Abstract: An expert system is used to perform surveillance, diagnosis and prognosis of a plant such as a nuclear reactor used in electrical power generation. The expert system uses one of current actual plant data, previously stored actual plant data or simulated plant data as plant condition data to perform an evaluation of operating conditions in the plant. When current actual plant data is used, a data acquisition system supplies the plant condition data. The expert system accesses the plant condition data, regardless of its source, periodically as a surveillance frame is triggered by the rules in the root frame controlling the expert system. Other frames in the expert system perform diagnosis and prognosis of the plant condition.