6:15 PM - 7:30 PM
[MAG38-P05] Atmospheric Radio-Sr and -Cs Depositions at Mt. Haruna
Keywords:the Fukushima Daiichi Nuclear Power Plant accident, Radioactive deposition, Strontium, Cesium, Resuspension
At Mt. Haruna (Takasaki, Gunma, 36?28'N, 138?52'E, 1,370m above sea level), monthly 90Sr and 137Cs deposition observations have been performed. The observation started at the end of 2006. Precipitation samples were filtered by a sieve of 2 mm mesh and 100 μm mesh to remove fallen leaves, wooden piece, large particles in size, etc. and processed to concentrate the whole sample. The desiccated sample was subjected to γ-spectrometry with a Ge detector to measure radio-Cs. Sr-90 was then measured by 2 π gas-flow β-counter after that strontium carbonate was radiochemically separated and radio-equilibrium was attained. Each analytical process was quality-controlled by using the reference fallout sample that was prepared by the Meteorological Research Institute.
Monthly 137Cs deposition in March, 2011 was 14±0.5 kBq/m2 which is 5 to 6 orders of magnitude higher than the level before the FDNPP disaster. However, this amount was approximately 60% of that observed at the MRI, Tsukuba. The cumulative 137Cs deposition at Mt. Haruna was 18.8 kBq/m2 for the year 2011, which was also approximately 70% of that in Tsukuba. Almost the same amount of 134Cs was simultaneously deposited with 137Cs. Thus, the total cesium deposition at Mt. Haruna reached about 40 kBq/m2. It was revealed that the current observation site had a relatively lighter pollution than those found in Tsukuba. Although Mt. Haruna was found within the so-called hotspot region distributed over northern Gunma, and heavier radio-Cs pollution (60-100 kBq/m2-level) was observed around Lake Haruna by the airborne observation mapping conducted by the MEXT. The current observation site might avoid pollution of wet deposition because of higher altitude than cloud layer height. On the other hand, monthly 90Sr deposition in March, 2011 was as small as 3.11±0.03 Bq/m2, which was about 1/4500 of the deposited 137Cs amount. This 90Sr deposition was also approximately 60% of the value in Tsukuba. Deposition observations at Mt. Haruna confirmed that the pollution impacts of 90Sr are relatively minor compared with radio-Cs over the Kanto district.