Tritium in Japanese precipitation following the March 2011 Fukushima Daiichi Nuclear Plant accident

Tritium concentrations in Japanese precipitation samples collected after the March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) were measured. Values exceeding the pre-accident background were detected at three out of seven localities (Tsukuba, Kashiwa and Hongo) southwest of the FNPP1 at distances varying between 170 and 220 km from the source. The highest tritium content was found in the first rainfall in Tsukuba after the accident; however concentrations were 500 times less than the regulatory limit for tritium in drinking water. Tritium concentrations decreased steadily and rapidly with time, becoming indistinguishable from the pre-accident values within five weeks. The atmospheric tritium activities in the vicinity of the FNPP1 during the earliest stage of the accident was estimated to be 1.5 × 103 Bq/m3, which is potentially capable of producing rainwater exceeding the regulatory limit, but only in the immediate vicinity of the source.

► We measured the 3H contents of Japanese rain collected after the Fukushima accident.
► 3H level became 30 times higher than pre-accident level in the first rain at Tsukuba.
► Some locality within 220 km from the source showed elevated 3H levels.
► These high 3H signals disappear in a few weeks.
► Atmospheric 3H level at the source during the earliest stage was estimated to be 1500 Bq/m3.