Ya got me...
Hi All
Very good point GB - thermalising gammas in a nuke pulse unit is very tricky. The USAF has been looking at hafnium as a way of powering UAVs, so maybe they've figured something out that's more efficient than what I can imagine.
However my other suggestion was using the Hf to heat a reaction mass of something suitable - which might be the better option. Not as much Isp but still good.
Adam