriched uranium is used in civil applications are for research reactor fuels and in fission radioisotope (eg. ⁹⁹Mo) targets.  In fact, both applications are very similar, since a radioisotope production target, in essence, is just a small size fuel plate, down to the actual fissile material.  With the transition of research reactor fuels to low-enriched fissile materials, higher density uranium compounds have come into use, including silicide (U₃Si₂) and uranium alloy (U(Mo)) based fuels.  Both are not useful for the production of radioisotope targets, since they are incompatible with the radioisotope extraction processes.  This means that for the conversion of the radioisotope targets to low-enriched uranium, other methods will have to be used to assure a minimal loss of productivity with the reduction of the ²³⁵U enrichment.  From a historical perspective, this is a new situation for the radioisotope producers, since they have in the past been able to use the existing qualifications of research reactor fuels to operate their targets, as the conditions in which these targets are irradiated are very largely encompassed by the domain of operation of the fuels.  For the first time, we are in a situation where a specific development and qualification of a radioisotope production target will have to be performed.  This project aims to study some of the existing and new solutions to this problem in detail, with a technological finality.

The state of the art of the low-enriched radioisotope target development is described by two designs that are currently in an advanced state : the UAl₂ based LEU dispersion targets and the U foil targets.

The UAl₂ dispersion targets are based on an evolution of the existing UAl_x targets, in which the uranium loading is increased from ~1.1 to ~2.5 gU/cc and in which the stoichiometry of the fuel particles is controlled more stringently to achieve the highest uranium density.  These loadings, in association with optimisations in target geometry, allow achieving ²³⁵U loadings per target near to what was available in the HEU targets.

The U foil targets are an important deviation from the existing target technology, both from a manufacturing and a processing point of view, but even also from the irradiation standpoint.  They consist of a pure U foil mount in between 2 concentric aluminium cilinders.  Although they certainly have a number of advantages, no full qualification has been achieved yet and the required modifications to the radioisotope extraction process still pose an important hurdle.

Besides these 2 current designs, others can be envisaged.  A number of compounds, perhaps in the past deemed unsuitable for use as research reactor fuel materials (requiring high burnups and high powers), may be perfectly suitable in the conditions in which the targets are irradiated (low burnup and average power).  As such, they should be revisited bearing in mind the constraints of manufacturing of the targets, the target processing and final product purity.

This project associates SCK•CEN, as a centre of excellence in research reactor fuel development/qualification and in radioisotope production, and IRE, a global supplier of radiopharmaceuticals, not in the least fission generated ⁹⁹Mo.  Besides support for the ongoing development of UAl₂ based targets in existing projects at SCK•CEN, the PhD project aims at a systematic study of available and perhaps also novel uranium compounds as candidates for radioisotope targets, studying the production and processing characteristics on fresh materials.  With the constraints of target production and processing in mind, the project aims at an irradiation of a set of selected candidate targets in the BR2 reactor and subsequent post-irradiation examinations and radioisotope extraction tests.  In the process of attempting to reach these technological goals, basic properties of the candidate materials (crystal structures, thermodynamic properties, transformation kinetics, etc.), with their intrinsic scientific importance and merit, will need to be carefully assessed through the use of a variety of available techniques (eg. XRD, TEM, EPMA, thermal interaction, etc.).