NDF President Hajimu Yamana proposed it as a new technical strategy plan to provide the technical basis necessary to realize the “mid-and-long-term roadmap” toward the NPPs’ decommissioning. The roadmap was originally issued by both the Japanese government and the Tokyo Electric Power Co. (TEPCO). The current version of plan—originally released in July 2016—will be revised soon based on the results of investigations of the interiors of the primary containment vessels (PCVs) and other locations at the Fukushima reactors.
Dr. Yamana began with the fundamental recognition that the decommissioning of Fukushima Daiichi was a project with great inherent uncertainty. Even in the face of that uncertainty, he said, the NDF would push ahead affirmatively and flexibly, while making use of past experiences and knowledge, and placing top priority on safety. He then enumerated several adjectives characterizing the effort, as follows: safe, certain, rational, prompt and practical on-site.
Concerning fuel debris, he described the conditions inside the pedestals (located below the reactor pressure vessels) at the bottoms of the PCVs at Units 1 to 3 at Fukushima Daiichi, comparing them with each other. He said that fuel debris existed mostly on the floor, though the amount was small, and that more fuel debris was on the floor than in Unit 2.
Yamana went on to talk about the evaluations of the feasibility of fuel debris removal measures. He said that although the conditions inside the reactors were not fully understood, it was possible to address removal possibilities using calculations and/or simulations of the reactor core meltdowns, as well as observations using muon measurement technology and other methods, while also estimating those interior reactor conditions that could be deemed most certain.
Accordingly, comprehensive evaluations were first made of measures taken up in the technical strategy plan released in 2016, namely: (1) water-covered, with removal of fuel debris from the upper part of the containment vessel (CV), (2) uncovered, with removal from the upper part of the CV, and (3) uncovered with removal from the side of the CV. The evaluations primarily focused on such aspects as water levels and access routes.
Based on the knowledge accumulated so far, as well as the time needed to reach the fuel debris, the conclusion was that it would be most practical to first remove the debris lying at the PCV bottom from the side of the CV.
In addition, Yamana stressed the importance of project management in the proposed new technical strategy plan, particularly on such matters as waste measures, R&D promotion, and strengthening international cooperation. He said that was because highly difficult activities in technical terms—such as the removal of fuel debris—will soon be in full swing.
