Non-destructive measurement of fuel rods inner pressure and composition: Numerical consideration of different spring dimensions

The technique presented in this paper enables non-destructive measurement of both the internal pressure and gas composition within fuel rods used in pressurised water reactors. The sensor consists of a piezoelectric element shaped as a tile, which can be applied directly on the fuel rod cladding in the plenum region, where fission gases accumulate. Acoustic waves generated by the sensor propagate inside the cladding and the reflections occurring within the gas are detected. The time-of-flight between measured echoes is used to retrieve the composition of the internal gas mixture. Pressure is related to the amplitude of the acoustic signal and is determined through a calibration process that links theoretical amplitude to experimental measurements. The developed theoretical model significantly reduces the calibration time by taking into account the losses and signal attenuation caused by the presence of a spring located within the measurement zone. This paper presents the construction of a theoretical model that considers the effects of the pressure and gas composition, as well as the shadowing and diffraction phenomena induced by the spring, all of which influence the amplitude of the measured acoustic signal.