Nuclear power reactors generate highly radioactive spent fuel assemblies. Initially, the spent fuel assemblies are stored for a period of several years in an on-site storage facility to allow the radioactivity levels of the assemblies to decay. As the radioactive fission product isotopes in the fuel decay, they generate significant amounts of thermal energy producing high temperatures in the spent fuel. The spent fuel from nuclear power plants will eventually have to be transferred to a federal geologic repository in a spent fuel transportation casks. The purpose of this research project is to characterize the relative importance of the heat transfer mechanisms of radiation, conduction, and convection in a dry horizontally-oriented nuclear spent fuel assembly, for eventual application in spent fuel transportation cask design. To determine the relative importance of each heat transfer mode, an experiment was designed and operated to characterize the heat transfer in an 8x8 square heater rod array (similar to a Boiling Water Reactor fuel assembly) in a horizontal orientation. The experimental apparatus was operated with the following variable parameters and their ranges: Power to Heater Rods (Controlling Temperatures from 40'C to 250'C); Heater Transfer Medium (Air, Nitrogen, Argon, and Helium); Pressure of the Heat Transfer Medium (15 psig, 0 psig, 24 inches of mercury); Power to Boundary Condition Box (not controlled). The experiment was designed, fabricated, and operated under the Sandia National Laboratories-approved MIT Nuclear Engineering Department Quality Assurance Program developed in this work specifically for this project. The test data obtained from the experimental apparatus was analyzed with the lumped keff/hedge model developed by R.D. Manteufel at MIT, in related work on this research project, and the Wooten-Epstein relationship developed at Battelle Memorial Institute. The test data was used to validate the lumped keff/hedge model. Good agreement was found between the lumped keff/hedge model and the test data in each Test Campaign with the exception of Below Atmospheric Pressure data. Both experimental and theoretical sources for the discrepancy are discussed. However, the full reason for the deviation is not know.