The NDT method using electromagnetic waves has availability for crack detection in large pipes or structures in nuclear power plants. The purpose of this study is then, to develop a new method to determine the crack size and location with high speed and accuracy. Through this study, it has been shown that the transmitted wave carries a lot of information on crack depth and the obtained signals of reflected wave below the cut-off frequency depend on the crack position. Experimental results indicate that there exists attractive possibility in the new NDT method.

In the case of using rare-earth barium copper oxide (REBCO) tape as a current-limiting element, there are three requirements: high normal state resistance, high thermal stability, and quick recovery to the superconducting state. In this study, a new type of REBCO tape with a metal porous medium as a stabilizer (a porous-stabilized REBCO tape) was proposed, which has high resistance compared to bulk material and prevents film boiling due to its strong capillary force. First, a numerical simulation was performed to evaluate the current-limiting performance of the porous-stabilized REBCO tape with a Ni-Cr porous medium. The results showed that the current-limiting performance was dramatically improved by placing indium partially on the REBCO tape to join the porous medium compared to placing indium wholly. Then, the porous-stabilized REBCO tapes with various arrays of indium were manufactured, and tested to confirm its recovery characteristics. The experimental results indicated that the partially-joined Ni-Cr porous-stabilized REBCO tape reduced the recovery time by 69% compared to that of the stabilizer-free REBCO tape.

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Ultrasonic welding (UW) is a candidate for easy and simple fabrication of lap joints of high-temperature superconducting (HTS) tapes. Our previous study applied UW to both silver-sheathed BSCCO tapes and copper-stabilized REBCO tapes with indium foil inserted in-between. However, the fabrication was performed with limited joining conditions and critical current, I C, degradation was confirmed in some of the BSCCO tape joints. In this study, we evaluated the joining-condition dependence of joint resistance and I C to improve UW-joined HTS tape performance. Various UW samples using BSCCO and REBCO tapes were prepared with different thicknesses of indium foil, ultrasonic vibration amplitude, joining pressure, joining time, and metal plate arrangement. The results showed that HTS tapes layered between metal plates on the top and bottom can avoid I C degradation even with higher joining energy, however thinner indium foil induced degradation in the BSCCO samples. Through this study, we found the value of joint resistivity only depended on the joining energy per unit joint area. The achieved joint resistivities were 14.0 nΩcm2 for the silver-sheathed BSCCO tapes and 36.1 nΩcm2 for the copper-stabilized REBCO tapes though it was strongly influenced by interface resistivity of the REBCO tape itself.

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