TRITIUM PERMEATION THROUGH HELIUM-HEATED STEAM-GENERATORS OF CERAMIC BREEDER BLANKETS FOR DEMO

The potential sources of tritium contamination of the helium coolant o f ceramic breeder blankets have previously been evaluated for the spec ific case of the European BIT DEMO blanket. This confirmed that the co ntrol of tritium losses to the steam circuit is a critical issue which demands development concerning (a) permeation barriers, (b) tritium r ecovery processes maintaining a very low tritium activity in the coola nt, and (c) control of the coolant chemistry. The specifications of th ese developments required the evaluation of the tritium losses through the steam generators, and includes the definition of their operating conditions by thermodynamic cycle calculations, and their thermal-hydr aulic design. For both tasks, specific computer tools were developed. The geometry obtained, the surface area and the temperature profiles a long the heat-exchanger tubes were then used to estimate the daily tri tium permeation into the steam cycle. Steam-oxidized Incoloy 800 auste nitic stainless steel was identified as the best-suited existing mater ial. Our results indicate that in nominal steady-state operation the t ritium escape into the steam cycle could be restricted to less than 10 Ci per day. The conditions for this are specified, but their feasibil ity demands, in particular, the resolution of certain gas chemistry pr oblems, and their validation in the more stringent environment of an o perating blanket. Tritium permeation during temperature and pressure t ransients in the steam generator (destruction and possible self-healin g of the permeation barrier) was identified as bearing a large tritium release potential. The problems associated with such transients are d iscussed and possible solutions are proposed.