Accidents occurring in nuclear power plants cannot be depended upon to train experts of nuclear operators. Generally, this expertise is developed through the simulator training. In this training course, only limited varieties of malfunctions can be presented. Thus, it would be impossible for operators to cope with any other possible anomalies of diverse varieties. Operators have to reorganize knowledge and experiences obtained in the training. Of course, prescribed procedures are far from covering them. This paper presents the structure and contents of a "operator's mental model" and it can deal with flexible operations under any situation. Based on the record of the simulator experiments, in-depth analyses were conducted by interviewing with experienced operators in order to model operators' thinking patterns and also to arrive at the aims behind their actions and utterances. Summarizing the findings obtained, fundamental functions of the mental model in coping with anomalies resulted in the following: (1) suggesting suitable preventive measures by envisioning the ongoing (future) scenario of plant dynamics; (2) identifying causes by investigating symptoms and implying causal remedies to eliminate them or to avoid influences from them and (3) adopting immediate responses simply formulated by "if alarm A then action B". These functions, as well as the mental model itself, can be made available by getting information on plant status and operator's structured knowledge together. The substances and structures of the mental models could be proposed, including implications of how to create it for a specific event, and finally they were synthesized into a more generalized format.
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