CFD analysis of the natural circulation of helium in the DEMO cryostat during a leak accident

After the ingress of helium into the DEMO cryostat the natural convection of the leaked helium is established inside the cryostat, driven by the temperature difference between the cold magnets and warm structures (vacuum vessel and cryostat). The cryostat is a large thin-walled vacuum vessel whose thermal contraction is constrained by its supports to the bioshield. The design of these supports depends on the temperature distribution on the inner side of cryostat walls. The loss of liquid helium from the superconducting coils into the cryostat is studied at both normal and baking conditions of the vacuum vessel. The studied helium leak accident considers the temperature of the magnet coils preserved at 59 K to represent the most severe case. Assuming a quasi steady-state condition to establish the natural convection of helium in the DEMO cryostat, a CFD code was used to determine the temperatures and the heat transfer coefficient on the cryostat inner wall.