If you only have one reactor, sure... if you only have one solar cell, even a bird can stop electricity production. But, if you have multiple/many nuclear powerplants (such as france does), you can turn off one reactore and the others will take the load, fix the first, turn it back on, turn the second one off, do whatever, bring it back on,.... With solar, when it's night, it's night for all the solar cells in your country, and when there is very little or no wind, this can be true for whole regios or even your whole country again.