Monitoring of Low-Level Radioactive Liquid Effluent in Tokai Reprocessing Plant

摘要

The Tokai reprocessing plant (TRP), the first reprocessing plant in Japan, has discharged low-level liquid wastes to the Pacific Ocean since the start of its operation in 1977. We have performed liquid effluent monitoring to realize an appropriate radioactive discharge control. Comparing simple and rapid analytical methods with labor-intensive radiochemical analyses demonstrated that the gross-alpha and gross-beta activities agreed well with the total activities of plutonium isotopes (~(238)Pu and ~(239+240)Pu) and major beta emitters (e.g., ~(90)Sr and ~(137)Cs), respectively. The records of the radioactive liquid discharge from the TRP showed that the normalized discharges of all nuclides, except for ~3H, were three or four orders of magnitude lower than those from the Sellafield and La Hague reprocessing plants. This was probably due to the installation of multistage evaporators in the liquid waste treatment process in 1980. The annual public doses for a hypothetical person were estimated to be less than 0.2 μSv y~(-1) from the aquatic pathway. Plutonium radioactivity ratios (~(238)pu/~(239+240)Pu) of liquid effluents were determined to be 1.3-3.7, while those of the seabed sediment samples collected around the discharge point were 0.003-0.059, indicating no remarkable accumulation of plutonium in the regional aquatic environment. Thus, we concluded that there were no significant radiological effects on the public and the aquatic environment during the past 30-year operation of the TRP.