Fatigue degrades human performance. The degree to which fatigue affects an individual can range from slight to catastrophic. Unlike alcohol or drugs which can be detected by biochemical tests, fatigue is more difficult to prove as a cause of accidents – typically fatigue must be inferred from the context of the situation. Fatigue is a personal experience, a function of the individual’s aspirations, achievements, self-evaluations and present and previous circumstances. Despite this limitation, increasingly, fatigue has been claimed as the primary cause of many major accidents. For example the incidents of Bhopal, Exxon Valdez, TMI, and Chernobyl list fatigue as a root cause.
Another less serious incident where fatigue was involved was the routine, but unannounced, inspection in 1987 of the Peach Bottom Atomic Energy Plant, U.S. Nuclear Regulatory Commission (NRC) inspectors found control room operators asleep in their chairs at the control panel. Consequently the NRC ordered the Power Station shut down, a first in American nuclear power history. Despite being a state-of-the-art high efficiency plant, management had overlooked the effects of human limitations in 24-hour operations. Even though no nuclear accident occurred, the plant closure, which resulted from human inattention, produced enormous costs. The plant shutdown was a “wake-up call” for the nuclear power industry.
The current NRC procedures include only prescriptive guidelines on working hours and shifts. They are general rules and guidelines on limiting worker fatigue. As an example, 10-CFR 26.20, which states that licensees provide “reasonable assurance that nuclear power plant personnel … are not … mentally or physically impaired from any cause, which in any way adversely affects their ability to … perform their duties.” The rules involving fatigue management need to be more specific and take a risk-informed approach. The NRC policy statement addresses only working hours and shift-limit recommendations, not the fundamental issue of fatigue and its effects on overall human error probability (HEP).
The task itself and the task environment can influence whether an individual feels alert or drowsy. The physiological effects of fatigue, however, are predominantly influenced by how long an individual has been awake, the duration of recent sleep, and the point at which the individual is in his or her circadian cycle of alertness. Fatigue as caused by extended work hours or circadian rhythms can significantly increase the likelihood of human error.
Studies of worker fatigue indicate a high correlation between worker fatigue and performance errors. A refinement in HRA is needed to model more accurately the phenomena of human performance by the inclusion of fatigue effects. This will lead to an improvement in HRA methods resulting in better PRA outcomes. An improved PRA will allow for better plant designs and plant personnel schedules by an increased understanding of the safe operating bounds of human behavior in the twenty-four hour economy world.
Current methods do not adequately include situation context and do not quantify the effects of fatigue. The quantitative and qualitative aspects of fatigue are not adequately addressed in current HRA models. In the SPAR-H model only eight PSFs are used, they are: time available, stress, complexity, experience & training, ergonomics, work process, fitness for duty, and procedures. This study investigates whether fatigue has such an overwhelming effect on human error rates and the boundary limits of human performance that it should be a factor onto itself. Many of the factors that produce fatigue are outside the control of regulators and industry, such as social and home life, i.e. the demands and needs of family, circadian rhythms, and sleep/rest period. Among factors that can be controlled by regulators and industry, some that influence fatigue are length of continuous work and length of time away from work. Current procedures and shift schedule guidelines do not give strong importance to these factors.
This research will incorporate industry, military, and psychological data on fatigue which will be used to supply data for HRA databases such as Human Event Repository and Analysis (HERA) and Human Factors Information System (HFIS). The retrospective data collection will also use databases such as:
The data collected from the above databases and literature review will be used to develop a methodology to include fatigue as a performance shaping factor and move the study of fatigue from implicit to explicit. The interaction of fatigue with other PSFs will also be investigated, in the SPAR-H model the PSFs used do not have an orthogonal relationship. The final step will be to demonstrate how this knowledge can be used to improve the HRA and PRA of Nuclear Power Plants.
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