Measurement of the gas velocity in a water-air mixture in crocus by neutron noise technique

The possibility to measure the gas phase velocity in a two-component mixture with neutron noise techniques is demonstrated in the zero-power reactor CROCUS of the Ecole Polytechnique Federale de Lausanne. It is the first step toward the experimental validation of a theoretical method aiming at the reconstruction of the void profile in a BWR channel. For this experiment, a channel is installed in the water reflector of CROCUS and two-component mixtures are generated inside the channel through injection of air at various flow rates. The signal fluctuations of two neutron detectors located at different elevations next to the channel are recorded and their Cross Power Spectral Density analyzed with various techniques to determine the transit time of the gas phase and its velocity. Experimental results are compared with predictions obtained with the TRACE thermal-hydraulic code. Results disagree in their magnitudes but the evolution of the gas velocity with the air injection rate are similar.