Time-Dependent Accident Sequences Including Human Actions

During an accident, transitions between plant states can occur due to operator intervention and the failure of systems while running. The latter cause of transition is much less likely than the first, which includes errors of commission and omission as well as recovery of lost functions. A methodology has been developed to model these transitions in the time domain. As an example, it is applied to the analysis of Three-Mile-Island-type accidents. Statistical evidence is collected and used in assessing the frequency of stuck-open power-operated relief valves at Babcock & Wilcox plants as well as the frequency of misdiagnosis. Statistical data are also used in modeling the timing of operator actions during the accident, i.e., turning off and on the high-pressure injection system and closing the block valves.