Observation of radioiodine-131 in brown algae in northeast coastal area of Japan and consideration of its source

A small amount of ¹³¹I has been detected in the seaweed 'Eisenia bicyclis (arame)' collected since after July 2006 in the sea area in front of the Tohoku Electric Power Onagawa Nuclear Power Station. In order to investigate the cause, the collection location of 'arame' was expanded more than usual, and the regional distribution and temporal fluctuation of radionuclide concentration were investigated. As a result, ¹³¹I has been continuously detected in 'arame' collected from afar, where the effects of the power plant would not reach. No ⁶⁰Co peculiar to the power plant was detected. From the detection state and distribution of these artificial radionuclides, it was considered extremely unlikely that the detected ¹³¹I was caused by the power plant. Therefore, the authors investigated the relationship with the usage state of ¹³¹I for medical use in Japan. (A.O.)