Information Notice No. 93-35: Insights from Common-Cause Failure Events

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                                May 12, 1993



All holders of operating licenses or construction permits for nuclear power
plants (NPPs).


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to inform addressees about the publication of an NRC report on common-
mode and common-cause failure events that could occur at nuclear power plants. 
It is expected that recipients will review the information for applicability
to their facilities and consider actions, as appropriate, to avoid similar
problems.  However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is


The Office for Analysis and Evaluation of Operational Data (AEOD) published a
report, "Insights from Common-Mode Failure Events," AEOD/E92-02, in June 1992
and issued Supplement 1 in February 1993.*  The staff reviewed 62 selected
licensee event reports that discussed actual or potential common-cause
failures.  Of the 62 events reviewed, 16 also were identified as precursors in
the accident sequence precursor program (ASP).  Common-cause failures may
significantly degrade safety since they can defeat the additional benefit
normally provided by redundant systems.  Furthermore, common-cause failure has
been cited as a major uncertainty in probabilistic risk assessments of nuclear
power plants.  

Most of the reports used in the AEOD study were prompted by events that
occurred in 1990.  In the study, the staff used reports of events where the
failed equipment was considered neither "recoverable" nor "self-revealing." 
A failure that is recoverable is one in which the plant condition could be
restored by taking a corrective action during the event.  However, failure in
a location inaccessible during an accident, such as inside the containment; a
failure of equipment needed for a rapidly developing event such as a loss-of-
coolant accident; a challenge to pressure vessel integrity; or a backwards
installation of a necessary valve are considered failures that are not
recoverable.  For a self-revealing condition, the licensee would presumably

*Copies of this report and its supplement are available for inspection 
 and copying in the NRC Public Document Room, 2120 L Street N.W., 
 Washington, D.C. 20037


                                                            IN 93-35
                                                            May 12, 1993
                                                            Page 2 of 2

correct the situation before the event occurred that required the equipment. 
The staff excluded such conditions from this study.  

The AEOD report attempted to identify the actions that would be most effective
in precluding or reducing the likelihood of common-cause failures at operating
nuclear power plants.  Each of the events was reviewed against a set of eight
potential actions:  comprehensive testing, staggered testing, post-maintenance
testing, using equipment from different manufacturers in redundant trains
(diverse equipment), using equipment with larger design margin, increasing
frequency of testing, providing adequate separation of trains, and using
different maintenance personnel on separate trains.  No single action would
have alleviated all the common-cause failures in the study.  Considering all
the events, the action that had the highest potential benefit was using
equipment with larger design margins.  This action alone would have corrected
about 56 percent of the situations examined in the AEOD study.  Performing
comprehensive systems tests, ensuring adequate train separation, and using
diverse equipment each had a potential benefit in about 27 percent of the
cases.  The use of staggered surveillance testing would have benefited 
20 percent of the events.  

The staff noted that about two-thirds of the events were related to design,
fabrication, and installation errors that may be undetected for long periods
of time.  The remaining events were associated with maintenance deficiencies,
either preventive or corrective.  The major alleviating action identified for
these maintenance-related events was staggered surveillance testing, which is
assumed to result in a shorter time during which the common-mode failure is

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please call one
of the technical contacts listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                       ORIGINAL SIGNED BY

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  S. Israel, AEOD
                    (301) 492-4437

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