There is a concept similar to this called a <a href="http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Gas_Core--Open_Cycle--Nuclear_Salt_Water">Nuclear Salt Water Rocket (NSWR)</A> 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:  
[![NSWR][1]][1]

20% UTB
<pre>
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</pre>

90% UTB
<pre>
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</pre>

Far more information available at the link provided above.

  [1]: https://i.sstatic.net/zBQmV.jpg