[en] In high temperature gas-cooled reactors (HTGRs), Tri-isotropic (TRISO)-coated fuel particles are employed as fuel. In the high burnup coated fuel particle, stress due to fission gas pressure and irradiation-induced pyrolytic carbon (PyC) shrinkage is introduced into the coating layers and consequently the stress could cause failure of coating layers under high burnup irradiation condition. A failure model has 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. Therefore, failure probability is strongly dependent on the irradiation characteristics of PyC. However, it is difficult to obtain new irradiation data of PyC by experiments. This paper describes the outline of the failure model and the issues to determine PyC characteristics. (author)

[en] 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

No abstract available

[en] To enhance the current core catcher technology and therefore the safety of light water reactors, it is crucial to deepen the knowledge on corium spreading resulting from a severe accident and to be able to simulate it properly. In the following study, we focus our attention on the three-dimensional Volume of Fluid (VOF) multiphase model towards the spreading of molten metal, which is released from the crucible vertically falling onto the flat metal plate. Unlike the previous simulations of the topic, we try to simulate a spreading phenomenon closer to the severe accident scenario and fully driven by the gravity force alone. From that point forward, the simulation results were compared to the available data from a previous set of experiments done by Ogura et al (2018). This kind of comparison allows us to see the drawbacks and short coming of the simulations under such severe condition and provide characterizations for the simulation improvement. As the simulations are driven closer to the real case scenario, experiments to help that extension were built to work parallelly with the available database. (author)

[en] An estimation of coated fuel particle behavior under accident conditions was carried out to reveal important points for High Temperature Gas-cooled Reactor (HTGR) design. Core heat-up and oxidization accidents were chosen in this study. Parameter calculations were carried out with a coated fuel particle failure model, which was developed based on core heat-up simulation tests, to evaluate the fuel failure fraction during core heat-up accident. The results showed that the failure fraction will decrease about an order by thicker SiC coating layer. For an evaluation of fuel behavior under oxidization accident, a fuel failure model based on thermodynamic analysis, which showed the active-to-passive transitions of oxidation of SiC layer, was introduced. Based on this model, an additional failure fraction during depressurization accident of the High Temperature Engineering Test Reactor (HTTR) was evaluated to be about 0.02%. These results obtained from this consideration will be useful for the fuel and safety design of future HTGRs. (author)

[en] Irradiation experiments for the HTTR fuel development were performed mostly by using Oarai Gas Loop-1 (OGL-1) and capsules in Japan Material Test Reactor (JMTR) of JAERI. Various kinds of researches have been carried out to confirm the integrity of the HTTR fuel. Present status and future plan of the HTTR project were also outlined

[en] In high temperature gas-cooled reactors (HTGRs), a small amount of fission products are released from fuel elements during normal operation. Then condensable fission products plateout on the inner surface of primary cooling system components. In accident conditions such as rapid depressurization events, plated-out fission products would be re-entrained into the coolant by chemical and/or mechanical forces. The re-entrainment process is called liftoff. Since this process is very complicated phenomenon, a quantitative model for analysis has not been established. Therefore, experiments were carried out to investigate the behavior of fission products and graphite dusts under the rapid depressurization condition caused by large-scale pipe rupture accident. One experiment was focused on fission products plated-out on metal surface or on/in oxide film and another was focused on the graphite dusts behavior. In this paper, applicability of turbulent burst model to graphite dusts and fission products liftoff models to the experimental data was investigated. (author)

[en] The very high temperature gas-cooled reactor (VHTR) can supply nearly 1000degC of reactor outlet temperature and is nominated as a candidate of Generation-4 reactor system. The development of new fuel, which can be used up to higher temperature and burnup comparing SiC-coated fuel particle, is expected to improve VHTR performance. The Japan Atomic Energy Agency proceeds development of fabrication and characterization technologies for ZrC-coated particle. This report describes present status of the development. (author)

[en] A modular high-temperature gas-cooled reactor (MHTGR) is expected to be one of the best energy sources in the near future because it can supply high-temperature heat and have high thermal efficiency and sufficient safety features. The safety evaluation of the future MHTGR should be performed based on the experience obtained from the safety evaluation of the High-Temperature Engineering Test Reactor (HTTR). The safety evaluation of the HTTR was performed considering the specific safety design features of the HTGR and is applicable to the future MHTGR. Before the detailed safety evaluation of the future MHTGR, the safety evaluation method and results of the HTTR should be reviewed, and newly established acceptance criteria and methods for selecting evaluation events must be clarified. This paper describes in detail the method and results of the safety evaluation of the HTTR

[en] The ZrC coating layer is a candidate to replace the SiC coating layer of the Triso-coated fuel particle. To compare the irradiation performance of the ZrC Triso-coated fuel particles with that of the normal Triso-coated fuel particles at high temperatures, a capsule irradiation experiment was performed, where both types of the coated fuel particles were irradiated under identical conditions. The burnup was 4.5% FIMA and the irradiation temperature was 1,400 to 1,650 C. The postirradiation measurement of the through-coating failure fractions of both types of coated fuel particles revealed better irradiation performance of the ZrC Triso-coated fuel particles. The optical microscopy and electron probe microanalysis on the polished cross section of the ZrC Triso-coated fuel particles revealed no interaction of palladium with the ZrC coating layer nor accumulation of palladium at the inner surface of the ZrC coating layer, whereas severe corrosion of the SiC coating layer was observed in the normal Triso-coated fuel particles. Although no corrosion of the ZrC coating layer was observed, additional evaluations need to be made of this layer's ability to satisfactorily retain the fission product palladium