PhD Studentship: Creep Behaviour of High-temperature Materials in Helium Environments of Future Nuclear Fission Plants

Structural materials in nuclear power plants are commonly operating at elevated temperatures in aggressive local environments and complex mechanical stress conditions. The upper temperature limit for the safe operation of a given reactor material or component is dictated primarily by its resistance against thermal creep, i.e. the increasing material’s straining over long periods of time at constant mechanical loads, and potentially by other effects and synergies, such as helium embrittlement or material-coolant interactions. The ability of operating a power plant safely at higher temperatures brings along the attractive benefits of higher coolant outlet temperatures and thermal efficiencies, yielding enhanced plant power and economic outputs. Therefore, a mechanistic understanding of creep deformation in structural materials, and the additional impact of local service environments on the structural integrity of nuclear materials and components, are critical for the design and safe operation of high temperature fission nuclear power plants. Those include the UK’s existing fleet of Advanced Gas-cooled Reactors, and looking forward into new plant designs and builds, the future Advanced Modular Reactor (AMR) and Generation IV (Gen IV) designs which will operate at significantly higher temperatures aiming to co-generate electricity and hydrogen.

At the expected local temperatures within the nuclear plant, creep will be the major deformation process for structural alloys used in critical components in the pressure vessel and coolant loops in those reactors. The UK is in the initial phases to develop the High-Temperature Gas-cooled Reactor (HTGR), a type of AMR, which will operate in the temperature range of approx. 750-950 °C and cooled through helium gas. Those are unprecedented conditions for the current gas-cooled nuclear fission plants in the UK, operating at maximum temperatures of approx. 650 °C and using CO2 instead as coolant. An in-depth understanding of the effect of helium gas environment on microstructural degradation and subsequent creep performance of candidate structural alloys for high temperatures is currently very limited. A significant challenge in deploying HTGR will be the evaluation of the performance of high-temperature structural alloys, which is reliant on empirical testing to establish both the metallurgical and mechanical behaviour including creep properties, both in air and helium environments.

This PhD project aims to accelerate testing activities through the development of a novel creep testing methodology using full-field digital image correlation (DIC) and innovative test specimen geometries, enabling the determination of creep properties from a lesser number of tests than traditionally required. This technique will be applied to investigate the creep deformation of high-temperature Fe-Ni-Cr alloys, that will be exposed to a helium gas environment at different temperatures and test durations.

The project will provide the PhD student with a unique set of skills including mechanical test methodology development and design, programming methods and creep property analyses, as well as advanced microstructure characterisation using electron microscopy. This project is a collaborative initiative between the University of Birmingham and Jacobs, with co-supervision by Jacobs, and provides the student with the unique opportunity to work for defined periods within Jacobs’ world-class high-temperature facilities in an industrial environment.

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