Characterising materials for walls of fusion reactors

Abstract

Experiments in which a permeation flux through a membrane specimen is modulated have been shown to provide valuable insight into permeation processes. In particular, surface and bulk effects can be separately identified from the experimental data. This approach requires an analytical model of the process in terms of absorption steps and bulk transport. A basic model has been developed to account for permeation through a binary membrane comprising a coating and a substrate. The interface reaction kinetics including trapping are taken into account in terms of rate constants. It is shown that given adequate knowledge of the substrate then the permeation parameters of the coating can be deduced. This greatly extends the range of materials which can be examined, particularly those materials which cannot be obtained as foils such as ceramics. Some strategies for first wall reactor materials combine a low diffusivity ceramic coating on a mechanical substrate; such binary (and even more complex) structures can now be assessed through the modulated permeation method, taking full account of the interface. As an example, recent measurements on a titanium carbide–molybdenum alloy system (TiC–TZM) are discussed.