Bernhard Dafferner

Tungsten as a Structural Divertor Material

Refractory materials, in particular tungsten base materials are considered as primary candidates for structural high heat load applications in future nuclear fusion power plants. Promising helium-cooled divertor design outlines make use of their high heat conductivity and strength. The upper operating temperature limit is mainly defined by the onset of recrystallization but also by loss of creep strength. The lower operating temperature range is restricted by the use of steel parts for the in- and outlets as well as for the back-bone. Therefore, the most critical issue of tungsten materials in connection with structural divertor applications is the ductile-to-brittle transition. Another problem consists in the fact that especially refractory alloys show a strong correlation between microstructure and their manufacturing history. Since physical and mechanical properties are influenced by the underlying microstructure, refractory alloys can behave quite different, even if their chemical composition is the same. Therefore, creep and thermal conductivity have been investigated using typical commercial tungsten materials. Moreover, the fracture behavior of different tungsten based semi-finished products was characterized by standard Charpy tests which have been performed up to 1100 °C in vacuum. Due to their fabrication history (powder mixing, pressing, sintering, rolling, forging, or swaging) these materials have specific microstructures which lead different fracture modes. The influence of the microstructure characteristics like grain size, anisotropy, texture, or chemical composition has been studied.

The Impact of Refractory Material Properties on the Helium Cooled Divertor Design

Many divertor design studies for future fusion reactors rely on helium gas cooling. In these concepts, pressurized tubes or channels had to be operated at maximum temperatures between 1000 °C and 1300 °C while the lowest operating temperature is preset by the coolant inlet or by specific start-up and maintenance conditions. At such extreme temperature regimes, the only reduced activation material that would provide enough strength, paired with the necessary heat conductivity, is tungsten. Therefore, various tungsten materials and alloys are often publicized as candidate material for structural divertor applications. However, there are also clear limitations. Therefore, an intensive study on the influence of microstructure and chemical composition on the fracture behavior of industrially produced tungsten materials has been perfomed. This paper reviews the results and some other relevant properties of tungsten materials with respect to possible applications for structural divertor parts. Drawbacks and possible alternatives are discussed.

Creep-Fatigue Interaction in Eurofer 3 Electron Beam Welds

Abstract In future nuclear fusion reactors, structural materials will undergo a large thermal cycling due to pulsed operation and the occurrence of several maintenance periods. Therefore, the investigation of the combined role of creep and fatigue loading is of major importance. In this study, we focused on Eurofer 3 electron beam welds. Two different post-welding heat treatments were carried out: a two-step heat treatment (30 minutes at 980°C followed by 2 hours at 750°C) and a one-step heat treatment (2 hours at 750°C). Fatigue, creep and creep-fatigue tests were performed. A 550°C test temperature was chosen, corresponding to the upper operation temperature currently foreseen for this material. Creep-fatigue experiments were achieved by interrupting a fatigue test and then applying a creep loading until the fracture of the specimen. Several fatigue pre-stress conditions were studied. The post-weld heat treatment influence was analyzed. The damage contributions of fatigue and creep were studied using electron microscopy. The results were compared to previous results obtained on base material.

Assessment of copper based materials for the Water-Cooled Divertor concept of the DEMO European Fusion reactor

High Heat Flux component fabrication for the DEMO European Fusion reactor requires the development of structural materials that exhibit high thermal conductivity, strength and radiation resistance. Depending on the cooling concept used (Water or Helium), several different materials are typical candidates for the divertor structural application, such as Tungsten alloys, Copper alloys, or Reduced Activation Ferritic Martensitic (RAFM) steels. This paper focuses on the use of Copper materials for Water-Cooled Divertor concepts. In this work, several possible solutions based on alternative copper material development were investigated. Reinforcement routes based on alloying, dispersion strengthening and composite material were studied. Material production was performed by several conventional melting processes on a laboratory scale. The produced materials were then characterized and compared using metallography, mechanical and thermal properties.