The acoustic background noise of a liquid metal fast breeder reactor

Absolute measuements of the background noise in the upper plenum of an operating liquid metal fast breeder reactor (LMFBR), the Experimental Breeder Reactor II (EBR‐II), have been performed over a 17‐month period. The temporal spectral density characteristics of the data were found to be representative of a weakly stationary random process. Cross‐power spectral density analyses established that below 5 kHz the spectral characteristics were dominated by pump‐induced structural vibrations and the excitation of acoustic resonances in the upper plenum volume. The spectral characteristic of the data above 5 kHz were found to be consistent with sonic power generation at the reactor subassembly exit nozzles and the statistical response of the cylindrical plenum volume. The production of sound by the exit flow from the subassembly nozzles was found to produce a broadband pressure level of 16l dB re 1 μPa, corresponding to an acoustic efficiency of 6×10⁻⁸. This compares favorably with a theoretical (Lighthill) value of 159 dB re 1 μPa and an acoustic efficiency of 9×10⁻⁹ for a submerged water jet. These measurements show that the noise field in the plenum of a fast reactor is a weakly stationary random process with identifiable sources of sound.