POWER October 2014 - 44

OPERATIONS & MAINTENANCE
Prepare Your Nuclear Plant for
Cold Weather Operations
Nuclear plant structures are designed to withstand extreme cold weather conditions.
However, a winterization program, such as those shared by Byron
and D.C. Cook power plants, can ensure critical operating systems are
prioritized and protected from freezing during low ambient temperature
conditions.
James M. Hylko
D
uring the Jan. 3-12, 2014, polar vortex
that brought record-setting cold temperatures
and severe winter weather to
much of the U.S., nuclear plants not only survived,
but thrived. According to the Nuclear
Energy Institute, the U.S. nuclear fleet maintained
a 95% average capacity factor-the
ratio of electrical power actually produced
by a generating unit to its rated full power capacity.
That percentage was greater than for
any other source of electricity during the period.
Still, nuclear plants are not completely
invulnerable to cold-weather issues, so well
before winter, nuclear plant operators should
have winterization on their minds.
Designed with Climate in Mind
Nuclear plants are built to withstand extreme
weather conditions. In the U.S. Nuclear Regulatory
Commission (NRC) regulations, Title
10, Code of Federal Regulations (10 CFR),
Appendix A to Part 50-General Design Criteria
for Nuclear Power Plants, 55 design
requirements are listed.
For instance, Criterion 2 covers Design
Bases for Protection Against Natural Phenomena.
Sufficient margins are included for
any limited accuracy, quantity, and period of
time involving historical environmental data.
In addition, 10 CFR Part 100-Reactor Site
Criteria considers the location and physical
site characteristics, including meteorology.
That information is used in conjunction
with 10 CFR 50 to determine the suitability
of a proposed site and characteristics that
may have an impact upon plant design. The
plant's design basis identifies the specific
functions, operating values, and bounding
parameters to demonstrate compliance with
these regulations.
Early Lessons Learned
Cold weather impacts started coming to the
forefront of the nuclear industry with the
44
NRC's IE Bulletin No. 79-24: Frozen Lines,
published in 1979. The bulletin discusses
how heat tracing was unable to prevent lines
from freezing at one northern U.S. plant due
to inadequate design when exposed to prolonged
sub-freezing temperatures. Although
the equipment had been working properly,
placement of temperature-sensing elements
in areas not exposed to the coldest temperatures
resulted in water freezing in a recirculation
line.
A total of 15 plants had experienced frozen
lines during a five-year period, which
prompted the NRC to issue the bulletin. All
licensees and construction permit holders
who received the bulletin were requested to
confirm that adequate protective measures
had been taken to ensure that safety-related
process, instrument, and sampling lines did
not freeze during extremely cold weather.
To prevent recurrence, thermostat temperature
settings for the heat tracing on recirculation
lines were increased, and heated
enclosures were built to shield against inclement
weather.
In January 1998, the NRC published Information
Notice No. 98-02: Nuclear Power
Plant Cold Weather Problems and Protective
Measures, because extreme cold weather
conditions continued to affect the industry.
Lack of design oversight, incomplete review
of operating experience, and insufficient attention
to cold weather preparations were responsible
for problems such as: ice formation
inside a diesel generator service water pump
column; frazil ice formation in the cooling
water intakes of three plants; a frozen safety
injection recirculation line, making the system
inoperable; inadequate insulation of refueling
water and condensate storage tank
level transmitter lines; emergency diesel generator
oil and grease viscosity problems; and
inadequately sealed electrical conduit and
cabinets, causing partial losses of onsite or
www.powermag.com
offsite power.
The NRC has continued to update its regulatory
documents, such as the Standard Review
Plan for the Review of Safety Analysis
Reports for Nuclear Power Plants, NUREG0800,
Section 2.4.7, Ice Effects, which was
revised as recently as 2007. In addition, the
NRC's Regulatory Guide 1.151, Instrument
Sensing Lines, first published in July 1983
and revised in July 2010, addresses the prevention
of freezing in safety-related instrument
sensing lines, and includes such design
issues as diversity, independence, monitoring,
and alarms.
The NRC's Inspection Procedure 71111,
Reactor Safety-Initiating Events, Mitigating
Systems, Barrier Integrity includes an
attachment on " Adverse Weather Protection. "
NRC resident inspectors use this procedure
to check readiness for cold weather,
freezing temperatures, and other extreme
weather conditions-including drought and
flooding-which could lead to loss of offsite
power, redundant equipment, and decay-heat
removal systems.
Walkdowns are used to verify that the
physical condition of weather protection features
is monitored to ensure structure, system,
and component operability. In addition,
the adequacy of heat tracing and space heaters
for cold weather protection of piping and
equipment is checked, and sensing lines and
fire suppression systems are inspected. These
reviews usually occur near the beginning of
each season.
Site-Specific Preparations Are Still
Needed
The purpose of a plant's winter readiness program
is to ensure that the station is protected
from freezing during extreme cold weather
conditions. POWER collected profiles from
two nuclear plants: Byron and D.C. Cook.
The Byron Generating Station, located in
POWER | October 2014
http://www.powermag.com
POWER October 2014

Table of Contents for the Digital Edition of POWER October 2014
