Under irradiation issues of the CSZ–based inert matrix fuels from IFA-652 Halden experiment

The renewal of interest in Fast Reactors (FR) deployment as possible answer to the future perspective of natural uranium shortage has led to reconsider the plutonium as an energy source with a consequent radical change in its management. The transition to a closed fuel cycle is estimated to occur in a period ranging from 2030 to 2050. In the near term, Inert Matrix Fuels (IMF) are currently viable options, capable to contribute in reducing the proliferation risk associated to nowadays separated plutonium stockpiles in a business-as-usual nuclear energy scenario where a prominent role is still played by Light Water Reactors (LWR) in a once-through fuel cycle strategy. Double-strata strategy with Accelerator Driven Systems (ADS) machines to transmute minor actinides also support the hypothesis of inert matrix heterogeneous fuels. These reasons justify the current interest in IMF. A high burning efficiency achieved 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. Amongst other options, Calcia-Stabilised Zirconia (CSZ) and Yttria-Stabilised Zirconia (YSZ) fulfil these criteria standing as the most promising matrices to host plutonium. ENEA has conceived an in-pile testing (IFA-652 experiment) of viable inert matrices concepts: CSZ and thoria. This experiment was performed in the Halden Heavy Boiling Water Reactor (HBWR), a joint project of the OECD Nuclear Energy Agency.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 matrix, the fissile phase is High Enriched Uranium (HEU) oxide (UO2 93% 235U enriched). IMT is a ternary fuel composed of CSZ + thoria matrix and HEU oxide as fissile phase. Thoria is added in IMT fuel mainly to improve the low IM reactivity feedback coefficients. The discharge burnup, under typical LWR conditions, was, for all pins, 90–95% of the planned 45 MWd/kgU. Pins are instrumented providing fuel centreline temperature, pin inner pressure and fuel stack elongation measurements. On the basis of the experimental dataset under power ramping, a possible correlation of the FGR onset of CSZ-based fuels with the Halden Vitanza curve apparently exists. The TRANSURANUS (TU) fuel performance code predictions proved to be in nice agreement with the experimental findings concerning the thermal performance of IMF whilst, regarding FGR and densification, a satisfactory modelling is still needed.