Water vapor rises hundreds of feet into the sky above the Palo Verde Nuclear Generation Station in the Arizona desert.
The white cloud looks like smoke, but it contains no toxins or radiation.
Below the cloud, nuclear fission takes place in three dome-shaped reactors. The heat produced inside the reactors is used to make steam, which drives massive turbines connected to generators that make electricity.
It is the same process used in coal-fired power plants, the only difference is the fuel used to make heat.
Unlike coal plants which spew mercury, carbon dioxide and nitrogen oxide into the atmosphere nuclear plants are largely emissions-free.
There are no greenhouse gases or other gases that you would get from the operation of a fossil-fired unit, said Mark Fallon, spokesman for Palo Verde.
Still, many environmentalists remain opposed to nuclear power.
The mining and milling of uranium ore can be destructive and dangerous to human health. There is no long-term solution for permanent storage of radioactive waste. And the consequences of a catastrophic mishap at a nuclear plant are too high to justify its continued use, said Dan Randolph, executive director of the San Juan Citizens Alliance in Durango.
Were not willing to trade off one devil for another, he said. It is a human endeavour, and humans make mistakes. When the consequences are so high, it is a bad proposition to go forward.
He added: We firmly believe that renewable energy is what we need to move to.
Palo Verde is the closest operating nuclear plant to Durango, about 400 miles to the southwest and 50 miles west of Phoenix in the Sonoran Desert.
It produces more electricity than any other power plant in the United States 3,800 megawatts per hour enough to serve about 4 million people.
The electricity feeds the Western Interconnection grid, which stretches from Western Canada, south to Baja California in Mexico, and eastward over the Rocky Mountains and the Great Plains.
The Arizona Public Service Co., which owns the largest share of Palo Verde (29.1 percent), also owns a majority share of the Four Corners Power Plant, the No. 1 emitter of nitrogen oxide in the United States.
The 50-year-old plant, which generates 2,040 megawatts, is 15 miles southwest of Farmington and is about 15 miles from the San Juan Generating Station, another coal-fired power plant in the Four Corners that produces 1,800 megawatts. Together, they emit a constant haze.
The air down here continues to be pretty bad a large brown visible cloud, said Mike Eisenfeld, New Mexico energy coordinator with the San Juan Citizens Alliance.
No greenhouse gases
Nuclear power supplies 20 percent of Americas electricity. Other power sources include coal (45 percent), natural gas (24 percent), hydro (6 percent), renewables (4 percent) and oil (1 percent).
Of the power sources that do not emit greenhouse gases, nuclear power makes up 69 percent of the output, said Tom Kauffman, spokesman for the Nuclear Energy Institute, a lobbying group for the nuclear industry.
Nuclear energy is by far the single largest source of clean electricity in the United States, he said.
But that doesnt take into account the mining and milling of uranium or the radioactive byproduct that is produced, which is hazardous to most forms of life and takes tens of thousands of years to diminish.
While it may be very clean at the reactor, its very dirty in the field where the fuel is produced and where the waste will remain forever, said Travis Stills, a lawyer with Energy & Conservation Law in Durango.
Uranium mining has all the challenges as other mining endeavours, including impacts to air, roads, habitat and ground water, Stills said.
But uranium mining has the added problem of radioactive emissions that need to be stabilized in perpetuity to prevent radiation from being released into the air or leached into ground water, he said.
Its an extraordinary challenge to handle these uranium mines in a safe manner, Stills said. And usually what that means is its really expensive to do it right.
Nuclear waste presents another challenge.
The United States has about 65,000 tons of nuclear waste in temporary storage waiting to be put into geologic storage. But finding a waste repository has proved difficult. A proposal to bury the waste in Nevadas Yucca Mountain was scrapped in 2009 after years of controversy and legal wrangling.
At Palo Verde, nuclear waste is stored inside massive concrete cylinders, called dry casks, that are stacked side by side on-site awaiting permanent burial. The plant has filled 94 of these casks in the last 25 years.
Safety a concern
The potential of a nuclear meltdown is low, but the results can be catastrophic, which is why the nuclear industry is one of the most regulated in the world.
A nuclear meltdown can release radioactive gases, resulting in poisoning of nearby people, animals and plants.
Last years Fukushima Daiichi disaster in Japan released large amounts of radioactive isotopes into the Pacific Ocean as well as trace amounts around the world.
The Chernobyl disaster of 1986 released a plume of radioactive smoke fallout that drifted over large parts of the western Soviet Union and Europe. The accident is blamed for thousands of cancer cases.
Nuclear power plants take every technological precaution necessary to ensure safety, but they also must guard against terrorist attacks.
At Palo Verde, private security guards search every entering vehicle. Visitors and employees must show identification, even to go to the visitors center. A second layer of security exists around the reactors, which include cameras, motion sensors and high barbed-wire fences.
Officials said they are prepared to handle terrorist attacks from the air, but they wouldnt elaborate on what precautions are in place.
Palo Verde, located in an area with a low earthquake hazard, is the only nuclear plant in the world not located next to a lake, ocean or river.
Instead, a 36-mile pipeline carries wastewater from Phoenix to the plant, where a reclamation system can recycle up to 90 million gallons a day. Huge reservoirs on-site hold a years supply of water should anything happen to the piped-in water.
Diverse power a plus
There are 442 nuclear reactors operating worldwide, including 104 in the United States. Many have been shutdown since Fukushima, but they are still considered operational until they are decommissioned.
More than 60 nuclear reactors are under construction, mostly in China. A few have been approved recently in the United States.
Because renewables such as wind and solar can be intermittent in their production, it is good have a variety of energy sources that feed the grid, said Fallon, the spokesman at Palo Verde.
Its got to be a good blend of sources, he said. They really sort of complement each other.
An earlier version of this story said Palo Verde produces 3,800 megawatts per year. It produces about 3,800 megawatts per hour.