Development of cooling strategies and advanced numerical approaches for heat transfer in nuclear fusion reactor components under extreme heat loads

World's energy demand is increasing at a rate of about 2% per annum. 87% of this demand is met by fossil fuels, with production of CO2 and polluting gases. While it is imperative to reduce the dependence on fossil fuels, new, carbon-free energy sources have to be found to tackle the climate change and meet the increasing demand. Nuclear fusion is emission-free, produces no long-lasting radioactive scores, and can generate high power densities. However, while a net energy gain seems not-too-far to be proved, there is a technological need to protect the walls and components from the extreme heat loads generated in a fusion reactor (millions degrees and heat fluxes of order of 20 MW/m2). Conventional water-cooling is very limited under such extreme heat loads and leads to issues like cavitation, local evaporation and strong pressurization needed to prevent these. The development of new technology that allows to extract and redistribute this heat is thus fundamental for the future employment of nuclear fusion in industrial cycles.

In this project the viability of liquid metals will be assessed. Liquid metals have the advantage to have strong heat transfer coefficients and diffusivity, so they can in principle extract more heat and redistribute it faster. Moreover, they do not rely on high pressure to remain in liquid form and their flow can be driven by the magnetic field within the nuclear reactor. Nevertheless, their behaviour under strong heat loads and magnetic field is complex and not well understood, also due to the lack of experiments. Also, higher temperatures are needed to keep the metal in liquid form, which can counteract the effect of the high heat transfer coefficient. A recent preliminary conjugate heat transfer analysis conducted using liquid lithium has revealed that only under extreme heat loads the liquid metal outperforms water for cooling purposes. Evaporation and magnetic effects were not considered. This PhD project will employ high fidelity, large eddy simulation to further analyse the performance of liquid metals under extreme heat loads. The high-fidelity simulation approach will involve conjugate heat transfer to assess the effect of the coolant on the structure in a meaningful way, the modelling of the unsteady effect of the magnetic field on the liquid metal and the localised evaporation (phase change) in regions of heat peaks. These are relatively unexplored phenomena and the investigation will take advantage from experimental campaign to be run at UKAEA in order to obtain data for model validation.