Ok, I think the original calculations here are still correct, if you design your rocket to directly emit the fission material at high speeds. This is a paper that proposes such a rocket design:
Dusty Plasma Based Fission Fragment Nuclear Reactor
We propose an innovative nuclear power generation system design using dusty radioactive (fissile or not) material plasma as a fuel. The fission fragments or decay products accelerated during the disintegration process to velocities of 3–5% of the speed of light are trapped and collected in a simple combination of electric and magnetic fields resulting in a highly efficient (90%), non-Carnot, DC power supply. In a conventional nuclear reactor this high kinetic energy of the fission fragments is dissipated by collisions to generate heat, which is converted to electrical power with efficiencies of no more than 50%. Alternatively, the fission fragments produced in our dusty plasma reactor can be used directly for providing thrust. The highly directional fission fragment exhaust can produce a specific impulse of one million seconds resulting in burnout velocities several thousand times those attainable today. Previous concepts suffered from impractical or inadequate methods to cool the fission fuel. In this work the heating problem is overcome by dividing the solid fuel into small dust particles and thereby increasing the surface to volume ratio of the fuel. The small size of the fuel particle allows adequate cooling to occur by the emission of thermal radiation.
Ok, I think the original calculations here are still correct, if you design your rocket to directly emit the fission material at high speeds. This is a paper that proposes such a rocket design:
Further comments on this rocket design:
https://forum.nasaspaceflight.com/index.php?PHPSESSID=3omp04d25qbe0qj5l5n1qfl0hp&topic=47693.20