ITER Pre-Compression Ring Tightening Analysis

The Pre-Compression Ring (PCR) is an important component of ITER Magnet System. It is designed to generate a radial inward force on every Toroidal Field (TF) coil, putting in compression the shear keys, contrasting the bending of the inner leg and improving the stress distribution on the wedge surface. Stretching the PCR by tensioning 16 bolts on 4 counter flanges generates a total force of 47.8 MN on each coil. The application of such high preload, together with the large number of bolts to be tighten, the limited space available, and obviously the need of preserving the structural integrity of all the components, make this phase of the tokamak assembly far to be trivial. For these reasons, an in-depth analysis of the tightening procedure has been performed by means of FEM analysis, investigating the possible patterns and multi-step sequences to achieve the desired preload. The study led to a deep comprehension of the behavior of the system and permitted to identify and control the main critical aspects, i.e., the increase of stress in the PCR due to the ‘ripple’ and the variation of preload in some portion of the rings when the preload is applied in a different area, both due to the need of operating on a limited number of bolts at each time. The outcome of the investigation is the detailed definition of two multi-step tightening procedures suitable to be adopted for the assembly.

I. Introduction

The ITER Pre-Compression Ring (PCR) is designed to generate a radial inward force on every Toroidal Field Coil (TFC), in order to put in compression the shear keys (IIS), contrasting the bending of the inner leg also improving the stress distribution on the wedge surface [1]. The pre-tensioning of the PCRs is obtained by tightening a set of 4 bolts (2 M80 + 2 M110) connecting the Counter Flanges (CF, four for each coil) that push on the inner surface of the rings. Two sets of three PCRs are provided for each coil, in the bottom and top area of the inner leg. The total preload for each coil is 47.8 MN.