Experiments have been carried out to investigate the boiling phenomena in the downcomer, and RELAP5/MOD3.2 has been assessed with the present experimental data. A heated wall with a thickness of 8.2 cm and a height of 32.5 cm is used. The wall is made of the same material as the prototype (APR1400) with chrome coating to protect against rusting. From the experiment, we visually observed strong liquid recirculation and vapor jetting near the heated wall. These phenomena arose from axial migration of voids located only in the thin layer of the heated wall, whereas there was little bubble migration to the bulk region. The size of the thin layer is below 4 cm, which is used for the determination of the radial nodal size in radial double-node schemes. The RELAP5 calculations using three different nodal schemes are compared with experimental data in terms of water level, void fraction, wall temperatures, and phase velocities. The radial single-node scheme produces no liquid recirculation, resulting in a sudden level drop due to a sudden increase in void fraction. The double-node scheme with top-bottom radial connections yields strong circulation, eliminating the sudden level drop. As a result, the scheme produces better results than the radial single-node scheme and a double-node scheme with all radial connections. Based on the information from measurement of the local liquid velocity profile and visual observations, a drift velocity model is developed for application into a downcomer with a large gap and a vertical heated wall. The proposed drift velocity model has been implemented into RELAP5 and verified with experimental results.