HIGH-TEMPERATURE REACTOR-FUEL FISSION-PRODUCT RELEASE AND DISTRIBUTION AT 1600-DEGREES-C TO 1800-DEGREES-C

The essential feature of small, modular high-temperature reactors (HTRs) is the inherent limitation in maximum accident temperature to below 1600-degrees-C combined with the ability of coated particle fuel to retain all safety-relevant fission products under these conditions. To demonstrate this ability, spherical fuel elements with modern TRISO particles are irradiated and subjected to heating tests. Even after extended heating times at 1600-degrees-C, fission product release does not exceed the already low values projected for normal operating conditions. Details of fission product distribution within spherical fuel elements heated at constant temperatures of 1600, 1700, and 1800-degrees-C are presented. The measurements confirm the silicon carbide (SiC) coating layer as the most important fission product barrier up to 1800-degrees-C. If the SiC fails (or is defective), the following transport properties at 1600 to 1800-degrees-C can be observed: cesium shows the fastest release from the UO2 kernel but is highly sorbed in the buffer layer of the particle and in the matrix graphite of the sphere; strontium is retained strongly both in UO2 kernels and in matrix graphite, but can penetrate SiC in some cases where cesium is still completely retained; only if all coating layers are breached can iodine and noble gases be released. For the first 100 h at 1600-degrees-C (enveloping all possible accident scenarios of small HTRs), these fission products are almost completely retained in the coated particles.