The problem is that high demand periods (i.e. for air conditioning) correlate with conditions (high temperatures) that are making the nuclear plants difficult to operate.
It doesn't matter so much for the summer demand if nuclear runs at full capacity in cold weather. Part of the issue is that these plants are using rivers for cooling, which by definition have less thermal mass than larger bodies of water like oceans.
The long term issue is that climate change is pushing these river temperatures consistently higher over time.
One of the reasons why nuclear goes so well together with renewables like PV. Actually said that we have more coal plants than nuclear ones in Germany. But that is hibdsight, at the time the decisions were almost reasonable.
It doesn't matter so much for the summer demand if nuclear runs at full capacity in cold weather. Part of the issue is that these plants are using rivers for cooling, which by definition have less thermal mass than larger bodies of water like oceans.
The long term issue is that climate change is pushing these river temperatures consistently higher over time.