In this paper, we tried to summarize the practical experience of information security audits of nuclear power plants' automated process control system (I&C). The article presents a methodology for auditing the information security of instrumentation and control systems for nuclear power plants. The methodology was developed taking into account international and national Russian norms and rules and standards. The audit taxonomy, classification lifecycle are described. The taxonomy of information security audits shows that form, objectives of the I&C information security audit, and procedures can vary widely. A conceptual program is considered and discussed in details. The distinctive feature of the methodology is the mandatory consideration of the impact of information security on nuclear safety.