Development of a Coated Fuel Particle Failure Model under High Burnup Irradiation

抄録

In high temperature gas-cooled reactors (HTGRs), coated particles are used as fuels. For upgrading HTGR technologies, Japan Atomic Energy Research Institute has been developing high burnup Tri-isotropic (TRISO) coated fuel particles. The TRISO coatings consist of a low-density, porous pyrolytic carbon (PyC) buffer layer adjacent to the spherical fuel kernel, follwed by an isotropic PyC layer, a SiC layer and a final PyC layer. In safety design of HTGR fuels, it is important to retain fission products within the particles so that their release to primary coolant does not exceed an acceptable level. Therefore the basic design criteria for the fuel are to minimize the failure fraction of as-fabricated fuel coating layers and to prevent significant additional fuel failures during operation. In the high burnup coated fuel particle, stress due to fission gas pressure and irradiation-induced PyC shrinkage is introduced into the coating layers and consequently the stress could cause failure of coating layers under high burnup irradiation condition. Some models have been developed to evaluate failure fraction of coated fuel particles, however, they are regarded as a guideline for fuel particle design rather than as a predictive tool for coated fuel particle performance. Then the behavior of coated fuel particles has been examined only by many irradiation experiments and development of reliable model has been needed. A failure model is newly developed to predict failure fraction of TRISO-coated particle under high burnup irradiation. In the model, it is assumed that the failure fraction depends not only on failure of the SiC layer but also on that of the PyC layers. The failure fractions of through-coatings failed particles and the SiC-failed particles are calculated based on the failure probability of each coating layer. Based on the model developed here, parameter calculations for the first-loading-fuel of the High Temperature Engineering Test Reactor and for the high burnup fuel were carried out.