The tritium proof-of-principle (TPOP) experiment was designed and built by Oak Ridge National Laboratory (ORNL) to demonstrate the formation and acceleration of the world's first tritium pellets for fueling of future fusion reactors. Many parameters measured during the course of the experiment have been used to evaluate the physical properties of solid tritium. Pellet size was measured as a function of equilibrium fill pressure. A model was developed to predict this information from values of thermal conductivity, vapor pressure, and density reported in the literature. Good agreement between theory and experiment was found for both deuterium and tritium pellets. Evaluation of breakaway pressure data for deuterium pellets indicates that the shear strength of deuterium is about equal to its ultimate tensile strength. Tritium shear strength appears to be about twice that of deuterium at temperatures around 8 K. The reduction in pellet diameter due to barrel erosion for deuterium was about twice that for tritium pellets at a given velocity. This was also indicative of the greater strength of tritium relative to deuterium.