The widespread use of medical linacs for cancer treatment and seismic isolation structures in Japan led to the construction of an increasing number of buildings in which medical linac rooms are located directly above the seismic isolation layer. Extending the control area in the seismic isolation layer is advantageous for reducing the cost of floor shielding. However, establishing a simple calculation method is necessary to evaluate the effective dose in the seismic isolation layer for shielding design. We proposed and verified a simple calculation method that can be applied to evaluate the effective dose in the seismic isolation layer of 10 MeV linac facilities. The parameters of the method were investigated when applied to 6 and 10 MeV linacs and extensively validated by comparing the results with the Monte Carlo calculation. For the seismic isolation layer in which the height of the space is ≤2 m, the results of the method and the Monte Carlo calculation agree within 28% and 21% at points of half-height and ≥3 m horizontally from the linac source, respectively. Furthermore, the measured doses were compared with the proposed method values in the seismic isolation layer of a 10 MeV linac facility.

We estimate the monthly discharged inventory of ⁹⁰Sr from the port of Fukushima Daiichi Nuclear Power Plant (1F) from Jun. 2013 to Mar. 2022 by the Voronoi tessellation method inside the port, following the monitoring of ⁹⁰Sr seawater radioactivity concentration inside the port that started in Jun. 2013. From the estimation results, we find that closing the Seaside Impermeable Wall on Oct. 2015 was the most effective method in reducing the discharged inventory in the period. In addition, after its closure, we confirm that a major source of the discharged inventory is narrowed down to the drainage channels flowing inside the 1F site. As for the nonmonitoring period from Apr. 2011 to May 2013, using the technique proposed by the authors, i.e., the activity ratio of ⁹⁰Sr to ¹³⁷Cs of stagnant water measured at the beginning of the accident and the ratio of the discharged inventory for the period until the Seaside Impermeable Wall closure, in which both radionuclides ⁹⁰Sr and ¹³⁷Cs were measured, we estimate the monthly discharged inventory of ⁹⁰Sr and report it from the initial month including the direct discharge accident (Apr. 2011) to the latest month (Mar. 2022) for 11 years in addition to that of ⁸⁹Sr only for the initial three months. Moreover, we compare the temporal variation of ⁹⁰Sr with those of ¹³⁷Cs and tritium. The comparisons reveal that the discharged inventory of ⁹⁰Sr is the most effectively reduced by closing the Seaside Impermeable Wall and its temporal variation is the most sensitively dependent on precipitation and seasonal changes compared with ¹³⁷Cs and tritium. Since the riverine input of ⁹⁰Sr into the sea is negligible compared with those of ¹³⁷Cs and tritium, we can evaluate the impact of the discharged inventory of only ⁹⁰Sr from the 1F port in the coastal and offshore area centered on 1F. The results reveal the rough monthly discharged inventory required to observe the visible enhancement of the sea radioactivity concentration from the background level in each area. Such an outcome is significant for considering the environmental impact on the planned future release of the treated water accumulated in the 1F site.

The hierarchization of initiating events (IEs) has been widely applied in seismic probabilistic risk assessments of nuclear facilities. In our previous study, we proposed the Initiating Event Matrix Method (IEMM) with a focus on IEs involving multiple failures of structures, systems, and components; such IEs are normally ignored in hierarchization for simplicity. In this study, we applied the IEMM to a case study to evaluate the seismic response correlation effect in a hypothetical fuel cycle facility to validate the applicability of the IEMM to such a case. This case study is suitable for this purpose since it is well known that the occurrence frequencies of IEs with multiple failures may increase significantly when the seismic response correlation is considered. As a result of the case study, the increase in severe-accident frequency due to the correlation effect was reasonably evaluated by the IEMM, and therefore, we confirmed the applicability of the IEMM to cases involving such an effect. Additionally, it was found that the event tree of IEs could be greatly simplified by contraction in the IEMM depending on the event trees of mitigation systems following the IEs.

Dose reduction factors of Japanese houses are important for estimating the levels of external exposure of returning residents. In 2019, a total of 19 wooden houses were surveyed in Iitate Village and Namie Town using a gamma plotter that can continuously measure the air dose rate, with six houses selected for analysis. The characteristics of the dose reduction factor were investigated from the measured air dose rates in and around each house. In the vicinity of a house, uncontaminated land exists beneath the structure, and paved surfaces such as asphalt roads occupy a high fraction of the land around the structure. In pavements, the radiation source tends to be washed away rapidly. Therefore, the air dose rate near each house was relatively low. Within a radius of 50 m from the center of a house, air dose rates above unpaved surfaces were higher and had larger variations than those above paved surfaces. The dose reduction factor was widely distributed even for one house when the ratio of the air dose rates observed inside and outside the building was considered. It is thus suggested that a representative reduction factor may not be obtained if it is based on a small number of measurements that do not capture the full variation of the radiation field around a house.