ITER TF Magnet System Analyses in Faulted Conditions

The ITER toroidal field (TF) magnet system must be designed, built, and operated, so that failures, which could occur under off-normal conditions, cannot cause damage to the confinement barriers. In fact, even if the magnets are not part of the barrier, the coils store a relevant amount of energy, which, in case of failure, may lead to large displacement of the coils, with the risk of a crash against confinement structures, which cannot be damaged. In this paper, the outcome of a structural analysis of the ITER TF coil magnet system, simulating possible faulted condition scenarios, is described. Two possible electrical failures of the TF coil system are considered: a short, resulting in a relevant current peak in one coil, and a quench without the discharge of the other ones. Since in both scenarios the assumption of cyclic symmetry condition is not valid anymore, a complete 18-coil finite-element (FE) model of the whole ITER TF magnet system has been developed for this purpose. By means of FE models, both electromagnetic and mechanical behaviors have been simulated, determining the magnetic field distribution; the forces acting on the coils and, consequently, stress; the displacement; and the forces on the interfaces. In particular, the mechanical model features a detailed description of all the subsystems which play a structural role in the TF magnet system, such as the intercoil structures, the poloidal shear keys, the TF wedged vault, the TF gravity support, the precompression ring system, and the poloidal field support structures. The model also includes all the bolted connection and all the nonlinearities (pretension, contact, etc.) characterizing the mechanical system. The results show that the deformations of the structure and the loads on the subsystems are within the allowable limits at off-normal conditions, thus demonstrating the soundness of the design, even in the failure scenarios.