Fissioning uranium plasmas are the gaseous fuel in high-temperature cavity reactors, originally conceived for nuclear rocket propulsion in space. A predominantly pragmatic research effort, sponsored by the National Aeronautics and Space Administration, has led to the determination of the most important characteristics of the uranium nuclear fireball in gaseous core reactors. For achieving thrust at a specific impulse up to 5000 sec, the nuclear fuel must bum at a temperature in excess of 10 000 K. For criticality the uranium particle density must be not less than the molecular density of gases at standard conditions, which, in combination with the high temperature, results in a uranium plasma pressure of several hundred atmospheres. The plasma is confined by a peripherally injected propellant flow, which simultaneously intercepts the thermal radiation from the nuclear fireball and provides for an effective mechanism for heat transfer. Results of extensive research indicate that the plasma core reactor scheme is feasible. In these investigations it was assumed that because of the high pressure the fissioning plasma is optically thick. It is now believed that in gases, the energy release of fissions can lead to distributions of ionized and excited states that deviate from Maxwell-Boltzmann distributions. In that case, the fissioning plasma, or gas, exists in a nonequilibrium state and is optically thin. This condition can be exploited for the direct conversion of fission fragment energy into coherent light, that is, for the nuclear-pumped lasers. In current research, the nonequilibrium conditions of fissioning plasmas and gases are emphasized, culminating in the first successful demonstrations of experimental nuclear-pumped lasers, and in a program of gaseous fuel reactor experiments with enriched uranium hexafluoride. A variety of applications of plasma core reactors and nuclear-pumped lasers is now envisioned for benefits in space and on earth. Such benefits include advanced propulsion in space, terrestrial power generation approaching 70% efficiency, the possibility of nuclear bumup of transuranium actinides wastes, and the breeding of 233U from thorium. The research on gaseous fuel reactors and nuclear-pumped lasers predominantly requires expertise in nuclear engineering, plasma, atomic, and molecular physics, and fluid mechanics and chemistry. A multidisciplinary effort is seen as a logical approach.
