Daily photo by Gary Lloyd|
Mist rises from one of the cooling towers at Browns Ferry Nuclear Plant on Wednesday. Unit 2 shut down last week because of high river temperatures.
Will warming temperatures
make nuclear power unfeasible?
World debates Browns Ferry
By Eric Fleischauer
The shutdown of a Browns Ferry reactor last week because of high Tennessee River temperatures was international news.
In the mainstream press from Britain to Germany to Malaysia, on National Public Radio, and in anti-nuclear blogs worldwide, people who could not find Alabama on a map became experts on Browns Ferry.
The interest stems from a raging environmental debate that deemed last week’s shutdown of Unit 2, and the power reductions in Units 1 and 3, as relevant to the viability of nuclear power.
Nuclear power has created a schism among environmentalists.
On the one hand, the world has yet to find a satisfactory solution for disposing of nuclear waste, and the risk of a catastrophic failure is alarming.
On the other hand, an operating nuclear plant creates essentially none of the greenhouse gases that most experts believe are contributing to climate change.
The shutdown dropped Browns Ferry into the middle of the debate. What use is nuclear power if plants become inoperable as global temperatures rise?
“I think both sides of the issue tend to overstate their case,” said Dave Lochbaum, director of nuclear safety at the Union of Concerned Scientists.
He was a reactor engineer at Browns Ferry Unit 1 from 1980 to 1983.
Global warming aside, the issue is significant for the Tennessee Valley. The ongoing heat wave escalates demand for electricity.
It also heats the Tennessee River. Thus Browns Ferry is prone to shut down when demand is highest.
High Tennessee River temperatures cause two problems for Browns Ferry. Problem No. 1 involves environmental regulations.
Problem No. 2 involves engineering issues.
Browns Ferry generates electricity when high-pressure steam passes through turbines, which look like big fan blades. After the steam passes through the turbines, cool water from the Tennessee River — circulating in condenser pipes — converts the steam into water. In the process, the circulating river water gets hot.
23 degrees hotter
With rare exceptions, the river water passes through the pipes and returns to the river. When it returns to the river, according to TVA’s senior manager of environmental policy, Jack Brellenthin, it is about 23 degrees hotter than when it entered the plant.
That’s why fish — and fishermen — are at Browns Ferry during cold months. A hike in temperature is bad for some aquatic species, though, and it’s particularly harmful when river temperatures are already high.
State environmental regulations, specific to Browns Ferry, impose limitations on how much the plant can raise the river’s temperature.
“When the water gets this warm,” explained Brellenthin, “there’s the potential for stress on aquatic resources.”
TVA has sensors upstream and 2,400 feet downstream. The plant can raise the river temperature but only up to 90 degrees. If the upstream temperature is already 90 degrees or more — which it often has been this month — TVA cannot increase the temperature.
“Last week into this week was a challenge, and it hasn’t let up much,” Brellenthin said Wednesday. “It’s still a challenge for us.”
Browns Ferry has a collection of wood cooling towers it rarely uses. It is using them now.
Typically, Brellenthin said, the plant ups the river’s temperature by about 5 degrees.
It’s a big river, and it is hard to conceptualize how any discharge from Browns Ferry could significantly change its temperature. At the plant, the river’s main channel is about 30 feet deep and 2,000 feet wide.
Disguised by the placid waters around the plant, however, is the massive amount of water it digests. When all units are online, more than 3 billion gallons of river water run through the plant per day, mainly through the condenser pipes. That’s enough to fill three Olympic-size swimming pools ... per minute.
The Unit 2 shutdown, and the power reduction of Units 1 and 3, was necessitated by environmental regulations. By reducing power output from the reactors, TVA reduced steam through the turbines. That meant that river water — even at a heat-wave-induced 90.4 degrees — could condense the steam and, after traveling through the cooling towers, enter the Tennessee River without violating downstream temperature limitations.
Having to reduce power output during peak demand — especially after investing $1.8 billion into the restart of Unit 3 — is enough to make TVA officials sweat.
“We’re starting an investigation today to see what the issues were last week and how we can learn from it,” Brellenthin, who worked at Browns Ferry for 12 years, said. “We need both to meet our need to produce power and comply with our permit limitations.”
The 90-degree cap imposed by environmental regulations makes the engineering issues — confronted when intake water exceeds 95 degrees — irrelevant to Browns Ferry.
The issues are relevant, however, to the reason Browns Ferry’s shutdown received global attention.
Jones explained that Browns Ferry can operate at 100 percent power until the temperature of the Tennessee River exceeds 95 degrees. That temperature is unlikely, unless global warming is factored in.
Lochbaum explains the engineering problem presented by high river temperatures.
As high-pressure steam enters the turbine chamber, it begins losing energy. It gets a major assist from a vacuum pulling it through the turbines. The condensation process creates the vacuum. As cool river-water condenses nuclear-heated steam, air pressure drops. That means high pressure is pushing the steam into the turbines, and low pressure is drawing it out.
As river temperatures rise, however, the river water’s ability to condense the turbine-turning steam plummets. The vacuum diminishes, and the steam struggles to make it through the turbines. If the intake water exceeded 95 degrees, TVA would have to reduce reactor power.
The problem is not unique to nuclear power plants, Lochbaum said. High water temperatures hamper any power plants — even some biomass plants — that rely on steam turbines.
At Browns Ferry, the cooling towers do nothing to resolve the problem. Water from the towers goes straight to the river.
Anti-nuclear activists from Malaysia to Britain have it part wrong, part right. Last week’s Browns Ferry shutdown had nothing to do with the limitations of nuclear power.
“Even the hottest day we’ve had this year,” Jones said, “we’ve not been limited by any vacuum issues.”
Accept the global-warming premise, though, and the headlines had merit. If climate change pushes river temperatures up 5 degrees, engineering issues — not environmental ones — would hamper Browns Ferry’s ability to produce electricity. And it would do so when the demand for electricity is highest.
Brellenthin’s not too worried. Even the most pessimistic evaluations of global warming suggest Browns Ferry is fine.
“The climate’s warming up,” he said, “but we don’t see a big enough change to where it’s going to affect our plans. The time scales for climate change are longer than the forecast life of the plant.”
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