Iterative design and preliminary assessment of the DTT vacuum vessel interface to divertor

The upcoming Divertor Tokamak Test (DTT) facility, whose construction is starting at ENEA Frascati Research Centre in Italy, plays a key role in the EUROfusion roadmap to achieve nuclear fusion. It aims at bridging the gap between ITER and DEMO, testing several divertor configurations and thus boosting the research on managing power exhaust in fusion reactors. The DTT divertor system, in the first configuration under development, is made of 54 cassettes. Each cassette is connected to the Vacuum Vessel (VV) through a dedicated fixation system, that allows the cassette for aligning with the operative position and locking, during assembly and Remote Handling operations. The interface between the VV and the cassettes fixation systems is realized through dedicated components, the divertor toroidal rails, whose primary aims are to sustain the divertor cassettes, transferring their loads to the VV, to feature the functional surfaces driving the cassettes in their correct operative position during the locking procedure, and eventually to allow the movement of the Cassette Toroidal Mover (CTM) during Remote Handling operations. This paper describes the activities of concept design and preliminary structural assessment of the divertor toroidal rails. After a state-of-the art study and the analysis and definition of the system requirements, a conceptual solution with some similarities with the ITER design has been developed. Then, it has been preliminarily structurally verified, through Finite Element (FE) analyses, against several load conditions (i.e., assembly condition, plasma operation, disruptive events, maintenance). Particular attention has been paid to properly study and design the mechanical coupling between the divertor toroidal rails and the interfacing systems: the VV, the divertor cassettes and the Remote Handling devices. In this sense, the activity focuses on two main aspects: finding the optimal welding points for connecting the divertor toroidal rails to the VV inner shell and designing the functional surfaces driving the locking of the cassette on the divertor toroidal rails and guaranteeing their reliable coupling during plasma operation.