Since the Fukushima Daiichi nuclear power plant accident caused by the Tohoku Region Pacific Coast Earthquake on March 11, 2011, decontamination work has been conducted in the surrounding environment within the Fukushima prefecture. Removed contaminants including soil, grass and trees are to be stored safely at temporary storage facilities for up to three years, after which they will be transferred to a planned interim storage facility. The decontamination pilot project was carried out in both the restricted and planned evacuation areas in order to assess decontamination methods and demonstrate measures for radiation protection of workers. Fourteen temporary storage facilities of different technical specifications were designed and constructed under various topographic conditions and land use. In order to support the design, construction and monitoring of temporary storage facilities for removed contaminants during the full-scale decontamination within the prefecture of Fukushima, technical know-how obtained during the decontamination pilot project has been identified and summarized in this paper.

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  This paper shows surveys of current Japanese regulations relevant to nuclear security, including regulations for nuclear damage compensation. It also shows the regulatory issues still left unresolved, particularly the question of whether existing laws are broad enough, or whether new legislation will be needed. The largest unresolved issue is how to establish a system of physical protection of nuclear materials for broader nuclear security purposes within a system that was originally introduced only to control nuclear proliferation. Such a system will necessitate a radical revision of the relevant law. Moreover, the current regulations have left unresolved the physical protection of radioactive materials used in research, medical, and non-nuclear-power-related industrial operations. Japanese legislation has already authorized an integrated Nuclear Regulatory Commission. Its mandate should include introduction of a physical protection system for currently unregulated radioactive materials. This will also necessitate a radical revision of the relevant law. Comprehensive consideration should also be given to transportation of nuclear materials away from the site of nuclear material processing businesses. Current regulation also leaves this issue unresolved. There is a possibility that consistent protective measures may not be possible across transportation modes even under the recent legislation that authorized reorganization of nuclear regulatory authorities.

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  Ethanol pool fire phenomena were investigated in a real-scale enclosed space under limited ventilation by a blower at an exhaust vent. From the experimental result, unlike a general compartment fire, it was found that the enclosed space consisted of one layer, and there were extinguishing conditions under which fire was naturally extinguished by insufficient oxygen. The conditions were determined using an equation related to the amount of ventilation. Then, to carry out a computer simulation of fire with insufficient oxygen, we developed a method of determining analysis conditions. The analysis result obtained using a fire dynamics simulator (FDS) with this method agrees closely with the measurements.

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  The critical heat flux (CHF) enhancement effect by nanofluid is widely known, and it is expected that nanofluid can be applied to in-vessel retention (IVR) under severe accident. The purpose of this study is to investigate the mechanism of CHF enhancement by nanofluid. In this study, an atmospheric saturated pool boiling test was performed. TiO₂ nanoparticles were used. To observe the boiling phenomenon in nanofluid, a Pt wire was boiled in nanofluid and coated with TiO₂ nanoparticles, and the coated wire was tested in pure water. The following conclusions can be obtained from test results: (1) Nucleation site density was decreased using the Pt wire coated with TiO₂ nanoparticles. (2) CHF enhancement in this study was about 200%. (3) CHF asymptotically increased with an increase in boiling time in nanofluid. (4) Nucleation site density had a strong effect on CHF enhancement.

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  In the design of a Japanese sodium-cooled fast reactor (JSFR), a design measure (fuel subassembly with an inner duct structure; FAIDUS) is considered to prevent severe recriticality events even in the case of core disruptive accidents by molten fuel ejection out of the core region through the duct equipped within the fuel subassembly. Confirming the principle effectiveness of such a design measure is important. In this study, the systematic heat transfer behavior in the ID1 test, which was conducted in the impulse graphite reactor (IGR) in Republic of Kazakhstan, was evaluated by applying a heat conduction code TAC2D and a reactor safety analysis code SIMMER-III focusing on the clarification of heat transfer from a high-temperature mixture of molten fuel and steel to the duct. As a result, the duct failure caused by high heat flux from the mixture was identified as one of an important mechanisms of early duct failure in FAIDUS. It was also suggested from this study that the high heat flux from the mixture is caused by the direct contact of molten steel in the absence of fuel crust on the duct wall. Based on these findings, it is judged that the mechanism of early duct failure with high heat flux obtained in the ID1 test satisfies the required condition for FAIDUS, i.e., the inner duct of FAIDUS should fail at an early phase of core disruptive accident in advance of wrapper tube failure so that the produced molten fuel can escape from the core region, which supports the feasibility of the FAIDUS concept.

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  The polycrystalline fine bulk samples of δ-phase Zr hydrides with various Hf contents were prepared and their thermal and mechanical properties were investigated. In the temperature range from room temperature to 973 K, the phase states were examined by high-temperature X-ray diffraction and thermogravimetry/differential thermal analyses. In the temperature range from room temperature to 673 K, the coefficient of linear thermal expansion, specific heat capacity, and thermal conductivity were evaluated. The Vickers hardness and sound velocity were measured at room temperature, and the elastic modulus was evaluated. The effects of Hf on the thermal and mechanical properties of Zr hydrides were studied.

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  A pyrochemical process using alkali molybdate melts has been studied as a candidate reprocessing process for spent FBR oxide fuels, but it was confirmed that SUS316 cladding tubes were corroded in Na₂MoO₄-Na₂Mo₂O₇ melts in our previous study. Moreover, in the vitrification process of HLLW, the molten phase containing alkali molybdates was generated in a borosilicate glass melt after mixing HLLW, and it is thought that the structural materials such as Ni-Cr alloys suffer from severe corrosion in the molten phase. Therefore, it is necessary to investigate the corrosion resistance of these materials in alkali molybdate melts. In order to understand the corrosion behavior of stainless steel and Ni-Cr alloys in sodium molybdate melts, the corrosion tests of Fe, Cr, Ni, SUS316, 55Ni-45Cr, and Inconel 690 specimens were carried out in Na₂MoO₄ and Na₂Mo₂O₇ melts at 750°C for 2 h. As a result, it was found that the corrosion rate decreased with increasing chromium content in these materials. The Cr₂O₃ layer was formed on the surface of Cr, 55Ni-45Cr, and Inconel 690 specimens by immersing them in the Na₂Mo₂O₇ melt, and these materials with the Cr₂O₃ layer showed a lower corrosion rate. We concluded that the corrosion behavior of these materials in the melt was controlled by the Cr₂O₃ layer.

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  Japan Atomic Energy Agency has undertaken the Mizunami Underground Research Laboratory (MIU) Project at Gifu Pref., central Japan. The aims of the project are to establish methodologies for investigation, analysis, and assessment of the deep geological environment, and to develop engineering techniques for deep-underground applications. The MIU consists of two vertical shafts and a number of horizontal tunnels connecting the two shafts. In this study, hydrochemical properties were monitored continuously to evaluate the hydrochemical changes during the MIU construction. The study demonstrated that groundwater salinity changed in relation to the variation of groundwater-flow conditions as a result of the shaft construction. In particular, “up-coning” of the salinity profile was observed at the bottom of the shafts owing to the upward movement of deeper groundwater. The intensity of up-coning was irregular, and depended on the hydrogeological heterogeneity of the groundwater system. Although pH rose to around 12 due to interaction with the concrete lining the shafts and with the grout injected into the adjacent host rock, it decreased to below 10 in less than 2 years. The redox condition remained reducing, which is considered to be controlled by Fe²⁺/Fe(OH)_3(am) and/or SO₄ ²⁻/FeS₂ couples. These findings can be summarized in a conceptual model.

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  Modified ant colony optimization (ACO) was applied to the in-core fuel loading pattern (LP) optimization problem to minimize the power peaking factor (PPF) in the modeled 1/4 symmetry PWR core. Loading order was found to be important in ACO. Three different loading orders with and without the adjacent effect between fuel assemblies (FAs) were compared, and it was found that the loading order from the central core is preferable because many selections of FAs to be inserted are available in the core center region. LPs were determined from pheromone trail and heuristic information, which is a priori knowledge based on the feature of the problem. Three types of heuristic information were compared to obtain the desirable performance of searching LPs with low PPF. Moreover, mutation operation, such as the genetic algorithm (GA), was introduced into the ACO algorithm to avoid searching similar LPs because heuristic information used in ACO tends to localize the searching space in the LP problem. The performance of ACO with some improvement was compared with those of simulated annealing and GA. In conclusion, good performance can be achieved by setting proper heuristic information and mutation operation parameter in ACO.

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