Issue Number:FDR-213
Phase: Design
Decommissioning Process Fuel Debris Retrieval
Investigation Subject Strategy and risk
Issue Fuel debris retrieval policy

Needs

① Confirming the details of fuel debris retrieval work and the feasibility of the system
Time Axis: Fuel Debris Retrieval [Short-Term]

Desired state and reasons for it

In order to implement fuel debris retrieval in a sustainable manner, it is desirable to confirm the contents of the work at each stage of fuel debris retrieval and the feasibility of the system through consulting and technical reviews.

Current state against ideal

TEPCO is currently conducting a conceptual study on further expansion of the scale of fuel debris retrieval, with Unit 3 being the first to go, and fuel debris retrieval scenarios as well as methods.
Among these, TEPCO has been investigating the methods by the end of FY2021, and since many issues and risks with high difficulty levels have been identified for each of the methods that have been discussed, TEPCO is to confirm the on-site applicability and technological feasibility of the measures to deal with these issues and risks from FY2022 onward.

Issues to be resolved

At the Fukushima Daiichi NPP, where uncertainties still exist, the uncertainty of the PCV internal conditions is an obstacle to the investigation, forcing it to proceed with a set of preconditions. For future determination of technological feasibility, it is important to clarify and investigate the requirements (boundary conditions) and constraints (site utilization area, interaction with existing facilities, etc.) for methods and systems, such as criticality control, dust confinement, shielding, heat removal, etc.
② Investigating alternative methods for fuel debris retrieval
Time Axis: Fuel Debris Retrieval [Mid-Term]

Desired state and reasons for it

In order to prepare a backup for the fuel debris retrieval method, it is desirable to consider in advance a method different from the existing one.
It is desirable to select the most appropriate method according to the condition or the safest method under the condition.

Current state against ideal

Regarding the retrieval process, the methods are classified according to the work environment, access direction, primary boundary, etc., and then the partial submersion method and the submersion method are being discussed.
The partial submersion method is a combination of top entry method and side access method, both of which were investigated heretofore, rather than choosing either one.
The concept of submersion method differs from that of the existing method. The existing method (i.e., PCV covered with water) has advantages in terms of radiation shielding effect, etc., though, it was judged to be less feasible considering the technological difficulty of stopping the water at the top of the PCV and the amount of radiation exposure during the work. Therefore, in Mid-and-Long-Term Roadmap for 2017 and 2019, it was decided to proceed with the partial submersion method, and the submersion method would be investigated again in the future, considering the progress of research and development. The new submersion method differs from the PCV submersion method described above and is created with a new concept to submerge the reactor building by enclosing the entire building with a new structure called a hull structure as a boundary.

Issues to be resolved

In order to proceed the decommissioning of nuclear power plants safely and stably, which have many uncertain factors, it is important to develop a system and establish a method that allows planning to be carried out based on the information and assumptions currently available, while at the same time incorporating new information and various findings obtained as work progresses in a timely manner and flexibly revising the plan according to the condition as it becomes clear.

Relevant Issues

References

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