Fully closed loop is probably not viable. But I suspect that's not the extreme GP might have meant.

If for the sake or discussion we simplify cooling to four options:

1: just passing river water through a heat exchanger for cooling

2: ingesting river water, using it for cooling, then passing it through a cooling tower before returning it to the river

3: pass the water output from the cooling tower directly back in the cooling loop, only ingest enough water to replace evaporation

4: fully closed loop via direct heat exchange with air

Then 4 is not very viable. But a lot of nuclear plants are stuck at 1 or 2, and each step up the ladder would allow them to operate in worse conditions. This one seems to be at version one