Depends on a few factors...
Hi gb
NERVA style NTRs had pathetic heat transfer characteristics. If the NTR used a DUMBO system then things probably improve out of sight. I'll have to work it out, but there's a couple of unknowns like water's heat flow characteristics at high temperatures that I'll have to data mine. Zubrin and others have has written in a few places that water should work just fine so long as oxide fuel is being used.
Thanks for the puzzle.
Adam
