United States Nuclear Regulatory Commission - Protecting People and the Environment

Maintainability of Digital Systems: Technical Basis and Human Factors Review Guidance (NUREG/CR-6636)

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Publication Information

Manuscript Completed: January 2000
Date Published: March 2000

Prepared by:
W. F. Stubler, J. C. Higgins/BNL
J. Kramer/NRC

Brookhaven National Laboratory
Upton, NY 11973

J. Kramer, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code J6012

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Abstract

There is currently a trend in nuclear power plants (NPPs) toward introducing digital technology into safety and non-safety systems. However, this equipment has characteristics different from older analog equipment and is susceptible to additional failure modes. Inadequate integration of digital systems into operating and maintenance practices, and inadequate understanding of the intricacies of software-based digital systems on the part of technicians and operators, can result in failures that render systems inoperable. Digital systems impose new demands on personnel for the testing, troubleshooting, servicing, and repair of hardware and software. This may become increasingly important as licensees, using the on-line maintenance capabilities of digital systems, perform more maintenance while the plant is at-power. The objective of this study was to establish human factors review guidance for the maintainability of digital systems based on a technically valid methodology. To support this objective, a characterization was developed for describing design features and practices important to maintaining digital systems. Then, technical information related to human performance in maintenance was reviewed. Information was drawn from nuclear power, process control, and aerospace domains and included reviews of maintenance practices and digital system failures. This information provided the technical basis on which guidelines were developed for reviewing design features that support maintenance. For some aspects the technical basis was insufficient to develop guidance; these were identified as issues to be addressed in future research.

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