Palo Verde Nuclear Generating Station

Palo Verde Nuclear Generating Station is a three-unit commercial nuclear power plant in the northeastern Sonoran Desert, approximately 45 miles west of downtown Phoenix, Arizona. Arizona Public Service operates the facility and holds a 29.1 percent ownership interest alongside six other public and investor-owned utilities. The station supplies electricity to a broad area of the American Southwest, including major load centers in Arizona, Southern California, and New Mexico. The plant is notable for its location. Unlike most large nuclear facilities, Palo Verde is not situated beside a major lake, river, or coastline. It uses treated municipal wastewater as the source for cooling water. Wastewater from nearby communities is further treated and stored in on-site reservoirs before being circulated through wet cooling towers. This arrangement allows a very large thermal power facility to operate in an arid environment while reducing reliance on scarce freshwater resources, although the plant remains a major consumer of reclaimed water through evaporation. Palo Verde consists of three pressurized-water reactors originally rated at approximately 1.27 gigawatts each. Subsequent power uprates increased their electrical output, bringing the units to roughly 1.4 gigawatts of generating capability each. Units 1 and 2 entered commercial operation in 1986, while Unit 3 followed in 1988. The station was built on approximately 4,000 acres and became fully operational after a twelve-year construction program. Its annual generation has been described as approximately 32 million megawatt-hours, with average output around 3.3 gigawatts, although production varies with maintenance outages, market conditions, and operating conditions. The facility is a major component of the western U.S. electrical grid. Its 500-kilovolt switchyard connects the plant to transmission paths serving Phoenix, Tucson, Albuquerque, Las Vegas, Los Angeles, San Diego, and surrounding regions. The Western Electricity Coordinating Council has treated the simultaneous loss of two of the three units as a particularly severe contingency for system stability because of the plant's size and transmission connections. Palo Verde's engineering and construction were led initially by the Arizona Nuclear Power Project, a joint effort associated with Arizona Public Service and Salt River Project, and later managed primarily by Arizona Public Service. Bechtel Power Corporation served as architect, engineer, and constructor. The reactors use Combustion Engineering's System 80 design, while General Electric supplied the principal turbine generators. Each unit operates largely as an independent generating station, sharing comparatively few major systems with the other units. The station has also been a recurring subject of regulatory and security attention. In the mid-2000s, equipment failures and other findings led the Institute of Nuclear Power Operations to place Palo Verde under heightened monitoring. The Nuclear Regulatory Commission later determined that performance had improved sufficiently to reduce the plant's level of special oversight. Separate incidents have included a 2007 security lockdown after suspected explosive material was found in a contractor vehicle and unexplained drone incursions over the facility in 2019. Palo Verde remains active under renewed operating licenses that permit Unit 1 to operate through 2045, Unit 2 through 2046, and Unit 3 through 2047.

Palo Verde emerged from planning for the Arizona Nuclear Power Project, an initiative associated with Arizona Public Service and Salt River Project to build major nuclear generating capacity for the rapidly growing Southwest. The selected site was in the Sonoran Desert near Tonopah, west of Phoenix. Its inland location created an unusual engineering challenge because the plant could not depend on a large natural body of water for condenser cooling. The solution was to obtain treated sewage water from nearby municipalities, process it further, and store it in reservoirs at the plant for use in cooling towers. The site selection generated controversy. Critics questioned the choice of a desert location, the prevailing winds toward the Phoenix metropolitan area, the limited local water supply, and alleged relationships involving land ownership and project decision-makers. The proposed development was nevertheless advanced, and Bechtel Power Corporation was selected as the project's architect, engineer, and constructor. Edwin E. Van Brunt was a key Arizona Public Service executive during engineering, construction, and the transition to operations. William G. Bingham served as Bechtel's chief engineer, while Arthur von Boennighausen was identified as an owner's representative. Construction lasted approximately twelve years. The completed facility occupies about 4,000 acres and contains three pressurized-water reactors using Combustion Engineering's System 80 design. Each unit was designed as a largely independent power station, with separate major systems and its own containment structure. General Electric supplied the turbine generators, which were among the largest 60-hertz turbine generators when installed. Units 1 and 2 began commercial operation in 1986, and Unit 3 entered service in 1988. Applications for two additional units were considered in the late 1970s, but those projects were cancelled before construction permits were issued because of economic risk. During operation, Palo Verde became a central source of electricity for Arizona and the wider western grid. Its 500-kilovolt switchyard supports transmission routes toward Southern California and other major southwestern markets. The station has often ranked among the highest-generating individual power plants in the United States and has been described as the country's largest nuclear power plant by capacity. Its output supplies a substantial share of Arizona's electricity and supports demand in metropolitan areas including Phoenix, Tucson, Albuquerque, Las Vegas, Los Angeles, and San Diego. The reactors underwent power uprates approved by the NRC in 2005, increasing their rated electrical output above the original approximately 1.27-gigawatt level. In 2011, the NRC renewed all three operating licenses for an additional twenty years beyond the original forty-year terms. The resulting expiration dates are 2045 for Unit 1, 2046 for Unit 2, and 2047 for Unit 3. Palo Verde's operating history has included periods of regulatory scrutiny. Findings in the 2000s included a dry pipe associated with the emergency core-cooling system and failures involving electrical relays on a diesel generator. These issues contributed to heightened industry monitoring and additional regulatory attention. In 2009, the NRC concluded that performance had improved sufficiently to reduce the plant's special oversight status, while emphasizing continued vigilance. Security has remained an important feature of the facility's operation because of its strategic role in the regional power system. National Guard troops were deployed to protect the plant during the start of the Iraq War in 2003. A 2007 vehicle-screening incident involving suspected explosive material led to a temporary lockdown. In 2019, several unidentified drones entered restricted airspace over the station on two nights, prompting investigations by multiple government agencies. Palo Verde continues to operate as a major nuclear generating asset using reclaimed municipal wastewater in an arid region.

1. 2019Drone incursions investigated

   Unidentified drones were observed over the plant on two successive nights, leading to a multi-agency investigation.

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2. 2011Operating licenses renewed

   The NRC renewed the operating licenses of all three units for twenty additional years beyond their original license terms.

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3. 2009Special oversight reduced

   The NRC returned Palo Verde to its normal Action Matrix column after finding sufficient performance improvement.

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4. 2005Reactor power uprates approved

   The NRC approved increases to the electrical output of Palo Verde reactors.

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5. 2003National Guard security deployment

   National Guard troops were sent to protect the plant amid heightened security concerns associated with the start of the Iraq War.

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6. 1988Unit 3 enters commercial operation

   The third reactor began commercial operation, completing the three-unit station.

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7. 1986Units 1 and 2 begin commercial operation

   The first two Palo Verde reactors entered commercial service, establishing the station as a major source of electricity for Arizona and the Southwest.

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A large-scale, low-carbon baseload electricity generator distinguished by its use of treated municipal wastewater for cooling in an inland desert setting.

• Unit 1Pressurized-water reactor1986

  The first Palo Verde reactor is a commercial pressurized-water reactor using the Combustion Engineering System 80 design. It originally had a net electrical capacity of approximately 1.27 gigawatts before an NRC-approved power uprate increased its rated output. Unit 1 forms part of the station's independently operated generating fleet and is licensed to operate through 2045.

• Unit 2Pressurized-water reactor1986

  Unit 2 is the second commercial reactor at Palo Verde and shares the station's reclaimed-water cooling system and transmission infrastructure. Like the other units, it uses a large pressurized primary loop, steam generators, and a high-capacity turbine generator. Its renewed operating license extends through 2046.

• Unit 3Pressurized-water reactor1988

  Unit 3 completed the station's three-reactor configuration. It uses the same broad System 80 design family and has an independently functioning reactor, steam, and turbine system. A power uprate increased its output above the original design rating, and its renewed operating license extends through 2047.

• Reclaimed-water cooling systemIndustrial water and cooling infrastructure1986

  Palo Verde's cooling system uses treated municipal wastewater rather than water from a nearby large natural body. Wastewater is treated, stored in on-site reservoirs, and circulated through wet cooling towers to condense steam after electricity generation. This design is the defining operational feature of the plant's desert location.

• 2011Renew all three operating licensesOther

  The original operating licenses for the three reactors were based on forty-year service periods.

  What changed. The NRC granted twenty-year license extensions for Units 1, 2, and 3.

  Aftermath. The authorized operating periods were extended through 2045, 2046, and 2047 respectively.

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• 2009Return to normal NRC oversightStrategy

  Earlier safety findings and equipment failures had resulted in heightened regulatory and industry monitoring.

  What changed. After reviewing reported performance improvements, the NRC cleared the Confirmatory Action Letter and returned the station to Column 1 of its Action Matrix.

  Aftermath. Special oversight was reduced, although the NRC stated that it would continue monitoring the plant closely.

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• 2005Increase reactor generating capacityGeneration change

  The original reactor ratings limited the amount of electricity that could be produced from the existing nuclear units.

  What changed. The NRC approved power uprates for Palo Verde reactors, increasing their rated net electrical output above approximately 1.27 gigawatts per unit.

  Aftermath. The uprates increased the station's generating capability without constructing additional reactors.

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• 2019Unidentified drone incursionsControversy

  Multiple drones were reported flying near the plant, including around Unit 3, on two nights in September. The NRC, FBI, Department of Homeland Security, FAA, and local authorities participated in investigations, but the operators and purpose were not publicly identified.

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• 2007Contractor vehicle explosives incidentControversy

  During routine vehicle screening, personnel found a pipe containing gunpowder residue in a contract worker's pickup truck. Police confirmed the material contained explosives, and Arizona Public Service initiated a security lockdown lasting several hours. The plant issued a Notification of Unusual Event, the lowest level in the U.S. nuclear emergency classification system.

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• 2011NRC renews licenses for all three Palo Verde units

  The Nuclear Regulatory Commission renewed the operating licenses for the three reactors, extending their authorized operating lives from 40 to 60 years.

  Regulation
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• 2009NRC reduces special oversight after performance improvements

  The NRC cleared a Confirmatory Action Letter and returned Palo Verde to the first column of its Action Matrix after concluding that plant performance had improved.

  Regulation
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• 2007Palo Verde placed under heightened nuclear-industry monitoring

  The Institute of Nuclear Power Operations assigned Palo Verde to a heightened monitoring category after failures involving diesel-generator electrical relays and a history of other safety findings.

  RegulationOther
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• 2005NRC approves power uprates for Palo Verde reactors

  The Nuclear Regulatory Commission approved power uprates that increased the rated net capacity of Palo Verde units, including increases reported for Units 1 and 3.

  Product generationRegulation
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1. Palo Verde Nuclear Generating Station