Al_ said:
a dusty plasma design :
https://www.nasa.gov/pdf/718391main_Werka_2011_PhI_FFRE.pdf
has nuclear fuel held in place by electromagnetic fields. It uses a very massive moderator around the fuel to slow and reflect neutrons back to the fuel, to enable continued fission. But, there is one kind of fuel that does not need a moderator: U235. Could this be controlled by expanding the plasma density, and prevented from going critical and exploding? Saving 30 tonnes by having no moderator, would make the engine much lighter, and saving the need to cool the moderator with huge radiators.
There's a lot wrong with the concepts mentioned.
The design of a FFRE, instead, allows these same heavy fission products to escape from the reactor, traveling at up to 5% of light speed. Theoretically, heavy fission products traveling at up to 5% of light speed produce thrust at a specific impulse of one million seconds (over 200 times better than electric engines). The efficiency of a FFRE, as measured by the quantity of fission fragments that escape as a beam rather than remain inside the reactor and produce waste heat, in this study was about 11%.
I don't believe that performance claim.
The sub-micron sized dust, composed of Uranium Dioxide, melts at over3000 Kelvinsand enables operating the FFRE at apower of approximately 1000 MW thermal. Fission fragments that travel forward, rather than aft, are reflected by the superconducting mirror magnet and pass twice through the core on their way to escape. This "double jeopardy" reduces the fraction that escapes and reduces the average exhaust velocity to about 1.7 percent. This FFRE configuration was estimated to produce almost 10 lbf of thrust at a delivered specific impulse of 527,000 seconds.
How sub-micron? UO
2!? U-hydride would be better. I don't believe the 5% or 1.7% of speed of light. Not all fissile atoms will fission simultaneously, but rather a fraction will fission, so fission products will necessarily collide with other atoms (fuel and other fission products) and slow down. Range of fission products is on the order of microns in solid UO
2. Moving a powder of sub-micron particles can be challenging.
One would have to determine the atomic density and multiply by the flux, or actually integrate the product of atomic density of fissile atoms and energy dependent cross-section as a function of energy-dependent flux over the energy spectrum. Based on the claims, I doubt the authors did some serious computation on this system.
I've seen various gas core concepts, and a lot of wacky ideas.
The authors mention Pu-239 early on, then U-235. Pu-239 is better for small cores. One can do a fast, thermal or mixed spectrum reactor. Fast reactors usually require highly enriched fuel in order to offset the low cross-sections at fast energies. Fast neutrons have a long mean free path, so that's a motivation to moderate. A moderated reactor, which could be epithermal, requires lower enrichment.
For a gas core, one would want to basically do a coupled core with a solid core driving the gas fuel.
Another challenge with highly enriched material is storing the fuel in a subcritical configuration.