Since the 2011 Tohoku-Oki Earthquake, evaluations based on a tsunami simulation approach have had a very important role in promoting tsunami disaster prevention measures in the case of mega-thrust earthquakes. When considering tsunami disaster prevention measures based on the knowledge obtained from tsunami simulations, it is important to carefully examine the type of tsunami source model. In current tsunami simulations, there are various ways to set the tsunami source model, and a considerable difference in tsunami behavior can be expected among the tsunami source models. In this study, we carry out a tsunami simulation of the 2011 Tohoku-Oki Earthquake around Fukushima Daiichi (I) Nuclear Power Plant and Fukushima Daini (II) Nuclear Power Plant in Fukushima Prefecture, Japan, using several tsunami source models, and evaluate the difference in the tsunami behavior in the tsunami inundation process. The results show that for an incoming tsunami inundating an inland region, there are considerable relative differences in the maximum tsunami height and wave pressure. This suggests that there could be false information used in promoting tsunami disaster prevention measures in the case of mega-thrust earthquakes, depending on the tsunami source model.

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　A filtered containment venting system was developed for nuclear power plants. In the Fukushima Dai-ichi Nuclear Accident, widespread land contamination was caused by cesium-137. This system was developed to filter aerosol particles and reduce the amount of radioactive particle release, while protecting the primary containment vessel from over pressure by venting gas from the vessel. Performance tests were conducted under various vent gas flow rate conditions to ascertain decontamination factors for aerosol particles with various diameters. It was observed through the tests that aerodynamic diameter was a good index for characterizing various aerosol particles for the filtered containment venting system. Test results showed that the decontamination factors were well over 1,000 for aerosol particles with aerodynamic diameters larger than 0.4 μm. For aerosol particles with aerodynamic diameters larger than 0.2 μm, the decontamination factors significantly increased with the diameter. This suggested that inertial deposition was the governing mechanism for filtering aerosols in this system. The decontamination factor of the water scrubber section of the filter increased with the increase in Stokes number.

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　A stress corrosion cracking (SCC) test was conducted using reverse U-bend specimens of MA alloy 600 and the uniaxial constant load test technique for alloy X-750 in simulated or modified PWR primary water under dissolved hydrogen (DH) of 5, 15 and 30 ml/kg at 290℃ and 320℃ because there is very little data from SCC initiation tests (particularly at 290℃) worldwide. It is well known that the SCC in primary water (PWSCC) growth rate for a Ni-based alloy shows a peak depending on the DH concentration due to the chemical stability of Ni and NiO. On the other hand, the dependence on DH is not clear for the PWSCC initiation time. It was anticipated that the PWSCC initiation time under DH of 5 ml/kg at 290℃ would be longer (better) than that at a higher DH concentration according to previous literature data. However, this study showed the shortest PWSCC initiation time under DH of 5 ml/kg at 290℃; thus, it matched the dependence on DH of the PWSCC growth rate. These results were considered to be due to the high stress applied to the specimens.

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 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” was broken and bent onto the in-vessel storage rack as an obstacle, damaging the upper core structure (UCS). In this paper, we describe the in-vessel repair techniques for UCS replacement, which are developed in Joyo. The UCS replacement was conducted in the following four stages: (1) jack-up of the existing damaged UCS, (2) retrieval of the existing damaged UCS, (3) installation of the O-ring, and (4) insertion of the new UCS. Since the UCS replacement was not anticipated in the original design, the work conditions at Joyo were carefully investigated, and the obtained results were applied to the design of special handling equipment. The UCS replacement was successfully completed in 2014. In-vessel repair techniques for sodium-cooled fast reactors (SFRs) are important in confirming the safety and integrity of SFRs. However, the techniques demonstrated in the actual reactor environment with high temperature, high radiation dose, and remaining sodium are insufficient to secure the reliability of these techniques. The experience and knowledge accumulated in the UCS replacement provide valuable insights into further improvements of in-vessel repair techniques for SFRs.

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 The redox condition of simulated high-level waste (HLW) glass was investigated by thermodynamic calculations for the phase equilibrium of crystalline phases composed of platinum group elements (PGE, Pd, Rh, and Ru) based on the thermodynamic data for Pd-Rh-Ru and (Ru,Rh)O₂ solid solutions. Phase equilibrium calculation indicates that the stability field of the Pd-rich metal phase with an FCC structure and the (Ru,Rh)O₂ phase is nearly independent of the relative abundances of Pd, Rh, and Ru. Thermodynamic calculation for rhodium partitioning between Pd-Rh-Ru and (Ru,Rh)O₂ can provide the temperature and oxygen fugacity conditions under which these phases are stable. The redox condition of HLW glasses retrieved from a full-scale mock-up melter (KMOC) in JAEA, Tokai, Japan, was evaluated by comparing the results of chemical composition analysis for PGE crystalline phases in the glasses with those of phase equilibrium and/or thermodynamic calculations. The obtained results suggest that the oxygen fugacity is higher than the atmospheric condition (f_O2>0.21) for the glass retrieved in a normal melter operation, while it is lower at 10^－1.5-10^－2.0 for the PGE-rich residual glass retrieved from the melter after glass draining. The calculated f_O2 values are consistent with the redox condition estimated from Ce³⁺/Ce⁴⁺ ratios of KMOC glasses reported previously.

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