Application of Synthetic Aperture Focusing Technique for Estimation of Width Using Pulse Echo Ultrasonic Testing

The condition of bowing of fuel rods in nuclear reactors is often detected through imaging of sub-assembly tops using C-Scans obtained from Pulse Echo Ultrasonic technique. The detection of bowing by means of post processing of A-Scan signals is discussed in this paper. Techniques have been initially developed on hollow cylinders, which mimic the sub-assembly tops. A-Scans are collected by placing the unfocussed ultrasonic transducer at the top of the hollow cylinder, kept in the water bath. The beam spread causes the object to be detected even when the transducer is not exactly above the object. This results in over sizing of the object. The objective is to estimate the width precisely by means of which, condition of bowing could be detected. Synthetic Aperture Focusing Technique is applied on these A-Scans to estimate the width. The width of the object can be estimated at different positions on the top surface of the object, from which centre points at each position can be obtained. If these centre points form an ellipse or a deformed circle, then bowing condition is detected, else normal condition is indicated. Two hollow cylinders of known width 5 mm and 3 mm have been used here. Application of this technique with a synthetic aperture of 14.4 mm, has given a minimal error of 1.4 mm and 0.3 mm for the 5 mm and 3 mm cylinders respectively.