Artificial radionuclides in surface air in Finland following the Fukushima Dai-ichi nuclear power plant accident

We present observations of radionuclides released during the Fukushima Dai-ichi nuclear power plant accident in ambient air and in deposition made in Finland during March–May 2011. The first observed fission product was 131I, which arrived in Finland 8–9 days after the accident. Detections of 137Cs and 134Cs were made 2–3 days after the first 131I observations. The highest concentrations of fission products in Finland were observed during March 31st and April 1st. The highest observed concentrations of the following isotopes were: 131I (10.6 ± 0.4 mBq/m3), 134Cs (0.397 ± 0.020 mBq/m3), 137Cs (0.405 ± 0.017 mBq/m3), 136Cs (28 ± 2 μBq/m3), 129Te (129 ± 9 μBq/m3), 129mTe (234 ± 20 μBq/m3), 132Te (51 ± 3 μBq/m3) and 132I (54 ± 3 μBq/m3). Generally, higher concentrations of fission product were observed in Southern Finland than in Northern Finland. The variations in the 137Cs and 134Cs activity concentration data suggest that three separate plumes passed over Finland with decreasing concentrations. The first plume, with highest cesium concentrations, passed over Finland during March 31st – April 2nd, the second plume during April 4th – 6th and the third and smallest one during April 10th – April 11th. Both aerosol and gaseous iodine fractions were sampled simultaneously and thus an accurate view of the behaviour of aerosol and gaseous fractions was obtained. Large variations between different fractions were observed with the gaseous fraction representing 65–98% of the total 131I. The 134Cs/137Cs ratio was determined to be 0.99 ± 0.10, which indicates a fuel burnup of approximately 30 MWd/t. The 136Cs/137Cs and 129mTe/132Te ratios were used to estimate the time lapse after the accident. The differences between true time lapse and the ones deduced from the isotope ratios were from the correct time lapse to 0–3 days for 136Cs/137Cs and 5 days for 129mTe/132Te, respectively. Radionuclides from the Fukuhisma Dai-ichi nuclear power plant were also observed in deposition samples. In Norther Finland, the total deposition of 0.28–0.62 Bq/m2 for 137Cs and 0.21–0.57 Bq/m2 for 134Cs was determined during March–May 2011. For 131I the deposition of 8.5 ± 2.9 Bq/m2 was determined at Rovaniemi from the samples from the sample collected during April 1st – 12th. -- Highlights: • Concentrations of 131,132I, 134,136,137Cs and 129,129m,132Te were observed in aerosol samples. • Strong variation between the gaseous and particle bound phases of 131I was observed. • In April–June deposition samples, deposition of 134,137Cs and 131I was determined. • The 136Cs/137Cs and 129mTe/132Te ratios were used as radiochronometers