Metallographic and Bending Strain Property Analysis of Reinforced Nb₃Sn Strand Candidates for Japanese DEMO

The influence of a large electromagnetic force on the conductor of a demonstration power plant (DEMO) reactor magnet is a crucial issue. The present study summarizes the metallographic and bending strain property analysis of DEMO-candidate reinforced Nb3Sn wires in Japan. Here, EBSD strain analysis was used as a new microscopic method to evaluate the material properties of Nb3Sn wire components. Two reinforcement options have been considered: (1) the use of a brass matrix for distributed-tin (DT) wire (made by Kobe Steel) and (2) the use of CuNb reinforcement in the outer sheath of a bronze-route Nb3Sn wire (made by Furukawa Electric). Both the reinforced wires exhibit better stress tolerance against bending load because of their higher mechanical strength than Cu. The high elasticity of the brass matrix of the DT wire and the CuNb area in the bronze-route Nb3Sn wire have been confirmed through electron backscatter diffraction (EBSD) strain analysis. The results strongly support the use of reinforced Nb3Sn wires to maintain their high Ic performance in Japan's DEMO designed with a larger operating current and higher magnetic field than International Thermonuclear Experimental Reactor (ITER).