Behavior of W-based materials in hot helium gas

Abstract

Materials for the plasma facing components of future fusion reactors will be subjected to complex loading and various forms of interaction with low Z species (hydrogen isotopes and helium). The divertor components will be among the most intensely loaded, as they will have to transfer heat loads up to 10–20 MW/m2. Besides the plasma facing surface being irradiated by highly energetic deuterium, tritium and helium particles from the burning plasma, the opposite surface will be exposed to a cooling medium at elevated temperature. Helium- and water-based cooling systems are currently being considered. While tungsten is the prime candidate material for the plasma facing components, in the helium-cooled divertor designs, it is also foreseen as a structural material, together with ferritic–martensitic steels. The behavior of these materials in He atmosphere at elevated temperatures has been little studied thus far, and therefore is the subject of the current work. A number of W-based materials (pure tungsten and some of its alloys) prepared by powder metallurgy techniques was exposed to He atmosphere at 720 C and 500 kPa for 500 h. Morphological surface changes were observed by SEM, chemical and phase composition was analyzed by EDS and XRD, respectively. The internal microstructure was observed by a combination of SEM, FIB and TEM techniques. Mechanical properties were determined by instrumented indentation. Some alloys developed a thin oxide layer, in some cases new morphological features were observed, while some samples remained mostly intact. The observed changes are correlated with specific compositions and microstructures.Materials for the plasma facing components of future fusion reactors will be subjected to complex loading and various forms of interaction with low Z species (hydrogen isotopes and helium). The divertor components will be among the most intensely loaded, as they will have to transfer heat loads up to 10–20 MW/m2. Besides the plasma facing surface being irradiated by highly energetic deuterium, tritium and helium particles from the burning plasma, the opposite surface will be exposed to a cooling medium at elevated temperature. Helium- and water-based cooling systems are currently being considered. While tungsten is the prime candidate material for the plasma facing components, in the helium-cooled divertor designs, it is also foreseen as a structural material, together with ferritic–martensitic steels. The behavior of these materials in He atmosphere at elevated temperatures has been little studied thus far, and therefore is the subject of the current work. A number of W-based materials (pure tungsten and some of its alloys) prepared by powder metallurgy techniques was exposed to He atmosphere at 720 C and 500 kPa for 500 h. Morphological surface changes were observed by SEM, chemical and phase composition was analyzed by EDS and XRD, respectively. The internal microstructure was observed by a combination of SEM, FIB and TEM techniques. Mechanical properties were determined by instrumented indentation. Some alloys developed a thin oxide layer, in some cases new morphological features were observed, while some samples remained mostly intact. The observed changes are correlated with specific compositions and microstructures.