MAGNET SAFETY ASSESSMENT FOR ITER

The ITER magnet system consists of structurally linked sets of toroida l (TF) and poloidal (PF) field coils, central solenoid (CS), and vario us support structures. The coils are superconducting, force flow Heliu m cooled with a Kapton-Glass-Epoxy multilayer insulation system. The s tored magnetic energy is about 100GJ in the TF system and 20GJ in the PF-CS. Coils and structure are maintained at 4 K by enclosing them in a vacuum cryostat. The cryostat, comprising an outer envelope to the m agnets, forms most of the second radioactivity confinement barrier. Th e inner primary barrier is formed by the vacuum vessel, its ports and their extensions. To keep the machine size within acceptable bounds, i t is essential that the magnets are in close proximity to both of the nuclear confinement barriers. The objective of the magnet design is th at, although local damage to one of the barriers may occur in very exc eptional circumstances, large scale magnet structural or thermal failu re leading to simultaneous breaching of both barriers is not credible. Magnet accidents fall into three categories: thermal (which includes arcing arising from insulation failure and local overheating due to di scharge failure in the event of a superconductor quench), structural ( which includes component mechanical failure arising from material inad equacies, design errors and exceptional force patterns arising from co il shorts or control failures), and fluid (Helium release due to cooli ng line failure). After a preliminary survey to select initial faults conceivable within the present design, these faults are systematically analyzed to provide an assessment of the damage potential. The result s of this damage assessment together with an assessment of the reliabi lity of the monitoring and protective systems, shows that the magnets can operate with the required safety condition.