I actually really like your suggestion 
for the gas coolant, helium should be the one used as it is being tested by a couple of modern designs =P
Thermal reactors aren’t bad, but they are dirtier (produce radioactive waste). My biggest concern honestly is that I don’t want to see Greg implement a Chernobyl-style light water reactor only because that’s the only type people know about. The new designs are so much cooler!