PhD Studentship: Synergistic Effects in Creep Behaviour of Advanced Reduced-activation Steels for Fusion Breeders
The deployment of future nuclear fusion power plants and related technologies, including the UKAEA programme STEP (Spherical Tokamak for Energy Production) as a demonstrator of net energy generation and plant sustainability, relies on the availability of qualified structural materials in industrial-scale amounts that can withstand the harsh local environments in the vicinity of the hot Deuterium (D)-Tritium (T) fusion plasma. In particular, the tritium breeder module components within the first-wall blanket will be exposed to elevated temperatures, variable thermo-mechanical stresses, and exposure to 14 MeV neutrons, liquid lithium media, and unstable tritium.
Amongst those potential material candidates for first-wall fusion applications, body-centred cubic Ferritic/Martensitic (FM) steels are regarded as frontrunners due to their maturity and potential for industrial scalability, together with their enhanced resistance to radiation-induced void swelling and their mechanical strength. The high-temperature limit for safe operation of FM steels of approx. 550C is governed by thermal creep strain effects and helium embrittlement. There is currently a strong thrive to develop novel Reduced Activation FM steel grades to push that temperature limit to 650C by a complex alloy chemistry and thermo-mechanical treatments, so as to attain significant gains in thermal efficiency and energy output of the fusion power plant in the future.
The thermal creep behaviour of the steel is governed by the interaction of mobile dislocations with second phase particles, such as nano-scale carbides in RAFM steels, and other pre-existing lattice defects such as grain boundaries. However, the exposure of the material to fusion-relevant radiation fluxes at temperatures below the material’s upper limit, causes damage in the microstructure, initially in the form of self-interstitials and vacancies within the displacement cascade, that evolve into larger lattice defects such as dislocation structures, nano-scale voids, bubbles or cavities. The particle bombardment of steels can affect the initial carbide particle distribution and stability.
There is already a body of knowledge and experimental data about the thermal creep behaviour and independently about the radiation damage structures of conventional RAFM steels (such as Eurofer of F82H). However, the potential synergistic effects of radiation damage and thermal creep in RAFM steels, and in general in structural alloys for the nuclear industry, remains unexplored. It is worth emphasizing that creep testing over long periods of time, is a key step to be able to qualify new structural materials for the nuclear sector. It is therefore the aim of this PhD to investigate the creep behaviour of RAFM steels at temperatures close to their upper limit for safe operation, with particular emphasis on simultaneous radiation damage and thermal creep straining interactions, thereupon decoupling thermal creep effects from irradiation creep, and identifying the dominant irradiation creep mechanisms in these steels. This will be done by using medium-energy proton beams to irradiate steel creep specimens through thickness while monitoring their creep strains, coupled with advanced electron microscopy post-mortem to evaluate the microstructure response to the irradiation and the applied mechanical load at elevated temperatures.
A 3.5-year PhD studentship is available in the group of Prof. Enrique Jimenez-Melero within the School of Metallurgy and Materials at the UoB, with a stipend of at least £18,622 per year. This project is funded by the UK Atomic Energy Authority, and is embedded within a multi-partner consortium to deliver a scalable RAFM steel for the tritium mock-up within the UK Fusion Technology development.