Issue Number:FDR-105
Phase: Preparation
Decommissioning Process Fuel Debris Retrieval
Investigation Subject Understanding status inside PCV
Issue Collection of knowledge on conditions inside PCV

Needs

① Consolidating obtained knowledge and comprehensively understanding the conditions in the reactor

Desired state and reasons for it

In order to formulate a fuel debris retrieval plan, it is desirable to reorganize information on the in-core condition obtained from actual equipment investigations, experiments, knowledge, accident progression analyses, or other sources and to improve the estimation of the condition of each unit.
Since it is not possible to confirm the entire inside of the PCV and the RPV, it is desirable to estimate the condition of parts that cannot be visually confirmed, by using analysis and evaluation technologies. These require not only the acquisition and detailed analysis of a portion of the debris, but also evaluation that pays attention to consistency with objective data. In order to maintain technical capability and human resources, it is important to conduct research such as experiments and analyses, and to constantly improve the accuracy of predictions through the exchange of opinions among experts on the content of analysis data and analysis evaluations.
The fuel debris retrieval method, the process management, and the integrity evaluation of structure are important issues because they may lead to improve work efficiency and safety. For this reason, it is important to understand the condition in the reactor that overlook these decommissioning processes (decommissioning management). Mainly during the fuel debris retrieval period and the PCV/RPV/building dismantling period, the in-core status should be updated on an ad-hoc basis based on the results of internal investigations and accident progression analyses and it needs to function as a hub (information collection point) for outputting to the decommissioning process.

Current state against ideal

TEPCO has been continuously conducting estimation of conditions inside RPVs and PCVs. The results were organized in 2021 in “Estimation of Conditions in the Reactor Pressure Vessel and Containment Vessel after the Accident at the Fukushima Daiichi Nuclear Power Plant”.
In addition, as of FY2025, the knowledge obtained from PCV internal investigations and other sources has been consolidated into the 2D/3D In-core Status Estimation Maps developed under the Decommissioning, Contaminated Water, and Treated Water Management project “Development of Analysis and Estimation Technologies for Understanding Fuel Debris Characteristics”, as well as into the debrisWiki, and is visualized through debrisEye.

Issues to be resolved

The future issue is how to reorganize information on fuel debris and FPs and on the status of structures inside the PCV and the RPV, which are listed in FDR101 to FDR104, for the purpose of contributing to the fuel debris retrieval method and system design.
In particular, it is necessary to integrate the “Properties List,” which compiles the results of fuel debris characterization under the Decommissioning, Contaminated Water, and Treated Water Management project “Development of Analysis and Estimation Technologies for Characterization of Fuel Debris,”, and the analysis and evaluation results of fuel debris and related materials that will be obtained in the future through fuel debris retrieval and other activities.
Since information will be continuously collected and its volume will keep growing, it is necessary to consider an efficient and effective data management method.

Relevant Issues

References

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