What happened?

Managers of a big Texas nuclear power station told U.S. Nuclear Regulatory Commission (NRC) staff Thursday they do not know what caused a leak in the plant's reactor, a discovery that could set off safety shutdowns at dozens of other plants.

The South Texas 1 plant, 90 miles southwest of Houston, has been shut since late March when a routine inspection turned up two tiny boric acid deposits on the underside of its pressurized reactor vessel. Boric acid, used in reactor coolant water to control its radioactivity, is highly corrosive in open air and can eat through metal.

An Unexplained Leak

Plant operators said the boric acid deposits, smaller than an aspirin tablet, could be the result of a leak which may have begun 4 years ago of up to 700 liters of coolant from the reactor. Plant officials said the leak poses no public health threat.

One possible explanation is that a small, intermittent leak developed around insulated tubes that pierce the reactor's metal hull to allow instruments to measure its inner workings. An inspection in November found no problems.

Industry-Wide Implications

But the mysterious deposits have raised concerns about possible leaks at other U.S. reactors of similar design. Those 69 pressurized water reactors, more than half of the 103-unit U.S. fleet, generate about 10 percent of the nation's electricity. Cracks and minor leaks around the reactor vessel have been discovered in several of them related to a widely used nickel alloy that has been found to be susceptible to cracks.

The NRC ordered owners of all U.S. pressurized water reactors, like the South Texas unit, to inspect their reactor vessel heads after finding the worst case of severe corrosion in 2002 at FirstEnergy Corp.'s Davis-Besse unit in Ohio. Leaking boric acid ate a cantaloupe-sized hole almost entirely through the 6-inch-thick heavy steel cap atop that reactor's outer hull.

Since then, several U.S. utilities including FirstEnergy have spent millions of dollars to replace faulty reactors. Prior to the South Texas plant inspection, leaks at the bottom of reactor vessels had not been a concern.

What needs to be done?

Senior NRC staff planned to meet later on Thursday to discuss the problem in more detail, Barrett said.

Managers of the South Texas Project Nuclear Operating Co., which runs the plant's two reactors on behalf of its four owners, told the NRC they will disassemble the plant's reactor to find the "root cause" of the leak.

Operators of South Texas Project's troubled Unit 1 reactor have narrowed their repair options down to one that would allow potentially damaged instruments to continue to function. Of three repair techniques considered by engineers, all of which are risky and complicated given the high radioactivity of the bottom of the reactor vessel, the one called a "half-nozzle" repair is favored. Half-nozzle repair involves cutting the tube in half and welding a new tube over it. This technique has the advantages of preserving instruments that monitor the reactor core and enabling some of the work to be done remotely, reducing worker exposure to radiation.

Engineers are in the process of removing the fuel and instruments from the reactor core to enable them to inspect the inner surface of the vessel before they begin the repairs. They will likely leave water used to cool the reactor inside the vessel to act as a shield against radiation, but the water will make the work more difficult. Engineers might use a video camera to visually examine the inner surface of the welds around the tubes.

Finding the cracks that would explain how telltale deposits of boric acid collected around two tubes that allow instruments to enter the reactor is essential to establishing whether the problem could be more widespread in the U.S. nuclear fleet.

Repair work could begin in mid-June, pending the NRC's approval, and a return to service could occur in late summer. The NRC has sent an inspection team to the plant to monitor the inspections and repairs. It will be up to the team to determine whether the agency needs to alert other reactor owners to the possibility of leaking from bottom-mounted instruments.

How Long and How Much?

The 1,250-megawatt Texas plant, which generates enough power to supply more than a million homes, could remain shut down until late summer, months longer than expected. Early estimates put the cost of the South Texas outage at about $5 million to $6 million.

The South Texas Project Operating Co. operates the two-unit plant for Texas Genco, which is 81% owned by CenterPoint Energy . CenterPoint, the City of San Antonio, American Electric Power unit Central Power & Light, and the City of Austin own the plant itself.

FirstEnergy estimates the repair job at Davis-Besse, including buying replacement power from other generators, will cost more than $400 million and keep the plant out of service through most of the summer.

More Plants in Trouble

Two huge Southeast nuclear power plants have been added to the growing list of U.S. reactors plagued by cracking or leaking reactor vessels, the Nuclear Regulatory Commission said on May 5. The latest additions are FPL Group Inc.'s Unit 2 at the St. Lucie nuclear facility in Florida and Duke Energy Corp.'s Oconee 3 unit in South Carolina. Both reactors are currently down for refueling and pose no public safety hazards.

FPL said the 839-megawatt St. Lucie unit, which generates enough electricity for about 800,000 homes, would receive temporary repairs and the reactor vessel head would not be replaced at this time. After inspecting 80 of the reactor vessel lid's 102 nozzles, cracks measuring about an inch in one and three inches in another were found in two nozzles. Inspection of the remaining 22 nozzles is expected to be completed within the next day or two.

FPL has ordered replacement heads for all four of its Florida nuclear units and will begin replacements next year during planned refueling outages. The first unit to receive a new vessel head will be the Turkey Point 3 unit, the oldest of the four units.

The reactor vessel head at Duke's Oconee unit, shut since about April 26, was scheduled to be replaced during the current outage. The unit was tentatively set to return to the power grid in early June. Duke first detected cracks in the vessel heads at its three Oconee reactors in late 2000, ordered replacement heads, and scheduled their installation over the next 2 years to coincide with scheduled refueling outages.

Reactors typically need refueling every 18 to 24 months.

Oconee 3 is the first of the three identical 846 megawatt Duke units to receive a new vessel head as part of a $60 million replacement program at the Oconee power station. -- Edited from articles at ENN, Dow Jones Newswires, and Yahoo! Finance

Davis-Besse Ohio Atomic Plant Is Investigated Over Acid Leak

Federal regulators in late August investigated accusations that FirstEnergy Corporation altered records about the damage. The Nuclear Regulatory Commission has said the leak that caused it should have been spotted as long as four years ago. A coalition of 14 environmental and nuclear watchdog groups is urging the agency to order an independent review of the plant. A coalition spokesman said investigators told him that the agency was studying whether FirstEnergy backdated videotapes, falsified documents and withheld a photograph to make damage seem less severe than it was. -- Edited from the full article (and many others) at Mindfully.org

Nuclear Regulators Dozed as Acid ate Davis-Besse Reactor Head

Many long suspected that FirstEnergy Corp. wasn't the only one hibernating while boric acid dined on Davis-Besse's steel-lined reactor head. Not until March of this year did Davis-Besse officials - and thus the NRC - discover that boric acid had burrowed through 6 1/2 inches of the Toledo area reactor's head. That is an unprecedented event. A few inches more and radioactive coolant could have spewed into the reactor building, perhaps precipitating a disaster. The NRC signed off on a company corrosion prevention program in 1990 that its own officials believed was weak. In 2000, the cleaning crew reported that boric acid deposits were so thick that they had to use crowbars to pry them off. -- Edited from the article at The Plain Dealer

Nuclear safety: Uh-oh in Ohio

Davis-Besse is a 25-year-old, single-unit pressurized water reactor located in Oak Harbor on the shore of Lake Erie, about 30 miles east of Toledo. Inside its containment building is the reactor pressure vessel, and inside that is the core. Sixty-nine hollow nozzles stretch through the vessel head down into the core; through the nozzles, control rods can be lowered to stop reactions. The carbon steel walls of the reactor vessel are 6 inches thick and internally lined with noncorrosive stainless steel cladding, and designed to withstand up to 2,500 pounds of pressure per square inch. On the vessel head, boric acid corrosion had eaten through the 6 inches of carbon steel and carved out a cavity 4 inches by 7 inches wide. Between 35-40 pounds of carbon steel were simply missing, and the only thing that contained the radioactive, highly pressurized coolant water inside the vessel was the thin skin of stainless steel cladding. Not designed to endure such pressure, the lining had started to bulge outward. If the lining had been breached, a loss of coolant accident would have resulted. This could have led to a severe accident. Simple boric acid accumulation on reactor vessel heads is not terribly meaningful. A tiny amount of boric acid, between 1,000-2,000 parts per million, is dissolved into reactor coolant water, which is not corrosive. Because coolant leaks at high temperatures (500-600 degrees Fahrenheit) as steam, and because the vessel head is also very hot, the steam quickly evaporates and leaves behind boric acid crystals, which look like sugar. It is common for pressurized water reactors to have small amounts of boric acid crystals on vessel heads. But at Davis-Besse, the boric acid was allowed to accumulate until it became "substantive." Beginning in approximately 1996, the boric acid deposits grew and changed in consistency until about 70 percent of the vessel head was covered with a hard, lava-like coating ranging from 1-3 inches thick. Somehow, in this environment-no one understands the mechanics of it yet-a pit in the carbon steel began to corrode rapidly. -- Edited from the Bulletin of the Atomic Scientists