There is a concept similar to this called a Nuclear Salt Water Rocket (NSWR) that was proposed by an SF writer who is also a physicist.
The fuel is a 2% solution of 20% enriched Uranium Tetrabromide in water. A Plutonium salt can also be used.
Just to make things clear, there are two percentages here. The fuel is a 2% solution of uranium tetrabromide and water. That is, 2 molecules of uranium tetrabromide per 100 molecules of water.
But the uranium tetrabromide can be 20% enriched. This means that out of every 100 atoms of uranium (or molecules of uranium tetrabromide), 20 are fissionable Uranium-235 and 80 are non-fissionable uranium. If it is 90% enriched, then 90 atoms are Uranium-235 and 10 atoms are non-fissionable. As a side note, 90% enriched is considered "weapons-grade".
The fuel tanks are a bundle of pipes coated with a layer of boron carbide neutron damper. The damper prevents a chain reaction. The fuel is injected into a long cylindrical plenum pipe of large diameter, which terminates in a rocket nozzle. Free of the neutron damper, a critical mass of uranium soon develops. The energy release vaporizes the water, and the blast of steam carries the still reacting uranium out the nozzle.
It is basically a continuously detonating Orion type drive with water as propellant. Although Zubrin puts it like this:
As the solution continues to pour into the plenum from the borated storage pipes, a steady-state conditions of a moving detonating fluid can be set up within the plenum.
Also, just to be clear this is a concept that could work in theory but working out the engineering details will be incredibly difficult (and other scientists do not think it will ever be practical).
NSWR:
20% UTB
Exhaust Velocity 66,000 m/s
Specific Impulse 6,728 s
Thrust 12,900,000 N
Thrust Power 425.7 GW
Mass Flow 195 kg/s
Total Engine Mass 33,000 kg
90% UTB
Exhaust Velocity 4,700,000 m/s
Specific Impulse 479,103 s
Thrust 13,000,000 N
Thrust Power 30.6 TW
Mass Flow 3 kg/s
Far more information available at the link provided above.