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MELCOR, SCDAP/RELAP5, MAAP4 are three representative U.S. computer codes that are being most widely used for the integral analysis of core melt accident progression and the resultant lower head response expected at the reactor lower plenum such as the accident that took place in the TMI-2: the first two are NRC codes and the last one is an industrial code. However, their capabilities in predicting core melt accident progression and lower head response differ substantially at each phase of the lower plenum accident progression, due to the different modeling assumptions involved in various initial and boundary conditions, different debris bed configurations and heat transfer correlations, different model parameters and numerical schemes. The main objectives of this paper are to give (1) some insights into the aforementioned questions through a qualitative comparison of functional aspects of the lower head models employed in the three U.S. severe accident codes and (2) some potential areas that should be taken into account for further modeling of the present lower head models. [Copyright &y& Elsevier]