During January this year, nuclear contributed about 62TWh of the electricity generated in the EU while in June this had fallen to 43.7TWh - about a 30% decrease. The deficit was largely filled by a surge of solar generation as well as burning more gas.
The drop was due to both scheduled and unscheduled downtime. There is no such thing as a non-intermittent source - even without unscheduled outages, a typical nuclear plant spends over 4 weeks offline a year. You need to be able to handle that also - i.e. with redundancy and backup.
Also because nuclear (like coal - the other thermal steam generation tech) requires huge scale to be profitable, it requires huge transmission investment. Contrast this with solar, for example, which is economic to scale down and can deployed in a more distributed pattern.