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| Fig. 1: Aerial View of Palo Verde Generating Station (Source: Wikimedia Commons) |
Even after living in Arizona for my entire life, I never realized how much my home state depends on nuclear energy. The biggest source of energy generation within the state comes from the nation's largest power producer 30 years running, Palo Verde Nuclear Generating Station (Fig. 1). I am shocked that I hadn't heard about the Palo Verde station. This may speak to the fear that surrounds nuclear power and the propensity of grid operators to keep it hush hush. With that said, the power plant has served Arizona well and has not to date had any major accidents.
The Palo Verde power plant generates more than 32 million megawatt-hours (1.15 × 1017 Joules) annually, which is enough power for more than 4 million people. [1] The plant supplies more than 70 percent of Arizona's clean-air energy, and does so with an innovative design that also reduces the station's water impact. Unlike every other nuclear plant in the nation, Palo Verde is not located on a body of water . It instead recycles wastewater from surrounding municipalities to cool the plant. [2] This is critical to sustaining such a large scale operation in the middle of the desert where water is a scarce resource.
Diving deeper into its design, Palo Verde generating station is a pressurized water reactor (PWR) which has three separate fluid systems: the reactor coolant system, the working fluid system, and the condenser coolant. This system has significant advantages over Boiling Water reactors (BWR) when it comes to the possibility of a contamination breach because BWRs only have two water loops which means the contaminated water leaves the containment structure to turn the turbine. This is not the case for Palo Verde, where each of the three nuclear reactors have their own bulbous concrete sarcophagus and operate nearly independently of one another. This allows the plant operators to take reactors offline for refueling and maintenance without entirely shutting down the plant. [3] Once the uranium fuel rods are spent, the nuclear waste is stored in earthquake proof, on site steel-and-concrete columns.
As many cities and states have made plans to eliminate their reliance on fossil fuels within the next couple of decades, Palo Verde Generation station has surprisingly been mentioned in cities outside of Arizona. For example, in collaboration with the National Renewable Energy Lab, two scenarios in Los Angeles plans to eliminate fossil fuels by 2035 highlight the city's continued 5.7% stake in the Palo Verde nuclear plant. This makes it clear that for a carbon-free grid to be possible, national leaders still believe that nuclear energy will play a critical role. [4]
The major risk with the Palo Verde Generating Station is the on-premise storage of the spent nuclear fuel rods. Because there has been no progress on creating a long term storage facility for nuclear waste in the United States, all of Palo Verde's nuclear waste remains on site. Assuming that a fuelrod is pure UO2 and that 5% of the Uranium mass in a fuel rod is burned, the thermal energy released per kilogram of fuel rod is
| Eth/M | = | 0.05 0.270 kg mole-1 |
× 6.022 × 1023 atoms mole-1 |
| × 2.3 × 108 eV atom-1 × 1.602 × 10-19 J eV-1 | |||
| = | 4.10 × 1012 J kg-1 | ||
Palo Verde Generating station produces 1.17 × 1017 Joules of electricity per year. Since only 30% of the thermal energy produced by fission is turned into electricity, the plant is generating over 95 tonnes of high-level waste per year. This is a scary amount of waste that will accumulate with time and poses a major security risk should terrorists get access to the casks.
Though there has never been an accident, Palo Verde's operator, Arizona Public Service, has come under some scrutiny for its violations of NRC regulations. These violations included APS's failure to perform a written evaluation for a change to a dry cask storage system for spent fuel as well as an inadequate analysis of the consequences of a hypothetical tip-over accident on the plants spent fuel pad. [5] This does raise concerns because when there has been nuclear accidents historically, many have occurred because of human error and non-compliance with regulation. Therefore, following regulations assiduously is of the utmost importance in such a critical operation. If a solution cannot be made long term, Palo Verde Generating Station will need to shut down.
© Samuel Beskind. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Economic Benefits of Palo Verde Nuclear Generation Station," Nuclear Energy Institute, Nov 04.
[2] C. Chandler II, "Palo Verde Generating Station," Physics 241, Stanford University, Winter 2016.
[3] D. Oberhaus, "How to Refuel a Nuclear Power Plant During a Pandemic," Wired Magazine, 6 Apr 20.
[4] K. Pirehpour, "Palo Verde Generator Helps Southwest Meet Climate Goals, But Future of Nuclear is Debated," Cronkite News, 19 Apr 21.
[5] "Confirmatory Order on Palo Verde Nuclear Generating Station," U.S. Nuclear Regulatory Commission, 17 Nov 20.
