The Great East Japan Earthquake and subsequent tsunami that struck the Fukushima Daiichi Nuclear Power Station gave ground to a set of events that resulted in an unprecedented severe accident occurring simultaneously in multiple reactors of one nuclear power station and significant release of radioactive materials into the environment. Following these events, Tokyo Electric Power Company in cooperation with domestic and international partners has made continuous efforts to clarify in detail the progression of the accident utilizing both analytical tools and robotic inspections of the damaged units. Recently, multiple inspections utilizing submersible remotely operated vehicles were conducted between December 2022 and March 2023, including the first entry into the pedestal area below the damaged reactor pressure vessel of Unit 1. This paper discusses the findings of these inspections in light of the evolving understanding of the accident scenario, leveraging insights gained through experimental and analytical studies on ex-vessel severe accident phenomena. The outcomes of these investigations have yielded new knowledge and highlighted existing gaps in our understanding. The identified knowledge gaps provide direction for future studies, ultimately advancing the level of nuclear safety.
