Information Notice No. 96-36: Degradation of Cooling Water Systems due to Icing

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                          WASHINGTON, DC  20555-0001

                                 June 12, 1996



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 degradation of facility water intake
systems (circulating, service, and fire water) due to icing conditions.  
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

Description of Circumstances

Wolf Creek  

Between 1:45 a.m. and 2:45 a.m. (all times central standard time) on 
January 30, 1996, operators at Wolf Creek received alarms indicating that the
circulating water system traveling screens were becoming blocked.  A visual
inspection showed that the traveling screens for bays 1 and 3 were frozen and
that water levels in these bays were approximately 2.5 m [8 ft] below normal. 
The emergency service water system was started with the intent to separate
this system from the service water system.  However, the emergency service
water system was incorrectly aligned; flow was directed to the service water
system and warming flow to the emergency service water system suction bays was
restricted.  Operators also shifted to circulating water pump B.  At approxi-
mately 3:30 a.m., operators received a service water pressure alarm and an
electric fire pump started on low service water pressure.  The shift super-
visor then directed a manual reactor/turbine trip.  Circulating water system
bays were subsequently determined to be at 3.5 m [12 feet] below normal.  The
level loss was caused by water from the spray wash system freezing and block-
ing the traveling screens.  

The Train A emergency service water system pump was tripped and declared
inoperable at 7:47 a.m. because of low discharge pressure and high strainer
differential pressure.  At about 8:00 a.m., the supervising operator coming on
shift noted the incorrect alignment of the emergency service water system and
took action to correct it.  At about 5:45 p.m., the operators declared Train A

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operable on the basis of an engineering evaluation and placed it in service. 
However, the pump was again stopped 1-1/2 hours later at approximately
7:30 p.m. when the pump exhibited further oscillations in flow and pressure. 
At approximately 8:00 p.m., operators noted that emergency service water
system Train B suction bay level was 4.5 m [15 ft] below normal and decreasing
slowly.  Operators placed additional heat loads on Train B and the suction bay
levels subsequently recovered.  At 10:14 p.m., the operators again started
Train A emergency service water system and secured it at 10:27 p.m. due to
decreasing flow and pressure.

At about 10:00 a.m. on January 31, divers inspected the suction bay of Train A
and noted complete blockage of the trash racks by frazil ice.  Train B was not
inspected because the pump was running.  The ice blockage was cleared by
4:00 p.m. by sparging the trash racks with air.  The emergency service water
system was designed to have warming flow injected in front of trash racks to
increase bulk water temperature and prevent the formation of frazil ice.  Due
to calculational errors by the architect-engineer and the as-built system
configuration, the emergency service water system warming flow was
insufficient to prevent frazil ice from forming at the Train A trash racks.  


On February 25, 1993, at 1:25 a.m., the electric fire pump started on low fire
header pressure.  After verifying normal fire header pressure, operators
secured the electric fire pump, and the diesel fire pumps subsequently started
on low header pressure.  It was later determined that the fire jockey pump had
lost suction because of the decreasing screenwell level.  
Over the next several minutes, operators noted an increase in circulating
water system motor amperage (which is consistent with decreased suction
pressure).  Reactor power was also reduced and one circulating water pump was
secured.  At 1:40 a.m., an operator reported that the screenwell level was
approximately 3 m [10 ft] below normal and the reactor was manually scrammed. 
After a second circulating water pump was subsequently secured, the screenwell
water level quickly recovered.  The licensee concluded that the reduced
screenwell level had been caused by ice partially blocking the lake intake
structure.  Either frazil ice formed around or in front of the heated intake
bar racks, or slush ice was present in front of the bar racks.


On February 5, 1996, at approximately 2:30 p.m., diesel generator service
water pump C failed to develop normal discharge pressure, flow, and motor
current during a surveillance.  This made Division I emergency diesel
generator (EDG) 12 inoperable.  After several unsuccessful attempts to start
the pump, an air purge of the pump column on the discharge of the pump was
initiated in an attempt to clear any obstruction in the column or the pump
inlet.  The licensee detected blockage when it tried to blow air through the
pipe.  At 8:21 p.m., the pump was started and after 3 to 5 seconds an erratic
discharge pressure was noted.  In a short time, pump flow, discharge pressure,
and motor amps were normal.  The following day, on February 6, diesel
generator service water pump B was started.  The pump, which cools Division II.                                                            IN 96-36
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EDG 13, showed no flow, no pressure, and low amperage for the first 
30 seconds.  After approximately 90 seconds, normal flow discharge pressure
and motor current were achieved.  The remaining safety-related pumps were
tested satisfactorily.

The diesel generator service water pumps are all deep-draft pumps and take
suction from the ultimate heat sink reservoir which is a segregated pool.  The
licensee evaluation concluded that although the reservoir temperatures are
maintained above 5 �C [41 �F], part of the pump column and two of the linear
guide bearings are located above the water level and are exposed to ambient
air temperature conditions.  The failure of the diesel generator service water
pump C was attributed to ice buildup around the shaft and spider bearing from
leakage past the discharge check valve and the cold weather.  The licensee
also concluded that under some credible meteorological conditions, the
functions of both divisions could have been affected.


Frazil icing is a phenomenon that affects the operation of intake structures
in regions that experience cold weather.  The accumulation of frazil ice on
intake trash racks can completely block the flow of water into the intake. 
The process starts when the water flowing into the intake is supercooled (a
condition where the water is below the freezing point).  The supercooling may
be very small, on the order of a few hundredths of a degree.

The supercooling occurs with a loss of heat from a large surface area such as
a lake with open water and clear nights.  High winds contribute to the problem
by providing mixing of the supercooled water to depths as great as 6 to 9 m
[20 to 30 ft].  The frazil ice, which is composed of very small crystals 
(1-15 mm) with little buoyancy because of their size, is carried along in the
water and mixed all through the supercooled water.

The suction of the supercooled water and the suspended frazil ice crystals
through an intake structure brings the frazil ice crystals in contact with the
trash rack bars.  Frazil ice crystals easily adhere to any object with which
they collide.  The ice collects first on the upstream side of the trash racks,
then steadily grows until the space between the trash racks is bridged.  This
bridging rapidly blocks the trash racks.  The accumulation of ice can with-
stand high differential pressures; effectively damming the intake suction.

Facility vulnerability to icing events is a function of plant design.  Frazil
and other ice formation is dependent on specific environmental conditions and
represents a potential common-mode failure that can cause the loss or degrada-
tion of multiple cooling water systems, including the potential loss of the
ultimate heat sink.
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Related Information

NUREG/CR-0548, "Ice Blockage of Water Intakes."

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 project manager. 

                                       signed by

                                    Brian K. Grimes, Acting Director 
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical Contacts:  John R. Tappert, NRR       
                     (301) 415-1167             

                     Bruce Jorgensen, RIII
                     (708) 829-9615

Page Last Reviewed/Updated Thursday, March 25, 2021