Thermal transient effects in fusion front wall/breeder blanket components

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The high operating temperatures and nuclear radiation within a fusion reactor can significantly change the microstructure of its components, which in turn can change the material properties and mechanical behaviour. The materials chosen for fusion reactors have been specifically engineered to reduce the amount of induced radiation, but it is also important to understand how such materials will behave when exposed to the high temperatures of the fusion plasma. The plasma in a fusion reactor can reach temperatures of millions of degrees, and is kept controlled during operation using focused magnetic fields for containment. However, in the event of a loss of this containment it is possible for the plasma to contact with wall, leading to dramatic increases in temperature over short periods of time. The thermal transients will heat reactor materials above their desired operating conditions for very short spells but many times during operation. This could produce undesired and unpredicted microstructural evolution in components. This project would build on work at Bristol that has shown that even short periods high temperature (~750C) exposure can cause significant microstructural degradation of the Eurofer-97 stainless steel material used for structural support and cooling pipes [1]. This could affect mechanical properties and corrosion resistance, in turn leading to a shortening of component life. This PhD project will study the effect of very short-term thermal excursions on fusion front wall, divertor and breeder blanket materials and assess their effect on the microstructure and corrosion behaviour of the material.The student will build on existing experimental research at Bristol using a custom design vacuum laser exposure rig, that can expose materials to controlled thermal pulses for short (1-10s) thermal spikes using a CO2 laser. The student will adapt this experimental rig to enable faster cooling rates and shorter thermal spikes to more closely simulate the conditions of a plasma excursion. The system will then be used to study the microstructural evolution behaviour of fusion materials after repeated short-term thermal exposure. The project will investigate the thermal evolution and potential impacts from high numbers (reactor relevant) of thermal cycling above the desired operating temperatures on FW materials (e.g. ODS steel) and/or breeder substrate (Eu97) close to FW and interface joint. To getthe appropriate cycling rates (with fast cooling rates), investment in a new rig will likely by required. But such a rig could be used for other fusion relevant projects.Thermally exposed specimens will be characterised using scanning and transmission electron microscopy, x-ray diffraction and tomography to observe the change in the material microstructure with increasing heat exposure, and determine if any embrittlement or change in mechanical properties occur that might cause problems if used in a fusion reactor. The student will also compare experimental results to computational simulations of the phase diagram using CALPHAD-based modelling. There may also be opportunities to combine thermal exposures with the effects of irradiation and stress, and to assess similar conditions in weldments.