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.
During 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 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 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, NUREG-0800, 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 Byron, Ill., consists of two 1,200-MW Westinghouse four-loop pressurized water reactors (PWRs). Units 1 and 2 received their operating licenses in 1985 and 1987, respectively. The plant is currently owned and operated by Exelon Corp.
The D.C. Cook Nuclear Plant is located in Bridgman, Mich., on the eastern shore of Lake Michigan. This plant also consists of two Westinghouse four-loop PWRs. Unit 1 is rated at 1,030 MW and received its operating license in 1974. Unit 2 is rated at 1,100 MW and received its operating license in 1977. The plant is owned by American Electric Power (AEP) and is operated by its subsidiary Indiana Michigan Power.
Each winterization program is customized for the plant and its location. Although both plants are located in the Midwest, Byron’s cooling water is from the Rock River, while D.C. Cook draws water from Lake Michigan.
For Gary Dudek, work management director at the Byron nuclear plant, the winterization process starts in April. The work management department is responsible for the seasonal readiness program, which covers both winter and summer seasons for the site. The system engineering department identifies systems and components that are critical to winter operations. Winter and summer readiness coordinators supervise the work teams. The D.C. Cook plant has a comparable schedule (see sidebar “D.C. Cook Winter Preparation Timeline”).
D.C. Cook Winter Preparation Timeline
March/April: The seasonal readiness coordinator (SRC) manages the winterization challenge board, and a “Historical Freeze List” consisting of ongoing lists maintained by operations staff of expected structures, systems, and components that may be affected by cold weather conditions. The SRC, system engineering manager, appropriate system managers, and operations and work control representatives conduct challenge board meetings to discuss results of material condition reviews and to evaluate if all needed reliability improvements have been identified.
March/April: The SRC performs the initial winterization walkdown, focusing on problem areas from the most recent winter period, and from previous years, to verify if corrective actions have been initiated for all known deficiencies.
April: Systems needed only for winter are secured during the summer, such as heating systems. The system engineers and the SRC perform walkdowns to identify maintenance requirements for the systems removed from service.
March/May: The SRC reviews open work orders (WOs) for items requiring completion prior to winter, and ensures that items identified by these reviews are incorporated into the work planning schedule for resolution prior to Nov. 1. From this point forward, the SRC continues to screen emergent issues and WOs for winter readiness applicability, tracks completion of deficiency resolutions, and ensures all items are completed by the Nov. 1 deadline. All identified work must be added to the schedule, which for winter readiness is the second week of May each year.
October/November: The SRC performs the final winterization walkdown with engineering, maintenance, fire protection, security, and facilities representatives to verify that the site is ready for the upcoming winter season.
Systems of Interest
Bobby Norrick, seasonal readiness coordinator (SRC) and eSOMS (enterprise Shift Operations Management System) administrator at the D.C. Cook plant conducts winterization reviews in conjunction with the systems engineering group. The team identifies all actions needed to ensure systems will function properly and reliably through the winter (Figure 1). In addition, the Work Assessment Group identifies and prioritizes new equipment deficiencies and resolutions that affect winter readiness during the daily screening meeting.
|1. Winter wonderland. While the typical winter struggles, such as snow and ice in the parking lot, are common to many plants, personnel at D.C. Cook also focus on specific plant systems during winter preparations. Courtesy: American Electric Power|
The following systems are reviewed for winter readiness:
- Auxiliary steam
- Alternate plant heating boiler
- Boric acid heat tracing
- Fire protection
- Circulating water
- Heating steam
- Refueling water storage tanks freeze protection
- Outside heat trace
- Turbine building
- Auxiliary building
Additional activities consist of verifying that all steam heaters in the vicinity of exterior plant walls, exterior rollup doors, auxiliary building crane bay, main steam stop enclosures, and auxiliary building supply ventilation units are operating during the initial winterization walkdown (March/April) and then during the final winterization walkdown (October/November).
Furthermore, the auxiliary building, screenhouse, and turbine building are inspected for damaged doors, windows, and vent louvers that do not close completely, including holes, gaps, and cracks in building siding, and missing weather stripping. The goal of the heater and building inspections is to verify that the systems supporting safe operation of the reactors, turbines, and generators are protected from freezing. The SRC and tour operators—at the discretion of the shift manager—can increase touring frequencies during severe winter weather conditions or high summer heat conditions.
At the Byron plant, the heating system and multiple ventilation systems are important for insulating and circulating heat generated in areas that have little heat load during normal plant operations. These systems are evaluated by engineering and operations to identify maintenance needs for pumps, valves, and heating coils. In addition, a winter surveillance is performed by operations to ensure that insulation is installed on selected outside louvers and to validate readiness of tank heating systems.
To provide clear direction on this subject, a corporate seasonal readiness procedure was created from past industry experiences and NRC notices. It specifically references the following Institute of Nuclear Power Operations (INPO) documents addressing winterization:
- INPO Just-In-Time Operating Experience Report, “Cold Weather Preparations.”
- INPO Significant Operating Experience Report (SOER) 82-15, “Freezing of Safety Related Equipment.”
- INPO SOER 07-2, “Intake Cooling Water Blockage.”
A corporate procedure and many other internal Exelon procedures drive all aspects of seasonal readiness and include various attachments and checklists to complete. The procedure details the roles and responsibilities for the winter readiness coordinator, and the other disciplines, along with spring, summer, and fall milestones. It culminates with a site certification letter stating that the plant is ready for the upcoming winter season (Figure 2).
|2. Readiness upgrades. Byron finished refurbishing its natural draft cooling towers in 2013, including the extension of a metal barrier along the bottom of the towers to help prevent icing and improve winter readiness. Courtesy: Exelon Corp.|
Don’t Get Frazzled
An unusual phenomenon that some power plants must be alert to is the formation of “frazil ice.” The ice forms below the surface of a body of turbulent water when the sustained air temperature is 18F or lower. Frazil ice crystals appear as elongated disks. These disks gradually stick together to form large ice floes and jams. Frazil ice can be drawn into cooling water intakes, where it blocks flow by adhering to screens designed to prevent trash from entering the intakes.
The Byron station has experienced issues with frazil ice in the past. To correct this issue, a 4-inch warming line was installed off the 10-inch gravity-fed circulating water blowdown line. Blowdown water is discharged on the outer bar grate, where the makeup pumps take suction from the Rock River. This warm water increases the temperature of the intake water to prevent frazil ice.
Although D.C. Cook’s corrective action program has not recorded instances of frazil ice blocking the cooling water intakes, as a preventative measure, it uses the following factors to determine when it is necessary to switch its circulating water system to de-ice mode:
- Circulating water temperature is less than 35F.
- There is greater than 1 foot of water drawdown in the forebay, which is not caused by starting a circulating water pump. (The forebay sits below the screenhouse and is the source of water for the circulating water pumps and essential/non-essential service water pumps.)
- Traveling screens’ high, or extreme high, differential pressure alarms are annunciating with little or no trash collection in the screen wash trash baskets.
The successful Byron and D.C. Cook winterization programs utilize team approaches. They also provide a framework for maintaining plant reliability while preventing cold weather disruptions.
Despite snowstorms and frigid temperatures dominating the first month of 2014, Exelon’s nuclear fleet produced electricity around the clock and was unaffected by ice. Exelon’s 10 nuclear power plants in Illinois, Pennsylvania, and New Jersey operated continuously throughout January to help meet the region’s electricity demand, which peaked in the PJM transmission system on Jan. 7 at 141,312 MW—a wintertime demand record at the time. Its plants continued to operate with the same reliability through a second blast of polar air that began Jan. 21.
The D.C. Cook seasonal readiness program also proved its value during the 2013–2014 winter season as southwest Michigan experienced one of its coldest winters. Both units operated successfully without any de-rates or unit trips, and they maintained 100% power throughout AEP’s winterization period. ■
— James M. Hylko is a POWER contributing editor.