“WASHINGTON — The discovery of water flowing across the floor of a building at the Indian Point 2 nuclear plant in Buchanan, N.Y., traced to a leak in a buried pipe, is stirring concern about the plant’s underground pipes and those of other aging reactors across the country.

A one-and-a-half-inch hole caused by corrosion allowed about 100,000 gallons of water to escape from the main system that keeps the reactor cool immediately after any shutdown, according to nuclear experts. The leak was discovered on Feb. 16, according to the plant’s owner, Entergy Nuclear Northeast, a subsidiary of the Entergy Corporation.

Entergy and the federal Nuclear Regulatory Commission emphasized that the Indian Point reactor could still have been shut down safely with either of two other backup systems, although operators generally avoid using both.

They also stressed that the supply pipe was quickly repaired after the leak was found and that the water itself, which is cleaner than tap water, posed no environmental threat. Yet the leak’s discovery has prompted Entergy and the regulatory commission to begin studying how the chief system for cooling during shutdowns, so important that the Indian Point 2 has three pumps in place to do the same job, could be endangered by the failure of a single part.

More broadly, it has raised concerns about the monitoring of decades-old buried pipes at the nation’s nuclear plants, many of which are applying for renewal of their operating licenses. Indian Point 2, whose 40-year operating license expires in 2013, already faces harsh criticism from New York State and county officials who want it shut down.

This week Representative Edward J. Markey, the Massachusetts Democrat who heads a House subcommittee on energy and the environment, said the leak raised serious questions about Entergy’s and the regulatory commission’s oversight.

“This leak may demonstrate a systemic failure of the licensee and the commission to inspect critical buried pipes in a manner sufficient to guarantee the public health and safety,” he wrote to the commission’s chairman, Dale Klein, in a letter on Thursday. The letter was also signed by Representative John J. Hall, whose district includes the plant. The congressmen said they were “shocked” that a leak that big could develop without detection and called the system for detecting such problems “profoundly inadequate.”

One argument raised by New York State in opposing extension of the license of Indian Point 2 or the adjacent Indian Point 3 reactor is that crucial components are aging in ways that the operators may not anticipate or understand.

The supply pipe at issue, measuring eight inches in diameter, is used to fill a 600,000-gallon tank that is employed whenever the plant “trips,” or shuts down because of an equipment malfunction. Such shutdowns are not unusual; one occurred on April 3, roughly a month after the pipe was fixed.

James F. Steets, a spokesman for Indian Point, said it was unclear when the leak began. The company initially said the pipe was losing 18 gallons a minute but later amended that to 12; either number is small relative to the 600,000-gallon tank, he said.

Mr. Steets said that the water level in the tank offered no clue that the supply pipe was leaking. The tank has an alarm to indicate its water level is falling, he said, but it did not sound because an automatic system was topping off the tank with purified water.

At a nuclear plant, a central water system takes heat from the reactor in the form of steam and turns it into electricity. During a shutdown at Indian Point 2, that system often turns off and a pipe measuring 12 inches in diameter carries water from the tank into the cooling system to carry off excess heat.

The buried portion of neither the eight-inch supply pipe nor the 12-inch pipe connecting the tank to the reactor cooling system has been visually inspected since the reactor began operating in August 1973, according to the Nuclear Regulatory Commission. Nor does the commission require such inspections.

Paul Blanch, an electrical engineer and nuclear safety expert who worked at Indian Point in 2001 and 2002, said that because neither pipe had been inspected, except for a short section that was replaced when the hole was located in February, “they shouldn’t be operating right now.”

He said the plant could be operating with a backup system that is ready to fail.

Mel Gray, a branch chief at the Nuclear Regulatory Commission who oversees inspections at Indian Point, confirmed in a telephone interview that inspectors “have not dug up and laid eyes visually” on the pipes. But he said that experts routinely conduct “surveillance tests,” measuring the tank level and the flow through the pumps that direct water from the tank to the reactor.

“If you had a gross leak, you’d detect its going somewhere else,” he said, referring, for example, to a leak large enough to drain the tank quickly.

Mr. Gray acknowledged that the 12-inch line that delivers water from the 600,000 gallon tank during a shutdown might be rusted in places, too, but he said it was unlikely to fail suddenly when called on. But Mr. Blanch warned that if gravel or dirt leaked into the 12-inch supply pipe when the pumps started up, that could make them shut down.

Mr. Steets of Entergy said that if the tank were disabled, a tank filled from Buchanan’s municipal water system could be used to deliver water during a shutdown.

But Mr. Blanch and the letter from the two congressmen faulted the system that relies on city water.

Plant operators dislike using such water because city tap water is not as clean as reactor water. And critics point out that the system is not safety-rated, meaning it is not certified to work in adverse conditions like blackouts and earthquakes and is not maintained as carefully.

Another potential solution proposed by the Nuclear Regulatory Commission involves using the reactor’s emergency core cooling system during a shutdown. But cooling water can be inserted only after the pressure in the reactor is reduced, which causes the water to boil. Letting the water boil can lead to core damage.

Buried pipes are emerging as an endemic problem as reactors age, although so far most of the attention has been to the substance that is leaked — not to a pipe’s role in ensuring the reactor’s safe operation over all.

Reactor water includes tritium, a radioactive form of hydrogen that can occur naturally but is also made in reactors. Leaks of water with tritium have been discovered in underground piping at the Byron, Braidwood and Dresden twin-reactor plants in Illinois, and at a three-unit plant in Arizona, Palo Verde. Indian Point also leaked water with tritium from its spent fuel pool in 2005.

While experts at the Nuclear Regulatory Commission said in interviews that additional pipe leaks like the one found in February would not pose a big challenge to reactor operators, they acknowledged that it was something new.

“We were not aware of a problem before with underground pipe,” Mr. Gray said. “Now that we have one, it’s got our focused attention.”

“We’re not done,” he said.”

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