NewsApril 10, 2011

In Tennessee, Heat Waves Diminish Nuclear Power Output

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Alyson Kenward

By Alyson Kenward

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On July 8, 2010, as the temperature in downtown Decatur, Alabama climbed to a sweltering 98°F, 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°F and forced Browns Ferry to run at only half of their 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 electicity demand from air conditions and fans was at its peak. The total cost of the lost power over that time? More than $50 million dollars, all of which was paid for by TVA’s customers in Tennessee. 

The Browns Ferry nuclear plant, located on the Wheeler Reservoir along the Tennessee River near Athens, Alabama. It has three reactors, each producing about 1000 megawatts of electricity. Credit: Nuclear Regulatory Commission.

“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 spent $1 million more than usual to purchase power from somewhere else, he says.

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 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.

The March-August 2010 was the warmest such period on record in the Southeast. Studies show that by the end of the century, the number of hot days in the summer could double for this region. Credit: NOAA NCDC.

Power companies like the TVA can’t control the weather. Nevertheless, plant operators are bound by environmental guidelines that are meant to keep temperatures at a safe level for fish in the river. For example, the Alabama Department of Environmental Management (ADEM) stipulates Browns Ferry cannot release water back into the Tennessee River that is above 90°F.

“I know this past summer the TVA was worried about exceeding their permits,” says Scott Hughes from the ADEM. “But they adjusted their operations and stayed within the limits.”

The 90°F ceiling has been especially problematic for the TVA because in the past five years, the river water has, on several occasions, warmed that much on its own. And each time, Browns Ferry has been forced to reduce electricity production. This puts a pinch in the electricity supply for the more than 2.1 million homes and businesses that depend on electricity from Browns Ferry. More importantly, the problem gets transferred to the pocketbooks of TVA’s customers.

“When we can’t generate that power from our nuclear plant, we have to go elsewhere in the energy market to get it,” says the TVA's Golden “In some cases we have to increase the production from some of our other plants, including coal plants, and in other cases we go to other companies and buy power.”

In addition to finding power from other sources, last summer the TVA called upon its customers to cut down on their electricity use throughout July and August. But the request came at the hottest time of the year, when electricity demand is usually at its highest.

Other large-scale power plants, including coal and biomass plants, are also vulnerable to heat waves, and they are subject to the same kind of environmental permits for hot water. But according the TVA's Golden, while “it is also a problem for coal-fired power plants, the size of nuclear power plants is much larger so the cooling problem is bigger.” That's because cooling at many nuclear power plants isn't as efficient as at coal-fired power plants, and the nuclear power plants usually require more water for their cooling than other types of plants. 

Heat Waves are on the Rise

What happened last summer at Browns Ferry may be a sign of what people living in the Southeast can expect in the future. As average global temperatures rise, studies show the risk of heat waves also increases. New research suggests extreme heat will become a more regular occurrence across the U.S..

“One of the things that is happening is that the heat wave season, the time over which heat waves might occur, is actually getting longer,” says Kenneth Kunkel, a climate scientist from the Cooperative Institute for Climate and Satellites in North Carolina. “Consequently, you can get heat waves a lot earlier in the year, and the season can also extend a lot longer.”

Kunkel and his colleagues have recently modeled the future of heat waves across the United States, depending on what global greenhouse gas emissions are like during the rest of this century. In the Southeast, they found that by 2100, every year there could be between 60 and 80 more days with heat wave-level temperatures than there are currently. More frequent heat waves will mean higher Tennessee River water temperatures.

A separate Climate Central analysis shows similar trends for the region. For example, each summer between June and September, there is an average of 44 days when the temperature is above 90°F in Athens, Ala., a location nearby to the Browns Ferry nuclear power plant. By the end of the century, however, Athens should expect to see about 80 summer days above 90°F. That 80-day estimate is based on a future climate scenario with relatively low greenhouse gas emissions; if atmospheric carbon dioxide emissions continue to climb at the current rate or higher, Athens could see even more of those exceptionally hot days. 

“It may be that humans are able to adapt to the higher temperatures,” says Kunkel, “but of course, a nuclear power plant is just going to have to deal with the conditions.”

When Nuclear is the Primary Power, Extreme Heat Can Hit Hard

Temperature departures from average during the 2003 summer heat wave in Europe. In France, more than 200 reactor days were lost because of shut-downs or reduced power productions across the country. Credit: NASA Earth Observatory.

The vulnerability of nuclear power to heat waves isn’t restricted to the Southeast. In the summer of 2003, during a record-breaking heat wave in Western Europe, millions of people across France and Italy suffered through an extended power shortage after the French network of 19 nuclear power plants had to reduce their operations. In France, over 70 percent of the country’s electricity comes from nuclear power, and Italy also purchases about a third of its electricity from French nuclear providers.

During the heat wave, France took some of the pressure off its electrical grid by purchasing power from other sources, promoting energy conservation among citizens and industry, and by exporting less to Italy — causing many Italian towns to endure blackouts.

Although the energy shortage can’t be solely blamed for the thousands of heat wave-related deaths in France and Italy that summer, it put a strain on people who lost air conditioning, as well as hospitals.

In Illinois, where a larger portion of electricity comes from nuclear power than any other state, plants have also fallen prey to summer heat waves. Back in 1988, which featured an unusually hot and dry summer, several reactors were reduced to just one-third of their maximum power output during a 90-day bout of abnormally hot weather.

But while inland nuclear power plants everywhere are threatened by heat waves, the dilemma may be growing worse in the Southeast. Last summer was the hottest on record for the region. An early season heat wave in May warmed the Tennessee River more than usual for that time of year. And then more hot weather settled in a few months later and sent water temperatures soaring; August 4, 2010 marked the hottest single day in the TVA region in more than 50 years – temperatures in Nashville climbed as high as 111°F, for example.

The power lost at Browns Ferry during the late summer heat wave of 2010 was enough to catch the TVA’s attention. In late August, the company decided to invest in more cooling infrastructure at their biggest nuclear power plant. “At Browns Ferry, we’re spending about $160 million on retrofits to improve the cooling,” says Golden. “It’s an awful lot of money but the project should pay for itself in just a few short years — especially if there are more heat waves.” The upgrade has added a larger cooling tower to the nuclear plant than the one originally there, which helps bring down the temperature of the water before it is sent back into the river. A few more similar improvements will be made in the next two years, says Golden.

Other power companies may have to explore similar options in the years to come. Installing better cooling to combat high water temperatures, and designing more efficient closed-loop systems that don’t constantly demand fresh water are technically feasible, says Golden, but they could prove to be prohibitively expensive upgrades for older power plants.

The Browns Ferry upgrades will be enough to combat heat waves similar to those seen in 2010. Whether they are able to withstand the earlier and more intense heat waves of the future, on the other hand, isn’t something the TVA can tell just yet. The repairs may end up being just a short-term solution for a long-term problem the nuclear industry is facing. 

Senior Scientist Claudia Tebaldi conducted heat wave calculations for this story.