The Third 2021 Fukushima Research Conference

International workshop “OECD/NEA TCOFF final workshop” was successfully held in Dec. 14-16, 2021 as a Fukushima research Conference (FRC) hosted by Japan Atomic Energy Agency (JAEA). The workshop was operated using a remote system and 121 experts on accident analysis and debris characterization from 17 countries participated to the workshop. TCOFF (Thermodynamic Characterization of Fuel Debris and Fission Products based on Scenario Analysis of Severe accident Progression at Fukushima Daiichi Nuclear Power Station) project was carried out in the NSC (Nuclear Science Committee) of the OECD/NEA (Organization for Economic Co-operation and Development/Nuclear Energy Agency) in the period between June 2017 and July 2020, based on the acceleration plan of decommissioning R&D for FDNPS (Fukushima-Daiichi Nuclear Power Station) which had been proposed by the MEXT (Ministry of Education, Culture, Sports, Science and Technology) of Japan. The MEXT has suggested the OECD/NEA to establish a wider synergy between basic research experts (who had not been involved in decommissioning research at that time) and applied research experts such as decommissioning and severe accident analysis. In response to this proposal, the OECD / NEA launched the TCOFF project. Nuclear research institutes and academic research institutes such as universities over 10 countries + EC participated to the TCOFF project, which aimed at the characterization of fuel debris and fission products (FP) based on the scenario analysis of the FDNPS accident.
At the beginning of the workshop, two plenary lecturers were open for public, such as (1) Updated understanding on the FDNPS accident scenario by Dr. M. Cibula (TEPCO) and (2) Decommissioning R&D necessary in the next decade by prof. K. Okamoto (JAEA). Then, Dr. T. Ivanova (OECD/NEA) showed appreciation to the TCOFF project and expectation to the phase 2. The project achievements were outlined by the TCOFF project members, which include fuel debris and FP behaviors and the background thermodynamic databases. After these lecturers, fruitful discussion was carried out among the workshop participants. The participants reached following consensus on debris and FP behaviour during the FDNPS accident, i.e. (1) failure mode of lower head of the reactor pressure vessel and relocation mechanism of fuel debris from the failure points to pedestal region in the FDNPS accident are different from previous understanding based on the prototypic accident scenarios. (2) Cs transportation route and the chemical conditions are a key issue to improve FP-behaviour models. (3) To improve the understanding of these phenomena, the comprehensive evaluation based on hydro-thermic evaluation and materials science characterization are extremely important. Furthermore, the workshop participants discussed on the ATF (Accident Tolerant Fuel), which is being developed in many countries after the FDNPS accident. We reached our consensus that for the proper development of these new materials, synergetic discussion among wide experts like the TCOFF project is highly expected. In the last day, lots of participants showed their appreciation to this informative workshop. Based on the discussion at the workshop, the OECD/NEA will coordinate the phase 2 of the TCOFF project, for which several OECD/NEA member countries will give the financial support.

Outline

This international workshop is held to summarize the results of the TCOFF project (TCOFF: Thermodynamic TEXT of Fuel Debris and Fission Products based on Scenario Analysis of Severe accident Progression at Fukushima Daiichi Nuclear Power Station), which had been carried between 2017.6 and 2020.7 in Nuclear Science Committee (NSC) of the OECD/NEA. Based on the decommissioning research acceleration plan proposed in 2014.6, the Ministry of Education, Culture, Sports, Science and Technology has suggested the OECD/NEA to establish a wider synergy between basic research experts (who had not been involved in decommissioning research at that time) and applied research experts such as decommissioning and severe accident analysis. In response to this proposal, the OECD / NEA launched the TCOFF project. Nuclear research institutes and academic research institutes such as universities over 10 countries + EC participated to the TCOFF project, which aimed at the characterization of fuel debris and fission products (FP) based on the scenario analysis of the TEPCO Fukushima Daiichi Nuclear Power Station accident (1F accident).

In the TCOFF project, based on the agreement of the participating organizations, following investigations were carried out: (i) materials interaction models and the thermodynamic database implemented in the severe accident analysis codes and identification of knowledge gap, (ii) simulation tests to validate the core/fuel damage models and identification of knowledge gap, (iii) materials science reaction mechanism in fuel and core degradation in the 1F accident scenarios, (iv) a survey of FP chemical database and identification of knowledge gap, and (v) accumulation of knowledge on FP leaching from fuel debris. In addition, about 50% of the TCOFF project fund was used for international R&D call, by which valuable fundamental data for fuel debris and FP were accumulated. In this workshop, TCOFF project experts will introduce their achievements and then discussion for further utilization of the TCOFF achievements will be carried out. In addition, TCOFF activities based on the synergies of a wide range of experts are expected not only to accelerate decommissioning research but also to safety research on next-generation fuels. Hence, TCOFF Phase 2 is currently under consideration at the OECD/NEA. In this workshop, we will also exchange opinions for the implementation of TCOFF Phase 2.

In this workshop, experts will exchange detailed scientific/technical opinions, but in principle anyone can attend by letting us know your affiliation and status by pre-registration system. Especially, the participation of university students, next generation researchers are highly welcome. In addition, two plenary lectures are planned on the first day of the workshop;
(i) Dr. Shinya Mizokami, Fellow of TEPCO's Decommissioning Company, will outline the progress of the accident analysis and core status of 1F and
(ii) Prof. Koji Okamoto, Director of the Collaborative Laboratories for Advanced Decommissioning Science, will introduce the expectation to decommissioning R&D in coming decades.
This will be open to the public as FRC according to the following outline.