Introduction

Immersion ultrasound system for the automated non-destructive weld control of the, so-called, “Plasma Facing Units” (PFUs) of the ITER  (International Thermonuclear Experimental Reactor) fusion reactor divertor. The PFUs act as water heat exchangers, they consist of curved pipes coated with high temperature withstanding materials. Scanning takes place from inside the tube with a helical path of the probe.

Technical features

The system is used to identify and size any defects in the Plasma Facing Units, PFUs, of the fusion reactors obtained through a diffusion welding process, through ultrasonic scanning of the welding between the tube and the coating material from inside the reduced diameter. It solves the problem of scanning times, which are halved compared to traditional scanning systems for generators. The probe is placed at the end of a flexible shaft which allows scanning of both curved and straight tubes. The particular shaft, having minimal torsion, allows an accurate identification of the position of the probe ensuring high accuracy in the positioning and sizing of defects. Unlike such systems, the probe must perform a high number of turns. For this reason, rolling contacts have been used to transmit the signals to the fixed echograph. These electrical contacts, compared to the commercial sliding ones, have shown not to introduce noise which is particularly harmful for the high frequency measurements necessary for reduced thicknesses and high resolutions.

Possible Applications

• Defect detection inside pipes even if curved;
• Non-destructive control of joints even in curved pipes;
• Defect detection in heat exchangers of nuclear plants.

Advantages

• Defect detection with UT technique from inside small diameter pipes (11 ÷ 15 mm);
• Low noise transmission of high frequency signal from a rotating shaft;
• Reduced scan times compared to other techniques;
• High resolution in sizing and positioning of defects.