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