Japan is one of the countries with abundant active volcanoes. In the field of nuclear energy, it is necessary to assess safety against volcanic hazards, and voluntary guidelines were published as JEAG4625 in 2009 to set up the requirements of site assessments and the basic designs of nuclear power plants (NPPs). Revised guidelines were published in 2014, which stated the need to examine volcanic phenomena and concrete countermeasures in the detailed designs of NPPs. A further revision included the requirement of the safety assessment of equipment used for measures in a severe accident. This paper focuses on the background and technical basis of the revision of the voluntary guidelines and shows the basic policy on the concept of defence-in-depth for protecting NPPs against volcanic phenomena and the requirements of equipment to perform measures in the case of a severe accident to prevent nuclear hazards due to volcanic phenomena and the maintenance program after an volcanic eruption.

 Seawater was injected into the reactors during the accident at TEPCO’s Fukushima Daiichi NPS. However, the effects of the seawater on the cooling performance of the fuel rods and fuel debris are not clear. As possible effects, the change in the physical properties of the coolant and the sea salt deposition on a heat transfer surface and in the coolant are considered. We conducted thermal-hydraulic experiments using an internally heated annulus to determine the effects of seawater under conditions without boiling. The same experiments for water and sodium chloride (NaCl) solution were also conducted for the purpose of comparison with the artificial seawater. In these experiments, considering the physical properties of the artificial seawater, the thermal-hydraulic behaviors of the artificial seawater under forced convection (Re>2300 [－]) was estimated from the Dittus-Boelter correlation although sea salt was deposited in the fluid. According to the results of particle image velocimetry (PIV), the velocity distribution in the artificial seawater was NOT different from that in the water and the NaCl solution. For a mixed convection regime, the Nusselt number of the artificial seawater was obtained from the correlation of the Grasholf number, Reynolds number and Prandtl number, as well as those for the water and the NaCl solution. Therefore, considering the physical properties of the artificial seawater, the thermal-hydraulic behavior of the seawater in single-phase flow can be estimated from the conventional thermal-hydraulic correlations for a single-phase flow.

 A design for an organic iodine filter was developed for filtered containment venting systems of nuclear power plants. After the Fukushima Dai-ichi nuclear accident, filtered containment venting systems were developed and installed at nuclear power plants in Japan employing aerosol filters, which are typically combinations of alkaline water scrubbers and metal fiber filters. Using silver zeolite, the newly designed filter absorbs organic iodine, which cannot be captured with alkaline water scrubbers and metal fiber filters. Methyl iodide absorption performance tests were conducted with various gas velocities and compositions, which covered the operating conditions of filtered containment venting systems. The test results demonstrated that the decontamination factors of the silver zeolite were over 50 if an appropriate gas contact time was ensured, even though steam condensation and the existence of hydrogen during vent startup had negative effects on the absorption performance. It was also revealed from the tests that intermittent venting might dampen the silver zeolite and reduce its absorption performance. Flow control of the vent gas was found to be an effective countermeasure to maintain the performance even under such conditions. Filter unit tests and numerical analyses of the gas flow within the filter vessel confirmed that the gas contact time was uniform in the filter and appropriate for achieving a decontamination factor of 50.

 In the experimental fast reactor Joyo, it was confirmed that the top of the irradiation test subassembly of the material testing rig named “MARICO-2” had been broken and bent onto the in-vessel storage rack as an obstacle and had damaged the upper core structure (UCS). As a result, to restart Joyo, it was necessary to replace the damaged UCS and to retrieve the bent subassembly. This paper describes the in-vessel repair techniques performed in the retrieval of the obstacle inside the reactor vessel. The retrieval was mainly conducted in five stages: (1) installation of a temporary shielding plug and the formation of a temporary boundary, (2) installation of the retrieval device, (3) retrieval of the bent subassembly, (4) storage of the bent subassembly in a cask, and (5) transportation of the cask to a postirradiation examination facility. The devices which were prepared for this work demonstrated the expected performance under the environmental conditions of a sodium-cooled fast reactor (SFR) such as a high temperature and radiation dose, and the work was completed in 2014. The successful operation to retrieve the damaged component inside the reactor vessel will contribute to the development of in-service inspections and repair techniques in SFRs.

 Fe₂B appears to have formed in the fuel debris at the Fukushima Daiichi nuclear disaster site through the reaction between stainless steel and the control material B₄C. In order to conduct defueling operations, understanding the physical properties of Fe₂B is of extreme importance. In the present study, we have prepared an Fe₂B bulk sample with 92.5% theoretical density by spark plasma sintering of Fe₂B powder, and we have evaluated the mechanical and thermal properties of the sample. The thermal diffusivity was measured by a laser flash method in an argon flow. The thermal conductivity was evaluated from the heat capacity, thermal diffusivity, and density. The electron thermal conductivity was calculated from measurement values of electric conductivity. The longitudinal and shear sound velocities were measured by a sing-around method at room temperature in air, which enables the elastic modulus and Debye temperature to be evaluated. The hardness measurements were performed at room temperature in air using a Vickers hardness tester. The fracture toughness was estimated from a crack produced by an indenter. The maximun Vickers hardness and fracture toughness were determined to be 12 GPa and 2.8 MPa m^0.5, respectively.