主催: The Japan Society of Mechanical Engineers
会議名: 福島廃炉研究国際会議2022(FDR2022)
開催日: 2022/10/14 - 2022/10/17
On 11th March 2011, the Great East Japan Earthquake and Tsunami hit the Fukushima Daiichi Nuclear Power Station. Units 1–3 lost all power supplies, which set in motion a chain of events that led to severe core damages and releases of radioactive contamination into the environment. Since then, TEPCO has made many efforts to investigate the status of the reactors and containment vessels utilizing remotely operated vehicles (ROV) providing visual information from inside the Primary Containment Vessels (PCV), including Reactor Pressure Vessel (RPV) pedestal areas of Units 2 and 3.
Through the previous investigations of Unit 1 PCV, presence of large amount of foreign materials (deposits) was confirmed on drywell (D/W) floor. To clarify the distribution and nature of these deposits, as well as the state inside the PCV, set of 6 ROVs with a specific purpose, namely installation of guide rings (A), visual observation (A2), 3D mapping of deposits (B), measurement of deposit height (C), fuel debris detection (D) and sampling (E) have been developed by International Research Institute for Nuclear Decommissioning (IRID). Through January to June 2022, the first half of the investigations (ROV-A, A2, C) has been successfully completed, revealing significant degradation of the pedestal wall concrete in the vicinity of the personnel access port (hereinafter referred to as pedestal opening), deposit shelves attached to concrete/metallic structures, various morphology of deposits, and height of the deposits ranging between 0.2 to 1.1 m from the D/W floor in the investigated range.
These results are examined in the light of the assessments by severe accident analysis codes and experimental results, and discrepancies between the expected and observed states are pointed out. The volume of the deposits outside of the pedestal, their distribution and extent of damage to structures suggest that the current knowledge of severe accident phenomena, such as ex-vessel accident progression, molten core-concrete interaction (MCCI) and others, is incapable of providing comprehensive explanation of the state inside the Unit 1 PCV.
