NEA-1925 TCOFF.

TCOFF Project

The decommissioning of the Fukushima-Daiichi Nuclear Power Station (FDNPS) is a long-term endeavour that needs a consistent interdisciplinary effort, integrating different approaches from science and technology. Among these approaches, improving the understanding of the materials science underlying the fuel/core degradation phenomena and the fission products (FPs) behaviour is crucial to develop a theoretical framework and to understand the observations accumulated during and after the FDNPS-accident as well as the severe accident phenomena. This also enables the development of more predictive models for the integral codes used in severe accident simulation. These aspects are important to an improved management of the FDNPS decommissioning, especially in the long term. This will ultimately lead to improved or newly developed severe accident mitigation strategies and enable improved design and operational safety.

The joint project on Thermodynamic Characterization of Fuel Debris and Fission Products based on Scenario Analysis of Severe Accident Progression at the FDNPS (TCOFF) was initiated within the Nuclear Science Committee (NSC) of Nuclear Energy Agency (NEA). One of the main drivers has been the importance of combining all available expertise, competences, and scientific resources to tackle the complexity of the FDNPS decommissioning and to improve future NPPs’ safety. This project has been particularly successful in fostering synergies of expertise by gathering strong competencies in a broad variety of domains. The project was launched by the Ministry of Education, Culture, Sports, Science and Technology - Japan (MEXT), which provided the entire financial funding (approximately 720 kEuros) over three years (June 2017 – June 2020).

The TCOFF project aimed at improving the interpretation of severe accident from the materials science point of view. The studies aimed to cover the in-vessel and ex-vessel phases of the three FDNPS units. They addressed: the fuel melting/relocation, FP-behaviour, fuel debris chemical and phase composition, and the formation mechanism of the FDNPS-samples. The practices and recommendations emerging from the project execution were built upon consensus among international experts and joint assessments of new evidence resulting from the R&D activities funded by the project. The project also enabled the evaluation of new data, and the improvement of reference thermodynamic databases and degradation models applied to FDNPS and other NPPs.

The R&D activity funded by the project was structured through two calls for R&D proposals that were organized in 2018 and 2019 respectively. These activities led to the release of technical reports, which make up this package.

The list of R&D proposals that were funded by the TCOFF project is given below.

R&D proposals selected for funding in 2018:

• P. Aleksandrov Scientific Research Technological Institute (NITI): Experimental investigation of the phase equilibria in the U-Zr-O and U-Zr-Fe-O systems;

• Delft University of Technology: Physical-chemistry of fission products phases with a potentially long-term radiological impact: experimental studies of the Ba-Sr-Cs-Mo-O system;

• Saint Petersburg State University: Vaporization and thermodynamics of Cs-bearing compounds in the Cs-Si-O, Cs-B-O, Cs-Mo-O, Cs-Cr-O, Cs-Fe-O systems studied by high temperature mass spectrometric method ;

• Khlopin Radium Institute (KRI): Water leaching tests of radionuclides from matrices of aged Chernobyl “lava” and corium;

• Tokyo Institute of Technology (TITECH): Application of chemical equilibrium techniques for the determination of thermodynamics of Cs and Si compounds.

R&D proposals selected for funding in 2019:

• P. Aleksandrov Scientific Research Technological Institute (NITI): Experimental investigation of the phase equilibria in the U-Zr-Fe-O-B-C systems;

• Khlopin Radium Institute (KRI): Water leaching tests of radionuclides from matrices of aged Chernobyl “lava” and corium (continuation);

• Tokyo Institute of Technology (TITECH): Application of chemical equilibrium techniques for the determination of thermodynamics of Cs and Si compounds (continuation);

• Joint Institute for High Temperature (JIHT): Vapour pressure data for Fe-Zr-O and Zr-O at high temperatures and mass spectrometry data in the vicinity of the melting temperature;

• IM2NP: Determination of cesium polymolybdates formation enthalpies by solution calorimetry.

Participating countries: Czech Republic, European Commission, France, Germany, Japan, Korea, Netherlands, Russian Federation, Sweden, Switzerland and United States.

Project period: June 2017 – June 2020

The distribution of this package is restricted and subject to prior approval.

For more detailed information visit https://www.oecd-nea.org/science/tcoff/

FSUE “Alexandrov NITI”, Sosnovy Bor LR, Russia

Delft University of Technology, The Netherlands

Saint Petersburg State University, Russia

V.G. Khlopin Radium Institute (KRI), Saint-Petersburg, Russia

Tokyo Institute of Technology (TITECH), Japan

Joint Institute for High Temperature (JIHT), Moscow, Russia

IM2NP (Institut Matériaux Microélectronique Nanosciences Provence), Aix Marseille Université, France

Secretariat :

Davide Costa

Division of Nuclear Science and Education of the OECD Nuclear Energy Agency (NEA)

46, quai Alphonse Le Gallo

92100 Boulogne-Billancourt

France