Executive summary 

The F-BRIDGE project, whose acronym stands for “Basic Research in support of Innovative Fuels Design for the GEn IV systems”, took place over the period 2008-2012. It sought to bridge the gap between basic research and technological applications for nuclear fuel systems. One of the challenges for the next generation of reactors is to significantly increase the efficiency in designing innovative fuels. The main objective of the F-BRIDGE project was therefore to complement the classical empirical approach by a more physically-based description of fuel and cladding materials which is essential to reach a better knowledge and by doing so to enable a better prediction, leading itself to a rationalization of the design process and a better selection of promising fuel systems. F-BRIDGE relied on the complementary expertise of 19 partners: nuclear and non-nuclear research organisations, universities, a nuclear engineering company, as well as technology and project management consultancy small and medium enterprises. F-BRIDGE produced extensive and consistent scientific and technical results.

One of the main objectives of F-BRIDGE was to perform basic research investigations on nuclear ceramic fuels and their cladding. It took advantage of the use of innovative experimental and modelling techniques in a consistent multiscale approach from the atomic to the macroscopic scale and made significant breakthroughs in the detailed knowledge of ceramic fuels and cladding behaviour for the performance and safety optimization. The project contributed to produce key basic data and identify relevant mechanisms that are to be used for a better prediction of fuel behaviour. Knowledge on the atomic transport properties of oxygen, fission products and helium in fuels, as well as of silver in SiC cladding, was considerably improved, and data were either updated or acquired through experimental characterisation studies. Micro-structural changes induced by gas bubble formation and growth, as well as damage produced by irradiation and their effect on transport properties were also investigated in depth. Regarding the atomic scale modelling of transport properties and micro-structural changes in carbides and nitrides, F-BRIDGE studies improved the description of the elementary mechanisms involved in the behaviour of these materials under irradiation and contributed to generate basic data. Finally, innovative experiments and advanced modelling using the CALPHAD method largely contributed to improve the knowledge on the thermodynamical stability of various fuel systems and their chemical interaction with their environment.

A significant contribution of F-BRIDGE was the integration effort between the various actors involved which improved the links between basic research and applied issues, scientist and end-users, investigations at various scales from atomic to macroscopic, modelling and experiments, experts and students. Data request lists gathering key technological issues, pending scientific questions and corresponding basic research investigations were built from the fruitful interaction between F-BRIDGE participants and industry representative members of the user group, showing the contribution basic research can make to solve key technological issues. Significant progress was made in the validation of atomic scale modelling methods and approximations used on fuel materials, which has contributed to improve their reliability. In addition, a special effort has been put to improve the link between scales. The mesoscopic scale was identified as a key scale since it bridges data and mechanisms obtained at lower scales to the fuel performance codes at macroscopic scale. The multiscale modelling exercise on transport properties in uranium dioxide, which demonstrated that electronic structure and empirical potential calculations can be used to obtain precise data to feed higher scale models and help interpret experiments on fuels, is one of the major successes of F-BRIDGE. The project also highlighted the importance of coupling experiments and modelling in an efficient way. Complementary to integral irradiations, separate effect studies performed in well controlled and various irradiation conditions, out of as well as in-pile, were identified as key experiments to contribute to a better understanding of phenomena occurring under irradiation and to produce data for the modelling validation. F-BRIDGE demonstrated a first success in updating existing fuel performance codes by introducing into them advanced material properties and models obtained from basic research and the multiscale modelling approach.

Applied research was moreover performed to assess and improve ‘sphere-pac’ fuel, a composite-ceramics concept which had shown promise. F-BRIDGE project has shown the interplay between modelling and technology, drawing on fundamental physics and chemical approaches and coupling them to detailed economic impact analysis to demonstrate the feasibility of an advanced fuel concept such a sphere-pac. Quality assurance has also been shown as a key component ensuring the reliability of reactor fuels. The F-BRIDGE project connected advanced experimental characterization methods operating at various size ranges (nm to mm), proving beyond doubt that a basic scientific approach is a key element to underpin quality of nuclear fuels. F- BRIDGE finally enabled the establishment of a joining technology for an advanced cladding material, with potential gains in the quest for a robust accident tolerant nuclear fuel, by establishing fundamental thermochemical properties of a soldering material to join SiC components.

F-BRIDGE has had a high level of knowledge dissemination and valorisation as demonstrated by the numerous peer-reviewed articles – approximately 80- and the comprehensive deliverables published during the project. F-BRIDGE was also very active in the international community by participating in the most important international conferences in the field (NuMat, MRS, MMSNF…) as well as in important instances as the OECD/NEA working party on multiscale modelling of nuclear fuels and materials. It had also close relationships with other European projects (CP-ESFR, PERFORM 60, GETMAT, ACTINET I3,…). A proof of the high quality of the work performed in F-BRIDGE and attractiveness to young scientists is the large number of PhD students and post doctorate associates – 34 – involved in the project. Regarding education and training activities, two successful schools were organized during the project and contributed to strengthen the links between senior scientists and students: the first one was dedicated to “Ceramic nuclear fuel and cladding materials” and the second one to “the synergy between modelling and experiments for the investigation of nuclear fuels and materials under irradiation”. Finally, a particular success of F-BRIDGE is the fruitful interaction with the international scientific advisory committee members which highlighted the quality of the work performed in the project and made constructive recommendations.

All the F-BRIDGE partners contributed effectively to lay the stones joining both end of a bridge between basic research and applied issues, improving knowledge and enabling a better prediction of fuel behaviour and selection of promising fuel systems. Even if a challenge for the future is the frequent use of such a bridge, its construction represents the exceptional success of the F-BRIDGE European project.