The issue of Tritium in DEMO coolant and mitigation strategies

A certain fraction of the tritium produced in DEMO blanket permeates into the primary coolant due to the high temperatures and the large metallic areas. From primary coolant, tritium can reach the working areas and the external environment via permeation and/or leaks. The mitigation of the tritium migration phenomenon can be accomplished by using appropriate anti-permeation barriers and by an opportune dimensioning of the coolant purification system. This work describes the technological status of the mitigation strategies under development for HCPB and WCLL blankets. The results of the tritium permeation analysis at system level reveal that the tritium permeation rate from blanket into primary coolant is much higher in WCLL than in HCPB. Therefore in HCPB the tritium concentration into primary coolant can be controlled exclusively with a Coolant Purification System (CPS) able to treat a by-pass of about 3 kg s−1. Two possible Helium-CPS processes are proposed and their pre-conceptual design is presented. Instead in WCLL due to the larger tritium permeation rate into coolant and the limit in water CPS size, the presence of appropriate coatings is required. Possible solutions are proposed and discussed based on the combination of feasible permeation reduction factors and water CPS size.