Nuclear rocket engine technology is being considered as a means of interplanetary vehicle propulsion for a manned mission to Mars. Significant technological research and development are required before nuclear-based rocket propulsion can be integrated into an interplanetary vehicle, including the firing of full-scale nuclear rocket engines in a test and evaluation facility. Testing of nuclear engines in the 1950s and 1960s was accomplished by directing engine exhaust gases into the atmosphere, a practice that is no longer acceptable. Testing nuclear rocket engines by injection of associated radioactive exhaust gases and water vapor into deep unsaturated zones may be a way to sequester radionuclides and will require comprehensive design of a nuclear engine test facility. We conducted numerical simulations to determine the ability of an unsaturated zone with the hydraulic properties of Yucca Flat alluvium at the Nevada National Security Site to contain gas-phase radionuclides. In these simulations, gas and water vapor (from water sprayed into the exhaust for cooling) were injected for two hours at a temperature of 600°C and with rates of 14.5 kg s-1 and 15 kg s-1 , respectively, in varying thicknesses of alluvium with an intrinsic permeability of 10-11 m2 and porosity of 0.35. These simulations suggest that following the test of an engine, gaseous radionuclides injected below 200 m will not migrate to the land surface. The simulations show that the gaseous/vapor injectate will cool and condense within several meters of the injection point, although there will be limited, if any, downward drainage of liquid. However, the nearly horizontal hydraulic groundwater gradient present in Yucca Flat should limit lateral migration of any condensate that may drain downward and reach the water table.