This paper presents basic ideas of an approach enabling to simulate multitask performance based on the single-channel theory of selective attention. The paper demonstrates the applicability and benefits of the use of computer discrete event simulation tools to imitate hazardous scenarios possibly taking place at a potentially hazardous installation with a focus on operators' performance. One of the outcomes of the modelling is the probability of action failure. A case study on the simulation of operators' performance under a medium Loss of Coolant Accident scenario at a nuclear power plant is briefly described in the paper.