R&D in support of the development of a Low-Enriched Uranium based radioisotope production target

Cea Andrew

Promoter

Pardoen Thomas, (UCL), thomas.pardoen@uclouvain.be

SCK•CEN Mentor

Van den Berghe Sven
sven.van.den.berghe@sckcen.be
+32 14 33 30 00

SCK•CEN Co-mentor

Leenaers Ann
ann.leenaers@sckcen.be
+32 14 33 30 44

Expert group

Microstructural and Non-destructive Analysis

PhD started

2016-10-01

Short project description

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 UAlx 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.

Objective

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 advancing the qualification of identified candidate U compounds (previous thesis work on this was interrupted, but has yielded promising results for certain compounds) for use as fissile materials in 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 fabrication of a set of selected candidate targets, with and without added Mo in the meat to simulate the fission Mo generated 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, TGA-DTA, etc.).

For the production of these targets, powder metallurgical methods are used, in which an alloy or compound is formed in an arc furnace, crushed and mixed with Al powder, pressed into a compact and rolled out between Al sheets to form a mini fuel plate. Many aspects of this processing were studied in the previously initiated PhD and rolled platelets are already available, but the methodologies will need to be perfected.  The established relation with Argonne National Lab will allow an exchange of know-how and a possibility for production of samples if necessary.  Eventually, the production of LEU based targets and test irradiations can be envisaged, but it may be more useful to work with IRE on the dissolution studies of as-produced targets with Mo addition.  Because of the short irradiation times, the differences in dissolution response is minimal, except for the decay heat input, which can be simulated using electrical heating.

Planning :

Year 1 : literature study, review of data from previous PhD and characterisation of available materials, compound synthesis and powder metallurgy up to compact

Year 2 : target rolling, advanced characterisation of compounds (TGA-DSC, crystallography, thermal interaction Al with uranium compound), initialisation of dissolution testing on compacts with heat treatment

Year 3 : continuation of dissolution studies, Mo addition to targets and extraction, characterisation of rolled targets and optimalisation of process

Year 4 : writing PhD, LEU target manufacturing