ASSESSMENT OF POSSIBLE FAILURE MODES AND NON-DESTRUCTIVE EXAMINATION OF THE ITER PRE-COMPRESSION RINGS

The pre-compression rings (PCRs) for the International Thermonuclear Experimental Reactor (ITER) represent one of the largest and most highly stressed composite structures ever designed for long term operation at 4K. Three rings, each 5m in diameter and 337 x 288 mm in cross-section, will be installed at the top and bottom of the eighteen "D" shaped Toroidal Field (TF) coils to apply a total centripetal load of 70 MN per TF coil. The interaction of the 68 kA conductor current circulating in the coil (for a total of 9.1 MA) with the required magnetic field to confine the plasma during operation will result in Lorentz forces that build in-plane and out-of-plane loads. The PCRs are essential to keep the stresses below the acceptable level for the ITER magnets structural materials. Each PCR will be pre-loaded at room temperature and cooled to 4K. They are designed to withstand various thermal cycles reaching up to 400 MPa hoop tension and up to 40 MPa radial compression without degradation. The rings will be fabricated (pre-dominantly) by hoop wound S-glass tow to result in a high glass content (>65% in volume) structure [ 1]. During manufacture emphasis will be placed on obtaining a structure with a low void content (<1.5%) and that is free from inclusions, faults, cracks and de-laminations. The biaxial loading conditions that exist in the PCRs can lead to complex behaviour patterns while the few existing studies on ring like structures have been limited to testing tubular specimens with axial loading and either internal pressure or a torsion load applied. This paper will study the possible failure modes of the PCRs and consider available non-destructive evaluation techniques that may be applied to the full scale rings after manufacture.