TOP PLANTS: Palo Verde Nuclear Generating Station, Wintersburg, Arizona

The nearly 4-GW, three-unit Palo Verde Nuclear Generating Station remains the largest generator of electricity in the U.S. for the 23rd consecutive year, producing more than 30 million MWh in 2014, for the 10th time (the only plant in the U.S. to do so), all while using only treated wastewater for cooling.

Palo Verde Nuclear Generating Station (Palo Verde), located on 4,000 acres deep in the Arizona desert about 50 miles west of Phoenix, serves the electricity needs of approximately four million people in Arizona (about 35% of its power needs), Southern California, New Mexico, and far west Texas. The plant, which began construction in 1976 and was completed in 1988 at a cost of $5.9 billion, features three units with-unlike most nuclear plants-very little common infrastructure between the units. Palo Verde has long been the largest U.S. nuclear power plant as measured by power generation.

Steam is produced by Combustion Engineering System 80 pressurized water reactors in a 2 x 4 configuration-four main reactor cooling pumps circulate 111,000 gpm of primary coolant through two steam generators. The reactors were originally licensed to operate in 1985, 1986, and 1987, and each was initially rated at 3,990 MWt. The General Electric generators remain the largest 60 Hz generators in worldwide service at a nuclear power plant.

Since 2005, the U.S. Nuclear Regulatory Commission (NRC) has approved increases in the net generating capacity of each unit to 1,311, 1,314, and 1,312 MW, respectively, as a result of plant upgrades. Operating license extensions for each of the three units were approved in 2011, extending plant operation until 2045, 2046, and 2047, respectively.

"Our mission is to safely and efficiently generate electricity for the long term," said Randy Edington, executive vice president and chief nuclear officer for Arizona Public Service Co. (APS), which operates the plant for the group of owners (listed at the top). "We have worked very hard to demonstrate to the NRC through extensive inspections and audits that Palo Verde is prepared to operate for an additional 20 years."

The plant employs about 3,000 workers and has an annual economic impact of more than $1.8 billion in Arizona, according to APS.

Rising Capacity Factors

Nuclear power plant capacity factors are rising across the industry. The Nuclear Energy Institute (NEI) reported that the average capacity factor of all U.S. nuclear power plants in June was 96.4%, the highest that it has been in six years (it was 91.7% in 2014). In fact, 90 of the 99 operating nuclear reactors averaged 90% or higher, and 62 operated at 100% or higher in June.

According to Platts' Megawatt Daily June 22 report, in 2014, Palo Verde Unit 3 generated more electricity than any single unit in the U.S., producing 12.2 million MWh, exceeded only by EDF's 1,560-MW Chooz-B2 reactor in France for worldwide honors. However, Palo Verde's 1,312-MW Unit 3 posted a higher annual capacity factor of 97.5%, compared to 94.1% at Chooz-B2.

Palo Verde Unit 3 continued the plant's history of outstanding operations by duplicating Unit 2's achievement the year before. In 2013, Unit 2 reported a 94.8% capacity factor, the highest of all plants in the world top 10 rankings, while Unit 1 was ranked third in the U.S. and seventh in the world.

Generating large amounts of electricity has been in the plant's DNA since it began operation. All three Palo Verde units are individually ranked among the top six producers in the U.S., according to industry data. "We take pride in regularly generating more electricity than any other power plant in the country, ensuring that people across Arizona and the Southwest can continue to enjoy reliable, low-cost electricity," said Edington.

For Palo Verde, 2014 was its 23rd consecutive year as the largest power generator in the U.S., producing 32.3 million MWh and breaking its own record of 31.9 million MWh set in 2012. Palo Verde is the only generating station of any technology to produce 30 million MWh in a single year, an achievement that it accomplished in 2014 for the 10th time, and in six of the past 10 years.

More Records

The Palo Verde staff is also well-practiced in the art and science of conducting short refueling outages and can turn around a unit in record time. In the spring of 2013, the staff completed its first sub-30 day refueling outage in plant history with a plant record-setting 29 days, 18 hours for Unit 1. In spring 2014, the staff bettered that record by refueling Unit 2 in 28 days, 22 hours. "This refueling outage is another example of the world-class performance we have come to expect from Palo Verde, where safety remains our highest priority," Edington said. At Palo Verde, the units are on an 18-month refueling cycle, with two refuelings scheduled each year-one in the spring and another in the fall.

In aggregate, the three units have been running very well. From April 28 until Oct. 5, 2013, a period of 160 days, all three units operated, the second-longest continuous run in plant history. The long run ended when Unit 3 was brought offline in order to begin a planned refueling and maintenance outage, although Units 1 and 2 continued to operate at 100%. During 2012, Unit 2 recorded the best performance in plant history with 518 consecutive days of operation, ending Oct. 5, 2012. Each of the three units has a recent continuous run exceeding 500 days.

Using Recycled Water

Palo Verde is the only U.S. nuclear power plant that is not located next to an ocean or other large body of water. It instead sits in the middle of Arizona's Sonoran Desert. Palo Verde was the first nuclear power plant in the world and remains the largest in the U.S. to use recycled municipal wastewater for condenser and other plant cooling needs (Figure 1).

1. Water reuse. The Palo Verde Nuclear Generating Station uses 100% recycled municipal wastewater from Phoenix and surrounding cities for condenser cooling. Shown are aeration ponds that are part of the plant's water reclamation and treatment facility. Courtesy: Arizona Public Service

APS concluded a landmark 40-year agreement in 2010 with the five cities in the greater Phoenix metropolitan area to provide an annual allotment of up to 26 billion gallons of treated municipal effluent to Palo Verde through 2050. The tertiary treated effluent originates from the 91st Avenue Wastewater Treatment plant in west Phoenix and is piped to Palo Verde, where it is further treated to meet the water quality standards established by the plant. The agreement was negotiated over several years and replaces the original water pact signed in 1973. Water deliveries under terms of that agreement began in 1982, when Unit 1 began operations, and was scheduled to expire in 2027. "Palo Verde provides substantial environmental benefits since it does not emit any greenhouse gases and because it makes the most efficient use of our limited water resources," said Edington. The pact also solves a problem faced by many municipalities-how to dispose of a potentially valuable byproduct that increases as the population grows.

Grey water effluent provided to Palo Verde is produced in three steps: solids removal, primary treatment to remove any remaining solids, and then secondary treatment in which biological or percolating filters break down organic material and purify the liquid. The treated effluent flows 28 miles downhill and then is pumped another 8 miles to the plant site, where it enters the Palo Verde Water Reclamation Facility. There the effluent is further treated before it is stored in a 760 million-gallon lined reservoir that covers 80 surface acres.

The closed loop condenser/cooling tower circuit uses water from this reservoir for plant cooling. Three mechanical forced-draft cooling towers are used for condenser cooling, one for each unit. The towers operate at 25 cycles of concentration, which produces a blowdown stream whose salinity approaches that of seawater. Once this concentration is reached, the water is discharged to evaporation ponds. Because of the corrosive nature of the effluent, the three-pressure, three-shell surface condensers were originally upgraded to titanium and the tube sheets are fabricated out of aluminum bronze with mechanically expanded tube joints. Mechanical scrapers are used to keep the tubes clean. The Marley condensers continue to provide reliable service using tertiary treatment grey water after almost 30 years of service. a-

- Dr. Robert Peltier, PE is POWER's consulting editor.

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