why it disrupts the operation of nuclear power plants

To operate, a nuclear power plant needs water to cool its turbines.
This water is drawn directly from the rivers or the sea that border these power stations. Then, once the system has cooled, in the case of an open circuit, the water is discharged at the source. Inevitably, this water is warmer. To preserve the ecosystem, to avoid the proliferation of algae or the disturbance of fish reproduction, temperature limits are imposed. This varies according to the power stations but, for example, for that of Blayais in Gironde, the discharged water cannot exceed 36°C.

These four plants needed a waiver because with rising water temperatures, these limits can no longer be met. Usually, what is decided is to reduce the power of the reactors to avoid discharging too hot water, this was precisely the case at Blayais and Saint-Alban, in Isère, in May and last June. But at the moment, 29 French reactors out of 56 are unavailable for corrosion problems or for maintenance. And the electricity market is very tight. So to avoid the shortage, EDF has asked for a temporary relaxation of these limitations. Four plants are therefore concerned: those of Saint-Alban, Blayais but also Golfech in Tarn-et-Garonne and Bugey in Ain. This derogation is valid until Sunday July 24 with reinforced environmental monitoring. The only time such an exemption was granted was in 2018 for the Golfech plant.

There is another constraint linked to the heat wave, it is the drought. Here too, the power stations must comply with a minimum flow rate to take water. It is still a standard imposed to limit the impact on the environment. It is especially at the end of the summer that you have to be vigilant, during the low water period, when the watercourse is at its lowest. For power stations by the sea, this does not pose a major problem, but for others, there may be production slowdowns.
In a context of risk of energy shortage, this is a parameter to be taken into account.