As part of the emergency core cooling studies for the SNR-300 reactor, the transient thermohydraulic behavior of the reactor inlet plenum was investigated. Using a simplified and scaled-down model of the plenum and using sodium as the working fluid, thermal shock experiments were performed for different mass flow rates and initial temperature differences. The measured temperature distributions of the test section were compared with calculated results of a computer program. Satisfactory agreement between theoretical and experimental results was obtained only if the influence of superimposed free convection on forced convection heat transfer was taken into account. The heat transfer coefficients used were theoretical results calculated by the computer program CØFFCØ. The experimental results consolidated the theoretical heat transfer coefficients for turbulent combined free and forced convection for a modified Rayleigh number ranging from 4 × 105 to 6 × 106.