Review on the EFDA Programme on Divertor Materials Technology and Science
M. Rieth1 and J. L. Boutard2
1Forschungszentrum Karlsruhe, Institut
für Materialforschung I,
2EFDA-Close Support Unit,
All the recent DEMO design studies for helium cooled divertors utilise tungsten materials and alloys, mainly due to their high temperature strength, good thermal conductivity, and comparably low activation under neutron irradiation. In principle, the most delicate design components are: (1) plasma facing shields, (2) high temperature cooling structures, and (3) high temperature backbone/support structures. For the shields, tungsten is used as armour material (also due to its high sputter resistance) to protect the subjacent cooling structure. Depending on the plasma operation conditions in tokamaks the shielding surface temperatures might, on the one hand, well exceed 1800 °C, and on the other hand, drop down to the maximum operation temperature of the cooling structure, which is between about 1200 and 1300 °C. The most important, and necessary, properties of the armour materials are therefore related to crack resistance and resistance to irradiation damage in general. Obviously, the backbone/support structures should be made of steel which restricts the operation temperature to about 600 °C for conventional steels and to 700 °C or possibly above for ODS steels (those still have to be developed). Therefore, the cooling structures are expected to work within the temperature interval of 600-1300 °C. In that temperature range the most critical requirements for possible structural tungsten materials are ductility, the creep strength, and resistance to recrystallization.
The long-term objective of the EFDA materials programme is to develop structural as well as armour materials in combination with the necessary production and fabrication technologies for future divertor concepts. The programmatic roadmap is structured into four engineering research parts which are (1) fabrication process development, (2) structural material development, (3) armour material optimisation, and (4) irradiation performance testing, which are complemented by a fundamental research programme on "Materials Science and Modelling".
This paper presents in the current research status of the EFDA experimental and testing programme for divertor applications, and gives a detailed overview on the latest results on fabrication, joining, gradient transition, high heat flux testing, plasticity, fracture mechanics, modelling, and validation experiments. Furthermore, the possible impact of innovative development of materials on the current design studies is discussed.
Dr. Michael Rieth
Forschungszentrum Karlsruhe, IMF I
Tel: +49 7247 82 2909, Fax: +49 7247 82 4567