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The control of the interaction between the U(Mo) fuel phase and the Al matrix is one of the challenges of dispersion fuel plate development for research reactors. Given the specific properties of this interaction layer, larger amounts of it in the meat could lead to a reduction of the plate mechanical integrity and thermal conductivity, eventually leading to pillowing. The SELENIUM project showed that by depositing a ZrN coating on the surface of the U(Mo) fuel particles, the amount of formed U(Mo)-Al interaction layer is limited but still present. Microstructural analysis performed on the as fabricated coating and fresh fuel plates containing ZrN coated U(Mo) dispersed in an Al matrix, revealed that the coating gets damaged during plate production. The post irradiation examinations (PIE) of the ZrN coated U(Mo) fuel plates, from the SELENIUM and SEMPER FIDELIS experiments, show how the U(Mo)-Al interaction layer is formed - only at those locations where the coating is missing or damaged - and the evolution of coating microstructure during irradiation. As a remedy, to further reduce the amount of interaction layer formed, the use of an Al-Si matrix was proposed based on the higher affinity of Si for U compared to the affinity of Al for U. PIE of a fuel plate consisting of ZrN coated U(Mo) dispersed in an Al-Si matrix irradiated in the SEMPER FIDELIS experiment, clearly demonstrates the benefit of adding Si to the matrix.

Although available data do not report significant thermal ageing of reactor pressure vessel materials, this should be reconsidered in the perspective of long term operation where exposure time is significantly higher than the available reported experimental data. One of the element that is reported to promote thermal ageing degradation is phosphorus. The thermal ageing embrittlement susceptibility is assessed by submitting reactor pressure vessel steels to a step cooling heat treatment known to induce P-segregation to grain boundaries resulting in intergranular rather than transgranular cleavage fracture. Thermal ageing susceptibility was experimentally assessed on two typical reactor pressure vessel steels with P-content of 0.017% and 0.006%. The experimental data including tensile, Charpy impact and fracture toughness tests, show a very consistent picture of the various mechanical properties. Upon step cooling, intergranular fracture is promoted and P-content has a major effect on the thermal ageing susceptibility. However, quantitative assessment shows that the effects of step cooling on the mechanical properties of both steels in their as received condition are not sufficiently high to induce thermal ageing embrittlement and therefore their susceptibility to thermal ageing is low. Consequently, given the low P-content of most Belgian RPV materials, it can be concluded that they will be not prone to thermal ageing in the perspective of long term operation.

Several irradiation experiments have confirmed the positive effect of adding Si to the matrix of an U(Mo) dispersion fuel plate on its in-pile irradiation behavior. E-FUTURE, the first experiment of the LEONIDAS program, was performed to select an optimum Si concentration and fuel plate heat treatment parameters for further qualification. It consisted of the irradiation of 4 distinct (regarding Si content and heat treatments), full size flat fuel plates in the BR2 reactor under bounding conditions (470 W/cm 2 peak BOL power, ∼70% peak burn-up). After the irradiation, the E-FUTURE plates were examined non-destructively and found to have pillowed in the highest burn-up positions. The destructive post-irradiation examination confirmed that the fuel evolves in a stable way up to a burn-up of 60% 235 U. Even in the deformed area (pillow) the U(Mo) fuel itself shows stable behavior and remaining matrix material was present. From the calculation of the volume fractions, the positive effect of a higher Si amount added to the matrix and the higher annealing temperature can be derived.

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