Ultrasonic testing methods and procedures for volumetric and surface inspection of CANDU reactor pressure tubes

Fuel channel pressure tubes are critical components at the heart of the CANDU reactor core which require periodic inspection to demonstrate continued fitness for service. A CANDU reactor consists of a large tank, the calandria, which is penetrated by several hundred horizontally mounted fuel channels containing natural uranium fuel bundles. The fuel channel consists of a 104 mm inside diameter, 4.3 mm thick Zirconium - Niobium pressure tube, inserted into slightly bigger calandria tube, and two stainless steel end fittings at the ends of the fuel channel. Ontario Power Generation; Inspection and Maintenance Services Division operates a variety of inspection systems to perform volumetric and surface nondestructive examinations of the pressure tubes. At the core of those systems is an ultrasonic probe module which contains several ultrasonic probes interrogating inspected tube from several angles and with various wave modes at the same time. The ultrasonic inspection system utilizes high frequency ultrasound (10 MHz and 20 MHz) in order to achieve high degree of detectability of small flaws and high accuracy of flaw sizing. The inspection is performed in two phases: - ultrasonic flaw detection; - sizing of detected flaws. Detection of the flaws is performed using several criteria, including signal amplitude criterion. Sizing is performed using a variety of B-scans acquired with various probes in both pulse echo and pitch catch configurations. The flaw depth sizing accuracies achieved are in 20 to 40 microns range for certain flaws. The pressure tube 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. (orig.)