Preliminary design and structural analyses of DEMO bioshield roof

Abstract

The bioshield is a reinforced concrete structure encasing the cryostat that fulfils three main functions: reduce the gamma radiation level to allow man access to external areas, provide access to the cryostat, and – as part of the tokamak building structure – provide support to the equipment and building structures on top of the bioshield roof. The bioshield roof may or may not have the additional function of supporting the cryostat top lid against external or internal pressure loads. Both the cryostat top lid as well as the bioshield roof must provide access to the vertical ports, i.e. large trapezoidal openings that are closed during plasma operation. Bioshield plugs must be inserted to maintain the bioshield’s function to protect the area above against radiation exposure. In addition, in case of a major failure in the early operational phase the bioshield roof shall be removable to allow full access to the tokamak via the overhead crane.The paper presents the results of the first ever design study of the DEMO bioshield roof. The circular symmetry roof construction that is a steel structure with concrete inserts arranged in three peripheral rows to allow full access to the tokamak via the overhead crane has been proposed. Using a flexible beam model, a number of options for the structural concept of the bioshield roof have been assessed. It has been proven that concepts with 16 innermost shield plugs as well as one innermost opening/plug are structurally feasible. The importance of the upper and lower toroidal girders of the steel load bearing structure has been identified. A mass assessment has shown that the weights of all parts of the bioshield roof are well within the lifting capacity of the overhead crane.