United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-37: Main Transformer Fault with Ensuing Oil Spill into Turbine Building

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

                                 June 20, 1997


NRC INFORMATION NOTICE 97-37:  MAIN TRANSFORMER FAULT WITH ENSUING OIL         
                               SPILL INTO TURBINE BUILDING

Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a scenario in which a large amount of
transformer insulating oil could bypass fire hazard control features, such as
oil impoundment pits, and spill into the turbine building and other areas of a
nuclear power plant.  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

On March 7, 1997, Boston Edison Company's Pilgrim Nuclear Power Station was in
a refueling outage with the core offloaded to the spent fuel pool.  The
startup transformer was out of service for maintenance.  Offsite power was
being supplied to the safety-related buses from the 345 kV system through the
main transformer and the unit auxiliary transformer with the main generator
disconnected.  A fault in the main transformer caused severe mechanical
agitation, breaking one of the low-voltage bushings and one of the high-
voltage lightning arresters.  Oil leaked out of the broken low-voltage bushing
and spilled into the turbine building via the isolated phase (iso-phase) bus
duct.  

Approximately 4,300 gallons of oil entered the turbine building.  Some oil
leaked under the doors to the A essential switchgear room and into some
nonsafety-related electrical cabinets but stopped short of safety-related
switchgear.  Because Pilgrim is a boiling water reactor, the turbine building
is divided by a concrete biological shield wall, which limited the area of the
oil spill.

Turbine building equipment potentially affected by the oil spill included the
iso-phase bus cooling unit, the generator hydrogen seal oil unit, the stator
winding liquid cooling unit, the low- pressure carbon dioxide storage unit,
the fire protection deluge valves for the 


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                                                            June 20, 1997
                                                            Page 2 of 3


transformer fire protection systems, and the trucklock bay (where trucks are
loaded and unloaded).  In addition, oil flowed down a stairwell and collected
in an area that included the radwaste monitoring tanks, the treated radwaste
holdup tanks, and the instrument air compressors.

As a result of the loss of offsite power caused by the main transformer fault,
the B train emergency diesel generator (EDG) started and picked up its safety-
related loads.  The A train EDG was out of service for maintenance.  The A
train safety loads were supplied by the secondary source of offsite power (the
23 kV system) via the shutdown transformer.

Discussion

The Pilgrim main transformer (700 MVA, 3-phase, 60 hz, shell type) is
configured so that the iso-phase bus ducts from the turbine building are
routed on the same horizontal plane as the low-voltage bushings, which
protrude from the side of the transformer, about 4 feet from the top.  The
iso-phase bus ducts are air-cooled ducts that surround each of the three
phases of electrical bus and connect the main generator to the main
transformer.  All three ducts terminate in a common junction box on the side
of the transformer where the iso-phase buses are connected to the low-voltage
(23 kV) bushings.  The high-voltage (345 kV) bushings then connect the
transformer to the 345 kV transmission lines to supply power from the unit
generator to the grid during normal operations.  During the outage, the
licensee was using the transformer as a step-down transformer (backscuttle
mode) to supply offsite power from the grid to the station auxiliary loads.

There is no standard configuration for large power transformers used at
nuclear power plants.  The configuration at Pilgrim may exist at other plants. 
Licensees may wish to consider the configuration not only of the main
transformer and duct work at their plants, but of any of the large power
transformers (such as the unit auxiliary or start up transformers) to see if
there are paths by which transformer insulating oil could leak into areas
where fire would pose a significant hazard to the plant. 

Faults of large power transformers at power plants are not uncommon.  The
event at Pilgrim demonstrates that a large amount of a combustible oil may be
introduced into the turbine building.  During power operations, a transformer
fault may cause a fire if the insulating oil is ignited by the arc that
results from the fault.  If the transformer fault at Pilgrim had occurred at
power, a serious turbine building fire could have ensued.

The combustibility of the mineral oil used as the transformer insulating oil
had a flash point of approximately 275�F.  A fire could have propagated into
the trucklock and down into the radwaste tank area.

Smoke and hot gases could have been transported via the open trucklock area to
elevations above the floor directly affected by the fire.  Smoke and hot gases
could have entered the B essential switchgear room (located on the next level
above the fire) via the ventilation wall opening.  Oil seeping under the door
to the A essential switchgear room could have allowed   .                                                            IN 97-37
                                                            June 20, 1997
                                                            Page 3 of 3


the fire to spread to that room and could have resulted in the loss of the A
switchgear as well.

The Pilgrim licensee enhanced the fire protection design in the turbine
building by installing containment curbs at the fire doors leading to the A
essential switchgear room and the stairway leading to the radwaste holding
tanks, and modified the iso-phase bus duct by installing an 8-inch diameter
downcomer drain line on each of the three phases.  Each drain line is routed
to drain into the oil leak retention pit, and will be equipped with a rupture
disc designed to open under 2 psig of static oil pressure in the drain line
down-comer.

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.


                                          signed by S.H. Weiss for

                                    Marylee M. Slosson, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Patrick Madden, NRR
                     301-415-2854
                     E-mail:  pmm@nrc.gov

                     David Skeen, NRR
                     301-415-1174
                     E-mail:  dls@nrc.gov
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