Research progress on the mechanical behavior of the cable in conduit conductor for the international thermonuclear experimental reactor project

Abstract

The international thermonuclear experimental reactor (ITER) program is of the largest and the most influential international cooperation project, and its purpose is to verify the scientific and technical feasibility of the magnetic constraint nuclear fusion reactor. The high temperature plasma with billion degrees Celsius can be constrained in a magnetic cage, which can provide a strong magnetic field by a Tokamak device wounded by cable in conduit conductor (CICC). In 2014, Dr. Devred A, a chairman of ITER magnet project, pointed out that the ITER CICC is mainly faced with three challenges: (1) During insertion into the jacket assembly, the cable exhibits a tendency to rotate under the action of the pulling force. That may increase the twist pitch, especially for the final one. Cable twist pitches must be controlled to prevent excessive AC losses in the CICC, which is a threaten to the stability of CICC. The elongation of the twist pitch must be settled. (2) The CICC′s current sharing temperature ( T cs) showing degradation after the electro-magnetic (EM) and thermal cycling load, that means the Tokamak can only run thousands of times, less than the original design of 30000 times, raising risks at the fusion project. (3) Find the way to fabricate the high-performance superconducting wire and the low resistance joint. As we all know, the central and the toroidal field solenoid superconducting cable of the Tokamak were made of Nb3Sn superconducting strands, and the superconducting cable will working at the mechanical–thermal–electrical–magnetic fields environment. Previous studies have shown that Nb3Sn superconductivity is sensitive to the mechanical deformation. The critical current of the superconductor wires will show a significant degradation with the deformation under the loads of tension, pressure and twist, which increases the risk of ITER Tokamak directly. Therefore, it′s important for the design of the Tokamak magnet system to investigate the equivalent mechanical parameters of the cable and its mechanical behavior under the action of multiple fields. In this paper, several key mechanical problems such as the equivalent mechanical parameters of the superconducting cable, the untwisting behavior in the process of insertion, and the T cs degradation under the thermo-electromagnetic cyclic loads have been briefly reviewed. Firstly, the stress-strain curve of the triplet was analyzed based on the thin-rod model and the tensile stiffness model of the triplet was established. Secondly, on the basis of the triplet model, a complex model of the tensile stiffness and the equivalent CTE of the 3×3 strand was build, and the derivation process was provided. Thirdly, the untwisting behavior of the cable in CICC fabrication was investigated. The bending stiffness model of the petal under the wrapping was established, and the tensile untwisting model of the superconducting cable was also build. Fourthly, a mechanical model was established for the T cs degradation mechanism of TF and CS CICC conductors under the action of thermo-electromagnetic cyclic load. Based on the compatible relationship between the transverse compressive strain and the axial elongation of the superconducting cable, the impacts of axial compressive stiffness and surface friction coefficient on the axial compression strain of the cable under the thermo-electromagnetic cyclic load were studied. Finally, we summarized the existing problems and the future research points on the basis of the previous research results, which will help our researchers to catch the progress of the ITER program. In the future, the CICCs will also be used in China′s large superconducting magnets.