Pressurized helium as coolant of fusion reactor breeding blankets: A focus on the purification technologies

Pressurized He is one of the candidate coolants for the breeding blanket of future fusion reactors as per the European Fusion Roadmap. As a matter of fact, two out of the four DEMO breeding blanket under development in Europe are cooled down by He at 8 MPa: they are the Helium Cooled Lithium Lead (HCLL) and Helium Cooled Pebble Bed (HCPB) concepts. In the Dual Cooled Lithium Lead (DCLL) concept, mainly developed in USA, the breeding blanket structural material is also cooled down by pressurized He. Among the R&D and design activities aimed at demonstrating that pressurized He is an effective coolant for fusion application, the development of a system with the main function to keep under control the tritium concentration and impurities level in the He coolant is of basic importance: the Coolant Purification System (CPS), directly connected with the He coolant, accomplishes these functions in normal operation. This paper first recalls the pros and cons related to the use of pressurized He as primary coolant, clarifying the reasons why a high-performance CPS is needed. Then the paper describes the current CPS design baseline as foreseen for HCLL and HCPB Test Blanket Systems (TBS), derived from the corresponding breeding blanker concepts, focusing on the proposed technologies and operative conditions, as well as the on the main challenges and required R&D activities. (author)