[EBH]

Two other units at the plant were operating, as well as towers to cool the water. But searing temperatures and a lack of cooler water in the upper part of the Tennessee River system made it too difficult to provide cool water for all three reactors. There was no safety threat posed by the shutdown.

It looks like you are correct. It is a cooling water issue and not a safety issue. The water is too hot coming into the system. They don't know when it can come back online due to the incoming water being too warm.

[libstripper]

I live in Knoxville, TN, about 35 miles west of where the French Broad and Holston rivers join to form the Tennessee. The problem isn’t just the very hot summer we’ve been having. We’ve had a year long drought (right now about 11 inches of rain short) that’s substantially reduced the amount of water coming into the river system. That, in turn, means the water that is in the system is easier for the sun to warm and we have a much warmer river than otherwise.