Japan Atomic Energy Agency (JAEA) has developed fabrication technology for oxidation-resistant fuel elements to improve the safety of high-temperature gas-cooled reactors in severe oxidation accidents on the basis of its previous research. Simulated fuel particles (alumina particles) were coated with a mixed powder of Si, C and a small amount of resin to form over-coated particles, which were molded and sintered by hot-pressing to form simulated oxidation-resistant fuel elements with a SiC/C mixed matrix, where the SiC was formed by reaction bonding. Simulated oxidation-resistant fuel elements with a matrix whose Si/C mole ratio (about 0.551) was three times as large as that in previous research were fabricated. No Si peak was detected by X-ray diffraction of the matrix. A monoaxial compressive fracture test was carried out, and the fracture stress was found to be more than three times as large as the standard for fuel compacts of High Temperature Engineering Test Reactor (HTTR). Better oxidation resistance than that of an ordinary fuel compact with a ordinary graphite matrix was confirmed by an oxidation test in 20% O2 at 1673 K. All simulated coated fuel particles remained in specimen after 10 h oxidation.
Following the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, fuel debris formed in the reactor. It is important to understand the thermal and mechanical properties of the fuel debris. The fuel debris mainly consists of oxide, boride, and metallic phases. Although the physical properties of the oxide and boride phases have been widely investigated, those of the metallic phase have not been deeply studied. Here, we focus on Fe2Zr, which is considered to be the main component of the metallic phase. Ingots of Fe2Zr were synthesized by arc melting, then the ingots were crushed to powders, which was followed by spark plasma sintering to obtain high-density polycrystalline bulk samples. The thermal and mechanical properties such as thermal expansion coefficient, specific heat capacity, thermal diffusivity, thermal conductivity, sound velocity, elastic moduli, and Vickers hardness were examined, and the obtained data were compared with the literature data on the oxide and boride phases to obtain an overall understanding of the properties of the fuel debris.
The wettability of liquid-state volatile fission products (FPs) for solid-state fuels is important for the evaluation of FP release behavior during severe accidents. Previously, we have reported that liquid CsI exhibits extremely high wettability with a contact angle of virtually 0° on a polycrystalline UO2 surface ［K. Kurosaki et al., Sci. Rep. 7, Article number: 11449 (2017)］ and a single-crystalline yttria-stabilized zirconia (YSZ) surface ［H. Ishii et al., J. Nucl. Sci. Technol. 55, No. 8, 838–842 (2018)］. Here, based on the previous studies, we investigate the wettability of liquid cesium halides (CsCl, CsBr) on single-crystalline YSZ, TiO2, and MgO. We observe the high wettability of liquid cesium halides on single-crystalline YSZ and TiO2 with contact angles of nearly 0° in all cases. However, liquid cesium halides exhibit completely different wettability on single-crystalline MgO, where the contact angles are measured to be 50°, 44°, and 25° for CsCl, CsBr, and CsI, respectively. Similarly to UO2, YSZ and TiO2 are non-stoichiometric compounds containing oxygen defects, while MgO is a stoichiometric line compound. Thus, it is assumed that the oxygen defects play a role in the high wettability of liquid CsI on solid UO2.
For the practical application of an optimization method for radioactive waste disposal facility design based on a probabilistic approach, we propose a procedure to set probability distributions for radionuclide migration parameters. First, from the initial and long-term viewpoints, it is necessary to extract factors affecting each radionuclide migration parameter and then the sources of uncertainty inherent to each factor. Taking the permeability coefficient of bentonite as an example, one of the factors is the montmorillonite content, whose initial uncertainty originates from the quality of the raw bentonite material. The long-term uncertainty originates from insufficient knowledge about when, how and how much montmorillonite is dissolved. Next, it must be considered how these uncertainties should be managed. The initial uncertainty can be managed by directly analyzing materials and facility structures. The long-term uncertainty can be managed by evaluation based on expert judgments. Finally, it is necessary to organize information on the engineering and economic costs associated with management of the uncertainties. This information is essential to select an optimal facility design in the examination of optimization based on the ALARA principle.
We developed an examination method that uses computational fluid dynamics (CFD) to investigate the effects of a complex pipe geometry on flow fields. Two kinds of pipe model with different geometries are simulated to test the developed method. The simulation models were split into several computational regions to reduce the computation time. The simulation results showed that the fluctuation of the flow rate depended on the pipe geometry, which qualitatively agreed well with the experimental results. The simulation results of one of the two models showed a swirling flow around the orifice with large fluctuations of the flow rate. It was found that the swirling flow caused velocity fluctuations in the recirculation zone around the tap positions, which resulted in the large fluctuations of the flow rate. We also investigated the mechanisms generating the swirling flow. The simulation results showed that the high velocity of the flow along the wall was caused by the valve and the bend pipe. The high-velocity flow then moves along the pipe wall of the tee, which causes the flow to swirl. These results show that the developed method can be used to evaluate the flow fields in piping systems.