Regional and global contributions of anthropogenic iodine-129 in monthly deposition samples collected in North East Japan between 2006 and 2015.

We measured the monthly atmospheric deposition flux of (129)I at Rokkasho, Aomori, Japan-the location of a commercial spent nuclear fuel reprocessing plant-from 2006 to 2015 to assess the impact of the plant on environmental (129)I levels. The plant is now under final safety assessment by a national authority after test operation using actual spent nuclear fuel. During cutting and chemical processing in test operations from April 2006 to October 2008, (129)I was discharged to the atmosphere and detected in our deposition samples. (129)I deposition fluxes largely followed the discharge pattern of (129)I from the plant to the atmosphere, and most of the deposited (129)I originated from the plant. In and after 2009, (129)I deposition fluxes decreased dramatically to reach the background level; the (129)I deposition fluxes at Rokkasho were almost the same as those at Hirosaki, where an additional sampling point was set up as a background site 85 km from the plant in 2011. The background (129)I deposition fluxes showed seasonal variation-high in winter and low in the other seasons-at both Rokkasho and Hirosaki. The results of a backward trajectory analysis of the air mass at Rokkasho suggested that reprocessing plants in Europe were the origins of the high (129)I flux in winter. The contribution of (129)I released from the Fukushima Dai-ichi Nuclear Power Plant accident to the (129)I deposition flux at Rokkasho in 2011 was small on the basis of the (129)I/(131)I activity ratio.