The use of Nb3Sn in fusion: Lessons learned from the ITER production including options for management of performance degradation

The overall 30 year Nb3Sn conductor program for ITER provides an unusual opportunity to look at the issues created by the application of novel Nb3Sn technology on a large scale. ITER design criteria have evolved to make use of the features of the industrialized material, exploiting advantages as well as managing the disadvantages. There are lessons from the successes and failures to be learned for the future application of very high current Nb3Sn strands in fusion and high energy physics. The behaviour of Nb3Sn strands in the large compound ITER conductors is still producing surprises after 25 years of development, industrial production and testing, with the Nb3Sn strain sensitivity and brittleness producing many more subtle design impacts than originally foreseen. The results now obtained at the end of the ITER production suggest that, as a novelty for Nb3Sn, there are options for conditioning the strands in the ITER conductors that can be applied during commissioning and which could reduce the impact of brittleness by as much as one third.