The Xe-135, I-131, I-132, I-133 and Te-132 concentrations in plumes at the monitoring posts in Fukushima prefecture in March 2011 were estimated using the pulse height distribution obtained from a NaI(Tl) detector, which were available to the public. Several corrections to the pulse height distribution were necessary owing to high count rates. The contribution to the count rates from each radionuclide except Xe-135 accumulated around each monitoring post was estimated using a method based on the time history of the peak count rate proposed by the authors. The concentration of each radionuclide in the plume was converted from the peak count rate using the response of the NaI(Tl) detector calculated with the egs5 code for a model of a plume containing a uniform distribution of radionuclides. The obtained time histories of Xe-135, I-131, I-132, I-133 and Te-132 concentrations in air at a fixed point in March 2011 were the first ones for Fukushima prefecture. The results at five monitoring posts near Fukushima Daiichi Nuclear Power Station were used to characterize radionuclides in the plume before March 15, soon after the accident. The results at three monitoring posts, Naraha-town Shoukan, Hirono-town Futatsunuma and Fukushima-city Momijiyama, which were analyzed during almost all of March, were used to characterize radionuclides in the plume in the period after March 14. It was fourd that Xe-135 was dominant on March 12 and Te-132 increased from March 13. For the radionuclides of iodine, I-131, I-132 and I-133 were detected with almost the same concentration for the first few days after the reactor shutdown.
This study provides a method of safety assessment for the geological disposal of HLW to evaluate the effects of uplift and erosion, which are widespread phenomena in Japan. This safety assessment method can account for different uplift and erosion rates while evaluating the repository depth and the time required for a repository to reach the weathered zone and the ground surface, and also considers the number of waste packages eroded on the basis of a landform evolution model. The landform evolution model uses altitude dispersion, which expresses the deviation of altitude from the topographical relief. The results of a trial analysis show that the maximum dose in the base case (uplift rate: 0.3 mm/y) is less than the target criterion suggested by an international organization even if the repository is close to the ground surface. Furthermore, the maximum dose can be reduced if the dispersion of the timing for the waste packages to reach the weathered zone due to heterogeneity in the altitude of the bottom of the weathered zone is considered. This method is applicable to evaluate the safety of geological disposal based on realistic uplift and erosion rates while considering the evolution of the topographical relief, and to quantify the safety margin and robustness of a geological disposal system.
In geological disposal, the direct effect of active faults on geological repositories is avoided at the stage of site characterization; however, uncertainty remains for the avoidance of faults derived from active faults, which are concealed deep under the ground and are difficult to detect by site investigation. In this research, the influence of the growth of undetected splay faults on a natural barrier in a geological disposal system due to the future action of faults was evaluated. We investigated examples of splay faults in Japan and set conditions for the growth of splay faults. Furthermore, we assumed a disposal site composed of sedimentary rock and made a hydrogeological model of the growth of splay faults. We carried out groundwater flow analyses, changing parameters such as the location and depth of the repository and the growth velocity of splay faults. The results indicate that the main flow path from the repository is changed into an upward flow along the splay fault due to its growth and that the average velocity to the ground surface becomes one or two orders of magnitude higher than that before its growth. The results also suggest that splay fault growth leads to the possibility of the downward flow of oxidizing groundwater from the ground surface area.
Since the Fukushima-Daiichi nuclear power station accident, the Japanese regulatory body has improved and upgraded the regulation of nuclear power plants, and continuous effort is required to enhance risk management in the mid- to long term. Earthquakes and tsunamis are considered as the most important risks, and the establishment of probabilistic risk assessment (PRA) methodologies for these events is a major issue of current PRA. The Nuclear Regulation Authority (NRA) addressed the PRA methodology for tsunamis induced by earthquakes, which is one of the methodologies that should be enhanced step by step for the improvement and maturity of PRA techniques. The AESJ standard for the procedure of seismic PRA for nuclear power plants in 2015 provides the basic concept of the methodology; however, details of the application to the actual plant PRA model have not been sufficiently provided. This study proposes a detailed PRA methodology for tsunamis induced by earthquakes using the DQFM methodology, which contributes to improving the safety of nuclear power plants. Furthermore, this study also states the issues which need more research.
After the Fukushima Daiichi nuclear power plant accident, public opinion of nuclear power generation changed. In this study, the differences in opinions of nuclear power generation between Japanese and Korean university students were examined. The surveys were conducted at three universities using paper questionnaires. It was found that students studying nuclear engineering in both countries had more favorable opinions of nuclear power than those studying other majors. In comparison, trust in the government’s management of nuclear power plants and the perceived danger of nuclear power plants also affected opinions of nuclear power plants in both countries. These results cannot be generalized because the number of respondents was limited.