Materials research towards a fusion reactor
Introduction
Energy generating systems based on nuclear fusion have the potential to provide a CO2 emission-free, sustainable, safe and clean energy option for the future. On the path of their realisation the choice of appropriate structural materials and their behaviour under specifically harmful loading conditions is a critical issue for the technical feasibility and a safe and economic operation. Favourable mechanical properties, good corrosion behaviour and compatibility, technical maturity and the potential for low activation are the conditions for a pre-selection. Furthermore, existing knowledge on material behaviour under high fission neutron irradiation helps to exclude those materials which show relevant weakness under neutron irradiation.
The development of appropriate structural materials follows at present two distinct lines: the first one is directed towards the construction of the next step machine, the international thermonuclear experimental reactor, ITER. This facility is—with regard to material issues—characterised by a strongly pulsed mode of operation, a very moderate neutron exposure and low operational temperature. It is expected that these demands can be fulfilled by the use of an austenitic stainless steel of type 316 LN-IG [1], a material, which already had been successfully applied in conventional fission reactor technology.
The second developmental line aims for materials which can withstand high neutron fluence and temperature ranges, typical for commercial fusion reactors, and which in addition have the potential for reduced or even low neutron-induced activation. In the last two decades three major material groups have evolved which eventually can fulfil the requirements as reactor materials. These are ferritic–martensitic steels, vanadium alloys and SiC/SiC ceramic composites. Recent assessments have shown that these alternatives have a different level of maturity and partially exhibit critical issues which have to be investigated with priority [2], [3], [4].
In this paper the status of development is shortly summarised and necessary future steps including the development of appropriate research tools are discussed.
Section snippets
Performance goals for reactor materials
Structural materials will have to satisfy stringent demands for the construction of competitive fusion reactors: For instance they should ideally guarantee a reliable and safe operation over a plant lifetime in the range of 30 full power years (FPys) with a high net plant efficiency in the order of about 35–45%. Such ambitious targets are at present achieved with modern fission reactors (35%) and ‘clean’ coal fired commercial steam power stations (45%) and are the result of a continuous
Materials requirements and nuclear-specific selection criteria
There are many requirements which have to be fulfilled by structural materials. For the design of FW/Breeding Blankets conventional material data like thermophysical and mechanical properties and the compatibility with cooling media and breeding/neutron multiplying materials are needed. They determine the appropriate window of application in terms of temperature and acceptable stress-levels for the envisaged lifetime. Such design exercises, which are driven by the wish to get an optimum
Short status of material development, critical issues and prioritary R&D-investigations
As mentioned above, advanced ferritic-martensitic steels, vanadium alloys and SiC-fibre reinforced SiC composites have been selected as major materials groups. Their selection is mainly based on favourable conventional properties and/or technical maturity, the potential for low activation and /or promising results under fission neutron irradiation. Opportunities for the consideration of alternatives could arise as a result of major advances of other materials groups, e.g. through exploratory
The need for an intense neutron source
The critical issue for all materials under investigation is their behaviour under fusion-specific conditions, i.e. under high-energetic neutrons with a peak intensity at 14 MeV. Since no appropriate 14 MeV neutron source presently exists, material performance is mainly studied in irradiation facilities like fission reactors and ion accelerators, where fusion conditions can partially be simulated by specific tricks. It is common understanding in the Materials Community that such experiments
Strategy for material development towards a fusion reactor
Recently the elements of a common strategy for materials development towards a fusion reactor have been discussed by an IEA-Workshop on Strategy and Planning of Fusion Materials R&D. They have been summarised and further developed in a strategy paper presented at the 9th International Conference on Fusion Reactor Materials in Colorado Springs, CO [20]. In those countries (Europe, Japan and Russia), where a mission-oriented and hence time-driven strategy exists to develop and achieve a
Summary and conclusions
The selection of structural materials for future fusion reactors is based on conventional data, nuclear specific primary damage parameters and experience in nuclear applications. It is also dependent on the combination with breeding-, coolant- and neutron multiplying materials in specific components like breeding blankets. Expected performance goal for the DEMO Breeding Blanket development in the EU is 70 dpa and a realistic target for prototypic materials lies in the range of 150 dpa.
The
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