Safety experiments for the demonstration of fission product retention in fully irradiated fuel elements

Since 1977, fuel elements from AVR operation and MTR irradiation tests have been investigated in accident simulation tests. In the first phase, the temperature margins were explored up to 2500degC where fission product release is massive. The second phase of the investigations concentrated on a new set of equipment enabling the measurement of minute fractions of fission product release in the temperature range from 1600deg-1800degC (KUEFA furnace). We were able to demonstrate that modern, TRISO coated, UO2 particles will retain all safety-relevant fission products up to 1600degC at a level not exceeding release under normal operating conditions which in itself is very low. The third phase of the investigation is dedicated to the study of fission product release under oxidising conditions. Again, special equipment designed for the purpose (the KORA furnace) enables the simulation of steam ingress, typically at 800degC air ingress exploring coated particle limitations in the range from 1300 to 1600degC. Steam ingress can lead to large releases of fission gases and iodine from defective and failed particles. Detailed results are presented and will be compared to in-pile experiments. While the original assumptions assumed complete particle failure at 1100degC during unimpeded air ingress, we can show that it takes several days at 1100degC to disintegrate the graphitic spherical fuel element still leaving the particles intact. Only prolonged air ingress at 1300degC or higher initiates the failure of particles. The first phase and the third phase were performed for hypothetical accident scenarios to enable the reliable prediction of accident consequences. The second phase concerns the design basis accident of small, modular HTRs. The results are useful provide a direct demonstration of the potential for a catastrophe-free form of nuclear energy. (J.P.N.)