Evaluation of Thermal Strain Induced in Components of Nb3Sn Strand During Cooling

Nb
₃
Sn strands, whose properties are very sensitive to stress/strain, are utilized for ITER cable-in-conduit conductor (CICC) of the central solenoids. The Nb
₃
Sn strands experience temperature range of ~1000 K from the temperature of the heat treatment with the initiation of the Nb
₃
Sn reaction to the operation temperature of ~4 K. Due to this large temperature range, large thermal strain is induced in the Nb
₃
Sn filaments due to the differences between the coefficients of thermal expansion and Young's moduli of the components of the strand. Therefore, it is considered that initial performance of the CICC is influenced by the thermal strain on the Nb
₃
Sn, and it is important to evaluate the strain state of the Nb
₃
Sn strand at low temperature. In this study, the thermal strain of the components of free Nb
₃
Sn strand was measured by neutron diffraction and stress/strain state was assessed from room temperature to low temperature. As the results of diffraction measurements, it was found that 0.111% and 0.209% compressive strain were generated in Nb
₃
Sn filaments at 300 and 10 K, respectively.