Stress analysis for the crack observation in cooling channels of the toroidal field coils in JT-60U

A possible mechanism of the crack formation observed in the cooling channels of the toroidal field (TF) coil in JT-60U is investigated. The compressive force (radial direction of the conductor of TF coil) on the cooling channel can produce localised stress above its yield strength near the corner of the cross-section of the channel. The cyclic load could form the initial crack to give rise to water leakage from the cooling channel. The overall stress analysis of the TF coil shows that the magnitude of the compressive force depends on the radial stiffness of multi-layered structure of the coil. Cracks are expected to be observed for tight-wounded coils and is consistent with the pre-load test result. The maximum stress in the conductor is analysed to be much lower than the yield strength of the copper conductor, therefore, the conductor itself is in good condition.