Gas core NTP

ToughSF has pointed to a couple of gas-core NERVA ideas. One is from Terry Kammash in June 1993 (pdf); arguing for a denser core to lessen the Kelvin-Heimholtz instability. He chooses Americium-242 of the metastable isomer. Another article from Albert Kascak 2012 posts an upper constraint on specific-impulse. This, based on ye olde thermodynamics on the cavity-wall and nozzle. Uranium-235 is assumed but I suspect not required.

Where has Kammash found his Americium-242m: from Plutonium-241. This (he says) is a wasteproduct of pressurised-water reactors. I don't know if we still want those, or supercritical CO₂, but hey; the nuclear fuel will be the same. From a 1993 perspective that fuel is uranium. I don't know what isotope of uranium; it may be U-235, which worries me, since we don't have a lot of that and we're not making more. U-238 (breeding to Pu-239) is more promising. As bonus, at a higher atomic number, it should produce Pu-241 faster.

Problem here is that here we're proliferating nucleides, which we normally don't like. My suggestion is to build this breeder-reactor where the space programme resides; hook it up to all those coilguns and SpinLaunch slings, so the wider electric grid doesn't have to.

There's also the law. I don't know if Am-242m gets to weapons-grade as easily as U-235; UltraSafe has been diluting their uranium to "LEU" (such that it's not gas). The law limits I_SP more than do the thermodynamics, around Earth.

UPDATE 3/24/23: Gerrit Bruhaug discusses thermal reactors.