Implementation of a Two-Phase Boiling Model into the RELAP5/MOD2 Computer Code to Predict Void Distribution in Low-Pressure Subcooled Boiling Flows

This paper demonstrates that the empirical models developed for subcooled flow boiling in RELAP5/MOD2 at high pressures are not valid for applications at low pressures. Modifications carried out in RELAP5/MOD2 to include better correlations of the interphase heat transfer and mean bubble diameter, and the wall heat flux partition model are shown to yield substantial improvements in the predictions of the axial void fraction distribution. When compared against experimental data covering a wide range of heat fluxes and flow rates, predicted axial void fraction profiles follow closely the measured data. Predictions made by the default subcooled boiling model show, however, an unacceptable margin of error with the experimental data.