Thermodynamic calculation of volatile radionuclides such as Cs, I and Te in a cladding failure accident of a sodium-cooled fast reactor has been conducted for the evaluation of an in-vessel source term. Important chemical reactions and chemical forms such as Cs, Cs₂, Te, CsI, Cs₂I₂, Cs₂Te, CsNa, NaI, Na₂I₂, Cs₂Te, Na₂Te, AgTe and Cs₂MoO₄ were suggested by the results of interaction among fuel, fission products, a cladding tube and Na coolant at high reduction rates inside the fast reactor vessel. As the cladding failure accident proceeds, it was confirmed that 15% and 60% of Cs was in gas phase at 1,155 K and 5.7 MPa and at 1,670 K and 10.5 MPa, respectively. 58% and 66% of Cs was in the gas phase at 1,155 K and 1,670 K at 0.1 MPa owing to cladding failure. Under Na mixing conditions, almost 100% of Cs became gas. Furthermore, we confirmed the same behavior as in light water reactors, that is, the release of Cs is suppressed at high pressures, but we observed that Cs rarely oxidizes and that the amount of Cs released increases with Na mixing in the fast reactor.

We estimate the inventory of tritium in two sea areas corresponding to coastal and offshore ones around Fukushima Daiichi Nuclear Power Plant (1F) on the basis of the measured seawater tritium concentrations monitored constantly from 2013 to Jan. 2021 by using the Voronoi tessellation scheme. The obtained results show that the offshore area inventory and its temporal variation amount correspond to approximately 1/5 and 1/50 of that of the treated water accumulated inside 1F, respectively. These results suggest that the amount of tritium included in seawater as the background is non-negligible in evaluating the environmental impact of releasing the accumulated treated water into the sea area. We also estimate the offshore area inventory before the 1F accident and find that it had exceeded the 1F stored inventory over 30 years from 1960s to 1980s, being approximately 4 times larger in the peak decade 1960s. This finding means that we had already experienced more contaminated situations over 30 years in the past compared with the conservative case emerged by just releasing whole the present 1F inventory. Here, it should also be emphasized that the past contamination situation was shared by the entire world. We further extend the estimation area to include the offshore area from Miyagi to Chiba Prefectures and find that the average area inventory is now comparable to half the present 1F inventory. Finally, we estimate the internal dose per year by ingesting fishes caught inside the area when the 1F inventory is added only inside the area and kept there for one year. The result indicates that it approximately corresponds to 1.0 × 10⁻⁶ of the dose from natural radiation sources. From these estimation results, we find that all the tritium inventories stored inside 1F never contribute to significant dose increment even when it is instantly released into the area.

Primary containment vessel (PCV) venting is one of the mitigating measures for severe accidents (SAs) on nuclear power plants. To prevent PCV failure, PCV venting will be conducted through judgement by responsible persons when the plant reaches relevant SA conditions. From the viewpoint of public safety, the judgement should be performed considering progress in public evacuation. This study is carried out to investigate effective PCV venting criteria in terms of public risk reduction and to propose protection measures against a delay in public evacuation caused by adverse conditions. To quantify the degree of judgement on PCV venting, the analysis method used here considers fuzzy inference, which makes it possible to formalize human reasoning. The analysis links both the thermal-hydraulic progression during an SA and protection measures to the discussion on interfaces between defense-in-depth levels 4 (control of SA conditions) and 5 (mitigation of a radiological consequence).

The possibility of data collection by a mesh-type LPWA was examined, expecting its use in radiation monitoring during complex disasters and unexpected circumstances. In this study, the Shimane Institute of Public Health and Environmental Science, located next to the Shimane Nuclear Environmental Center, was used as the base for the study. The mesh network connecting the monitoring points around Matsue City and Lake Shinji specified in the disaster prevention plan of Shimane Prefecture was flexibly deployed. As a result, stable communication and flexible network deployment were confirmed. On the basis of the results of this study, a proposal for a monitoring scheme using a mesh-type LPWA is introduced.