Realistic interplanetary space exploration depends critically upon the development of a high-speci cimpulse propulsion system. Previous studies indicate that the speci c impulse of an open-cycle gas-core nuclear rocket (OCGCNR) might approach 3000 s. Although the OCGCNR is deceptively simple in concept, it will be dif cult to develop in practice because the core is a uranium plasma that must be nearly totally con ned. Before constructing a more comprehensive model for this engine, there is a requirement to understand the limits of present full-scale simulation models and recent scaled experiments. In this scoping study we have used a two-dimensional, axisymmetric, nite difference code to investigate the formation and stability of a recirculation region observed in a scaled experiment. It has been proposed that such a recirculation region, or vortex, might provide improved con nement of the uranium fuel. Our simulation results indicate that a more comprehensive model must treat the rocket nozzle in a selfconsistent fashion to properly calculate the con nement of the uranium plasma. Under conditions that lead to vortex formation, the position of the vortex depends upon the inlet geometry and injection velocity, the nozzle position and subsonic convergence angle, the base-bleed injection rate, and turbulence. With a large base-bleed injection rate, a vortex forms but is then swept away through the nozzle, a result that resolves an inconsistency between a full-scale engine simulation model and recent scaled experiments.