Computer Simulation of Ultrasonic Measurement of In-Situ ´Bow´ of the Prototype Fast Breeder Reactor Fuel Sub-assembly

An Ultrasonic under-sodium scanner has been developed for use in Prototype Fast Breeder Reactor (PFBR), Kalpakkam, India. We have devised a scanning technique to determine the in-situ ´bow´, if any, of the fuel sub-assembly(FSA) in the reactor by any one of the three methods viz ´circle-fitting´, ´geometrical´ and ´Hough transform-based´. In this paper, we propose a computer simulation of the ultrasonic pulse-echo measurement technique adopted in an earlier under-water experiment to determine the distances of a mock-up FSA head located at distances of 135 to 161 mm from the scanner axis, simulating the ´bow´ up to 26mm. The simulation includes an acoustic model using Huygens approach to calculate the amplitudes of the ultrasonic echoes received by two downward viewing transducers (DVTs), while they scan over a simulated FSA top surface . We estimate the distances by the latter two of the above methods to an accuracy of ±2mm. These values, which are more accurate than the experimental values, are obtained after selecting for the simulation study a larger scale value for the distance to number of image pixels and a lower value for the incremental scanning angle (than those of the experiment), and by processing of ´direct-surface´ echoes.