Issue Number:CWM-101
Phase: Preparation
Decommissioning Process Contaminated Water Management
Investigation Subject Understanding current status
Issue Understanding the current status of the contamination sources

Needs

① Understanding the current status of the contamination sources
Time Axis: Contaminated Water Management [Short-Term]

Desired state and reasons for it

It is desirable to obtain information pertaining to the current status of contamination sources (the property of the contamination sources and the surrounding environment). This is because ultimately the contamination sources need to be removed and treated.
Specifically, if there is information on the property of a contamination source and the surrounding environment, it is possible to rationally investigate how to access the contamination source, how to remove the contamination source, and how to handle (treat) the removed contamination source afterwards.

Current state against ideal

In 2020, the treatment of the retained water in the buildings was completed except for the reactor buildings of Units 1-3, the main process building, and the high-temperature incinerator building.
Sources of contamination currently considered to be of particular importance include stagnant water at the bottom of the torus chamber in the reactor building and so-called "zeolite sandbags" that exist in high-dose conditions on the lowest floors of the main process building and the high-temperature incinerator building.

Residual water in the torus chamber

Residual water containing α-sludge and ionic α-nuclides exists at the bottom of the torus chamber in the reactor building, and relatively high concentrations of α-nuclides have been detected. In addition to α-nuclides, there is also highly concentrated stagnant water containing high concentrations of radioactive materials and salt, which is close to the condition immediately after the accident.
Currently, the properties of the retained water are being analyzed and removal measures are being investigated to prevent the spread of α-nuclides. The nuclides that can be significantly present in contaminated water before it is treated by ALPS, etc. have been verified, and 29 nuclides have been selected as target nuclides.

Zeolite sandbags

Zeolite sandbags are present in a high-dose state on the lowest floors of the main process building and the high-temperature incinerator building.
Prior to the treatment of retained water, regarding zeolite sandbags investigation is underway on "underwater recovery" method in which they are directly transported to the first floor by remote heavy equipment, ROV, etc., dewatered, and filled into containers.

Issues to be resolved

Residual water in the torus chamber

The chemical form of α-nuclides at the bottom of the torus chamber of the reactor building may vary depending on the water quality and coexisting materials. For reliable removal, it is necessary to collect samples from multiple locations to understand the variation in properties. For property analysis, it is necessary in the future to increase the number of samples, steadily expand data, and conduct data analysis. No major R&D issues are considered to exist here.
Since alpha sludge also contains high concentrations of Cs-137, it is required to investigate on measure to reduce radiation doses to workers and removal measures from the viewpoint of maintainability and t secondary waste. In addition, regarding the treatment process, it is desirable to investigate optimal treatment methods considering the properties of the stagnant water and the surrounding environment. Thus, R&D issues exist.

Zeolite sandbags

The location of zeolite sandbags and the surrounding environment have not been sufficiently clarified, and the recovery method has not yet been optimized. In the "underwater recovery" method for zeolite sandbags, securing visibility in muddy water and improving the recovery rate of zeolite are issues, and thus preliminary investigation using mockups are important. It is also necessary to investigate on evaluation of the impact and alternative measures to reduce the radiation dose in the case when some of the zeolite are not recovered.
Currently, preparations are being made to remove the zeolite sandbags and other materials that have been installed on the lowest floor.

Relevant Issues

References

Please note that this English text is a provisional translation of the Japanese original.