Ferritic steels for the first generation of breeder blankets

Materials development in nuclear fusion for in-vessel components, i.e. for breeder blankets and divertors, has a history of more than two decades. It is the specific in-service and loading conditions and the consequentially required properties in combination with safety standards and social-economic demands that create a unique set of specifications. Objectives of Fusion for Energy (F4E) include: 1) To provide Europe's contribution to the ITER international fusion energy project; 2) To implement the Broader Approach agreement between Euratom and Japan; 3) To prepare for the construction and demonstration of fusion reactors (DEMO). Consequently, activities in F4E focus on structural materials for the first generations of breeder blankets, i.e. ITER Test Blanket Modules (TBM) and DEMO, whereas a Fusion Materials Topical Group implemented under EFDA coordinates R and D on physically based modelling of irradiation effects and R and D in the longer term (new and /or higher risk materials). The paper focuses on martensitic-ferritic steels and (i) reviews briefly the challenges and the rationales for the decisions taken in the past, (ii) analyses the status of the main activities of development and qualification, (iii) indicates unresolved issues, and (iv) outlines future strategies and needs and their implications. Due to the exposure to intense high energy neutron flux, the main issue for breeder materials is high radiation resistance. The First Wall of a breeder blanket should survive 3-5 full power years or, respectively in terms of irradiation damage, typically 50-70 dpa for DEMO and double figures for a power plant. Even though the objective is to have the materials and key fabrication technologies needed for DEMO fully developed and qualified within the next two decades, a major part of the task has to be completed much earlier. Tritium breeding test blanket modules will be installed in ITER with the objective to test DEMO relevant technologies in fusion environment and conditions. Materials and materials technologies (fabrication, welding, joining) have to be fully qualified in front of a rigorous licensing process within the next decade. Therefore, materials development for DEMO is based on present technologies and knowledge with some reasonable extrapolation. The 9% Cr steel EUROFER steel is the primary EU candidate structural material. For increased thermal efficiency the temperature window of the structural materials needs to be enlarged. Various ODS (Oxide Dispersion Strengthened) Fe-Cr-steels are candidates for higher temperature application. The large fraction of high energy neutrons in the fusion neutron spectrum results in gaseous transmutations (He and H) that are more than one order of magnitude higher than in fission. Even though fission based material test reactors are the essential and indispensable pillar of the current and future irradiation qualification programme, they can not provide sufficient data for a successful licensing process towards DEMO. For this reason, the construction and use of a facility called IFMIF, designed for simulating as closely a possible the fusion neutron spectrum, is mandatory. Meantime, and complimentary, an enhanced material science programme should increase knowledge and understanding of radiation effects. The focus of this programme for the next decade should be on the development and validation of predictive capabilities for modelling micro-structural evolution and mechanical properties of EUROFER-type steels under fusion reactor relevant conditions, addressing in particular the Helium issue. In a longer term perspective, this should result in the implementation of an integrated approach involving modelling and model-oriented experimental validation into a strategy of accelerated development and testing of candidate fusion materials, material systems and material technologies. (author)