A methodology for modeling operator errors of commission in probabilistic risk assessment

Abstract This paper describes a methodology to incorporate operator errors of commission (EOCs) in nuclear power plant probabilistic risk assessments (PRAs). This is done by taking appropriate information from the plant PRA, operating procedures, information about plant configuration in terms of systems and functions, as well as physical and thermal-hydraulic information. This is combined with a set of performance influencing factors (PIFs), and results in an initial condition set which is fed into the primary tool for the methodology, called H uman I nteraction T ime LINE (HITLINE) to systematically generate sequences of human actions, including errors. Screening is applied to combinations of hardware failures, instrument failure, and PIFs to select the combinations that meet the criteria developed for the purpose. The criteria are based on the operator action or inaction causing a transition from one event tree (ET) branch to another. The strategy of utilizing mapping tables to accomplish all major steps of implementation decomposes the analysis into two separate parts. Values used in different scenario-dependent (error) likelihoods, and adjustors for these likelihoods to account for the influence of the scenario-independent PIFs, are separately assigned. While developing the HITLINE, the methodology then involves the use of mapping tables to generate a set of PIFs, given the relevant information about plant and emergency operating procedures (EOPs). This set is then used to select predetermined weights and adjustors to compute the final weight to be assigned to each branch at a single branching point. Quantification at each branching point is done through the multiple factors assigned to systematically assign the weights for different actions. Dependencies are carried from one branch point to another through the use of operator related variables such as operator diagnosis, and expectation about plant behavior. Size of HITLINE is managed by applying merging, truncation and termination rules at each time step. Similar end states with respect to the ET are terminated and their weights are combined. Combination is also applied among HITLINEs constructed for different initial condition sets for a given initiating event. Incorporating the results back into the ET either causes a reordering of sequence probabilities or including additional operator related top events to the ET. The methodology is demonstrated through a hypothetical example.