The effects of nuclear fusion plasma excursions on the performance of Eurofer-97 components

The high operating temperatures and nuclear radiation within a fusion reactor can significantly change the microstructure of its components, which in turn can change the material properties and mechanical behaviour . The materials chosen for fusion reactors have been specifically engineered to reduce the amount of induced radiation, but it is also important to understand how such materials will behave when exposed to the high temperatures of the fusion plasma.

Eurofer-97 is a reduced-activation ferritic-martensitic steel that is proposed for many of the structural and cooling components within the ITER and DEMO fusion reactors, including the cladding material and coolant pipes in the water-cooled lithium-lead breeder blanket, one of four cooling mechanisms being trialled at ITER. Generating power using a cooling system is a key step in achieving commercialisation of nuclear fusion power generation. The plasma at DEMO is expected to be around 100 million C, using focused magnetic fields for containment. However, in the event of a loss of this containment it is possible for the plasma to contact with wall, leading to dramatic increases in temperature over short timeframes.

Preliminary work in the Interface Analysis Centre (IAC), has shown that even a few hours of high temperature (- 1s 0 C) exposure can cause significant microstructural degradation of the Eurofer-97 stainless steel material used for structural support and cooling pipes. This could affect mechanical properties and corrosion resistance, in turn leading to a shortening of component life. This PhD project will study the effect of very short-term thermal excursions and assess their effect on the microstructure and corrosion behaviour of the material.

The student will use advanced computational modelling using the software package Comsol to explore the effect of thermal transients due to plasma excursions on the temperatures of components within the reactor. This temperature behaviour will then be input into Abaqus to investigate the structural integrity implications of repeated thermal cycling on component stress. Meanwhile, the impact of repeated thermal excursions on the microstructure will be explored using phase chemistry software such as Matcalc. The student will use a combination of these simulations results to predict the resistance of components to degradation mechanisms such as creep and corrosion as a factor of their proximity to thermal excursion locations.

In parallel with the computational work, the student will build on existing experimental research at Bristol studying the behaviour of Eurofer-97 after repeated short-term thermal exposure. This will include design of experiments for short-term thermal treatments. Exposed specimens will then be characterised using scanning and transmission electron microscopy, x-ray diffraction and tomography to observe the change in the steel's structure with increasing heat exposure and determine if any embrittlement or change in mechanical properties occur that might cause problems if used in a fusion reactor. There may also be opportunities to combine this work with the effects of irradiation and/or corrosion.
This project is a multi-technique approach to understand the structural integrity of an importar:,t material for fusion power. The student will have the opportunity to learn how to use advanced simulation packages and characterisation techniques, and will work closely with industry partners at the UKAEA and NNL.