Impact of the Test Device on Acoustic Emission Signals From Nuclear Safety Experiments: Contribution of Wave Propagation Modeling to Signal Processing

A strongly energetic spectral component has been observed in the operating noise of a research reactor before the beginning of reactivity-initiated accidents (RIAs). We identify the source of this component by means of analytical derivations of torsional wave propagation in the test device. Numerical simulations, performed with a spectral-element method, confirm that this component is a resonant frequency of the device, and allow to evaluate the impact of wave travelpath in the test device on the acoustic emission (AE) signals recorded during RIA experiments. The transfer function of the test device is strongly dependent on both the source and the receiver locations, which precludes signal processing by deconvolution when the location of the AE source mechanism is unknown. Moreover, the geometry and the configuration of the test device may not influence the signals in the low-frequency range and hence the signals generated by source mechanisms located on the fuel clad such as clad failures. Finally, replacing the heat transfer fluid (pressurized water) by sodium leads to an increase in the recorded signal amplitude and a small shift of the frequency content toward the high frequencies. The results obtained in this paper may be useful to improve the design of future RIA experiments. They are also useful to understand wave propagation in the core of the reactor and to choose appropriate signal processing tools for recovering the source of AE signals generated during RIA experiments.