Thorium nuclear engines?

[Kerbface]

I really don't know much about atomic engines for spacecraft at all, but I was reading up on what an incredibly efficient fuel source thorium would be on Earth and I was just wondering if it would also be effective for rocket engines? I couldn't find anything talking about it, for all I know it may be impossible...

[Kryten]

U²³³, which is what 'thorium' reactors actually use, works just the same way as U²³⁵ or Pu²³⁹ in reactors. There's no reason it wouldn't work in something like NERVA.

[Rune]

It needs a slightly larger core, doesn't it? I mean, NTR cores are ultra compact highly enriched reactors. They are really on the edge of what a fission reaction can be. I mean, how many commecial reactors are out there with neutron reflectors and GWs thermal output in a couple of tons? And thorium reactors need a slightly more massive core in commercial reactors, IIRC.

Rune. Not my thing either, but I am interested.

[architeuthis]

I agree Thorium breeds U233. IRC the Project Rover reactors all used 90% enriched U235.

[softweir]

Thorium reactors provide little advantage for mobile reactors such as those used in Nerva engines:- they are less compact and much heavier! There may be safety advantages for large nuclear vehicles such as aircraft carriers, but the extra weight is an issue.

Thorium reactors are safer than Uranium or Plutonium-based reactors: the fuel is liquid and can be pumped through the heat-exchangers, and it can be dumped out of the reactor core into a heavily-damped secondary chamber that kills the reaction in the event of serious reactor failure, thus preventing meltdowns. Additionally, in some states (such as India) Thorium is much more readily available than Uranium. Finally, there is only one isotope, so it doesn't need to be enriched before it can be used.

[Nuke]

molten salt reactors (forefather of lftr) have been demonstrated while installed in an aircraft. making lftr work in zero gravity would be a tough challenge. some of the safety features might not work. for example in the gravity well, when the reactor gets too hot, or the power goes out, all the fuel drains out to subcritical tanks. that would not work in space so well. perhaps draw it out of the reactor with ambient vacuum and a small amount of inert gas. on the other hand it would have similar benefits to the liquid metal reactors that have already been used in space. probibly in a fluoride with a liquid metal secondary loop, a thermionic converter and radiator panels, or i suppose to provide heat to a reaction engine. this kind of reactor is something i would use with nuclear-electric propulsion, rather than a nerva type engine though. it would be a lot more efficient than a nerva, with say mpd or vasmir type thruster (newtons to tens of newtons, and isp between 3500 and 6000), which are severely limited to what can be done with solar panels and rtgs (milinewton thusters are about all you are doing with those). and you would have power for more instruments and/or life support for manned missions, not to mention more powerful transmitters.

nerva does have one benefit of being extremely simple with fewer moving parts. and we are probibly better off saving the thorium for ground based applications anyway since its really good at burning up long term nuclear waste. for an interplanetary or interstellar mission where the reactor is as far away from habited areas as possible, and probibly wont be coming back, nuclear waste is less of an issue, so use the uranium/plutonium for those jobs. we will be seeing nervas long before nuclear-electric is used (barring other breakthrough propulsion technologies making either of them obsolete).

Edited May 30, 2013 by Nuke

[Kryten]

Why do people conflate liquid salt reactors and thorium reactors? All the thorium reactors actually in operation that aren't tiny research reactors are solid fuel, and liquid designs work just as well with more conventional fuel cycles.

[Nuke]

because lftr. its a really sexy reactor design. and i said it was a forefather, not the same, there is a difference.

Edited May 30, 2013 by Nuke