Design and verification of a non-self-supported cryostat for the DEMO tokamak

The paper presents a conceptual design and its structural verification of the cryostat for the DEMOnstration Fusion Reactor (DEMO). The cryostat is a large pressure vessel providing the vacuum required to operate the superconducting coils at cryogenic temperatures. Cryostats of existing machines typically are cylindrical and self-support the external pressure. In a nuclear machine, like DEMO, a massive bioshield will enclose the reactor providing radiological protection to maintenance areas in the primary building. The proposed design makes use of the bioshield to support the cryostat which allows substantially reducing the amount of steel needed for its construction. The cryostat is a conventional pressure vessel and designed according to ASME VIII, Div. 2. Linear and nonlinear structural and thermal-structural FEM assessments show that the proposed conceptual cryostat design provides both, the required membrane strength to withstand the external pressure as well as the required flexibility to allow the thermal contraction in case of a loss of vacuum event causing the cryostat to cool down. However, the relatively thin shell is not capable of bearing any significant internal overpressure. Therefore, a rupture disk in the cryostat to release Helium into the building in case of large internal leaks of liquid Helium is required.