United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-91: Load Sequencers for Emergency Diesel Generators

                                                         SSINS No.: 6835
                                                            IN 85-91       
                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              November 27, 1985



All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).


This information notice is provided to advise licensees and applicants of
potential design deficiencies that could bypass load sequencers, thereby,
causing loss of redundant emergency diesel generators (EDGs). Recipients are
expected to review the information for applicability to their facilities and
consider actions, if appropriate, to preclude similar problems occurring at
their facilities. However, suggestions contained in this information notice
do not constitute NRC requirements; therefore, no specific action or written
response is required.

Description of Circumstances:

On August 22, 1985, the licensee for the Duane Arnold nuclear plant
discovered that an accident signal and the loss of the standby transformer
(a source of offsite electric power) would cause engineered safety feature
(ESF) loads to be applied as a single block load onto the EDGs (the sources
of onsite electric power), which would likely cause loss of both EDGs.

Pending replacement of the unit auxiliary transformer (lost in a transformer
fire in October 1984), the licensee was operating the plant with the
nonsafety-related loads on the station startup transformer and the
safety-related loads on the station standby transformer. The plant design
objective was to sequence the ESF loads onto the EDGs if offsite power to the
ESF buses should be lost and an accident signal was present. The licensee's
training staff realized that the logic and sensors used to determine the
availability of offsite power were such that the offsite power feeder
breakers to the ESF buses could be tripped, but offsite power would be
indicated as being still available. Under these conditions the design would
cause the ESF diesel generator load sequencers to be bypassed.

To justify continued safe operation, the licensee has temporarily placed
certain sequencer test switches in the test position, which forces the
sequencers to function even though offsite power is sensed as being


                                                       IN 85-91         
                                                       November 27, 1985 
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For the longer term the licensee is developing a permanent design change
which is to be reviewed by the NRC.


The design of the electric power system at the Duane Arnold nuclear plant
includes features to sequence ESF loads onto the EDGs, but not to sequence
loads onto offsite power. In a sense, these design objectives are in
conflict; that is, one is for sequencing and the other is for not sequencing.
When design objectives are potentially conflicting, careful analysis is
necessary to ensure that failures of various types do not result in
implementation of the improper objective. In this case, the logic was
designed so that if any source of offsite power is "available" (such as at
either the standby transformer or the startup transformer) the ESF load
sequencers would be bypassed. Thus, if the standby transformer were lost,
causing a loss of power to the safety-related loads, the logic would still
indicate offsite power as available. This design was provided by Bechtel

The result was the potential for an interaction between the offsite electric
power system and the onsite electric power system that could have caused the
loss of redundant sources of onsite power. Such an interaction is
incompatible with the requirements of 10 CFR 50, Appendix A, General Design
Criterion No. 17, "Electric Power Systems." The Duane Arnold original design
was such that the availability of offsite electric power was determined
indirectly; that is, by an upstream measurement rather than directly at the
ESF buses. This deficiency existed in the original plant design and was not
discovered when the design was reviewed again by the licensee after the loss
of the unit auxiliary transformer in October 1984.

No specific action or written response is required by this information
notice. If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.

                                   Edward L. Jordan Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  J. T. Beard, NRR
                    (301) 492-7465

                    Eric Weiss, IE
                    (301) 492-9005

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