Is it possible (reasonable) to go back and retrofit some existing power plants for dry cooling?

Retrofitting is possible, however the plants electrical output would suffer. Thermodynamics of heat engines, of which steam turbines, diesel engines, and gas turbines are examples, state that the output is directly proportional to the heat flux times the difference between the input temperature and output temperature. For water it is difficult go higher than about 350º C on the hot end because of corrosion, high pressure and other engineering problems. On the low side we can go down to about 50º C if we can use cold sea water for cooling. For PWRs, (Pressurized Water Reactor) this limits us to about 34% efficiency. With a dry heat exchanger it would be difficult to get the low end temperature lower that about 110º C which would lower system efficiency to maybe 30%.

One large advantage of the LFTR and gas turbine is it can operate at between 650º- to 900º C on the hot end. With an air cooled heat exchanger on the low end running at 110º C we have an efficiency of over 50%, and lower cost.

The Lower Mississippi River has no foreseeable heat sink issues. The volume of water is incredibly vast.

Just the shipping channel at New Orleans is 900' wide and 100' deep moving at several knots at the summer minimum. Add the water underneath the shipping channel and to the sides.

Alan