A METHODOLOGY FOR MODELING OPERATOR ERRORS OF COMMISSION IN PROBABILISTIC RISK ASSESSMENT

This paper describes a methodology to incorporate operator errors of c ommission (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 t erms of systems and functions, as well as physical and thermal-hydraul ic 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 Human Interaction Ti meLINE (HITLINE) to systematically generate sequences of human actions , including errors. Screening is applied to combinations of hardware f ailures, 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 t ree (ET) branch to another. The strategy of utilizing mapping tables t o 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 fo r the influence of the scenario-independent PIFs, are separately assig ned. While developing the HITLINE, the methodology then involves the u se of mapping tables to generate a set of PIFs, given the relevant inf ormation about plant and emergency operating procedures (EOPs). This s et is then used to select predetermined weights and adjustors to compu te the final weight to be assigned to each branch at a single branchin g point. Quantification at each branching point is done through the mu ltiple factors assigned to systematically assign the weights for diffe rent actions. Dependencies are carried from one branch point to anothe r through the use of operator related variables such as operator diagn osis, and expectation about plant behavior. Size of HITLINE is managed by applying merging, truncation and termination rules at each time st ep. Similar end states with respect to the ET are terminated and their weights are combined. Combination is also applied among HITLINEs cons tructed for different initial condition sets for a given initiating ev ent. Incorporating the results back into the ET either causes a reorde ring of sequence probabilities or including additional operator relate d top events to the ET. The methodology is demonstrated through a hypo thetical example.