An adaptable and self-calibrating service robotic scanner for ultrasonic inspection of nuclear nozzle-vessel welds

In this paper the development of a robotic scanner for non destructive inspection of the nozzle to vessel weld of Boiling Water Reactors (BWR) nuclear power plant reactors is presented. The scanner carries and deploys a specially designed ultrasonic (UT) 2D matrix phased array probe. The 3 degrees of freedom (d.o.f.) robotic scanner is mounted on the nozzle and drives the UT probe over selected areas above the weld on the vessel at predefined paths, via a 2-link manipulator mounted on a carriage, sliding on a precision 360 degrees circular ring. The system is controlled using hybrid position-force control for 3D trajectory tracking. The scanner´s novelty lies in the very low setup time needed by operators, but mainly in its self-calibration capability by utilizing a variety of sensors, drastically minimizing the operators´ exposure to nuclear radiation. Moreover, it is able to fit on nozzles of various diameters, as well as to operate on non-vertically nozzles welded on the vessel. The UT probe was designed to electronically steer its beam between 35 to 80 degrees and achieve electronic skewing between ±10 degrees for improved detection capabilities of the inspection technique. The developed 2D matrix phased array probe is positioned in a local immersion bath and combined with a flexible membrane front, complying on any curvature variations along the nozzle circumference. The performance of the prototype was successfully validated on a replica of a nuclear power plant nozzle.