There are definite environmental concerns. Just this morning there's an article about how rising sea levels in the Marshall Islands and melting ice on Greenland will expose nuclear waste. As for heating rivers, this is absolutely true, but with this said, if the plants are small then the discharge will be small, relatively speaking. Also, to the point about location, it is as much about the efficiency of the turbines as it is the temperature of the water. Combining nuclear power with solar updraft towers might be solution.