Accident testing of high-temperature reactor fuel elements from the HFR-EU1bis irradiation

Four spherical high-temperature reactor fuel elements irradiated at the High Flux Reactor in Petten within the HFR-EU1bis irradiation campaign were transported to JRC-ITU for post-irradiation examination and accident testing. This article reports the accident tests performed on the first two fuel elements HFR-EU1bis/1 and HFR-EU1bis/3 with the Küfa device. The fuel elements were heated up to the irradiation temperature (1250 °C) and then to accident temperatures (1600 °C and 1700 °C, respectively) for durations of several hundred hours to simulate depressurization and loss-of-forced-circulation accidents (D-LOFC). The overall low fractional release of 85Kr and the absence of steps in the release curves suggest that no coated particles failed during the accident simulations. On the other hand, the fractional release of 134Cs and 137Cs was much higher than in previous experiments, indicating increased diffusion through the coating layers at the higher irradiation temperature, as confirmed with a finite-difference diffusion calculation. Finally, the calculation suggests that the high fractional silver and cesium releases at 1250 °C are due to a matrix contamination prior to the Küfa accident testing.