During the emergency response period of the accidents that took place at the Fukushima Daiichi nuclear power plant (NPP) in March of 2011, researchers at Oak Ridge National Laboratory (ORNL) conducted a number of studies using the MELCOR code to help understand what was occurring and what had occurred. During the postaccident period, the U.S. Department of Energy (DOE) and the U.S. Nuclear Regulatory Commission (NRC) jointly sponsored a study of the Fukushima Daiichi NPP accident with collaboration among ORNL, Sandia National Laboratories, and Idaho National Laboratory. The purpose of the study was to compile relevant data, reconstruct the accident progression using computer codes, assess the codes' predictive capabilities, and identify future data needs. The current paper summarizes some of the early MELCOR simulations and analyses conducted at ORNL of the Fukushima Daiichi NPP Unit 3 (1F3) accident. Extended analysis and discussion of the 1F3 accident are also presented taking into account new knowledge and modeling refinements made since the joint DOE-NRC study.