Evaluation of Accumulated Fission Products in the Contaminated Water at the Fukushima Daiichi Nuclear Power Plant

As a result of the mass balance analysis for fission products (FPs) in the contaminated water accumulated in the Fukushima Daiichi nuclear power plant in the aftermath of the accident, it was concluded that the short-term cesium source was 50% of the total amount of cesium accumulated in the reactor, which was estimated with the SAMPSON severe accident analysis code, while the short-term source of tritium was 26% of the total tritium that was released during the UO₂ fuel meltdown. The FP concentrations in the contaminated water during the 2 yr since the accident were determined by short-term FP sources, while their saturated concentrations, due to a balance between the release from the reactor and the cleanup, were determined by long-term FP sources. A multinuclei removal system, ALPS, has been operated at the plant to clean up the contaminated water to reduce FP concentrations to a level that is permitted for release to the environment. Tritium concentration in the contaminated water decreased a little during the 2 yr since the accident primarily from dilution by mixing groundwater rather than intentional removal. Industrial-scale removal of tritium from the huge amount of contaminated water is almost impossible using isotopic separation methods. Storage of tritiated water for a long period presents a high risk for leakage to the environment. As the most realistic procedure, the authors propose that the tritium be diluted with plenty of seawater to the natural background level and then be released into the ocean with continuous monitoring at the release point.