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.