Measurements of the void-fraction distribution in a simulated fuel assembly and the role of the void-fraction on the dynamics of a natural circulation loop

The role of the void-fraction in a natural circulation BWR has been investigated using the scaled natural circulation loop facility DESIRE. To this end measurements of the void-fraction have been performed on a subchannel basis using gamma-transmission tomography. Subcooled boiling and void slip were observed. Lateral transport of void between the subchannels was negligible. Also the effect of the void-fraction on natural circulation has been studied both in steady-state and dynamic situations using noise analysis techniques. Results of the steady-state and dynamic measurements are compared. The void-to-flow transfer function is strongly affected by transport of void from the core region to the riser section