During development of the UKAEA’S Spherical Tokamak for Energy Production (STEP), assessment of manufacturing strategies for the inboard shield identified several gaps in knowledge. Limits arose from the modularization of this component, geometric constraints of tungsten ceramic forming, joining with low-activation interlayers and dissimilar joining. These unknowns are addressed by tungsten and tungsten carbide manufacturing demonstrations for key features of the inboard shield. This began with an exploration into low activation interlayers an bonding techniques for tungsten, tungsten carbide and 316L joining.

Iron and vanadium and tantalum are all low-activation in a fusion environment, and it was found that these can produce good quality bonds for tungsten joining, tungsten carbide joining or dissimilar joining with 316L steel via hot isostatic pressing diffusion bonding. Vanadium at 1050°C, 105 MPa, 60-minutes, and tantalum at 1150°C, 105MPa, 60-minutes, can produce acceptable tungsten – tungsten bonds. Tantalum at 1150°C, 140MPa, 60 minutes is also compatible with tungsten carbide- tungsten carbide joints. Iron at 1050°C, 105 MPA, 60-minutes can produce good bonds for tungsten carbide-tungsten carbide joining, tungsten carbide-316L joining, and tungsten -316L joining. Higher temperature requirements for tantalum interlayers can compromise the steel microstructure for 316L dissimilar joints. Carbon-containing substrates were incompatible with vanadium interlayers due to brittle intermetallic formation at the interfaces.

It was concluded that the required joint quality between the relevant substrate materials can be achieved with the named low activation interlayers and hot isostatic pressing diffusion bonding. The results of this will inform a geometric demonstration which includes permanent and semi-permanent joints, dissimilar joints, bonded coolant pipes and cross-joint coolant channels.