Distribution of environmental radioactivity in the vicinity of lake Biwa

Currently (March, 2014), Ooi Nuclear Power Plant is situated in Kinki region and the only reactor which is operated currently in Japan since the accident of Fukushima Daiichi Nuclear Power Plant in 2011. Since we noticed that the increase of radioactivity of ¹³⁷Cs in soil at Shiga Prefecture by checking the 'Ministry of Education, Science and Culture' database, we investigated by ourselves. Our aim is to check where the ¹³⁷Cs is coming from, what the other radioactivities exist, and where any peculiar areas/regions of each radioactivity present along with the South region of lake Biwa for high population area and the North region of lake Biwa for nature-oriented forest area. We investigated the soil samples at Kibougaoka-bunka park, Touju shrine, Yamagami shrine, and Wakamiya shrine and obtained its contained radioactivity via HPGe semiconducting detector. Especially the one at Kibougaoka-bunka park is the same place where the Shiga Prefectural Institute of Public Health is monitoring its radioactivity in soil every year. Our measured ¹³⁷Cs activity of 13.6 Bq/kg and its inventory of fallout of 797 M Bq/km² are increasing, compared with the one by the Shiga Prefectural Institute of Public Health in the previous year. Touju shrine is also the same sampling point where Kanazawa University obtained its radioactivity and inventory of fallout in 2000. Thus, comparing our measured result in 2013 and the one by the Kanazawa University in 2000, we concluded that the culprit of this anthropogenic radionuclide (¹³⁷Cs) is mainly due to the remaining activity from nuclear weapon tests in the fifties, sixties and seventies, since its decrease in the activity between 2000 and 2013 is consistent with the radioactive decay of ¹³⁷Cs. The Touju shrine gave the highest air dose rate of 0.41 μ Sv/h and its radioactivity of ¹³⁷Cs in soil is the highest value of 21 Bq/kg among the current sampling points. The high value might be caused by its circumstance such as wind-blocking by house wall and accumulation by rain drops from above roof. (author)