Materials for Sustainable Nuclear Energy - The Strategic Research Agenda (SRA) of the Joint Programme on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA)

This Strategic Research Agenda (SRA) has been prepared by the EERA-JPNM, based on a wide consultation with the scientific and industrial community involved, to identify the research lines to be pursued in the EU to ensure that suitable structural and fuel materials are available for the design, licensing, construction and safe long-term operation of GenIV nuclear systems. Three Grand Challenges have been identified, namely: (i) Elaborate design correlations, assessment and test procedures for the structural and fuel materials that have been selected for the demonstrators under the service conditions expected; (ii) Develop physical models coupled to advanced microstructural characterization to achieve high-level understanding and predictive capability; (iii) Develop innovative materials solutions and fabrication processes of industrial application to achieve superior materials properties, to increase safety and improve efficiency and economy. For structural materials, the requirement of 60 years design lifetime for non-replaceable components is in perspective the most demanding requirement, which includes under its umbrella several issues related with the reasonable prediction of long-term degradation processes: high temperature processes (creep, fatigue, thermal ageing), compatibility with –especially- heavy liquid metal and helium coolants, and effects of low flux prolonged irradiation, with emphasis on welded components in all cases. In terms of testing, there is a need for standardization, especially for sub-size and miniature specimens. The modelling, supported by microstructural characterization, has as its main objective the development of suitable microstructure evolution models to be used as input to models for the mechanical behaviour under irradiation and at high temperature, eventually linking with fracture mechanics. Specific developments are required for coolant compatibility models, as well as for models in support of the use of charged particle irradiation for the screening of new materials solutions, such as those listed above. Concerning fuel materials, the properties and processes that govern its behaviour in pile, on which research effort is focused, are: margin to melting (establishment of phase diagrams and evolution of thermal properties), atomic transport properties and ensuing microstructural evolution, fission product (non-gaseous) and helium (gas) behaviour and transport, mechanical properties (their evolution, subsequent fragmentation and cracking, fuel-cladding mechanical interaction), and compatibility with cladding and coolant (internal cladding corrosion, chemical interactions especially in case of severe accident). These are all addressed from both an experimental and a modelling perspective. Besides the obvious need of adequate financial resources in order to address the research problems outlined in this SRA, as well as the necessary corollaries, four recommendations emerge that this document is intended to bring to the attention of stake-holders, particularly decision-makers: R1: Data from materials property measurements after exposure to relevant conditions are the essential ingredient for robust design curves and rules. Plenty of data were produced in the past that are now de facto unusable; this is either because they are covered by confidentiality or because they were not properly archived. Correct data management to guarantee availability for future re-assessment is therefore essential and should be encouraged and fostered. In particular, financially supported policies to foster data sharing and encourage old data disclosure should be implemented. R2: Some infrastructures are absolutely essential to enable the correct qualification of nuclear materials, not only irradiation facilities, but also suitable ‘hot’ cells where active materials can be safely handled and tested, nuclearized characterization techniques, loops and pools for compatibility experiments, etc. They are also crucial for education and training of young researchers and operators. These infrastructures are costly to build and maintain. Other research facilities are, on the other hand, more common and sometimes redundant. A rational and harmonised, pan-European management of infrastructures, based on joint programming, including trans-national infrastructure renewal planning and a scheme for facility sharing and exploitation, would be highly desirable and, at the end of the day, beneficial for all. R3: International cooperation with non-EU countries where research on nuclear materials is pursued can be very valuable for Europe. Quite clearly, the goals of this cooperation are in the end the same as in the case of internal European cooperation, namely coordination of activities, sharing of data, and access to infrastructures. Currently, however, the instruments available in Europe for international cooperation are not sufficiently attractive to motivate significant cooperation with non-EU researchers. Efforts should be made to improve their attractiveness and ease of access. International organisations such as OECD.NEA, IAEA, but also Euratom and JRC for the connection with GIF, have here a crucial role. R4: The nuclear materials research community in Europe is currently strongly integrated and engaged in thriving collaboration, in a bottom-up sense. This is in contrast with the inadequacy of the top-down instruments offered to make this integration efficient and functional. This SRA is largely the result of matching bottom-up research proposals with top-down strategies. The appropriate instrument to allow this community to deliver according to the SRA goals should provide the conditions to implement the agreed research agenda and to set up suitable E&T&M schemes that allow knowledge, data, and facility sharing. Since the financial support of Euratom will never be sufficient, earmarked funding from the MS dedicated to support integrated research on nuclear materials is crucial. In this sense, a co-fund instrument, such as a European Joint Programme, seems to be most suitable.JRC.G.I.4-Nuclear Reactor Safety and Emergency Preparednes

Thermal ratcheting of a P91 steel cylinder under an axial moving temperature distribution

International audienceThe progressive inelastic deformation, commonly referred as ratchetting, is a major concern in the design of components and structures submitted to mechanical and thermal cyclic loads in the plastic range. In the RCC-MRx code, the design assessment against ratcheting is performed by applying a simplified method based on the elastic analysis of the structure. The design rule uses a diagram, called efficiency diagram, elaborated essentially on the basis of the results of tension/torsion experiments.In this work the application of the RCC-MRx efficiency diagram to P91 steel has been investigated. It has been considered the thermal ratcheting of a cylinder subjected to a moving axial temperature gradient with no primary stresses applied. The thermal loads have been produced via hot liquid thermal shocks, by dipping a cylindrical mock-up in a molten eutectic mixture of sodium and potassium nitrates. The experimental results show that the present efficiency diagram in RCC-MRx is not suited for P91 steel. Its use for the design would foresee cumulative strains lower than those observed experimentally which is clearly not conservative. This result confirms the results obtained in previous tension-torsion ratcheting tests and provides additional data for the development of a modified efficiency diagram suited for 9Cr steel