PhD Studentship: Irradiation Performance of Electron Beam Steel Welds for Fusion Breeder Technologies

Structural steels based on body-centred cubic structures are a preferential option for first wall and breeder blanket components in magnetically-confined fusion tokamak designs, due to their enhanced resistance to radiation-induced void swelling, as compared to face-centred cubic materials, and to their high-temperature mechanical strength. In particular, the aim of the tritium breeder module is to generate sufficient tritium from Li-6 and the 14 MeV neutrons generated in the deuterium (D)-tritium (T) reaction in the plasma fuel of the reactor, to sustain the D-T reaction itself and the reactor operation over its service life.

Ahead of those potential fusion-grade steels are the Reduced Activation Ferritic/Martensitic (RAFM) steels (e.g. EUROFER97 RAFM steel). Traditional RAFM steels are able to operate with sufficient mechanical integrity up to temperatures of approx. 550 C. There is currently an intense effort in the UK, and also at international level, to develop advanced RAFM steels with adequate scalability to operate safely up to 650 C. This is to be achieved by alloy chemistry design and targeted thermo-mechanical treatments to achieve a fine martensitic microstructure, and even more importantly, by tailoring the chemistry, size and spatial distribution of nano-scale carbide particles.

The production of a tritium breeder mock-up, and future tritium breeder modules, require to join RAFM steel plates and components. Conventional fusion welding techniques melt a relatively large volume of the material. The melting and re-solidification process would result in a significantly sized region with a degraded particle dispersion, and consequently that region would act as a weak point for the mechanical integrity of the breeder component. Alternatively, Electron Beam (EB) welding is a radiant-energy joining process that uses a highly focused electron beam to volatise a very small region of material, opening the door to joining thick sections of RAFM steel in one single pass, with relatively short solidification times and narrow fusion zones (of the order of approx. 1 mm depending on the electron beam energy).

Even more importantly, EB welding offers the potential of reduced disruption of the particle distribution, and if needed, enhance the local weld microstructure by a post-weld heat treatment, since EB welding does not use a filler material. However, there is limited knowledge and experimental data in the literature about the impact of the EB welding process on the local RAFM steel microstructure and mechanical performance, esp. in the irradiated condition. The aim of this PhD project is therefore to assess the changes in local microstructure and particle distribution due to different EBW process and irradiation parameters, and ultimately decide if those RAFM steel welds are fit-for-purpose to act as structural materials in a tritium breeder blanket module. This will be done by producing EB welds of conventional RAFM steels (e.g. Eurofer, F82H), and characterise the microstructure in the base material and fusion zone using analytical electron microscopy. We will then be using medium-energy proton beams to irradiate EB welds through thickness, and assess the potential changes in particle distribution and in mechanical performance with respect to the base material. Based on the irradiation and mechanical tests, we can explore potential post-weld treatments to improve the weld stability under irradiation.

A 3.5-year PhD studentship is available in the group of Prof. Enrique Jimenez-Melero within the School of Metallurgy and Materials at UoB, with a stipend of at least £18,622 per year.

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