Prediction of RAFM Steel Components Performance for Nuclear Fusion

Background

Fusion has potential to become a key part of the future energy supply. In the next few years, both the UK’s Spherical Tokamak for Energy Production and the EUROfusion DEMO program will commence engineering design. However, to facilitate design of fusion power plants, significant materials and component design challenges must be addressed simultaneously.

A pivotal challenge is the development of reduced activation ferritic/martensitic (RAFM) steel for structural materials in breeding blanket components, essential for breeding tritium – one of the fuel sources for fusion, and thermal power generation. This project aimed to optimise the chemical composition and thermal-mechanical processing of current RAFM steels.

Despite international efforts to develop RAFM steels since the 1980s, progress remains limited, thus restricting the design of breeding blanket components for fusion reactors. Current RAFM steels have narrow design margins due to irradiation-induced hardening and embrittlement at lower-service temperatures between 250°C and 350°C, and loss of creep strength with embrittlement at high temperatures between 550°C and 650 °C.

About the Project

This project aimed firstly to optimise the chemical composition and thermal mechanical processing of RAFM steels; and secondly to develop an integrated approach merging materials development with component design by analysis, moving away from the traditional sequential path where materials development precedes component design.

The project includes three work packages, led respectively by the Universities of Sheffield, Manchester, and UK Atomic Energy Authority, with in-kind contributions and support from LIBERTY Steel Dalzell.

Project Details and Results

A suite of advanced alloys was developed based on the latest insights into RAFM steels for high temperature applications.

Detailed creep fatigue lifetime assessment of the Helium Cooled Pebble Bed breeding blanket showed wide variation in creep-fatigue life, with some regions enduring over one million cycles, while others lasted as few as ten cycles. This variability arises from temperature dependent material behaviour, local stress concentrations, joints, and potentially irradiation effects which were not assessed here.

This project established a methodology to integrate alloy development with breeding blanket design-by-analysis, enabling feedback between materials innovation and structural design.

One standout alloy was the “T alloy”, which showed approximately 15% improvement in ultimate tensile strength at all test temperatures – this improvement was attributed to a high dislocation density after heat treatment. This novel alloy design shows strong promise for improving structural integrity and extending fusion component service life.

Furthermore, a MATLAB based tool incorporating RCC MRx creep fatigue rules and EUROFER97 properties was developed allowing targeted evaluation of lifetime limiting regions and efficient identification of lifetime limiting factors.

The project helps UK’s steelmakers maintain leadership in the global steel industry by investigating scalable manufacturing parameters for advanced RAFM steels. It has positively impacted the wider fusion community by advancing multidisciplinary integration to address fusion engineering challenges. The tools and knowledge developed in this project will enable more efficient materials qualification and design assessments. The project outcomes also contribute to fusion-specific design rules, codes, and standards, providing a platform to accelerate innovation in advanced materials across UK industries.