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ANALYSIS AND EFFECTS OF FUSION RELEVANT DAMAGE IN TUNGSTEN
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
Tungsten is a candidate material for plasma facing components in the experimental fusion reactor, ITER and the demonstration fusion reactor, DEMO. It has been chosen due to its high melting point, low sputtering yield and resistance to oxidation. However, there are still some issues that need addressing, such as its response to 14 MeV neutrons and helium production, products of the deuterium-tritium reaction that will be used to harness fusion energy. Additionally, the high temperatures of operation are another factor to consider. There is no dedicated materials testing device that can concurrently recreate all the conditions expected in ITER or DEMO. Previous work has focussed on the use of ion beams and linear plasma devices to try and mimic the damage induced from the neutrons and helium. Following from this work, this thesis has focused on three methods in simulating the neutron and helium damage. The three methods used in this project are; helium plasma exposures at fusion relevant temperatures to tungsten and tungsten-rhenium alloys, where rhenium was used to mimic the transmutation induced by neutrons; heavy ion (tungsten and rhenium ions) irradiations in tungsten at 400 °C; and proton irradiations in tungsten at 400 °C and 800 °C. Analysis of the microstructural and micromechanical properties of the exposed samples has been carried out. The effect of rhenium on the formation of a nanostructure (fuzz) in tungsten induced by helium plasma exposures was studied. Rhenium was shown to generally inhibit fuzz formation. Mechanisms by which inhibition could occur were discussed. The most recent literature had suggested that an incubation fluence was required before fuzz growth could occur, but the research in this thesis pointed more towards an alternative incubation time theory. The simultaneous effects of transmutation and displacement cascade damage induced by neutrons were mimicked via irradiations with rhenium ions into tungsten, which were compared to tungsten ion irradiations. The effect on mechanical properties of tungsten was tested using nanoindentation. Up to levels of 40 dpa and 1600 appm, at temperatures of 400 °C, the difference in hardness increase between the two types of irradiations was negligible. Proton irradiations at 400 °C and 800 °C, up to levels of ~2 dpa in the first 2 µm of the samples were explored. The hardness increase observed at 400 °C was much greater than that seen in the heavy ion irradiations, most likely due to the larger volume of irradiated material and the large Bragg peak in the proton irradiated samples. In both the heavy ion and proton irradiated material, higher hardness increases were observed in annealed material, in comparison to an as-received material. Impurity concentrations were observed to be an important issue when carrying out irradiation experiments, particularly at raised temperatures.