The environmental behavior of radioactive Cs in the fallout from the accident of the Fukushima Daiichi Nuclear Power Plant has been studied by measuring its spatial distribution on/in trees, plants, and surface soil beneath the plants using autoradiography analysis. The results of autoradiography analysis showed that radioactive Cs was distributed on the branches and leaves of trees that were present during the accident and that only a small fraction of radioactive Cs was transported to new branches and leaves grown after the accident. Radioactive Cs was present on the grass and rice stubble on the soils, but not in the soils beneath the grass and rice stubble, indicating that the radioactive Cs was deposited on the grass and the rice plant. In addition, the ratio of the radioactive Cs that penetrated into the soil layer by weathering was very small two months after the accident. These results indicate that trees and other plants are the reservoir of the fallout Cs and function to retard the fallout Cs migration with rain water.

View full abstract

  The Fukushima Daiichi Nuclear Plant Unit 1 accident was investigated with the TRAC-BF1 code in order to confirm the effect of an isolation condenser (IC) on core cooling analytically. The analysis shows that the formation of two-phase natural-circulation flow is a key phenomenon for core cooling. Therefore, it is too late to cool fuel rods, once the core is heated up because of the lack of coolant. It also shows that the early start-up of the IC is essential for avoiding the core meltdown under station blackout conditions.

View full abstract

  After the severe accident in the Fukushima-1 nuclear power plant, a large amount of contaminated stagnant water has been produced in turbine buildings and surrounding areas. This rapid communication reports the calculation of the radionuclide inventory in the core, the collection of the measured inventory in the stagnant water, and the estimation of the radionuclide release ratios from the core to the stagnant water. The present evaluation is based on data obtained before June 3, 2011. It was revealed that the release ratios of tritium, iodine and cesium were several tens of percent, while those of strontium and barium were smaller by one or two orders of magnitude. These release ratios of the Fukushima accident were equivalent to those of the TMI-2 accident.

View full abstract

  The purpose of this study is to clarify public awareness of utilizing nuclear power in China and to determine the effects of the accident of Fukushima Daiichi nuclear power plants. Web online surveys were carried out before and after the accident of Fukushima Daiichi nuclear power plants. The online survey before the accident of Fukushima Daiichi nuclear power plants had 4,255 adult respondents consisting of 1,851 males and 2,404 females. The online survey after the accident had 721 respondents consisting of 406 males and 315 females. The two online surveys about the attitude toward nuclear power plants consisted of 37 items, such as the necessity of nuclear power plants, the reliability of safety, and government confidence. As a result, respondents of the online surveys in China consider that nuclear energy is more important than the anxiety of accident. On the other hand, women have sensation of fear for the accident of Fukushima Daiichi nuclear power plants and radiation.

View full abstract

  Thermodynamic equilibrium calculations using the FactSage program package were performed to investigate the oxidation behavior of a Mo-Ru-Rh-Pd alloy, which is well known as the undissolved residue in the reprocessing process. The influences of temperature, oxygen partial pressure, and Pd concentration on the equilibrium composition, melting temperature, and activities were evaluated. On the basis of the computed T-P_O2 phase diagram of Mo_0.3Ru_0.5Rh_0.1Pd_0.1, the stable compositions of Mo_0.3Ru_0.5Rh_0.1Pd_0.1 under the conditions of a high-burnup UO₂ fuel and a glass melter were evaluated. The computed phase diagram showed that under an oxygen pressure of 0.2 Pa, RuO₂, Rh₂O₃, the FCC phase, and MoO₃ (liquid or gas) are stable at T=1,000-1,200°C. The computed phase diagram also showed that precipitates in the glass provide information on the oxygen partial pressure and temperature of the glass in the melter. The computation of the phase diagram was also carried out for Mo_0.3Ru_0.5Pd_0.2. The resultant phase diagram was compared with that of Mo_0.3Ru_0.5Rh_0.1Pd_0.1 to investigate the effect of Pd concentration on the oxidation behavior of a Mo-Ru-Rh-Pd alloy. Based on the computed temperature-concentration phase diagram and activities of Mo, Ru, Rh, and Pd in the HCP phase, the effect of Pd concentration on the stability of each metallic species was discussed.

View full abstract

  In this paper, we consider the concept and the concrete direction of the institutional design concerning role-sharing between national and local governments with respect to nuclear safety regulation, and propose the following four institutional options: 1) strengthen the regulatory body's independence and clarify its accountability, 2) establish a local information committee in reference to the French CLI (Commission Locale d'Information), 3) institutionalize the environmental monitoring of local governments, and 4) institutionalize consultation among the regulatory body and local governments. Options 2, 3, and 4 are designed on the assumption that option 1 is implemented, because it is thought to be essential for fundamental improvements to enhance the regulatory body's ability as well as to reinforce bridges between safety regulation and society. Each option has some advantages and disadvantages, so there is room for further research into achieving reforms. However, it is believed that these institutional options can lead to the development of a trusting relationship between organizations responsible for safety regulation and siting areas and to the reduction in consensus-building costs in local areas.

View full abstract

  It is known that violent boiling (i.e., geysering) of liquid sometimes occurs in a device with a long, heated vertical tube. In this study, we experimentally investigated the mechanism of geysering in a vertical small-diameter glass tube heated by a cartridge heater. We also investigated the effects of the water temperature of the upper plenum, the heat input into the boiling vessel and the water depth of the upper plenum on the occurrence condition of geysering. The experimental results showed that when the water temperature of the boiling vessel reached saturation, which included the effect of the hydrostatic head, the superheated water in the vessel suddenly vaporized and geysering occurred. Measurements of differential pressure showed that in the process of geysering, the two-phase flow pressure drop in the vertical tube was almost constant. Although the period of geysering became quite irregular at a high heat input, the periods of regular geysering correlated with the delay time for boiling, τ, indicating the period required for the boiling of subcooled water flowing into the boiling vessel.

View full abstract

  Up to now, few studies have been devoted to investigate all the steps from the creation to the application of operation manuals and to consider human factors of operators in order to prevent the troubles caused by the problems of operation manuals. This paper deals with the steps and the human factors. It this study, the troubles caused by the problems are examined and the causes of these troubles are investigated. Here, the general procedure for creating an operation manual from the experiences of authors to create some operation manuals is first summarized and the method of analyzing the tendency of troubles and their causes is considered. From this consideration, it is concluded that there are several steps that cause a trouble in the creation of operation manuals and it is necessary to analyze the causes of a trouble from the viewpoints of creators and operators of operation manuals. Next, in this study, the troubles reported in Nuclear Information Archives (NUCIA) as an example of real trouble data bases are analyzed, a list of trouble causes related to the problems of operation manuals is made, and using this list, the causes of troubles reported in NUCIA are categorized. From analysis results, it is concluded that there are some steps that cause troubles in creating operation manuals, that the creation of a checklist and the description of necessary information are sometimes missing, and that operators take operations following operation manuals but sometimes do not understand the contents of operation manuals and/or have little knowledge for operations.

View full abstract

  In order to understand systematically the influence of an environmental factor on the intergranular stress corrosion cracking (IGSCC) susceptibility of cold-worked 316 stainless steel, constant extension rate tensile tests were carried out under simulated pressurized water reactor (PWR) primary water conditions while varying the boric acid, lithium hydroxide, and dissolved hydrogen concentrations and temperature. It was found that IGSCC susceptibility increased for a low boric acid concentration and high lithium hydroxide and dissolved hydrogen concentrations, and a high temperature. The influences of the water chemistry changes were recognized as the changes in pH and magnetite solubility in a manner that IGSCC was promoted for higher pH and magnetite solubility. On the other hand, IGSCC crack growth rates measured using compact-tension-type specimens revealed that boric acid, lithium hydroxide, and dissolved hydrogen did not affect IGSCC crack growth rate significantly.

View full abstract

  It is necessary to design a low level radioactive waste storage system so that the decay heat of radioactive waste does not breach the structural safety limit. Currently, this waste storage system is designed as a natural cooling system, which continuously cools the radioactive waste without an active device. It consists simply of a storage pit for radioactive waste and air inlet and outlet ducts. The radioactive waste is cooled by natural convective air flow, which is generated by the buoyancy of heated air due to the decay heat of radioactive waste. It is important to clarify the flow characteristics in the systems in order to evaluate the cooling performance. The air mass flow rate through the system is determined by the balance between the natural convective flow force and pressure loss within the system. Therefore, the pressure drop and flow pattern in the waste storage pit are important flow characteristics. In this study, the pressure drop and air temperature distribution, greatly influenced by the flow pattern in the pit, were measured using a 1/5 scale model and compared with the results obtained from CFD. Flow network analysis, which is a simple model that simulates the flow by nodes and junctions, was conducted and its validity was confirmed by experimental results and CFD.

View full abstract

  Gloveboxes used for plutonium fuel development and fabrication are eventually dismantled for replacement. Since equipment interior and the inner surface of gloveboxes are contaminated with radioactive materials, glovebox dismantling is performed by workers wearing an air fed suit with mechanical tools in a plastic enclosure system to control the spread of contamination. Various improvements of the enclosure system are implemented including the modification of the rooms to decontaminate and undress the air fed suit and the introduction of an inflammable filter and a safety film near the size reduction workspace against fire. We describe the countermeasures deployed in the enclosure system against potential hazards and how these devices work in the real dismantling activities.

View full abstract