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.