The U.S. nuclear industry is turning up the power on old reactors, spurring quiet debate over the safety of pushing aging equipment beyond its original specifications.
The little-publicized practice, known as uprating, has expanded the country's nuclear capacity without the financial risks, public anxiety and political obstacles that have halted the construction of new plants for the last 15 years.
The power boosts come from more potent fuel rods in the reactor core and, sometimes, more highly enriched uranium. As a result, the nuclear reactions generate more heat, which boils more water into steam to drive the turbines that make electricity.
Tiny uprates have long been common. But nuclear watchdogs and the U.S. Nuclear Regulatory Commission's own safety advisory panel have expressed concern over larger boosts — some by up to 20% — that the NRC began approving in 1998. Twenty of the nation's 104 reactors have undergone these "extended power uprates."
The safety discussions have largely escaped public attention, but they could become more prominent as the Japanese nuclear crisis focuses more scrutiny on U.S. reactors.
In an uprated reactor, more neutrons bombard the core, increasing stress on its steel shell. Core temperatures are higher, lengthening the time to cool it during a shutdown. Water and steam flow at higher pressures, increasing corrosion of pipes, valves and other parts.
"This trend is, in principle, detrimental to the stability characteristics of the reactor, inasmuch as it increases the probability of instability events and increases the severity of such events, if they were to occur," the Advisory Committee on Reactor Safeguards, which is mandated by Congress to advise the NRC, has warned.
Still, the committee has endorsed uprates, based on assurances that any increased risk falls within federal safety standards and is countered by additional safety measures such as plant modifications and more frequent inspections.
"You can always make a plant safer," said William Shack, a materials engineer and member of the safety committee. "The question is, when do I say I've made it safe enough?"
Computer models used to analyze risk suggest that a properly uprated reactor is no more vulnerable than one operating at its original capacity.
But critics of uprates point out that such analyses may fail to account for unforeseen accident scenarios.
"It's beyond the wit of mankind to identify all challenges to a nuclear plant," said John Large, a former researcher for the British atomic energy agency who runs a consulting company in London specializing in nuclear safety.
A case in point involved three uprated reactors in Illinois.
In 2002, both reactors at the Quad Cities Nuclear Plant were restarted after having their capacity boosted by 17.8%. Pipes began to shake, and cracks formed in a steam separator, which removes moisture from the steam before it enters the turbines. In one case, a 9-by-6-inch metal chunk broke off and disappeared.
Similar problems were discovered at the Dresden Nuclear Power Plant, about 60 miles southwest of Chicago, which had undergone a 17% uprate.
Broken parts were replaced, but the problem continued. Exelon Corp., which owns the three plants, and the NRC were mystified.
"The greatest concern is loose parts that you can't find," John Sieber, a nuclear engineer on the NRC advisory committee, said during a 2004 meeting. "Are they in the bottom of the reactor vessel? .... Is it floating around where it can damage internal parts of the core?"
Eventually the problem was uncovered: acoustic waves caused by the geometry of the steam pipes. The pipes were acting like a musical instrument. Their geometry was modified to "detune" them.
Plans to boost the power by 14.3% at three reactors in Athens, Ala., and 12.9% at a plant in Monticello, Minn., have been held up, in part, by NRC concerns over the steam separators.
Nuclear industry officials and regulators say that safety calculations are conservative and that even the biggest uprates fall far short of the power loads the reactors could actually handle.
Craig Nesbit, an Exelon spokesman, said that uprates "do not cut into the safety margins of these plants."
He and other industry officials note that uprates often require replacing turbines, transformers and other major equipment to accommodate higher water and steam flows.
But some things do not change, including the suppression pool, which is designed to soak up heat from the reactor core during some kinds of accidents, and the heat removal pumps, which deliver water from the pool into the core to prevent the fuel from melting down.
David Lochbaum, a nuclear engineer with the environmental group Union of Concerned Scientists, has argued that in some uprated reactors the pool may be too small and could become so hot that its contents could begin to vaporize, causing the pumps to lose suction.
Such a scenario is extremely unlikely, because the accident itself would create a pressure buildup in the surrounding containment vessel that would ensure that the pumps kept working. The system would risk failure only if the containment vessel itself were breached, allowing outside air to rush in.
The NRC allows companies to include that containment vessel pressure in their safety calculations. It has been used in 17 uprate approvals, according to NRC spokesman Scott Burnell.
But factoring in the pressure buildup "represents a decrease in the safety margin available to deal with a phenomenon subject to large uncertainties," the agency's safety advisory committee wrote in a March 18, 2009, letter to the agency. Forcing regulators to show that the safety system would work without the pressure buildup would offer an extra layer of protection against "potential melting of the core," the letter said.
The alternative would be requiring plant modifications so costly that companies say it would no longer make economic sense to uprate.
For the U.S. nuclear industry, which supplies a fifth of the nation's electricity, uprating is attractive because it is one of the cheapest ways to add power to the grid.
The 1979 partial meltdown at Three Mile Island eroded public confidence in nuclear power. Construction proceeded on many reactors that had already been approved — the last one went into operation in 1996 — but the industry was forced to look for ways to get more out of existing plants.
The biggest gains have been achieved by running reactors more efficiently — less downtime for fuel changes, for example.
But uprates have played an important role, adding the equivalent output of nearly five average-sized reactors since 1996. Regulators say they expect to approve boosts totaling 3 1/2 more reactors over the next four years.
Exelon, the nation's top nuclear provider, plans to spend $3.65 billion on power boosts equivalent to one new nuclear reactor over the next eight years, according to its filings for investors.