Session: 03-01: Major facilities experience in handling accidents and D/D&D

Paper Number: 109765

109765 - Investigation of Fukushima Daiichi Unit 1 Accident End-State in Primary Containment Vessel 

On March 11^th, 2011, the Great East Japan Earthquake and Tsunami hit the Fukushima Daiichi Nuclear Power Station, resulting in the loss of power supplies and ultimately to severe core damages at Units 1–3. Following the disaster, TEPCO has made numerous efforts to investigate the accident progression and the status of the reactors and primary containment vessels (PCV).

The degree of core damage and amount of materials relocated outside the reactor pressure vessel (RPV) is considered the highest for Unit 1, which experienced the earliest onset of loss of all cooling function and the longest period without any effective cooling water injection.

With the progress of decommissioning in three accident plants, multiple investigations utilizing remotely operated equipment provided valuable information from inside the PCVs, and some visual information of the RPV pedestal areas has been obtained for Units 2 and 3. On the other hand, for Unit 1's PCV, only the presence of large amounts of deposits has been confirmed on the drywell (D/W) floor without any successful entry into the pedestal area so far. Previously conducted muon tomography indicates that almost no fuel materials are remaining inside the RPV of Unit 1.

To clarify the distribution and nature of the deposits on the D/W floor, as well as the general state within the PCV, with the ultimate goal of entering and investigating the pedestal area, a set of 6 ROVs with specific purposes was developed and investigation was implemented since February 2022.

The investigation has so far revealed significant degradation of the pedestal wall concrete in the vicinity of the personnel access port (pedestal opening), which contrasts with the relatively small extent of damage to the reinforcing steel of the wall and pre-existing metallic structures inside the D/W. In some areas deposit shelves with cavities under/between them were found attached to concrete/metallic structures. The height of the deposits varies from 0.2 to approximately 1.5 meters from the original D/W floor level. The variety in the morphology and distribution of deposits suggest that there may be multiple origins and formation mechanisms. Neutron flux and gamma spectrum analyses suggest the presence of actinide elements within the deposits across the D/W.

The results of this investigation have so far provided important insights into the events that took place at the Fukushima Daiichi Nuclear Power Station. However, they have also revealed that knowledge of severe accident phenomena such as ex-vessel accident progression, molten core-concrete interaction (MCCI), and others, is currently insufficient to provide a comprehensive explanation of what happened inside Unit 1's PCV. In particular, the volume and spread of the deposits, as well as the distribution and extent of damage to structures, suggest that further research and investigation is necessary to fully understand the events that took place in 2011.

Recently obtained investigation results are critical in gaining a clear understanding of the end state of the molten core materials to assist with the safe and efficient decommissioning. Through understanding the distribution and characteristics of the damaged core materials, the elaboration of decommissioning plans can be better informed and optimized. The detailed analyses of the sampled deposits, visual information, 3D scans and radiation measurements are expected to provide a comprehensive view of the deposits' distribution and their characteristics. Moreover, the information gathered from these efforts is also expected to help to improve our understanding of the severe accident progression and the phenomena occurring in the molten core materials interactions with the containment systems, ultimately contributing to the improvement of the safety of existing and future nuclear power plants.

Presenting Author: Michal Cibula Tokyo Electric Power Company Holdings, Inc.

Presenting Author Biography: Dr. Michal Cibula is an expert in the field of nuclear technology. After earning his Master's degree from the Slovak University of Technology in 2014, and his PhD from the Tokyo Institute of Technology in 2019, he currently works for Tokyo Electric Power Company. where he is part of the Fuel Debris Retrieval Program Department of Fukushima Daiichi D&D Engineering Company. His main focus is on sample transportation, investigation, and accident analysis, with the aim of clarifying progression of accident in 2011.