Most of the nuclear power plants that Germany built were single-reactor plants and would shut down for refueling. Multi-reactor plants will stagger shutdowns so that the plant continues to generate at reduced power. It is the same in the US. Only a few reactor designs like CANDU are capable of online refueling without interruption. Planned shutdowns for refueling are scheduled for low-demand times of the year.

Here's a nuclear industry blog post explaining the practice: http://neinuclearnotes.blogspot.com/2012/05/what-happens-dur...