Design of Two Experimental Mock-Ups as Proof-of-Concept and Validation Test Rigs for the Enhanced EU DEMO HCPB Blanket

Within the framework of EUROfusion activities, the helium cooled pebble bed (HCPB) breeding blanket is under development in Karlsruhe Institute of Technology (KIT). The enhanced HCPB, using a fissionlike fuel breeder pin assembly configuration, is proposed as the near-term breeding blanket for the European Union DEMOnstration power plant (EU DEMO). The helium gas with a pressure of 8 MPa is used as the coolant, EUROFER is used as the structural material, advanced ceramic breeder pebbles are used as the tritium breeder, and Be₁₂Ti is used as the neutron multiplier material. In contrast to the former HCPB cooling plate configuration, the fuel breeder pin assemblies greatly reduce the pressure drop, reducing the circulating power to the level where state-of-the-art helium turbomachinery can be used. However, because of the reduced coolant velocity in the breeder zone, the heat transfer performance is compromised, especially in the annular channel of the fuel breeder pins. An increased surface roughness is therefore proposed as a heat transfer augmentation technique for the fuel breeder pins. Although heat transfer augmentation using artificial roughness is common, it is relatively novel for small annular gaps with moderate velocity as the ones in the fuel breeder pins. Currently, a dedicated correlation for the small annular rough-wall channel is available to predict the Nu number. This correlation is wished here to be benchmarked and validated experimentally. Therefore, an experimental investigation on a fuel breeder pin mock-up (mock-up 1) is planned. Additionally, based on computational fluid dynamics calculation, unsteady, nonuniform flow patterns were found at the return flow after the jet impingement in the first-wall region. Another upscaled mock-up (mock-up 2) to investigate the nonuniform flow patterns of the return flow in the annular channel of the fuel breeder pin is planned. The dedicated experimental campaigns are foreseen at the Helium Loop Karlsruhe (HELOKA) in KIT as validation and proof-of-concept test rigs for this enhanced pin design. In this paper, the motivation and the preliminary design of these two mock-ups of the enhanced EU DEMO HCPB blanket are shown, together with the plan for the foreseen experiments.