Over the past decade, Prof. Hibiki and a scientist at the Institute of Nuclear Safety Systems have engaged in international technical collaboration related to advancing the reactor system analysis code RELAP5. They developed a basic database through core-simulated rod bundle tests and constitutive equations for drift-flux models for core and downcomer analyses to refine the simulation code for two-phase flow behavior in the core and downcomer under the low-pressure and low-velocity conditions expected in a reactor accident. The constitutive equations of the drift flux model for core and down-comer section analyses were developed. These constitutive equations have been implemented in the RELAP5 code, and various test data have demonstrated their validity. Their technical development results are expected to improve the prediction accuracy of the safety analysis of nuclear reactors.