Note that this isn't some brand new exception, nor is it "waiving" the rules. It has been done multiple times, most notably during the 2003 and 2006 heat waves, so more has been learned from that. They are temporarily raising the rejected cooling water heat, forbidding the plants from raising the hourly average by more than 0.3 degrees Celsius. Nuclear power plants already work with the strictest minimum they can get to, and raising that isn't that much of a problem. Many industries do not have a limit to their emitted heat.