Revised calculations:
Doing a little more thinking here.
The reactor is inclosed in 1/2 a sphere. 50% of the energy is lost to space.
Only 1/2 the energy is used as a thrust vector. This gives 25% of the fuel into
thrust. Asuming 80% burn of mass this gives a 5:1 thrust ratio.
A 16Mw reactor will give 8Mw of heat and 3.2MW of thrust.