- The material has to be packed very densely, so that the low enrichment of uranium can be compensated.
- The nuclear fission generates gases that have to be retained in the fuel.
- The fuel must have good thermal conductivity to prevent overheating of the interior of the fuel and to be able to safely dissipate the heat generated during operation, even in the presence of small non-bonds.
- The fuel must remain mechanically stable under irradiation and must not bend.
- The fuel elements must be produced on an industrial scale at viable prices, i.e. the manufacturing complexity is limited. The same applies to the back-end.
Objective: Avoid diffusion layer and limit swelling
The most promising candidate for this fuel is an uranium-molybdenum (UMo) alloy, which is currently being tested and improved. As part of these tests, the materials are irradiated in research reactors. Under irradiation, the UMo alloys form an interdiffusion layer (IDL) at the junction of UMo with the surrounding aluminium shell. This diffusion layer retains the gases generated during fission only insufficiently, as opposed to the UMo fuel itself. Furthermore, it has a low density and poor thermal conductivity. These properties induced a very strong swelling in the first UMo-Al test fuel plates, so that they had to be considered as unsuitable for use in the reactor.
It is therefore one of the main goals of fuel development to avoid or delay the formation of the IDL. This is achieved by applying diffusion barriers at the transition of U-Mo/Al. Suitable elements for this purpose are Si, Nb, Zr, Ti ZrN or TiN and a few others.
Irradiation experiments with heavy ions
Recently, samples that realize this concept were irradiated with heavy ions (iodine at 80MeV). Heavy ions in this case serve as a quick and easy-to-handle substitute for lengthy and complicated irradiation tests in a reactor. It has been shown that by using a sufficiently thick silicon layer, the formation of an IDL can be significantly delayed and can even be largely prevented with the help of dense TiN and ZrN layers. Irradiation experiments in the Belgian material test reactor BR2 confirm these findings but also show that the metallurgical difficulties of the diffusion barrier are not yet fully solved at very high fuel irradiation levels. Clarifying the causes and finding logical solutions are challenges for future research.
In addition to heavy ion irradiation, the research group has also participated in a number of other measurements necessary for the fuel to be qualified. This includes, inter alia, determining the thermal conductivity of both unused and spent fuel and the characterization of the phase behaviour of UMo.