WASHINGTON -- The rupture of a steam generator tube that triggered Monday's accident at the Ginna nuclear plant in Ontario, N.Y., is part of a baffling problem plaguing almost a score of reactors around the country.
Steam generator tube failure has been a leading safety concern of the Nuclear Regulatory Commission for a decade and a formal staff study of the problem has been under way for a couple of years.
Leaks in steam generator tubes were found recently in the undamaged Three Mile Island 1 reactor, complicating attempts to restart the plant.
Virginia Electric & Power Co. already has had to replace steam generators at its Surry plant and Florida Power & Light is in the process of doing so at Turkey Point. Total replacement can cost as much as $300 million.
Less extensive repairs to the steam generator made recently at Ginna, San Onofre 1 in California, Palisades in Michigan, and Point Beach 1 in Wisconsin. Similar work is planned for Point Beach 2 and New York's Indian Point 3.
Chronic problems with the tubes, blamed on poorly understood water chemistry and hydraulic forces, range from ruptures and cracks to thinning, denting squeezing, pitting, stress corrosion and vibration wear.
'They've resolved a lot of problems, but others keep springing up,' said Frank Ingram, an NRC spokesman.
Dr. Fred Millard of the Environmental Policy Center in Washington, D.C., said the steam generator problem is the result of an immature technology being scaled up to commercial size too fast.
He said commission experts worry that a massive steam generator failure could be disastrous in the event of an accident in which core coolant is lost. They fear steam bubbles from the secondary loop could block the flow of primary coolant from emergency pumps, he said.
A steam generator is essentially a heat exchange between the radioactive and pressurized primary coolant water that bathes the nuclear fuel core and a secondary coolant loop that carries steam to the turbines.
It may be 20 stories tall, weigh 200 tons and contain as many as 15,000 small tubes that separate the fluid in the two loops and serve as the membrane for heat exchange.
When a tube breaks, the secondary coolant loop is contaminated with radioactivity from the primary coolant that is in contact with the core.
Early reports on the Ginna accident were unclear whether the radiation -- apparently a small amount -- that was vented into the outside air came from the secondary or primary loop.