In the new generation of nuclear reactors design, a series of passive safety systems are adopted to provide significant improvements in plant simplification, safety and reliability. However, when applying the RELAP5 code in predicting the passive residual heat removal system (PRHRS), the capability of RELAP5 code needs further validation. In the present work, in order to validate the RELAP5 code for simulating nucleate pool boiling heat transfer outside vertical tube bundle, a test facility of pool boiling heat transfer outside vertical tube bundle was designed and built. By considering the axial natural convection and the radial gas-liquid heat exchange in large water tank, two kinds of RELAP5 calculation models are adopted in this paper. With the comparison between calculated results and experimental data, it is found that the maximum deviation between the experimental and calculated boiling heat transfer coefficient exceed 50% in the test condition, and the boiling heat transfer coefficient change tendency between them is also obviously different with the increase of heat flux.
