Observation of fallout deposition in an outdoor swimming pool 50 km away from the Fukushima Daiichi nuclear power plant

• Deposition density of radiocesium was estimated at a swimming pool in Fukushima.
• The density was determined with a small standard uncertainty of approximately 10%.
• Water balance was simulated for estimating radioactivity budget in the pool.
• Detected gamma-emitting nuclide was 110mAg other than radiocesium.
• Radiocesium was much dominant compared with 89Sr, 90Sr, 110mAg, 238Pu and 239+240Pu.

After the accident at the Fukushima Daiichi nuclear power plant (NPP), outdoor school swimming pools at Fukushima were decontaminated to curb the redistribution of radioactivity into downstream farmlands. In the process, the radioactivity concentrations of the pool water and sediment substances (residue) were measured to estimate the deposition density of the fallout. At a pool situated 50 km away from the NPP, the average concentrations of radiocesium (134+137Cs) for the water and residue were quantified as 170 Bq L−1 and 3.6 × 105 Bq kg−1, respectively. Taking account of the radioactivity concentrations and of the water balance in and around the pool, the deposition density of radiocesium, as of August 2011, was precisely determined to be 0.32 ± 0.03 MBq m−2 (k = 1). The density corroborated the previous results obtained by other methods, i.e., airborne surveys, in-situ Ge surveys and soil samplings at neighboring locations. Other than radiocesium, the only gamma-emitting nuclide detected was 110mAg, with a concentration of 560 Bq kg−1 in the residue. The radioactivity concentrations of 89Sr, 90Sr, 238Pu and 239+240Pu in the water were all less than the minimum detectable activities – 2, 0.1, 0.002 and 0.002 Bq L−1, respectively.