The compatibility of stainless steels with low-sulphur UCS fuels

Sodium-bonded and vacuum-bonded compatibility tests at 600 and 7000C between stainless steels and UCS fuels of differing sulphur contents and stoichiometries revealed that, over the range of compositions studied, the amount of carburisation of the cladding is a linear function of only the total non-metal content (C+O+N+S) of the fuel, and does not depend on the actual sulphur content of the fuel. A UC and a UCS fuel with the same non-metal content therefore cause equal amounts of carburisation of the cladding. X-ray and microstructural investigations of the fuels further showed that a single-phase region does not exist at non-metal contents greater than 50 at%, and that sulphur probably behaves as a carbon equivalent in that it enters the UC lattice in solid solution, but displaces a carbon atom which is freed to form a higher carbide. The conclusion is therefore drawn that the addition of sulphur to UC fuels has no direct advantage at non-metal contents greater than 50 at % as far as the compatibility behaviour of the fuel towards the cladding is concerned. (Auth.)