In high temperature gas-cooled reactors (HTGRs), Tri-isotropic (TRISO)-coated fuel particles are employed as fuel. The TRISO coatings consist of a low-density, porous pyrolytic carbon (PyC) buffer layer adjacent to the spherical fuel kernel, followed by an isotropic PyC layer (inner PyC; IPyC), a silicon carbide (SIC) layer and a final PyC (outer PyC; OPyC) layer. In safety design of the HTGR fuels, it is important to retain fission products within particles so that their release to primary coolant does not exceed an acceptable level. From this point of view, the basic design criteria for the fuel are to prevent significant additional fuel failures during operation. In order to investigate fuel behavior under high burnup condition, irradiation tests were performed. The irradiation tests were carried out independently as 91F-1A capsule (at the Japan Materials Testing Reactor of Japan Atomic Energy Research Institute) and HRB-22 capsule (at the High Flux Isotope Reactor of Oak Ridge National Laboratory) irradiation tests. The fuel for the irradiation tests was called high burnup fuel, whose target burnup was 5 to 10%FIMA (% fission per initial metallic atom). In order to keep fuel integrity up to high burnup, thickness of buffer and SiC layers of the high burnup fuel particle were increased. During irradiation, fuel performance was monitored by fission gas concentration in sweep gas. The failure of coated particles was observed in both irradiation tests. The fuel failure behavior was analyzed by pressure vessel failure model, which evaluate failure probability of coating layers due to the internal pressure of fission gases and CO gas with burnup. The result showed that no tensile stresses acted on the SiC layers even at the end of irradiation and no pressure vessel failure occurred in the intact particles. This result implies that there were weak particles and they failed to the through-coatings failure during irradiation. The weak particles correspond to particles with defect in the SiC layer. The SiC-defected particle would be the particle with micro crack on the SiC layer surface. Since the high burnup fuel compact was high packing fraction, i.e., contained much coated fuel particles, to keep fissile materials, contact of the coated fuel particles would occur in fuel compaction process.
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