A gas nuclear reactor (or gas fueled reactor) limits the only temperature limiting materials in a reactor to the reactor walls. A limitation for conventional nuclear fission reactors is that if the nuclear fuel temperature were to rise too high in temperature, the Nuclear reactor core would melt. It may also be possible to confine gaseous fission fuel magnetically, electrostatically or electrodynamically in the reactor so that it would not touch (and melt) the reactor walls. A potential benefit of the gaseous reactor core concept is that instead of relying on the traditional rankine or brayton conversion cycles, it may be possible to extract electricity magnetohydrodynamically, or with simple direct electrostatic conversion of the charged particles.
Theory of operation
The vapor core reactor (VCR), also called a gas core reactor (GCR), has been studied for some time. It would have a gas or vapor core composed of UF4 with some 4He and/or 3He added to increase the electrical conductivity, the vapor core may also have tiny UF4 droplets in it. It has both terrestrial and space based applications. Since the space concept doesn’t necessarily have to be economical in the traditional sense, it allows the enrichment to exceed that which would be acceptable for a terrestrial system. It also allows for a higher ratio of UF4 to helium, which in the terrestrial version would be kept just high enough to ensure criticality in order to increase the efficiency of direct conversion. The terrestrial version is designed for a vapor core inlet temperature of about 1500 K and exit temperature of 2500 K and a UF4 to helium ratio of around 20% to 60%. It is thought that the outlet temperature could be raised to that of the 8000 K to 15000 K range where the exhaust would be a fission-generated non-equilibrium electron gas, which would be of much more importance for a rocket design. A terrestrial version of the VCR’s flow schematic can be found in reference 2 and in the summary of non-classical nuclear systems in the second external link. The space based concept would be cut off at the end of the MHD channel.
Reasoning for He-3 addition
3He may be used in increase the ability of the design to extract energy and be controlled. A few sentences from Anghaie et al. sheds light on the reasoning:
The spacecraft variant of the gaseous fission reactor is called the gas core reactor rocket. There are two approaches: the open and closed cycle. In the open cycle, the propellant, most likely hydrogen, is fed to the reactor, heated up by the nuclear reaction in the reactor, and exits out the other end. Unfortunately, the propellant will be contaminated by fuel and fission products, and although the problem can be mitigated by engineering the hydrodynamics within the reactor, it renders the rocket design completely unsuitable for use in atmosphere.
One might attempt to circumvent the problem by confining the fission fuel magnetically, in a manner similar to the fusion fuel in a tokamak. Unfortunately it is not likely that this arrangement will actually work to contain the fuel, since the ratio of ionization to particle momentum is not favourable. Whereas a tokamak would generally work to contain singly-ionized deuterium or tritium with a mass of two or three daltons, the uranium vapour would be at most triply-ionized with a mass of 235 dalton (unit). Since the force imparted by a magnetic field is proportional to the charge on the particle, and the acceleration is proportional to the force divided by the mass of the particle, the magnets required to contain uranium gas would be impractically large; most such designs have focussed on fuel cycles that do not depend upon retaining the fuel in the reactor.
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