Hi gb

The optimal exhaust velocity thing is an old result - it merely minimises the total energy expenditure of the rocket system. Thus for a given mission delta-vee the least amount of kinetic energy in the propellant is expended with an exhaust velocity of V/ln(4.92) = 0.627V... which is true, but not necessarily practical.

If we're talking antimatter rockets or fission rockets with very high burn-up fractions then it's meaningful, but with space power reactors? I'm not sure it's all that useful. Still... it takes a lot of energy to turn water into LH2/LOX - if we're using straight LH2 in an NTR it takes 120 MJ/kg just to make the stuff from water. If that was kinetic energy the kg would be doing 15.5 km/s.