Atmospheric radionuclides from the Fukushima Dai-ichi nuclear reactor accident observed in Vietnam

Radionuclides from the reactor accident at the Fukushima Dai-ichi Nuclear Power Plant were observed in the surface air at stations in Hanoi, Dalat, and Ho Chi Minh City (HCMC) in Vietnam, about 4500 km southwest of Japan, during the period from March 27 to April 22, 2011. The maximum activity concentrations in the air measured at those three sites were 193, 33, and 37 μBq m−3 for 131I, 134Cs, and 137Cs, respectively. Peaks of radionuclide concentrations in the air corresponded to arrival of the air mass from Fukushima to Vietnam after traveling for 8 d over the Pacific Ocean. Cesium-134 was detected with the 134Cs/137Cs activity ratio of about 0.85 in line with observations made elsewhere. The 131I/137Cs activity ratio was observed to decrease exponentially with time as expected from radioactive decay. The ratio at Dalat, where is 1500 m high, was higher than those at Hanoi and HCMC in low lands, indicating the relative enrichment of the iodine in comparison to cesium at high altitudes. The time-integrated surface air concentrations of the Fukushima-derived radionuclides in the Southeast Asia showed exponential decrease with distance from Fukushima.

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► Fukushima radionuclides were detected at three stations in Vietnam from late March.
► Peaks of radionuclides corresponded to direct arrival of air mass from Fukushima.
► 131I/137Cs activity ratios were higher in Dalat (1500 m high) than in Hanoi and HCMC.
► Integrated concentrations exponentially decreased with distance from Fukushima.