PhD Studentship: Lithium Corrosion Behaviour of RAFM Steels for Tritium Breeder Blankets in Fusion Energy Production

The production of vast amounts of nuclear fusion energy in tokamak and compact spherical reactor designs, as a replica of nuclear reactions in the sun, rely on the deuterium (D)-tritium (T) reaction and the hot D-T plasma confinement using intense superconducting magnets. Tritium is an unstable hydrogen isotope which is very scarce in nature. Therefore, for the sustainability of the fuel cycle in the fusion power plant, tritium is planned to be generated in the tritium blanket module (TBM) close to the plasma fuel, by the reaction of high-energy neutrons from the D-T reaction in the plasma with Li-6 present in the TBM.

Amongst those candidate materials for structural TBM components, Reduced Activation Ferritic Martensitic (RAFM) steels stand out as frontrunner candidates, due to their resistance to radiation-induced void swelling, reduced radiation activity, high-temperature mechanical strength, together with the relatively high thermal conductivity and low thermal expansion coefficient. TBM components are expected to operate in normal conditions in the temperature window of approx. 300-650 C, together with experiencing structural damage from the incoming fusion neutrons. Unfortunately, the upper temperature limit of operation of traditional RAFM steels lies at approx. 550 C.

There is currently an intense effort in developing advanced RAFM steels with adequate scalability to operate safely up to 650 C. Steel development strategies focus on alloy chemistry designs and targeted thermo-mechanical treatments to achieve a fine martensitic microstructure, and even more importantly, to tailor the chemistry, size and spatial distribution of thermally stable carbide nanoparticles. These TBM steel components will additionally experience the environmental degradation caused by their direct contact with the tritium breeder medium containing lithium, either solid or particularly in liquid form.

There are studies in the literature about the corrosion behaviour of conventional RAFM steels in contact with lithium-containing liquid breeders. The corrosion rate, surface degradation and damage localization over time depend on the steel microstructure, i.e. chemistry and location of fine carbides and type and density of grain boundaries, and also on the chemistry and impurity level of the corrosive media. It is also reported that the exposure to radiation fluxes at those elevated temperatures causes the destabilization of the carbide distribution in conventional RAFM steels. However, the impact on irradiation on the microstructure of novel RAFM steels, and even more importantly, the potential synergistic effects of irradiation and corrosion on those novel steel grades and microstructures, remain unexplored.

It is therefore the aim of this project to study the impact of irradiation and contact with liquid tritium breeders on the microstructure stability of novel RAFM steels, and ultimately their potential interactions and synergistic effects. This will be done by using intense beams of protons coming from our lab-based cyclotron facility, as surrogate of neutron damage, and by exposing the steels to potential breeder media containing lithium such as liquid lithium or lead-lithium, supported by advanced microstructural characterization using analytical electron microscopy. We ultimately aim to down select the best RAFM steels and processing conditions for a suitable performance in a tritium breeder module mock-up, as stepping stone for the future deployment of commercial fusion energy power plants.

The project will be supervised by Professor Enrique Jimenez-Melero (e.jimenez-melero@bham.ac.uk).

This project is funded by the UK Atomic Energy Authority. A three-year PhD studentship includes a stipend of at least £18,622 per year.

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