The review contained in NUREG-1560 determined human performance to be an
important contributor to risk. For example, in the pressurized water reactor (PWR) PRAs, switchover to sump recirculation was observed to account for 1 to 16% of CDF (average of 6%). Contribution to CDF for feed and bleed initiation was observed to range from 1-10% with an average of 4%. An overall impact of
the set of all modeled human actions was not provided as part of the report, but in some instances a single human action was involved in as much as 40% of the CDF. Generally, PRAs find that human performance is important in sequences that require operator actions to initiate or operate plant systems to mitigate the effects of an initiating event and subsequent equipment failures. Examples of
such actions include switchover to sump recirculation mode, initiation of “feed and bleed” or once through core cooling, and depressurization and cooldown.
In the events studied, both BWRs and PWRs were susceptible to the influence of latent errors. For example, known design problems for components and systems that have not been acted upon by the licensee are considered to be latent errors. Inadequate engineering evaluations, problems in configuration management, and poor work package preparation, are additional examples of latent errors. The distribution of significant events in this study follows the general percentages among BWRs and PWRs in the U.S.
Replaced/Superseded by document(s)
This report presents the findings of a study of the contributions of human performance to risk in operating events at commercial nuclear power plants. The Nuclear Regulatory Commission (NRC) Accident Sequence Precursor (ASP) Program and the Human
Performance Events Database (HPED) were used to identify safety significant events in which human performance was a major contributor to risk. Conditional core damage probabilities (CCDPs) were calculated for these events using Systems Analysis Programs for Hands-on Integrated Reliability Evaluation (SAPHIRE) software and Standardized Plant Analysis Risk (SPAR) models.
Forty-eight events described in licensee event reports and augmented inspection team reports were reviewed. Human performance did not play a role in 11 of the events, so they
were excluded from the sample. The remaining 37 events were qualitatively analyzed. Twenty-three of these 37 events were also analyzed using SPAR models and methods. Fourteen events were excluded from the SPAR analyses because they involved operating
modes or conditions outside the scope of the SPAR models.
The results showed that human performance contributed significantly to analyzed events. Two hundred and seventy human errors were identified in the events reviewed and multiple human errors were involved in every event. Latent errors (i.e., errors committed prior to the event whose effects are not discovered until an event occurs) were present four times more often than were active errors (i.e., those occurring during event response). The latent errors included failures to correct known problems and errors committed during design, maintenance, and operations activities. The results of this study indicate that multiple errors
in events contribute to the probabilistic risk assessment (PRA) basic events present in SPAR models and that the underlying models of dependency in HRA may warrant further attention.