Thermo nuclear Thruster
Two variants of thermo nuclear engines powered by water
Water is found on all celestial bodies. In addition, it is found as ice asteroids in the asteroid belt between Mars and Jupiter. A "refueling" of water is thus always possible.
The advantage is also that the uranium used can certainly be used for well over 50 years, as the engine is very often decommissioned.
The Steam Thruster as variant 1
As a steam engine, I imagine a uranium engine that produces water vapor from nuclear heat release. The water is converted to a very hot steam. This results in a very strong pressure. If the engine nozzle is calculated accurately, the water vapor could flow out in parallel to the rear under the extremely strong pressure and thus generate thrust.
Double bang thruster as variant 2
The double bang thruster requires heat resistant materials. It must be remembered that the water does not react until it reaches 4500° Fahrenheit (2500°C). This temperature can certainly be reached with a nuclear reaction, but must clearly take place in the center of the thruster. This heat would melt the surrounding metal.
So this is a very critical variant, but it would be very interesting.