This paper describes an approach to simulate operators diagnostic strategies under emergency situation of nuclear power plants. To assess operators cognitive workload during the diagnosis, we have developed a diagnostic model, which is an embedded module of a task simulation analyzer, SACOM. This model is based on a blackboard architecture and uses four types of diagnostic agents. The diagnostic agents can simulate typical diagnostic strategies used by human operators: i.e., data-driven search, symptomatic search, hypothesis-driven search, and topographic search. The model is developed using a G2 expert system tool.