Abstract: The RELAP5/MOD3.2 code was validated with experimental data for simulating pool boiling heat transfer outside vertical tube bundle. By considering the axial natural convection and the radial gas-liquid heat exchange in large water tank, the model using RELAP5/MOD3.2 code was properly created. Compared with the experimental data, it is found that the maximum relative deviation between the experimental and calculated boiling heat transfer coefficients exceeds 50% in the experimental range, and the boiling heat transfer coefficient change tendency between them is also obviously different with the increase of heat flux. Therefore, it is indicated that the Chen correlation is not applicable to calculate the pool boiling heat transfer outside vertical tube bundle. By contrast with the existing pool boiling heat transfer calculation correlations, the calculated results agree well with the experimental values for the Kutateladze “new” correlation or Rohsenow correlation.