In order to improve the containment function, it is desirable to identify leak paths in the PCV under a high dose environment.

The PCV and the reactor building (torus room) have been confirmed to have penetrations and to have been damaged by the earthquake. At present, the water level difference with the groundwater has been maintained to prevent contaminated water containing radioactive material from flowing out from the reactor building to the soil outside the reactor building.

In order to ensure the reliable confinement function, water stoppage by repairing the lower part of the PCV has been investigated. Based on the results of the previous investigations, it has become clear that it is difficult to completely stop water leakage by repairing the lower part of the PCV. Considering the PCV repair technology and the results of its actual scale test, the confinement function, including a system that combines leak suppression by applying water stoppage technology with a cooling water circulation and purification system are considered.

In Unit 1 and 3, a drop in the PCV water level was confirmed after the February 13, 2021 earthquake off the coast of Fukushima Prefecture. When conducting internal investigation of the Unit 1 PCV using a submersible ROV, the risk of interference with sediments, etc. increases when the water level is low, so it is being investigated increasing the water injection volume to raise the water level to the pre-earthquake level and then returning the increased water injection volume after the survey is completed to lower the water level to the current level.

As one of the debris removal methods, a flooding method (ship hull method) is being considered. In this method, the entire reactor building is surrounded by a new structure called a hull structure as a confinement barrier. This structure is useful for sealing the upper part of the PCV, which has been an issue with the conventional flooding method.

In order to improve the confinement function, it is desirable to identify leak paths in PCV in a high dose environment.

In order to improve the confinement function of the liquid phase in the lower part of the PCV, several methods, such as vent pipe sealing, downcomer sealing, and strainer sealing, have been investigated to control the leakage of cooling water into the torus room. Based on discussions in Government-led R&D Program on Decommissioning and Contaminated Water Management, the application of the current water stoppage technology involves difficult problems. If water stoppage work is to be performed, even partially, it is necessary to consider the water level in the PCV, the PCV circulation cooling system, and the water level control during the water stoppage work on the vent pipe. In addition, due to the small seismic margin of the S/C support column and the existence of leakage routes other than the S/C, investigation is under way on a circulating cooling system in combination with water level control in the PCV and the torus room.

In the Fukushima Daiichi NPS, the reactor building, the PCV, etc. were partially damaged by hydrogen explosions, degrading the confinement function. Therefore, for the construction of a dynamic confinement function by negative pressure control during fuel debris retrieval is considered. At present, nitrogen is injected into the PCV to maintain a nitrogen atmosphere in order to prevent hydrogen explosions due to hydrogen regularly generated by water radiolysis and to prevent corrosion by oxygen of structural materials (inactivation). Radioactive material release is controlled in this exhaust gas by the PCV gas control system, which removes radioactive material with filters and measures radioactivity.

Accessing the inside of the reactor for fuel debris retrieval will adversely affect the confinement state maintained currently. In addition, special operations and maintenance work in the fuel debris retrieval process will increase the radiation exposure of the workers engaged in the work.

During the drill opening of the inner door of the X-2 penetration for the internal investigation of the PCV of Unit 1, changes in dust concentration (in June 2019, the dust monitor for work monitoring reached the control value) were confirmed. At that time, the work was conducted with limited cutting volume, and based on the obtained data on dust diffusion characteristics, measures were taken such as cutting with controlling peak concentrations and the addition of a new dust monitor for work monitoring. Considering this, in Unit 2, preparations are underway for low-pressure water washing of sediments, spray curtains to the penetration outlet, and measures to reduce the pressure in the PCV.

In order to maintain negative pressure inside the PCV, an exhaust ability is required according to the damage status of the PCV. The exhaust ability is set based on the data of the nitrogen supply volume in the actual equipment and PCV pressure fluctuations, although the damaged parts have not been identified yet.

In case the primary confinement function with negative pressure control is lost and causes radioactive material to leak from the confinement boundary, the necessity of a secondary confinement function is considered, by installing a building cover or a container in the existing reactor building, that enables the reactor building to control at a slight negative pressure and collect and process radioactive material.

TEPCO is monitoring radiation levels at the boundary of the Fukushima Daiichi Nuclear Power Plant site and around the buildings.