A dynamic reliability technique for error assessment in man-machine systems

This paper presents a methodology still under development for the analysis of human errors, named DREAMS (Dynamic Reliability technique for Error Assessment in Man-Machine Systems), which is dedicated to human reliability analysis and which identifies the origin of human errors in the dynamic interaction of the operator and the plant control system. This distinctive aspect differentiates DREAMS from the most commonly applied techniques in the nuclear as well as the conventional industries. Indeed, in the proposed methodology, the human behaviour depends on the working environment in which the operator acts ("external world"), i.e. the control room, and on the "internal world", i.e. his psychological conditions, which are related to stress, emotional factors, fixations, as well as to lack of intrinsic knowledge. As a logical consequence of the dynamic interaction of the human with the plant under control, either the error tendency or the ability to recover from a critical situation may be enhanced. The probability of erroneous actions and of recovery is thus a function of a generic "stress-in-action" correlation, which represents the effect of the internal-external worlds on the operator behaviour. This leads to the evaluation of Instantaneous Human Error/Recovery Probabilities (IHEP and IHRP) which are dynamically evaluated during the unfolding of the sequence of the man-machine interaction. These can then be used for evaluating an overall probability measure of plant safety related to human erroneous actions. A study case of application of the methodology to a control system of a real nuclear power plant is given in the paper as a sample case.