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[en] Highlights: •We identify unreliable instrumentation and provide alternative signals. •Proposed the Empirical Parameter Network based on statistically related parameters. •Connectivity determines which parameters are most important from an I&C perspective. •Proposed method was demonstrated through various SBLOCA scenarios. -- Abstract: During the Fukushima Daiichi nuclear accident, the plant operators' ability to observe the status of the power plant using the instrumentation and control (I&C) system was severely hampered by breakdowns in the plant's network, caused by the earthquake and tsunami. Thus it was difficult to obtain essential information for monitoring the internal situation of the power plant. Also, missing and incorrect information on status caused confusion, which then led to an accident. Clearly, it is crucial that I&C systems maintain the ability to monitor the internal state of reactors, even in such an inferior working environment. Herein we propose a method to identify unreliable instrumentation and to provide alternative signals. Our method, called the Empirical Parameter Network (EPN), provides estimates to replace faulty information based on statistically related parameters, and includes visualizations and other tools to enable recognition of various scenarios. The EPN included essential parameters that were selected on the basis of a literature survey, and was based on statistical analysis of an array of simulated post-accident data. The behavior of each parameter was identified and a data visualization technique was developed to intuitively display parameter correlation information. Connectivity analysis to reveal associations was performed based on the data visualization results. By incorporating the concept of connectivity, we were able to determine which parameters were most important from an I&C perspective, allowing further construction of the EPN. This newly constructed EPN will propose an alternative signal when an incorrect input signal is generated, or even when a specific parameter is missing altogether. The proposed method was demonstrated through various scenarios originating from an initial SBLOCA event, which is considered to be one of the greatest contributors to overall severe accident risk. In this research, the relationships between parameters were confirmed based on analysis of connectivity during an accident. Overall, in the damaged network condition, the reliability of the monitoring system can be improved by using the relationships between the parameters. This research can be helpful in managing accidents.
[en] This chapter provides a short summary of the dispersion of radionuclides in the environment following the accident in the Fukushima Daiichi Nuclear Power Plant (FDNPP). A detailed description and analysis of the accident is given.
[en] Highlights: • For inspiration to others the special Nordic cooperation pathways in nuclear emergency preparedness are outlined. • Recent Nordic research work under NKS has produced international state-of-the art results on nuclear emergency preparedness. • Cooperation on nuclear emergency preparedness of smallish countries with similar cultural background can be cost-effective. • Internet links are provided from NKS activity discussions to all relevant NKS project reports for cost-free download. - Abstract: Contrary to most areas of Europe, the Nordic countries (Denmark, Finland, Iceland, Norway, Sweden, and the Faroe Islands) have for many years shared a regional research and development program on nuclear reactor safety and emergency preparedness - NKS. In spite of its project results having received great recognition and having been integrated in state-of-the-art emergency preparedness tools over the world, NKS as an organization does not seem well known outside the Nordic countries. Although the Fukushima accident had no health impact at all in Nordic areas, it taught a number of lessons of generic nature with respect to new R&D tasks that could further strengthen and secure future maintenance of the Nordic region's capability to effectively respond to such events. For broader inspiration, this paper briefly introduces the Nordic nuclear emergency preparedness cooperation channels and outlines the related NKS R&D project initiatives launched after the Fukushima accident, many of which should be of general interest also far outside the region. The paper is intended as an introduction to NKS with an invitation to explore its results. All project results are available cost-free on the NKS website.
[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.