This document describes a practical experimentation of safety assessment of safety-critical software used in nuclear power plants. To enhance the credibility of safety assessments and to optimize the safety justification costs, the Research and Development Branch of Electicit? de France (EDF) investigates the use of methods and tools for the semantic analysis of source code, so as to obtain indisputable proofs or so as to help assessors focus on the most critical points. Two tools based on different static analysis methods — Abstract Interpretation and Hoare Logic - were used for this experimentation. The use of two independent approaches is important since it confers a high level of confidence in the results. In addition, we found that the tools complement one another: the information provided by one tool was often used to improve the results of the other. We were able to prove formally that the application software of one of our shutdown systems is free from intrinsic run-time fault. The current versions of the tools were not mature enough for us to derive conclusions about the system software.