Performance of tungsten based materials and components under ITER and DEMO relevant steady-state thermal loads

摘要

In nuclear fusion devices the surfaces directly facing the plasma are irradiated with high energy fluxes. The most intense loads are deposited on the divertor located at the bottom of the plasma chamber, which has to withstand continuous heat loads with a power density of several MW/m² as well as transient events. These are much shorter (in the millisecond and sub-millisecond regime) but deposit a higher power densities of a few GW/m². The search for materials that can survive to those severe loading conditions led to the choice of tungsten which possesses advantageous attributes such as a high melting point, high thermal conductivity, low thermal expansion and an acceptable activation rate. These properties made it an attractive and promising candidate as armor material for divertors of future fusion devices such as ITER and DEMO. For the DEMO divertor, conceptual studies on helium-cooled tungsten plasma-facing components were performed. The concept was realized and tested under DEMO specific cyclic thermal loads. The examination of the plasma-facing components by microstructural analyses before and after thermal loading enabled to determine the mechanisms for components failure. Among others, it clearly showed the impact of the tungsten grade and the thermal stress induced crack formation on the performance of the armor material and in general of the plasma-facing component under high heat loads. A tungsten qualification program was launched to study the behaviour of various tungsten grades, in particular the crack formation, under fusion relevant steady-state thermal loads. In total, seven commercially available materials from two industrial suppliers were investigated. As the material's thermal response is strongly related to its microstructure, this program comprised different material geometries and manufacturing technologies. It also included the utilization of an actively cooled specimen holder which has been designed to perform sophisticated material tests at different surface temperatures. The steady-state thermal loading with superimposed transient thermal loading was induced by high frequency scanning of the electron beam. The steady-state thermal loading was performed with different power densities, surface temperatures and cycle numbers. The cracking threshold was investigated in a temperature range of 1000 to 1900°C. Once cracks occurred, the surface temperature had no impact on the crack network of the loaded surface. The cracks grew in depth with increasing the cycle number. However, under all loading conditions, crack depths were still limited in a shallow region, namely below 100 µm. One disadvantage of tungsten is its high brittleness at room temperature which makes the manufacturing of tungsten parts challenging as it requires suitable machining techniques. The examination of the helium-cooled tungsten plasma-facing components revealed cracks in as-machined surfaces. For a better understanding of the performance of plasma-facing components it was necessary to estimate the impact of pre-cracked surfaces on the components' degradation under high heat fluxes. Therefore, in the frame of the tungsten qualification program, specimens with defect-free and pre-cracked surfaces were exposed to high heat fluxes. Surface processing by electric discharge machining (EDM) led to pre-cracked surfaces and defect-free surfaces were achieved by polishing. EDM-pre-cracking resulted in a high crack density consisting of inter- and intra-granular cracks, which did not change after thermal loading. Even more, the cracks did not grow with the cycle number in contrast to thermo-mechanical induced cracks on polished surfaces which occurred at lower crack density.