Evaluation of Application of the Gas Core Nuclear Reactor to the Icarus Mission
by Adam Crowl
The gas core fission reactor is a concept for achieving improved performance relative to solid core reactors by operating at a temperature at which the nuclear fuel is a gas. Several possibilities exist for containing the fissile gas but, in the generally favored approach, the nuclear material is contained in the chamber by means of a stable vortex induced by tangential injection of the reaction mass fluid (usually hydrogen). The extreme temperature of the core allows heating of the reaction mass to much higher temperature than conventional reactors of the Nerva variety thus leading to higher exhaust velocity and specific impulse. While the stable vortex has been demonstrated in cold flow, no such critical assembly has ever been operated. A number of practical considerations stand in the way of actual use including, but not limited to, heat transfer to the reaction mass fluid, limitations on cooling of the structure of the engine and details such as how to start and stop the reactor. Even if the gas core reactor propulsion system lived up to its potential performance, this performance is entirely inadequate for an interstellar spacecraft. That said however, such a system might well be useful as secondary propulsion. Secondary propulsion might include course adjustments for the main vehicle or as main propulsion for probe vehicles that might be targeted for flybys of individual planets in the system being investigated. The performance of a gas core nuclear rocket has often been touted as being on the order of 3000 sec Isp and, optimistically, even higher. Practical considerations such as structural heating probably limit the achievable Isp to something closer to 1500 sec. This is still a substantial improvement over solid core nuclear rockets and probably makes the idea worth pursuing. A variation on the idea has the fissioning gas fully contained in a transparent envelope. The presence of the envelope inherently limits the gas temperature. The transmissibility of the envelope in the frequency range of interest is critical because any substantial absorption of the optical radiation will vaporize the envelope. The only candidate seems to be quartz. While the gas temperature limits render this approach unlikely for a rocket, it might show promise for an auxiliary power plant for the vehicle. The nuclear material used might be a vapor of U235 or possibly UF4 or UF6. The latter compounds are gases at room temperature and thus avoid the complexities of vaporizing the solid uranium and condensing it when the engine is shut down. Starting the reactor, assuming the nuclear fuel is already a gas, should be fairly straightforward since the vortex can be established with the reaction mass fluid and the fissile material then introduced into the core until criticality is reached. Shutdown may be far more difficult. One cannot simply turn off the reaction mass because the stabilizing effect of the vortex is lost and the hot fissile gas would be free to expand vaporizing everything in its path. It would immediately go subcritical of course but still, it is a mass of very hot plasma with great destructive potential. By throttling the reaction mass flow it might be possible to let the fissile material leak out into space in a somewhat controlled manner. This would preclude damage but would mean a new load of uranium for every start of the engine. If the engine is not intended for reuse, this might be acceptable operationally. The issue of local contamination by a highly radioactive substance must be considered however. Ideally, one would extract the hot fissile material from the core and return it to its original containers for reuse although the practicality of this is open to serious question for many obvious reasons. If one opens the neutron reflectors so that the reactor becomes subcritical, the fissile material will cool down but the flow of reaction mass must be continued to contain the material until it is cool enough to recapture. If the fuel burn percentage is large, there will be a large inventory of fission products and daughter products. The decay of these isotopes generates significant energy and cool down may take some time. The reaction mass expelled during this period will be much cooler than while the reactor is critical, reducing the effective specific impulse of the overall operation. It will be clear from this summary that the gas core nuclear rocket is a promising concept however serious operational and engineering issues arise when one contemplates using such a device. The currently on-going study is looking at the concept in much more detail.