The Nuclear Salt Water Reactor

Recently, from a YouTube video, I heard about Robert Zubrin's invention
of the Nuclear Salt Water Reactor. It is a kind of rocket motor with an
incredible specific impulse, such that if it could be made to work, it
would allow velocities of over 5 percent of the speed of light to be
attained, making sublight travel to other star systems practical.
I'm pretty sure that I had seen the NSWR mentioned before long ago, but
at
the time I paid it no mind, it was only after seeing that YouTube video
that
I realized what a revolutionary potential it had.
Except...
One thing I remember about the development of the atomic bomb is that
when
subcritical masses of fissionable material are brought together, they
have
to be brought together very quickly. Otherwise, there won't be an
explosion,
there will just be a "fizzle" that releases a lot of neutrons and other
deadly radiation.
I really don't see how an NSWR could possibly be made that would
actually
produce thrust instead of just a lot of radiation and a little heat,
because
spraying streams of liquid together is vastly slower than shooting
something
with a charge of high explosives. Explosion never, fizzle always.
So instead of having to solve the engineering problem of the great heat
of the reaction melting the thrust chamber, say with water cooling, it
looks like the NSWR is impossible on the face of it. Or maybe some
really
fast method of pulsed pumping of the fluid is possible...
John Savard