TCOFF-2 is the second phase of the NEA joint project entitled Thermodynamic Characterisation of Fuel Debris and Fission Products Based on Scenario Analysis of Severe Accident Progression running until 2026.
The first phase focused on analysis of aspects pertaining to the Fukushima Daiichi Nuclear Power Plant reactors. The current phase builds upon the knowledge acquired to also include studies on other reactor and fuel designs including advanced technology fuels (ATF). The project brings together 12 organisations from Canada, France, Germany, Japan, Sweden, United States and the European Commission.
Specifically, the aim of TCOFF-2 is to improve models and underpinning data on the material interactions and their role in the various potential phases of the severe accidents. The ultimate goal is the integration of such models into severe accident simulation codes to increase their accuracy including in respect of the extent of potential radioactive release. This improvement will be driven by state-of-the-art knowledge of the thermodynamic properties of the relevant combinations of materials.
To review progress in support of these aims, the TCOFF-2 Programme Review Group (PRG) and Management Board (MB) meetings were convened in Tokyo, Japan, on 19-20 May 2025 hosted by the Central Research Institute of Electric Power Industry (CRIEPI) and facilitated by PRG Chair Dr Masaki Kurata of the Japan Atomic Energy Agency (JAEA) and Kinya Nakamura (CRIEPI).
The meeting gathered 31 participants and progress was reported from each of the five principal tasks of TCOFF:
- Prioritisation of the material science issues related to severe accident study – Led by Dr Christophe Journeau (CEA) and Prof. Sevostian Bechta (KTH)
- Improvement of the materials science knowledge regarding fuel core degradation and fission product release (UO2-Zr and ATF) – Led by Marc Barrachin (ASNR) and Prof. Ayumi Itoh (Science Tokyo)
- Implementation of improved knowledge for the simulation of the accident behavior (UO2-Zr and ATF) – Led by Dr Andrew Morreale (CNL) and Dr David Luxat (SNL)
- Extension of the leaching experimental database to better evaluate mid and long-term stability of degraded cores – Led by Dr Daniel Serrano Purroy (JRC) and Takeshi Sonoda (CRIEPI)
- Training and Education – Led by Prof. Yoshinao Kobayashi (Science Tokyo) and Dr Stéphane Gossé (CEA)
Sessions then followed with updates from supplementary research and development projects currently being pursued:
- Thermochemistry of fission product phases with a long-term radiological impact: experimental studies of the Ba-Sr-Cs-Mo-O system – Prof. Anna Smith (TU Delft)
- Oxidation of Cr-coating layer by steam in accidental conditions: Gaseous phase of Cr-Cs-O system description using Knudsen effusion mass spectrometry + Thermodynamic evaluation” – Dr Ioana Nuta (CNRS)
- Phase relation of Eu-Zr-O system and the variation in reducing H2/H2O atmosphere – Prof. Shigeru UEDA (Tohoku University)
- Experimental characterisation of representative corium samples under severe accident conditions – Prof. Markus Piro (McMaster University)
- Modelling heavy actinide solute trapping during solidification with data from the CNL cold crucible experiments – Dr Mike Welland (McMaster University)
- In situ high-temperature XRD/XAFS study of phase equilibria for hypo-stoichiometric region in U-Zr-O system – Prof. Ayumi Itoh (Science Tokyo)
- The effect of FeO and ZrO2 on the phase equilibrium of CaO-SiO2-Al2O3 simulant of the SiO2-rich concrete at 1673 K – Dr Ryoto Nakazawa (Science Tokyo)
A guest technical lecture from industry on “Expectations for Fuel Material Science Research in Severe Accident Analysis” was provided by Dr Koji Nishida of the Institute of Nuclear Safety System (INSS).
On 20 May, a number of the participants travelled to the Shimane prefecture to attend a technical tour organised by JAEA of the Shimane Nuclear Power Station of the Energia Chugoku Electric Power Company.
Company representatives delivered a technical lecture on the implementation of the lessons learnt from the Fukushima Daiichi Nuclear Power Plant accident at the site. Participants also heard about the restart of the Unit 2 BWR (Boiling Water Reactor) in December 2024 and preparations to commence operation of the Unit 3 Advanced Boiling Water Reactor (ABWR).
Further details on TCOFF-2 can be found on the dedicated web page and the partnership with the NEA Nuclear Education, Skills and Technology (NEST) Framework page.
The next TCOFF project review meeting will be held in France on 12-14 November 2025.
