This is the original paper by R. Zubrin proposing the Nuclear Salt Water Rocket design.
Basically the design is that a capillar set of pipes store a uranium salt-water solution, inside a cadmium matrix, which helps to keep it below criticality. The fluid is simply flown at high speed into a combustion chamber where it reaches criticality, heats quickly and is expelled from some unspecified nozzle.
The paper is from 1991, and i'm sure that the design has been revisited many times, so there must be good critiques and discussion of the problems that would have to be addressed, but i could not find any serious follow-up literature about the viability of the design, just, you know, the usual blog and mailing list informal ranting that you can find from a simple google search.
Have there been any simulations or attempts to evaluate the design with more detail in the literature that i'm not aware? The original paper doesn't go into much discussion about the temperatures, which i presume are quite high, and most of the informal viability analysis i've read about focus on that aspect.