On July 8, 2010, as the temperature in downtown Decatur, Alabama, climbed to a sweltering 98 degrees Fahrenheit, operators at the Browns Ferry nuclear power plant a few miles outside of town realized they had only one option to avoid violating their environmental permit: turn down the reactors.
For days, the Tennessee Valley Authority (TVA), which owns the nuclear plant, had kept a watchful eye on the rising mercury, knowing that more heat outside could spell trouble inside the facility. When the Tennessee River, whose adjacent waters are used to cool the reactors, finally hit 90 degrees Fahrenheit and forced Browns Ferry to run at only half of its regular power output, the TVA hoped the hot spell would last just a few days.
Eight weeks of unrelenting heat later, the plant was still running at half its capacity, robbing the grid of power it desperately needed when electricity demand from air conditions and fans was at its peak.
The total cost of the lost power over that time? More than $50 million, all of which was paid for by TVA's customers in Tennessee.
"Last summer, the water in the Tennessee River warmed up early and stayed warm," says TVA spokesman Ray Golden. "When it got hot again in July and August, we were impacted by that and had to reduce power at the plant and get it from somewhere else."
With river water so warm, the nuclear plant couldn't draw in as much water as usual to cool the facility's three reactors, or else the water it pumped back into the river could be hot enough to harm the local ecosystem, says Golden.
But for every day that the Browns Ferry plant ran at 50 percent of its maximum output, the TVA had to spend $1 million more than usual to purchase power from somewhere else, he says.
No Threat of Meltdown, But Hefty Fines
What happened in northern Alabama last summer, at the largest of TVA's nuclear power plants, did not present a human safety concern. Operators knew there was never a risk of an explosion or nuclear meltdown, nor was there a threat of leaking radioactive material.
But the prolonged spell of hot weather put the TVA at risk of violating environmental permits, with hefty fines as one consequence and potential harm to the Tennessee River ecosystem as another.
It's not the first time high temperatures have affected the performance of the Browns Ferry plant, and extreme heat is a growing concern for power plant operators across the Southeast. While some nuclear plants can improve their cooling procedures to cope with the intake of warmer water, the upgrades can cost hundreds of millions of dollars and still don't offer an indefinite defense against extreme heat.
Because scientists say the Southeast (like many other parts of the world) can expect to see more frequent and intense heat waves by the end of this century, the problems for nuclear power and the people that rely on it for electricity may only be beginning.
Extreme Heat Limits Nuclear Energy Production
The disaster still unfolding at Japan's Fukushima Daiichi nuclear plant has refocused America's attention on nuclear power, calling into question its future role in the country's energy portfolio.
Many advocates of nuclear power say that we need to maintain — and even expand — nuclear power to get away from using fossil fuels such as coal, and to help lower greenhouse gas emissions.
But nuclear power has a paradoxical relationship with climate change. Even though it might help mitigate long-term global warming, nuclear power is already being challenged by rising temperatures and the increasing number of heat waves around the world. Throughout the last decade, several plants have had to reduce electricity production during heat waves, just when electricity demand typically reaches peak levels.
"It's a dilemma between mitigation of climate change and adaptation to it," says Natalie Kopytko, an energy policy doctoral student at the University of York in England. Having recently studied the ways in which climate change could have a negative impact on nuclear power, she says nuclear power is caught in the middle because it could be used to help lower greenhouse gas emissions, but global warming is making the technology less effective at providing electricity.
Most nuclear power plants draw water from nearby sources to help cool the reactors. Several American plants are on the coast and rely on ocean water, but the vast majority of nuclear reactors in this country (89 of the total 104) are inland, next to freshwater sources, and many of these are constantly cycling through river or lake water.
Normally, there isn't much difference between the water cooling process of inland and coastal facilities, but when hot weather strikes, a slow-moving and shallow river or a lake heats up a lot quicker than the ocean does. And when a nuclear power plant is drawing in such warm water, it can end up releasing unusually hot water back into the river. That's because the water gains heat while cycling through the plant.
Laws to Ensure Water Temps at Safe Levels