Improving chemical thermodynamics knowledge of severe accidents within the OECD-TCOFF2 Project

Journeau, C.*; Bechta, S.*; Komlev, A.*; Kurata, Masaki ; Ohgi, Hiroshi  ; Matsumoto, Toshinori ; Mohamad, A. B. ; Barrachin, M.*; Quaini, A.*; Bottomley, D.*; Serrano-Purroy, D.*; Ito, Ayumi*; Nakamura, Kinya*; Steinbrck, M.*; Dufresne, A.*

The OECD project TCOFF-2 (Thermodynamic Chemistry of Fission Products - Part 2) is an extension of the original project that was part of the near-term projects intended to support the Fukushima Dai-ichi decommissioning efforts. This second part continues to be mainly financed by Japanese Ministry (MEXT) with JAEA-CLADS support. TCOFF2 started in August 2022 and includes certain new material requirements compared to the first TCOFF project. These are increased emphasis on accident tolerant fuels (ATFs), certain actinides and fission products related to volatility/leachability but also certain combinations of the ceramic oxide systems important for MCCI behaviour. Following a PIRT review of phenomena in TCOFF 1, there was in TCOFF2 a Task 1 to prioritise current needs, particularly for relevance to severe accident phenomena and alternative materials (ATFs). This included a re-evaluation and ranking of the thermodynamic systems to make a Systems Identification and Ranking Table (SIRT) for the improvement of the thermodynamic database foreseen in TCOFF2. The further systems for evaluation were then proposed by the partners according to their particular requirements; this resulted in over 150 thermodynamic systems. In the mid-year meeting (June 2023) discussions were made to rationalise these into the most important 20 systems. The rationale for the reduction of systems to this limit will be explained in the talk, both those systems that were omitted as well as those finally included. These systems will then be used as a priority list of work for the experimental call to members that will be launched by the TCOFF-2 project towards the end of the year 2023 with the intention to initiate the first projects soon afterwards.