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[en] Highlights: • We examined trends in exchangeable 137Cs in Fukushima forest soils for 6 years. • The proportion of 137Cs in exchangeable form decreased significantly for 2–4 years. • The proportion of exchangeable 137Cs was almost constant in the following years. - Abstract: We investigated the changes in 137Cs exchangeable fraction in the soils of contaminated forest ecosystems until 6 years after the Fukushima radioactive atmospheric deposits. For this investigation, we performed chemical extractions of 137Cs from both organic and mineral soil layer sampled from two forest stands, Japanese cedar (Cryptomeria japonica) and broadleaf deciduous, mainly konara oak (Quercus serrata), which are located 26–27 km inland from the Fukushima Dai-ichi Nuclear Power Plant. The exchangeable 137Cs as a proportion of the total 137Cs inventory was only 10% in the organic layer and 6.1% in the mineral soil layer about 5 months after the accident at the Japanese cedar site. We observed an exponential decrease in the proportion of exchangeable 137Cs in both organic and mineral soil layer samples over the 6-years observation period at both sites. The proportion significantly decreased within 2–4 years after the accident, becoming almost constant (2–4%). These results support the interpretation that contaminated forests have entered a steady-state phase of 137Cs cycling, although we need more integration for an improved vision of the future of Fukushima forests.
[en] The fuel debris produced by the accident of the Fukushima Daiichi Nuclear Power Station is probably in a state of mixture of spent fuels with different burnups each other. In such a case, the mixing ratio of spent fuels in fuel debris would affect its criticality. This report shows computation results of criticality characteristics of fuel-debris compositions prepared by mixing nuclide compositions of spent fuels in various patterns based on a fuel loading pattern. The results indicate that fuel debris is potentially subcritical when 1-cycle fuels, whose average burnup is several GWd/t, are included homogeneously in fuel debris because remaining 155Gd and 157Gd in 1-cycle fuels works to reduce neutron multiplication. The results also indicate that 155,157Gd/235U ratio well characterize criticality of fuel debris. (authors)
[en] Safety Culture has traditionally been treated as an issue primarily related to the operators of nuclear (and other) installations. As the Fukushima accident clearly highlighted, though, it is not enough to focus merely on licensees. There is a need to adopt a broader view on the entire overall system of stakeholders and on how the participants in this system mutually influence each other. Among the stakeholders who play an important role in the overall system and interact with the licensees are the regulators. They are concerned with the safety culture of the organisations they oversee and develop approaches and tools for oversight in the domain of safety culture. However, this is only one perspective. The regulators also deeply impact the licensees’ safety culture with their own safety culture. Therefore, the regulatory body needs to take different perspectives on the issue of safety culture: (1) Safety culture as an oversight issue, with the need and challenge to develop suitable approaches and tools for oversight on the licensees’ safety culture, and (2) safety culture as an issue of self-reflection, in order to understand how the own (regulatory) safety culture influences the licensees’ safety culture and to develop and apply appropriate regulatory approaches capable of positively influencing the licensees’ safety culture. The paper illustrates how ENSI has embraced these two perspectives on safety culture. ENSI’s approach and practices on oversight of safety culture is presented, as well as ENSI’s project which has been conducted over three years after the Fukushima accident in order to initialise and institutionalize a self-reflection process on its own safety culture. (author)
[en] Interest in the need for systematic radiology education has increased since the Fukushima Daiichi nuclear disaster. Practice and attitude surveys on radiology education have been previously undertaken and indicate that there is currently limited quantitative knowledge on the use of radiation and radiological consequences. Although maintenance of an education system and measurements of its effects on knowledge levels have been conducted, no novel developments have been made in the methods of measurement. In this study, we measured the effect of the infographics to develop more effective educational materials and lecture methods in radiology education. In addition, we quantitatively evaluated the effects of using infographics on changes in knowledge levels using statistical analysis. (authors)
[en] Highlights: • Improved collection and measurement technology reviewed for legacy aerosol systems. • New capabilities enable the verification responsibilities of aerosol systems in high-level release scenarios. • Electrostatic precipitation improves sample air volume per watt. • Simple MDC analysis shows existing systems could benefit from alternative operations. • Shorter sampling intervals will enable better source location estimates. - Abstract: Pacific Northwest National Laboratory (PNNL) staff developed the Radionuclide Aerosol Sampler Analyzer (RASA) for worldwide aerosol monitoring in the 1990s. Recently, researchers at PNNL and Creare, LLC, have investigated possibilities for how RASA could be improved, based on lessons learned from more than 15 years of continuous operation, including during the Fukushima Daiichi Nuclear Power Plant disaster. Key themes addressed in upgrade possibilities include having a modular approach to additional radionuclide measurements, optimizing the sampling/analyzing times to improve detection location capabilities, and reducing power consumption by using electrostatic collection versus classic filtration collection. These individual efforts have been made in a modular context that might constitute retrofits to the existing RASA, modular components that could improve a manual monitoring approach, or a completely new RASA. Substantial optimization of the detection and location capabilities of an aerosol network is possible and new missions could be addressed by including additional measurements.
[en] In order to retrieve fuel debris in Fukushima Dai-ichi NPP, many retrieval methods are proposed. Geopolymer is the candidate material applied for an alternative retrieval method. In this concept, Geopolymer can be used to stabilize fuel debris and suppress the diffusion of radioactive materials. Objective: Evaluation of geopolymer applied for retrieving fuel debris. Slump Flow Test: Slump flow values decreased as the test temperature increased. In air condition, water molecules evaporated from geopolymer and the viscosity of geopolymer increased. In water condition, the same trends as in air condition were observed. Slump flow values of geopolymer also depended on the elapsed time after mixing. By adding water, the slump flow values of geopolymer became higher (Even at 80 deg. C). It is possible to adjust the solidification time (Setting time) of geopolymer. Solidification Process of Geopolymer: After the first layer solidified, the second layer was injected. At 300 deg. C, porous structure was observed in the first layer, while the second layer structure was intact. In the actual application, multilayered geopolymer may be used to stabilize fuel debris. Conclusion: Fluidity of geopolymer decreased with raising temperature. Setting time can be adjusted by adding water. Geopolymer may be applied by injecting multiple times on high temperature materials above 100 deg. C
[en] The residents of Town A have been taking shelter in various parts of Japan ever since the accident of Fukushima Daiichi Nuclear Power Plant. The staff members carry out their tasks in seven locations across the country under a great stress with the limited manpower. This study reports the present situation of the stress management for the disaster recovery support staff helping the evacuees living outside the prefecture. The subjects were the 19 members. The T-scores were calculated from Coping Inventory for Stressful Situations (CISS) by assessing their CISS before the mental health seminars commenced. There was a significant positive correlation between 'task-oriented coping' and 'avoidance-oriented coping' as well as 'task-oriented coping' and 'amusement'. Between the genders, the male subjects expressed a significantly higher 'amusement' value. (authors)
[en] In the decommissioning of Fukushima Daiichi Nuclear Power Station (NPS), the removal of fuel debris cannot be avoided. The fuel debris is currently considered to be sub-critical. Volume ratio of debris/water has strong sensitivity to the criticality. Thus, the criticality estimation is required in the debris removal process. While, the best estimation of the criticality is difficult due to the unclear properties and/or geometries of debris, therefore, random sampling with Monte Carlo calculation was used in current evaluation. However, the Monte Carlo calculation spend three days to one week for the evaluation due to its high calculation cost. Therefore, it is difficult that Monte Carlo apply to on-demand statistical criticality evaluation. For these background, multidimensional interpolation was applied to the statistical criticality evaluation. In this study, fundamental validation was examined by comparing with Monte Carlo results. The proposed method was 614,400 times faster than Monte Carlo calculation, additionally, difference of mean value was 0.9 % δk. As a result, proposed multidimensional interpolation showed good agreement with direct Monte Carlo calculation. In the future work, simplification of the model, evaluation of applicable limit and operation procedure will be examined. (authors)
[en] Fukushima TEPCO has to do intakes samples from the heart of the reactor. The aim is to understand how address the dismantling. It is the 1st sampling campaign. Mitsubishi Heavy Industries (MHI) is the 1st subcontractor of TEPCO, and architect of all of the intakes sample project. In the frame of the project a PADIRAC cask will be docked to a container, this last being himself accosted to the reactor. Samples are introduced into the PADIRAC then the cask is conveyed remotely to a cell with considered DPTE®Alpha 270. This cell is built beside the clean area, it is dedicated to manage the samples. CLEO is a French company (located in Marcoule area) is in charge of the design and delivery of a tele operated system that is destined to dock/undock and convey the PADIRAC from the clean area to the reactor core and back. It is a complex specific development, which will implement built-in cameras and cable remote controls. This cable is required for the robustness of the bindings and maintenance of the mechanical connection with the cold side. (author)
[en] Highlights: • A review of ongoing severe accident studies on corium behavior was presented. • A comparison of research priorities identified in different projects was performed. • An EU-Japan roadmap on research of corium behavior was suggested for bilateral collaborations. - Abstract: A joint research roadmap between Europe and Japan has been developed in severe accident field of light water reactors, focusing particularly on reactor core melt (corium) behavior. The development of this roadmap is one of the main targets of the ongoing EU project SAFEST. This paper presents information about ongoing severe accident studies in the area of corium behavior, rationales and comparison of research priorities identified in different projects and documents, expert ranking of safety issues, and finally the research areas and topics and their priorities suggested for the EU-Japan roadmap and future bilateral collaborations. These results provide useful guidelines for (i) assessment of long-term goals and proposals for experimental support needed for proper understanding, interpretation and learning lessons of the Fukushima accident; (ii) analysis of severe accident phenomena; (iii) development of accident prevention and mitigation strategies, and corresponding technical measures; (iv) study of corium samples in European and Japanese laboratories; and (v) preparation of Fukushima site decommissioning.