According to the current risk assessment for each unit, Unit 1 has no upper part of reactor building, and Unit 3 has only a fuel retrieval cover instead of the upper part of reactor building. On the other hand, in Unit 2, the reactor building remains as it used to be and most of the fuel debris is presumed to stay inside the RPV. Therefore, the degree of damage to the RPV in Unit 2 is small. Thus, there is a difference between the three units in terms of the importance of management importance. The fuel debris, which may affect the potential impact, may take various forms, ranging from near-powder to solid, but the form has not been specified at this time. Especially for Unit 2, most fuel debris is presumed to remain inside the RPV, and it is thought that the proportion of concrete reaction products in the molten core is smaller than that in Unit 1 and Unit 3, and that it maintains a stable form. Therefore, the potential impact on Unit 2 may be relatively low.

The partial submersion method is emphasized in the R&D. Side entry to the PCV bottom is prioritized in the fuel debris retrieval policy. Based on this policy, R&D of various technologies has been accelerated and emphasized, especially on the confinement function and the water level management technology in the PCV, assuming the presence of alpha nuclides. TEPCO is conducting a conceptual study to select a method for further expansion of the scale of retrieval, and it is planned to proceed with the investigation on the feasibility of each method FY2022 onward.

Investigations on the PCV internal investigations that have been conducted so far have provided various useful information for understanding the situation inside the PCV. On the other hand, the information about the situation inside the PCV is still limited, partly because further RPV internal investigations have not been conducted yet. PCV internal investigation is being planned to obtain more detailed information such as deposit and fuel debris distribution inside the PCV. RPV internal investigation is also planned to obtain information inside the RPV.

In addition, the airborne method ((RPV water injection), optional (RPV filling and solidification)) and the flooding method (vessel hull method) are examples of construction methods that are being evaluated and studied.

The proposed airborne method (RPV water injection) is a method to retrieve fuel debris by pouring water inside the RPV while the fuel debris is exposed in the air or immersed at a low water level.

The airborne method option (RPV filling and solidification) is a method in which the bottom of the pedestal, RPV, reactor well, etc. are physically stabilized by solidifying them with filling material and fuel debris is excavated and removed together with the filling material.

The flooding method (ship hull method) is a method of enclosing the entire reactor building with a new structure called a ship hull structure as a confinement barrier, thus flooding the reactor building to remove fuel debris.