Tri-isotropic Research Articles

Research and development on HTGR fuel in the HTTR project

In the high temperature gas-cooled reactors (HTGRs), refractory coated fuel particles are employed as fuel to permit high outlet coolant temperature. The high temperature engineering test reactor (HTTR) employs tri-isotropic (TRISO)-coated fuel particles in the prismatic fuel assembly. Research and development on the HTTR fuel has been carried out spread over about 30 years, in fuel fabrication technologies, fuel performance, and so on. Furthermore, for upgrading of HTGR technologies, an extended burnup TRISO-coated fuel particle and an advanced type of coated fuel particle, ZrC-coated fuel particle in order to keep the integrity at higher operating temperatures has been developed. The present paper provides experiences and current status of research and development works for the HTGR fuel in the HTTR Project.

Investigation of Irradiation Behavior of SiC-Coated Fuel Particle at Extended Burnup

In current high-temperature gas-cooled reactors (HTGRs), Tri-isotropic (TRISO)-coated fuel particles are employed as fuel. 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 minimize the failure fraction of as-fabricated fuel coating layers and to prevent significant additional fuel failures during operation. The maximum burnup of the first-loading fuel of the High Temperature Engineering Test Reactor (HTTR) is limited to 3.6%FIMA (% fission per initial metallic atom) to certify its integrity during the operation. In order to investigate fuel behavior under extended burnup condition, irradiation tests were performed. The irradiation was carried out as HRB-22 and 91F-1A capsule irradiation tests. The fuel for the irradiation tests was called extended burnup fuel, whose target burnup and fast neutron fluence were higher than those of the first-loading fuel of the HTTR. In order to keep fuel integrity up to over 5%FIMA, the thickness of buffer and SiC layers of fuel particle were increased. The fuel compacts were irradiated in the HRB-22 and the 91F-1A capsules at the High Flux Isotope Reactor of Oak Ridge National Laboratory and at the Japan Materials Testing Reactor of the Japan Atomic Energy Research Institute, respectively. The comparison of measured and calculated release rate-to-birth rate ratios showed that there were additional failures in both irradiation tests. A pressure vessel failure model analysis 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 even in a particle with thin buffer layer with failed OPyC layer. The presumed failure mechanisms are additional through-coatings failure of as-fabricated SiC-failed particles or an excessive increase of internal pressure by the accelerated irradiation. Further study is needed to clarify the failure mechanism.

Research and Development of Fuel Element for Chinese 10 MW High Temperature Gas-cooled Reactor

The fuel elements for Chinese 10 MW High Temperature Gas-cooled Reactor (HTR-10) are spherical all- ceramic fuel elements (SFE). TRISO (Tri-isotropic) coated fuel particles (CP) are uniformly dispersed in the graphite matrix of the fuel element. All radiological fission products are almost completely retained inside the SiC layer of the intact CP. The fabrication of SFE includes UO2 kernel preparation by sol-gel method, pyrolytic carbon (PyC) and SiC coating on the UO2 kernels by Chemical Vapor Deposition, manufacture of SFE by the quasi-isostatic pressing and the inspection of over 30 kinds of properties. This paper describes research and development (R & D) and design specification of the HTR-10 SEF, summarizes the fabrication technology and quality control mastered in R & D for HTR-10 SFE.

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

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.

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

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.