F-BRIDGE Project

The sustainability and safety demands for nuclear energy have led the Generation IV International Forum (GIF) to define requirements for the next generation of nuclear power plants. Amongst many other aspects which need to be addressed, significant innovation is necessary to design fuel-cladding systems that contribute to meeting these requirements. The support is allocated to these efforts throughout the world (Asia, United States…), and the European Union needs to keep a leading position in this field of major importance for its energy policy by contributing to select, define and manufacture fuel systems for the next generation of nuclear power.

Up to now fuel development and qualification has been a long and expensive process essentially based on an empirical approach. European experts currently have an adequate knowledge of conventional fuel manufacturing and its behaviour under operating conditions encountered over the 50 year period of industrial application and R&D activities. This knowledge is embedded in current design and fuel performance modelling codes and has yielded promising concepts. In particular, composite ceramic fuels and among them “sphere-pac fuels” obtained by sol-gel fabrication and vibrational compaction techniques have been developed. Further investigation of these fuel concepts has been proposed because they exhibit significant advantages for Generation IV prerequisites such as actinide recycling and high burn-ups.

To significantly improve the efficiency of present fuels and design innovative fuel systems, however, the empirical approach has reached its limit. In many respects the understanding of fuels remains empirical, and cannot be easily extrapolated to new materials, new environments, or new operating conditions because the basic underlying mechanisms governing manufacturing, behaviour and performance remain largely poorly understood. One of the challenges for the next years is to significantly increase the efficiency in designing innovative fuels to both improve present fuel systems and design tomorrow’s ones by a transition from an empirical approach to a sound physical description of fuel and cladding materials. The project proposed intends to develop a new approach to fuel development based on a fundamental understanding of fuel behaviour from the atomic to the macroscopic scale and to apply it to fuel design and in particular to the improvement of the sphere-pac concept. This approach will enable a rationalization of the design process, a better selection of promising fuel systems, and will therefore reduce significantly the time and costs currently required for developing new fuels, as well as contribute to improving safety features of new systems under all operational and accidental conditions.

Such a breakthrough in fuel design can be achieved by using basic research investigations which enable the generation of missing basic data, the identification of relevant mechanisms and the development of appropriate models. Moreover, basic research brings further insight into the knowledge of the physical, chemical, and mechanical behaviour of fuel materials under extreme conditions of temperature and radiation, basic phenomena involved being de-correlated and studied at a relevant level of detail. In particular, a multi-scale approach in both experimentation and modelling is needed to reach a correct description of the phenomena involved. This is now possible because atomic scale experimental characterization and modelling techniques have reached sufficient maturity to do so. There is, however, still a strong effort to be done to adapt these modelling and experimental tools and develop new methodologies best suited to fuel materials study. Furthermore, the key to successful multi-scale modelling is the effective translation and transfer of quantitative and qualitative information from one scale to another. Bridging the gaps in the multi-scale chain is necessary to improve the understanding and is expected to provide a solid basis for fuel performance codes to both enhance the sustainability of current fuel systems, and effectively develop innovative fuel systems within the Generation IV framework.

To be really effective and tackle the most important issues relative to the various advanced nuclear systems considered (Light Water Reactors, Sodium Fast Reactors, Gas Fast Reactors, High Temperature Reactors…), basic research investigations must also be strongly connected to their clients i.e. fuel designers and manufacturers. The transfer between technological issues and basic research can be effectively achieved by bringing together within the same project materials scientists and engineers that have a detailed knowledge of critical issues. These experts working together will ensure the integration of basic research results, leading to a direct impact and feedback on innovative fuel design, manufacturing, in pile behaviour prediction, and the optimisation of irradiation experiments. The translation of the technological issues into basic research items and of basic research results into useable qualitative and quantitative information is another key aspect. This requires an effort from all participants to look beyond the scope of their line of work. This will be addressed by internal educational programs and open inter-disciplinary discussions, ensuring mutual understanding and establishing a common vocabulary on the approach and issues the different disciplines are facing. Such an integration effort will, without any doubt, build a bridge from basic research to technological applications for Generation IV fuel systems and in particular “sphere-pac” fuels as presented in figure a.

Figure a: F-BRIDGE main objective: build a bridge from basic research to technological application to Generation IV fuel systems and in particular “sphere-pac” fuels

The F-BRIDGE project, Basic Research for Innovative Fuels Design for GEN IV systems, was proposed on this basis with the following objectives:

Objective 1: obtain data, mechanisms and models from basic research for an improved description of fuel and ceramic cladding under irradiation

The behaviour and performance of an as-fabricated fuel material under given operating conditions results from its chemical composition, its crystalline structure and its microstructure determined by the manufacturing process. During in-pile operation, these three aspects are strongly modified by high temperature, high temperature gradients, strong neutron irradiation, transmutations and formation of fission products: nuclear interactions with neutrons and fission products induce atomic displacements and disturb the crystalline structure; electronic excitations disturb the chemical bonds; transport phenomena assisted by temperature and irradiation modify the microstructure; fission and transmutation reactions alter the chemical composition. The fuel becomes a very complex material, and the challenge is to control and optimise the behaviour of this transformed material during its reactor life. Ceramic cladding is also affected by such transformations. Even if there is no drastic chemical change due to fission product formation inside the material itself, ceramic claddings must keep their physical and mechanical properties despite neutron irradiation, fission product implantation at the fuel-cladding interface and the diffusion of specific fission products which may change the material behaviour. The chemical interaction between fuel and cladding has also to be controlled adequately under normal operating conditions.

The figure b below shows a schematic picture of the main fuel properties that are affected under irradiation:

Figure b: Fuel properties affected under irradiation

Critical milestones of this part of the project include the supply of samples for the experiments, which implies uranium and/or plutonium carbide manufacturing, as well as the development of an effective capacity of characterization using various experimental techniques. In the same way, the advances and progress of the modelling tools constitute important milestones.

A success indicator will be the ability to transfer the experimental and modelling methodologies developed on uranium dioxide to innovative fuel, for instance uranium carbide. A second one will be to provide, by the end of the project, missing thermodynamics data and phase diagrams relating to oxide and carbide fuels containing minor actinides. A third indicator will be the ability to increment during the project a list of data, mechanisms and models yielded by the basic research which can be transferred to fuel performance codes and fuel design.

Objective 2: ensure transfer and integration between technological issues of Generation IV systems and basic research

A success indicator for this objective will be the ability to involve external experts from US-DOE, Japan, Russia, IAEA or OECD-NEA in the F-BRIDGE scientific advisory committee. Another one would be the success in connecting the multi-scale modelling exercise on uranium dioxide to the “work party” on “Multi-scale Modelling of Fuels and Structural Materials for Nuclear Systems” proposed within the framework of OCDE-NEA.

The degree of integration is a key performance indicator to the entire project. Continuous and critical monitoring should evaluate on the one hand whether the end-user issues are successfully apprehended and transferred to basic research activities and whether these activities are subsequently aimed at solving these issues, and on the other hand how much of the basic research results are transferred and translated back to the users, and whether they successfully contribute to effective fuel design and evaluation. Close monitoring will also enable integration obstacles to be identified at an early stage, such that solutions can be found quickly to pave the way continuously for the multi-scale approach to fuel development as envisaged at the beginning of the project.

Objective 3: assess the technological implications (benefits and drawbacks) of sphere-pac fuels for the GEN IV fuel systems

In interaction with the integration effort, provide an assessment of the drawbacks and benefits of the sphere-pac fuel application to various Generation IV systems. The success indicator will be the ability to assess the potential and feasibility of (thermally bonded) sphere-pac fuels to Generation IV systems.

Objective 4: ensure result dissemination, education and training

F-BRIDGE also aims at organizing education and training activities. Firstly, workshops will be organized for scientists, design engineers and end-users involved in the project to ensure the exchange of results and ideas among the participants in the project. Then, one or two summer schools will be proposed to young scientists to promote research in the field of fuel materials, thus preparing to meet tomorrow’s challenges. Universities as well as small companies that have experience in training activities will be involved side by side with nuclear organisations in this aspect of the project. Moreover, knowledge transmission between F-BRIDGE experts and young scientists will be operational through the training of Ph.D. students and post-doctoral associates involved in the project.

The main success criterion for this objective will be the number of participants to the various training activities. Success will also be measured by the exchanges between all actors of the project, in particular between experienced scientists and younger ones. Ideally, this would involve Ph.D.s or post-docs carrying out their research work at different laboratories involved in the project. These education and training activities will be complemented by knowledge dissemination actions through peer-reviewed publications, taking part in conferences and giving public communications. Collaborations will be sought with existing summer schools on related subjects or existing workshops in the field to avoid any useless duplication. Interaction with the European Nuclear Education Network (ENEN) will also be sought for mutual benefit.

The F-BRIDGE project is structured as a medium scale focused project, gathering the skills of European universities, nuclear organisations and industrials working on fuel behaviour to reach two common objectives: gain further insight into the basic phenomena involved in the behaviour under irradiation and apply it directly to improve fuel design and manufacturing. The outcome of the project will put the European Union in a position to play an active part in proposing innovative fuels within the framework of the Generation IV initiative, strengthening the Euratom position in GIF.