The limit is not how cold the water needs to be for the power plant, but how hot it can be for the river ecology to not take (too much) damage. Even in Germany power plants regularly need to be shut down.
> 2022 was another consecutive year in which water levels of major European rivers – such as the Rhine, the Danube, and the Rhône – were dangerously low and the water temperatures very high. This caused severe problems for the operation of nuclear power plants across continental Europe. Energy companies in France, Germany, Switzerland, Belgium and elsewhere had to shut down their nuclear power plants partly or fully because there was not enough cooling water available or, more commonly, because the cooling water that was returned to the river became too warm (Barber, 2022; Limb, 2022; Miller and Vladkov, 2022). Environmental regulations, designed to protect the riverine flora and fauna as far as possible, stipulated that nuclear power plants were not allowed to release cooling water above a certain temperature (European Parliament and European Council, 2000; IKSR, 2022b). The resulting unplanned outages — and the efforts by nuclear operators to avoid such disruptions — highlight pressing concerns about the sustainability of nuclear energy, particularly its impact on river ecosystems in an increasingly warming world (see Fig. 1).
https://www.sciencedirect.com/science/article/pii/S030142152...