Inert Matrix Fuels Analysis by Means of the TRANSURANUS Code: The Halden IFA-652 In-Pile Test

Inert matrix fuels are a possible option to reduce separated plutonium stockpiles by burning it in LWR fleet. A high burning efficiency targeted by preventing new plutonium build-up under irradiation (U-free fuel), a proved high radiation damage and leaching resistance are fundamental requirements when a once-through fuel cycle strategy is planned. Among other options, both calcia-stabilised zirconia (csz) and thoria fulfill these criteria standing as the most promising matrices to host plutonium. While several in-pile tests concerning thoria fuels are found, calcia-stabilised zirconia under-irradiation performance is still to be fully assessed, with this regard the thermal conductivity, markedly lower than UOX and MOX cases, plays a fundamental role. For this reason, ENEA has conceived a comparative in-pile testing of three different U-free inert matrix fuel concepts, that have been performed in the OECD Halden HBWR (IFA-652 experiment). The discharge burnup accomplished about 90–97% of the 45 MWd/kgU_eq target under typical LWR irradiation conditions. The test-rig is a six-rod bundle loaded with IM, IMT and T innovative fuels. IM and T fuels have, respectively, csz and thoria as matrix, the fissile phase being HEU oxide (UO₂ 93% ²³⁵U enriched). IMT is a ternary fuel composed by csz+thoria matrix and HEU oxide as fissile phase. Thoria is added in IMT fuel to improve the low IM reactivity feedback coefficients. Pins are instrumented providing fuel centerline temperature, pin inner pressure and fuel stack elongation measurements. Our purpose is to investigate the key processes of IMF under-irradiation behaviour by means of the TRANSURANUS code. Thermal conductivity and its degradation with burnup, densification-swelling response and FGR are tentatively modelled in the burnup domain of IFA-652. In particular it is pointed out the effects of pellet geometry and fuel microstructures in the IM and IMT cases. The consistency of our results is discussed aiming at understanding the in-pile response, as a fundamental step, in the perspective of future deployment of the nuclear fuels we are dealing with Notwithstanding this ambitious objective, it is clear, however, that these results rely on a limited data set and that, as TRANSURANUS is a semi-empirical code mostly tailored for commercial fuels, the modelling of the IMF is still a work in progress.