Dynamic Human Error Simulation of Work as Done – Modeling Procedure Deviations With Empirically Derived Failure Mechanisms

Abstract

Human Unimodel for Nuclear Technology to Enhance Reliability (HUNTER) is an Idaho National Laboratory software tool developed to support dynamic human reliability analysis (dHRA). The software performs Monte Carlo simulations of a virtual operator performing procedurally prescribed tasks within the context of a dynamic and coupled nuclear power plant model. Changes in the plant state impact what tasks the operator must perform and vice versus. HUNTER supports a suite of scenarios with models containing procedures and corresponding human performance context parameters such as loss of feedwater and steam generator tube rupture scenarios. The procedure module contains a single path of steps to mitigate the faults within the two scenarios. Failures occur if the dynamically calculated human error probability (HEP) value exceeds a chance value for any given task or if the elapsed time to complete a task exceeds the allowed time for that task. In reality, reactor operators deviate to the wrong path due to their errors. To improve accuracy, HUNTER needs to be able to allow the virtual operator to incorrectly deviate along the wrong procedure path due to a diagnostic or understanding errors in addition to the existing HEP and time failures. Procedure deviations are errors of commission which are quite challenging to model since there are theoretically infinite errors of commissions that could be made at any point in the simulation. Empirical data collected from a recently performed study evaluating computer-based procedures and failures to adhere to the prescribed procedure steps were used to realistically constrain the possible deviations to a manageable set that could be modelled within the HUNTER simulation. The process to analyse the empirical procedure adherence data and develop generalized forms of the empirically observed failure mechanisms are described along with their implementation within the HUNTER simulation. Future work aims to continue to validate these failure mechanisms outside of the specific context of the loss of feedwater and steam generator tube rupture scenarios to understand their generalizability to other scenarios and to more accurately model work as done within nuclear process control.