United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-24: Inadequate Maintenance of Uninterruptible Power Supplies and Inverters


  March 24, 1994

                         SUPPLIES AND INVERTERS


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential failures of uninterruptible power
supplies and inverters because of inadequate maintenance.  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 required.

Description of Circumstances

Nine Mile Point Nuclear Station Unit 2 (NMP-2)

On August 13, 1991, an internal failure in the main transformer caused a
degraded voltage which resulted in the simultaneous loss of power outputs from
five uninterruptible power supplies.  The power outputs from the five power
supplies were lost because of a combination of a wiring problem and the
failure of the internal batteries to supply control power.  The five power
supplies provide power to the main control room annunciator system and to
other systems important to safety.

On March 26, 1992, a non-safety-related power supply failed to provide power
to a radiation monitoring cabinet, causing an engineered safety features
actuation.  The licensee determined that an output breaker from the power
supply failed to close because grease in the breaker had dried out and

Susquehanna Steam Electric Station Unit 1 (SSES-1)

On July 31, 1991, a relay in the switchyard was operated incorrectly, causing
the loss of one of the offsite power sources to the plant.  The loss of this
power source, coupled with the failure of one uninterruptible power supply to
transfer to its backup power supply, resulted in a loss of power to some ac
instrument panels.  The power supply failed to transfer because of problems

9403180068.                                                IN 94-24
                                                March 24, 1994
                                                Page 2 of 3

with the internal batteries.  The loss of power to the instrument panels
caused the loss of numerous instruments and control room indications, as well
as some plant support equipment.


Uninterruptible power supply units are designed to supply reliable ac power to
their loads.  Designs for such power supplies use an inverter to convert dc
power from a battery to ac power.  Some designs also provide a rectifier in
combination with the inverter as one unit.  The rectifier converts station
power from ac to dc.  The inverter then converts either the output of the
rectifier (normal) or power from a battery (backup) from dc to ac.  Offsite ac
backs up the inverter ac output through a power transfer device.  Direct
current control power for the power supply is developed internally using power
from external power sources.  In some designs, back-up control power is
provided by an internal battery bank.

Major contributing factors for the power supply failures at NMP-2 on
August 13, 1991, and March 26, 1992, were inadequate maintenance of the
batteries which supply power to the control logic and inadequate maintenance
of the power supply output breakers respectively.  The cause of the failure at
Susquehanna was three failed cells in the power supply battery bank.

The NRC Office for Analysis and Evaluation of Operational Data (AEOD) recently
prepared engineering evaluation AEOD/E93-03, "Electrical Inverter Operating
Experience - 1985 to 1992," December 1993, to assess inverter operating
experience in the nuclear industry over the last 7 years.  In the report, the
staff concludes that the number of inverter failures has decreased, but some
problems still warrant attention.  Significant findings in the report are as

(1)Inverter failures are still causing a large number of inadvertent
engineered safety features actuations, reactor trips, and turbine

(2)Component failures are still the predominant root cause of the failures,
followed by human error.

(3)The components that failed most frequently were electrolytic capacitors,
followed by transformers, silicon-controlled rectifiers, and transfer

The staff concluded in this report that, based on a review of licensee event
reports, some licensees obtained a marked decrease in the frequency of
electrical inverter failures by (1) providing the inverter units with adequate
cooling and (2) implementing a preventive maintenance program for the inverter
units that periodically replaced selected components.  The staff also
concluded that the proper implementation of vendor recommendations for minor
and major maintenance activities would reduce the number of failures because
licensees would frequently identify and replace degraded components before
they failed.
.                                                IN 94-24
                                                March 24, 1994
                                                Page 3 of 3

Related Generic Communications

The NRC has issued the following related generic communications:  Information
Notice (IN) 93-26, "Grease Solidification Causes Molded-Case Circuit Breaker
Failure to Close," April 7, 1993, and Supplement 1 to IN 93-26,
January 31, 1994; IN 91-64, "Site Area Emergency Resulting From a Loss of
Non-Class 1E Uninterruptible Power Supplies," October 9, 1991, and Supplement
1 to IN 91-64, October 7, 1992; IN 88-57, "Potential Loss of Safe Shutdown
Equipment Due to Premature Silicone Controlled Rectifier Failure," August 8,
1988; and IN 87-24, "Operational Experience Involving Losses of Electrical
Inverters," June 4, 1987.

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

/s/'d by BKGrimes

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

Technical contacts:  J. G. Ibarra, AEOD
                 (301) 492-4441

                 F. S. Ashe, NRR
                 (301) 504-2785

List of Recently Issued NRC Information Notices

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