Chinese modular high temperature nuclear reactors reach full power and 42% thermal efficiency.

[John ONeill]

4. Air as reactor working fluid

Really like the Kairos concept - it should avoid the radioactive dust problem that the German HTTR had, due to the lubrication and buoyancy from the salt. Per Petersen says the stresses on the fuel pebbles are so low, it's like them relaxing in one of those sensory deprivation tanks that rich people allegedly pay to spend time in.
Wouldn't running air through a reactor core make nitrous oxide, from the heat, and carbon 14, from neutron reactions on the nitrogen? Otherwise, I'd thought autonomous nuclear drones, with minimal shielding, could stay aloft for a year or so, and tow passenger planes a safe distance behind them. That would cut out most of the greenhouse gases from aviation. Hooking on and unhooking would be automated, too. The aircraft would only need to carry enough fuel to get to cruising altitude, get back down, make a backup landing field in case of a mishap, and run the auxilliary power unit for house loads.
Considering the fallibility of pilots shown by 9-11, the German who took his passengers into the Alps, and probably the Malaysian Air jet that disappeared somewhere in the Indian Ocean, that might be a safer as well as cleaner option, though I'd probably have trouble convincing most people.