United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-21: Availability of Alternate AC Power Source Designed for Station Blackout Event

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                                April 18, 1997


NRC INFORMATION NOTICE 97-21:    AVAILABILITY OF ALTERNATE AC POWER SOURCE
                                 DESIGNED FOR STATION BLACKOUT EVENT 


Addressees

All holders of operating licenses for nuclear power reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential unavailability of an alternate ac
(AAC) power source during a station blackout (SBO) event.  It is expected that
recipients will review this 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

On March 11-29, and May 13-22, 1996, a special inspection team from the NRC
assessed the engineering and licensing activities at Millstone Nuclear Power
Station.  During a review of the Millstone Unit 3 SBO systems, the team
identified a concern about the design of the AAC power source, the SBO diesel
generator (DG), and its dedicated battery.  The AAC power source may not be
available if an SBO occurs as a result of a loss of onsite power, i.e., the
EDGs, following 1 hour of loss of offsite (preferred) power (LOOP).  The AAC
power source at Millstone Unit 3 is controlled and monitored by a computer. 
The AAC power system includes two batteries, one 48-volt battery for the
computer and one 125-volt battery for DG field flashing, oil pump, and breaker
control power.  The battery chargers for these batteries are fed from offsite
power when the AAC power is not operating.  If offsite power is lost for a
significant period, these batteries will be depleted as a result of the
connected loads (the computer for the 48-volt battery and the EDG oil pump for
the 125-volt battery).  If AAC power is needed more than 1 hour after the loss
of offsite power, the batteries will be so depleted that the SBO DG cannot be
started and therefore, will not be available if EDGs are lost.

On March 7, 1997, during a refueling outage at Pilgrim Unit 1 a main
transformer failure resulted in both safety buses losing power.  One safety
bus was energized from the shutdown transformer and the other from EDG "B." 
Approximately 6 hours into the event, operators attempted to start the SBO DG,
but failed.  The SBO-DG failed to start because the 

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DG support systems, particularly the lubricating oil system, was powered by a
nonsafety-related power supply and had been out for an extended period during
the loss of offsite power.  This extended loss of auxiliary power caused the
oil temperature to drop and that resulted in oil pressure below the minimum
pressure required for DG operation.  

Discussion

The Millstone licensee reviewed the design and operation of the AAC power
source at Millstone Unit 3 and determined that the AAC power source required
modification to assure availability of the SBO DG for no less than 8 hours
(the probable time needed to restore offsite power) following a postulated
LOOP event.  

The Pilgrim licensee discovered that the SBO DG was susceptible to a failure
to start as a result of low oil temperature if power was lost to the
lubricating oil keep-warm system for an extended period.  The licensee is
considering enhanced monitoring of SBO DG auxiliaries, procedural changes, or
modifications to SBO DG start logic.   

The SBO DG capability to start on demand depends on the availability of the
necessary support systems to fulfill their required function.  The SBO DG
support systems may require varying combinations of dc or ac power for varying
periods to maintain operational readiness.  Prompt operator actions to
conserve battery capacity, support system power supply modifications, use of
special procedures, or start logic modifications may be necessary to preserve
the operational readiness of the SBO DG under challenging environmental
conditions.  The staff is evaluating these events and the consequential need
for additional generic communications.

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.

                                         ORIGINAL SIGNED BY    

                                       
                                       Thomas T. Martin, Director
                                       Division of Reactor Program Management
                                       Office of Nuclear Reactor Regulation

Technical contacts:  A. Pal, NRR                      David Skeen
                     (301) 415-2760                   (301) 415-1174
                     E-mail:  anp@nrc.gov             E-mail:  dls@nrc.gov

                     Thomas Koshy
                     (301) 415-1176
                     E-mail:  txk@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013