Radiocarbon in aquatic biota samples in a brackish lake adjacent to a reprocessing plant in Rokkasho, Japan, from 2006 to 2022

The commercial spent-nuclear-fuel reprocessing plant in Rokkasho, Japan, is scheduled for completion in 2024. Its fuel rods were cut into segments and chemically processed between 2006 and 2008, as part of the plant's test operations. During the test, 14C (4.4 TBq) were discharged from the plant's main stack into the atmosphere. At reprocessing plants, 14C has the largest contribution to the effective dose to the public among the nuclides released into the atmosphere, and it important to understand the concentration distribution and impact of 14C in the surrounding environment. This study conducts a continuous monitoring of 14C in aquatic biota samples around the plant from 2006 to 2022. The 14C concentration in most samples was determined at the background level, and the effect of the plant was slightly recognized in some samples. Moreover, the specific radioactivity of 14C decreased faster than the theoretical decay from 0.242 to 0.225 Bq-g C-1.