Rationale for the selection of the operating temperature of the DEMO vacuum vessel

The vacuum vessel (VV) of DEMO has to fulfil various requirements. The main function is to provide the high vacuum environment essential for plasma operation. Likewise, it has important safety functions: it acts as the first and main nuclear confinement barrier and the main constituent of the neutron and radiation shielding function protecting other components in particular the superconducting coils and all maintenance areas. It has to remove the heat load from nuclear heating also in the unlikely event of a major technical malfunction. The VV is well designed to provide the mechanical support and attachment for the in-vessel components. This paper addresses the effect of the VV temperature on the functions and issues described, and the approach to define the best suitable temperature window. Following a qualitative assessment, the finding is that a lower operating temperature around 40 °C is advantageous for most of the issues.