Requirements specify the expected characteristics of a system. Requirements engineering is an activity that defines and manages requirements in upgrades and new-builds of nuclear power plants. In the VAHAYA project, we carried out a case study on requirements engineering focusing on safety-related automation systems in nuclear power plants. The results of the study indicate that requirements engineering is becoming an increasingly important activity in the nuclear area, e.g., due to the increased complexity of the used technology. Also, other application domains can provide good practices. However, the special characteristics of the nuclear domain add special challenges to requirements engineering.