Information Notice 1998-02, Nuclear Power Plant Cold Weather Problems and Protective Measures
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UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
January 21, 1998
NUCLEAR POWER PLANT COLD WEATHER
PROBLEMS AND PROTECTIVE MEASURES
Addressees
All holders of operating licenses for nuclear power reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees to potential common-cause failure mechanisms of safety-related systems and
systems important to safety caused by extremely cold weather. 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
Wolf Creek
IN 96-36, "Degradation of Cooling Water Systems Due to Icing," described the effects of icing on
intake trash racks and traveling screens which resulted in a manual reactor/turbine trip on
January 30, 1996, at Wolf Creek. In addition to the degradation of the circulating water and
essential service water systems, the turbine-driven auxiliary feedwater pump was degraded
during this event. The NRC identified the event as a precursor within the framework of the
NRC's accident sequence precursor program, and calculated a conditional core-damage
probability of 2.1 x 104.
Circulating water system degradation was initially attributed to water from the spray-wash system
freezing on the traveling screens, making them inoperable. To improve the reliability of the
circulating water system, the licensee revised plant procedures (1) to eliminate the requirement
to operate traveling screens continuously in slow manual mode during cold weather or under
unusual icing conditions and (2) to allow the screens to be operated in the automatic mode in
which the screens remained stationary without sprays until the system was started either by a
timer or a high differential level. The traveling screens were also enclosed in a heated
environment.
After IN 96-36 was issued, more information became available about the degradation of the
circulating water system. Specifically, on August 5, 1997, the licensee reported that during the
event, two of three air release valves on the circulating water warming line had been plugged.
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IN 98-02 January21, 1998 Failure of these valves caused air binding in the piping and caused the warming flow to be
nearly zero.
The effect of the resultant air binding was demonstrated during a 5-day test in January 1997 with 2.8 'C (37 °F) lake water, when warming line flow was reduced from 1388 liters per second
(22,000 gpm) to approximately 347 liters per second (5,500 gpm). This warming flow
degradation would have been more pronounced during the event because the temperature of
the lake water was approximately 0 OC (32 OF) during the event and, therefore, contained more
air in solution. Although air removal by the water box venting system should have significantly
reduced the potential for air binding, approximately one-half of the water box air release valves
had been isolated during the January. 1996 event.
In response to these findings, the three air release valves on the warming line were replaced
and incorporated into the preventive maintenance program. The licensee also revised
procedures to ensure (1) manual venting of the circulating water warming line when the inlet
temperature fell below 1.1 'C (34 0F), and (2) verification of the presence of circulating water
warming flow and the passing of air from the air release valves when the warming line valve
was opened in the fall. A procedural step was also added to verify that the water box air
release valves were not isolated.
Millstone Unit 2
On January 8, 1996, with the plant at 100-percent power, an ice plug formed in a horizontal, common, service water (SW) strainer backwash drain line that ran through the intake wall in a
trough toward a fish basket. This would have prevented automatic backwash of the SW
strainers. This pipe had been welded onto the end of the original vertical discharge leg in a
modification that had not undergone a formal engineering review. Minor leakage through the
strainer backwash isolation valves and an unusually long period of subfreezing temperatures
created the conditions needed to form the ice plug. Removal of the ice plug restored the SW
strainer backwash capability and operators backwashed the strainers every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> to ensure
that another ice plug would not form until the horizontal pipe was eliminated. Nonetheless, the
open end of the line was still susceptible to ice buildup. The licensee later reported that the
operators had failed to recognize that the ice plug that made the backwash functions of the SW
strainers inoperable also made both SW system trains inoperable. The licensee should have
declared the SW system inoperable during the event and entered Technical Specification
Limiting Condition for Operation 3.0.3, which required a plant shutdown.
On February 29, 1996, the licensee reported that the SW strainer backwash system was also
susceptible to a common-mode failure if the intake structure's nonvital heating system failed to
operate. The licensee changed an operating procedure to require (1) monitoring of the intake
structure temperature when temperature fell below 4.4 OC (40 OF) and (2) using portable space
heaters or manual operation of the strainers to prevent freezing. The licensee also proposed
replacing the common line with three independent backwash lines, locating the discharge points
to minimize the effect of outdoor weather conditions, and protecting differential pressure
instrumentation from freezing.
January 21, 1998 LaSalle Unit 2
On February 4, 1996, operators shut the plant down manually when the oil temperature in the
main transformer could not be maintained within design limits because of the loss of the
transformer cooling fan and cooling pump. Ice severed the transformer cooling logic cable in its
conduit where it entered the underground cable trough. Licensee corrective actions included
inspecting other transformer conduits, clearing water and ice from conduits, and sealing the
conduits.
McGuire Unit 2
On February 8, 1996, with the plant at 100 percent power, two of three refueling water storage
tank (RWST) level transmitters were found to be inoperable because of frozen impulse lines.
The lines froze because thermostat setpoints for the strip heaters were set too low for cold
weather conditions. The frozen impulse lines affected control room RWST level indication and
the ability to automatically switch to the emergency core cooling system sump. To correct this
problem, the licensee increased the thermostat setpoint and added inspection of the level
transmitter panels to its cold weather preventive maintenance procedure.
Discussion
The January 1996 event at Wolf Creek, described above, led the NRC Office for the Analysis
and Evaluation of Operational Data (AEOD) to review the extent of cold weather-related
problems at U.S. nuclear power plants over the past 6 years. The results of this study are
given in Engineering Evaluation Report AEOD/E97-03, "Nuclear Power Plant Cold Weather
Problems and Protective Measures." It contains a compendium of recent cold weather-related
events and corrective actions, as well as design, operations, and training lessons learned by
the nuclear industry and the U.S. Army Corps of Engineers Cold Regions Research and
Engineering Laboratory. The report noted 37 cold weather related events at 23 different sites
between 1991 and 1997. The study also reported an increasing trend in the number of these
events.
Licensees continue to find that icing and freezing from extreme cold weather conditions is a
common-cause failure mechanism that can quickly affect a variety of systems unless mitigating
actions are taken in a timely manner. The recent operating experience described in this study
suggests that, despite NRC and industry communications on this subject, some licensees have
not effectively protected components whose failure could degrade safety-related systems and
systems important to safety. Extreme cold weather conditions continued to affect intake
structures; process lines; instrument lines; emergency diesel generator oil and grease
viscosities; essential chillers; electrical systems; and heating, ventilation, and air conditioning
systems. Lack of design oversight, incomplete review of operating experience, and insufficient
attention to cold weather preparations were responsible for most of the events.
IN 98-02 January 21, 1998 This information notice requires no specific action or written response. However, recipients are
reminded that they are required by 10 CFR 50.65 to take industry-wide operating experience
(including information presented in NRC information notices) into consideration, where practical, when setting goals and performing periodic evaluations. 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.
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical Contacts:
John R. Tappert, NRR
301- 415-1167 E-mail: jrt@nrc.gov
L. Mark Padovan
301- 415-1423 E-mail: Imp@nrc.gov
Robert A. Spence
301- 415-6346 E-mail: ras2@nrc.gov
Attachment:
List of Recently Issued NRC Information Notices
"-S 98-02
January 21, 1998 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
98-01 Thefts of Portable Gauges
1/15/98
All portable gauge licensees
97-91
97-90
97-89
97-88
Recent Failures of Control
Cables Used on Amersham
Model 660 Posilock Radiography
Systems
Use of Nonconservative
Acceptance Criteria in
Safety-Related Pump
Surveillance Tests
Distribution of Sources and
Devices Without Authorization
Experiences During Recent
Steam Generator Inspections
Second Retrofit to
Industrial Nuclear Company
IR 100 Radiography Camera, to Correct Inconsistency in
10 CFR Part 34 Compatibility
Additional Controls for
Transport of the Amersham
Model No. 660 Series
Radiographic Exposure Devices
12/31/97
12/30/97
12/29/97
12/16/97
12/12/97
12/12/97
All industrial radiography
licensees
All holders of OLs for nuclear
power reactors except those
who have ceased operations
and have certified that fuel has
been permanently removed from
the vessel
All sealed source and device
manufacturers and distributors
All holders of OLs for pressurized- water reactors except those who
have permanently ceased
operations and have certified that
fuel has been permanently
removed from the reactor
All industrial radiography
licensees
Registered users of the Model
No. 660 series packages, and
Nuclear Regulatory Commission
industrial radiography licensees
97-87
97-86 OL = Operating License
CP = Construction Permit