Probabilistic analysis of operators actions at the ignalina nuclear power plant taking account of the specific conditions of accident sequences

Abstract

The procedure used at the Ignalina nuclear power plant for probabilistic analysis of the actions taken using the ASEP and THERP methods by operators during accidents is described. The method makes it possible to evaluate the probability of operator error at different stages of the action taken ‐ identification, making a decision, and taking action ‐ as well as correctly taking account of different factors influencing human actions ‐ emergency signaling, instrument indications, presence of instructions, preparation and experience, stress, time, and others. The procedure used in the qualitative and quantitative analysis is briefly reviewed and examples of the evaluation of real actions taken by operators are given. It is shown that it is important to take account of the specific nature of the development of an accident when evaluating the probabilities of operator error. An integral part of probabilistic safety analysis of nuclear power plants is the evaluation of operator error probabilities. The objective of such an analysis is to determine the operator actions, the factors having the greatest effect on the operators, and the operator error probabilities for each action which are most important for nuclear power plant safety. Human actions depend on many factors, such as training and experience, stress, and the availability of information. In turn, these factors can differ for different initial events and development scenarios of accidents. In this connection, one and the same operator action will have different error probabilities under the conditions of different accident scenarios. Operator Actions in an Emergency Situation. In an emergency situation, the operators of a nuclear power plant take actions to stop and cool down the reactor. These actions include monitoring the automatic actuation of the safety systems (or similar manual actions), monitoring the parameters of the power-generating unit, and controlling the operation of the safety systems. Operator actions are an integral part of the general course of liquidating an emergency situation; they are clearly defined and described in the emergency instructions. The importance of operator actions increases enormously if the accident develops in a manner different from the expected scenario, for example, safety systems are not actuated or additional failures occur. In such cases, the operators must duplicate the actuation of systems, use back-up systems or equipment instead of the failed equipment, and take additional measures to maintain the safety parameters of a plant within prescribes limits. Such actions include the evaluation of the situation, making decisions, acting on the decisions, and monitoring the results based on which subsequent decisions are made. The factors influencing operator actions are specific to different scenarios, i.e., their effect and correspondingly the error probability will be different for one and the same actions taken under different circumstances depending on the initial event and the course of the accident. Main Stages of an Accident. The operator actions which were the keys to the safety of the Ignalina nuclear power plant were determined at the first stage. The model of probabilistic safety analysis, operating and emergency instructions, and conferences with the operators served as the source of information. Next, the following were determined for each action: