Inspection of CANDU Reactor Pressure Tubes Using Ultrasonics

Fuel channel pressure tubes are critical components of the CANDU reactor core which require periodic inspection to demonstrate continued fitness for service. Ontario Power Generation; Inspection & Maintenance Services Division has been operating a variety of inspection systems to perform volumetric and surface nondestructive examinations of the pressure tubes. Since the early days, ultrasonics was selected as the preferred method of inspection. Several ultrasonic transducers are employed to perform inspection from different directions and at various angles at the same time. The ultrasonic inspection system utilizes immersion, focused transducers operating at high frequencies (10 MHz and 20 MHz) in order to achieve high degree of detectability of small flaws and high accuracy of flaw sizing. The inspection could be divided into the following phases: System amplitude calibration Ultrasonic flaw detection Sizing of detected flaws In addition to inspecting tube for flaws, data is collected allowing gauging measurements (diameter and wall thickness), pressure tube sag measurements and Garter Spring detection. Detection of flaws is performed using several criteria, including reflector amplitude criterion. Sizing is performed using a variety of fully digitized scans acquired with various probes in both pulse echo and pitch catch configurations. The sizing accuracies achieved are in 20 to 40 microns range for certain flaws. Flaw evaluation is performed off line, using a variety of methods to display and analyze collected data. The inspection systems in use by Ontario Power Generation and previously by Ontario Hydro have successfully inspected hundreds of pressure tubes in Canadian and foreign CANDU reactors over the last 20 years.