We tried the decontamination of surface soils for three types of agricultural land at Nagadoro district of Iitate-mura (village) in Fukushima Prefecture, which is highly contaminated by deposits of radionuclides from the plume released from the Fukushima Daiichi nuclear power plant. The decontamination method consisted of the peeling of surface soils solidified using a polyion complex, which was formed from a salt solution of polycations and polyanions. Two types of polyion complex solution were applied to an upland field in a plastic greenhouse, a pasture, and a paddy field. The decontamination efficiency of the surface soils reached 90%, and dust release was effectively suppressed during the removal of surface soils.
Zeolite is used for decontamination of radioactive water containing salts from seawater in the Fukushima Dai-ichi Nuclear Power Station. Evaluation of hydrogen production by water radiolysis during the process is important for a safe operation. Thus, hydrogen production from the mixture of zeolite and seawater was studied by γ-radiolysis experiment, and the hydrogen production during the process was evaluated. The measured yield of hydrogen from seawater was comparable to the primary yield in the γ-radiolysis of water. This result indicates that oxidation of hydrogen by radical products of water radiolysis is not effective in seawater. The measured yield from the mixture decreased at a high weight fraction of zeolite. However, the measured yield was higher than that expected from the direct radiolysis of water in the mixture, which would decrease proportionally to the weight fraction of water. This result suggests that the radiation energy deposited on zeolite is involved in hydrogen formation. From the measured yields, the hydrogen production rate was evaluated to be 3.6 mL/h per ton of radioactive water before the process and 1.5 L/h per ton of the waste adsorbent after the process.
The analysis of the long-term station blackout accident of BWR has been performed using the TRAC-BF1 code. The actuation of RCIC was assumed, and the results were compared with the data observed at the Fukushima Daiichi power plant unit 2 reactor. BWR-5 of 1,100 MW was analyzed, while the unit 2 reactor was BWR-4 of 780 MW. The reactor pressure and the core liquid level were, however, in good agreement with the observed data. It was confirmed that the quasi-steady state continued for a long time with the RCIC actuation. The timing of recovery action, which was composed of depressurization and coolant injection, necessary for the maximum clad temperature being less than 1,500 K was studied and compared with that of the unit 2 reactor.
The Central Research Institute of Electric Power Industry and Japan Atomic Energy Agency have fabricated the first sodium-bonded metallic fuel elements in Japan as a collaborative research for the irradiation test at the experimental fast test reactor Joyo. The U-20 wt.%Pu-10 wt.%Zr fuel slugs of 200 mm length and approximately 5 mm diameter were fabricated in a small-scale injection casting furnace using U metal, U-Pu alloy and Zr metal. The fuel slugs met the specifications determined on the basis of the specifications of the Experimental Breeder Reactor II driver fuels in the United States and the results of preliminary casting tests. Each fuel slug was loaded into a ferritic-martensitic stainless steel (PNC-FMS) cladding tube with a few pieces of sodium rods as thermal bonds, the U-10 wt.%Zr thermal insulator and the PNC-FMS reflector in a helium gas atmosphere glove box. After top-end plug welding to the cladding tube and the heat treatment of the welding area, each fuel element was subjected to sodium bonding to fill the annular gap between the fuel slug and the cladding with the melted sodium. After inspection, six metallic fuel elements were manufactured successfully and transferred to the Joyo site.
Imperfect maintenance planning was frequently identified in domestic nuclear power plants. To prevent such an event, we analyzed causal factors in maintenance planning stages and showed the directionality of countermeasures in this study. There is a pragmatic limit in finding the causal factors from the items based on report descriptions. Therefore, the idea of the systemic accident model, which is used to monitor the performance variability in normal circumstances, is taken as a new concept instead of investigating negative factors. As an actual method for analyzing usual activities, cognitive task analysis (CTA) was applied. Persons who experienced various maintenance activities at one electric power company were interviewed about sources related to decision making during maintenance planning, and then usual factors affecting planning were extracted as performance variability factors. The tendency of domestic events was analyzed using the classification item of those factors, and the directionality of countermeasures was shown. The following are critical for preventing imperfect maintenance planning: the persons in charge should fully understand the situation of the equipment for which they are responsible in the work planning and maintenance evaluation stages, and they should definitely understand, for example, the maintenance bases of that equipment.
In a “hiyarihatto” activity, workers report and discuss incident cases related to their work. Such an activity is particularly effective for cultivating participants' attitudes about safety. Nevertheless, a conventional face-to-face hiyarihatto activity includes features that are inappropriate for conduct in a nuclear power plant organization. For example, workers at nuclear power plants are geographically distributed and busy. Therefore, they have great difficulty in participating in a face-to-face hiyarihatto activity. Furthermore, workers' hesitation in discussing problems inhibits the continuation of their active participation. This study is conducted to propose a hiyarihatto activity with an asynchronous and distributed computer-mediated communication (CMC) for a nuclear power plant organization, with the demonstration of its effectiveness through field practice. The proposed method also involves the introduction of special participants who follow action guidelines for the promotion of the continuation of the activity. The method was used in an actual nuclear power plant organization. Results showed that the method is effective under some conditions, such as during periods without facility inspection. Special participants promoted the activity in some cases. Moreover, other factors affecting the activity and some improvements were identified.
A high-temperature gas-cooled reactor (HTGR) is a graphite-moderated and helium gas-cooled reactor. It is particularly attractive due to its capability of producing high-temperature helium gas, and its passive and inherent safety features. To enable nuclear energy application to a wide range of heat process industries, Japan Atomic Energy Agency (JAEA) has continued extensive effort for the development of the HTGR using the high-temperature engineering test reactor (HTTR), which is the first HTGR in Japan with a thermal power of 30 MW, and operates it at the site of the JAEA's Oarai Research and Development Center. The HTTR has successfully completed a full-power high-temperature (950°C) continuous operation for 50 days from January to March in 2010. Through this operation, the potential of a stable high-temperature heat supply to heat application systems, such as a hydrogen production system, was demonstrated. This paper presents the operation results including reactor characteristics.
The Monte Carlo code uses a sequence of pseudo-random numbers with a random number generator (RNG) to simulate particle histories. A pseudo-random number has its own period depending on its generation method and the period is desired to be long enough not to exceed the period during one Monte Carlo calculation to ensure the correctness especially for a standard deviation of results. The linear congruential generator (LCG) is widely used as Monte Carlo RNG and the period of LCG is not so long by considering the increasing rate of simulation histories in a Monte Carlo calculation according to the remarkable enhancement of computer performance. Recently, many kinds of RNG have been developed and some of their features are better than those of LCG. In this study, we investigate the appropriate RNG in a Monte Carlo code as an alternative to LCG especially for the case of enormous histories. It is found that xorshift1) has desirable features compared with LCG, and xorshift has a larger period, a comparable speed to generate random numbers, a better randomness, and good applicability to parallel calculation.
This paper discusses the reprocessing plant concept suitable for the transition period from the Light Water Reactors (LWRs) to the Fast Breeder Reactors (FBRs). This transition requires the reprocessing of spent fuels in order to supply an adequate volume of fissile plutonium (Pu-fissile) for the FBRs. The transition period would continue for more than 60 years, and the reprocessing plant should match with the change in the power generation plan during the transition period. The ability to supply Pu-fissile has been evaluated for two plant concepts. One is the independent-type concept, which contains two processes for reprocessing either LWR or FBR fuels. The other is the modularized-type concept, which contains only one process for reprocessing both the LWR and FBR fuels. The result showed the superiority of the modularized-type concept over the independent-type concept, because the former can enhance the ability to supply Pu-fissile with less reprocessing capacity. Therefore, the reprocessing plant suitable for the transition period is that based on the modularized-type concept.
A high-purity Ar gas atmosphere glove box accommodating injection casting and sodium-bonding apparatuses was newly installed in the Plutonium Fuel Research Facility of Oarai Research and Development Center, Japan Atomic Energy Agency, in which several nitride and carbide fuel pins were fabricated for irradiation tests. The experiences led to the establishment of the technological basis of the fabrication of U-Pu-Zr alloy fuel pins for the first time in Japan. After the injection casting of the U-Pu-Zr alloy, the metallic fuel pins were fabricated by welding upper and lower end plugs with cladding tubes of ferritic-martensitic steel. Subsequent to the sodium bonding for filling the annular gap region between the U-Pu-Zr alloy and the cladding tube with the melted sodium, the fuel pins for irradiation tests are inspected. This paper shows the apparatuses and the technological basis for the fabrication of U-Pu-Zr alloy fuel pins for the irradiation test planned at the experimental fast test reactor Joyo.
Taking into consideration the influence of the media coverage, this research aims to analyze the characteristics of the local newspapers that cover diverse events relevant to nuclear energy in regional areas where nuclear facilities are located (hereinafter called the “region”). According to the previous surveys, local residents in the region are more interested in the nuclear energy matters than those who live in urban areas. Plus, the local newspapers turn out to report more events of nuclear energy from a variety of angles. Through interviews with executives and journalists of the local newspaper companies in the regions, it is revealed that the local newspapers tend not to report news sensationally, but they would rather take a supportive stance toward the development in their regions. The interviewees hope that various activities of the nuclear industry will promote education, employment and cooperation among government, industry and academia. They also desire that the industry's activities will help to increase benefits in their regions. It appears that the interviewees' awareness reflects articles of the local newspapers. As a result of the surveys conducted for this research, it is considered that the journalists expect that their region will make particularly qualitative progress in the future.