Abstract-Automating the functional testing of reactive systems requires to provide a formal specification of the system environment which defines the admissible test inputs. It also requires a specification of the expected properties of the system in order to decide whether a test succeeds or fails. Engineering these formal specifications is a difficult task, as it is not part of the usual manual testing process. In this paper, we report some experiments that have been conducted within a project in collaboration with industrial developers of nuclear power plant control systems. In this project, automatic testing tools have been used for checking the correctness of reactive systems developed incrementally, using heterogeneous industrial engineering workbenches. But these tools appeared to be useful also for elaborating and refining formal, consistent, and accurate functional requirements.