Study of forced convection boiling of sodium at low heat fluxes in electrically heated seven-rod bundles

An experimental study of forced convection boiling of sodium in electrically heated seven-rod bundles of fast reactor subchannel geometry is made. These conditions are similar to those found in in-core gamma-heated sodium boiling experiments. Analysis of the experimental data shows that various boiling patterns can be generated, depending upon the degree of incipient superheat, the amount of inlet throttling and the energy storage potential of the system. The voiding patterns associated with these parameters are related to the observed experimental signals and explained in terms of the controlling physical processes. The behavior of pressure signals generated during boiling is found to be highly time dependent and is accounted for by variations in the noise source and the attenuation properties of an expanding two-phase region. Calculations with the SAS3A computer code (10) show good qualitative agreement with the experiments. For slow transients in small bundles at low power, the multi-bubble model used in the code is a suitable approximation. Discrepancies between the code predictions and experimental data are explained in terms of the computational model. The importance of adequately accounting for system heat capacity and external heat losses is demonstrated. The design and analysis of in-core, gamma-heated boiling generators are examined in the light of these results. Significantly different voiding scenarios can be produced by variations in bundle design or operation. The expected neutronic and acoustic signals for several idealized voiding patterns are considered