The nuclear salt water rocket(NSWR) uses a plutonium salt (at least to my understanding) dissolved in water as its fuel. The mixture is kept stable by filling the fuel tank with boron carbide, which absorbs any neutrons produced by the fissile material right up until it escapes from the injector, where it then undergoes nuclear fission and heats the water vapour to incredibly high temperatures, achieving an ISP of over 10,000. Achieving this temperature comes with the drawback of cooling however, and as far as I know of there is no way to practically solve this problem. Is there such a way to protect the nozzle from this heat, do we have the means to produce such a setup, and if so, could it be used in a mission, or is this exactly the reason we haven't seen any examples of the technology.

  • $\begingroup$ The [WP description}(en.wikipedia.org/wiki/Nuclear_salt-water_rocket) touches on this: "For example, Zubrin argues that if diluted nuclear fuel flows into the chamber at speed similar to diffusion speed of thermal neutrons, then nuclear reaction is confined in the chamber and does not damage the rest of the system (it is a nuclear analog of a gas burner)." - which seems to set aside that a Bunsen burner isn't a notable source of thrust. If it needs to eject the fuel at such pressure that the reaction doesn't propagate back to the engine then isn't it like a turbopump? $\endgroup$ Mar 15, 2021 at 7:14
  • $\begingroup$ @TomGoodfellow I thought the fission was inhibited by the boron carbide, and only generates heat once it has escaped the boron carbide-clad fuel tank/injector. this still leaves the problem that, once it has reacted, to extract thrust out of the system, the exhaust needs to be run through a nozzle of some sort, even if concession are made in the name of heat resistance, and unless the exhaust speed is nearing the speed of light,I can't see how it would avoid the thermal radiation. $\endgroup$
    – R. Hall
    Mar 15, 2021 at 7:28
  • $\begingroup$ I'm very skeptical as to whether there's been a real workup of the NSWR. $\endgroup$
    – ikrase
    Mar 15, 2021 at 17:25
  • $\begingroup$ @ikrase Indeed. As WP says: These claims have still not been proven due to no test of such a device having ever been conducted. [...] as one physicist wrote, "Writing the environmental impact statement for such tests [...] might present an interesting problem ..." $\endgroup$
    – PM 2Ring
    Mar 15, 2021 at 18:44
  • $\begingroup$ BTW, neutron absorption isn't a passive process. Each absorbed neutron causes a boron nucleus to fission into a lithium & a helium. This will slowly degrade the boron carbide in the fuel tank. $\endgroup$
    – PM 2Ring
    Mar 15, 2021 at 18:54

1 Answer 1


From one of Zubrin's own papers on the subject, NUCLEAR SALT WATER ROCKETS: HIGH THRUST AT 10,000 SEC ISP:

NSWR nozzle diagram

a layer of pure water would be sprayed around the perimeter of the column to form a moving neutron reflector and to protect the plenum walls and throat from the very high temperatures generated in the detonating solution.

How practical this is and what proportion of the propellant tankage would be this shielding water is not detailed in that paper.


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