Yucca Mountain has been designated as the nation's high-level radioactive waste repository, and the U.S. Department of Energy has been approved to apply to the U.S. Nuclear Regulatory Commission for a license to construct a repository. The temperature and humidity inside the emplacement drift will affect the degradation rate of the waste packages and waste forms as well as the quantity of water available to transport dissolved radionuclides out of the waste canister. Thermal radiation and turbulent natural convection are the main modes of heat transfer inside the drift. This paper presents the result of three-dimensional computational fluid dynamics simulations of a segment of emplacement drift. The model contained the three main types of waste packages and was run at the time that the peak waste package temperatures are expected. Results show that thermal radiation is the dominant mode of heat transfer inside the drift. Natural convection affects the variation in surface temperature on the hot waste packages and can account for a large fraction of the heat transfer for the colder waste packages. The paper also presents the sensitivity of model results to uncertainties in several input parameters. The sensitivity study shows that the uncertainty in peak waste package temperatures due to in-drift parameters is <3°C.