Engineers at a nuclear plant are examining residue, about half the size of an aspirin, apparently from reactor coolant fluid that leaked near the bottom of a reactor vessel.

The residue was found a week ago while the reactor was shut down for scheduled refueling and maintenance.

"At this point we don't know the root cause. We do have some seepage," said Ed Halpin, manager of the South Texas Project plant. He said engineers and chemists had found no additional residue but the reactor would remain shut down until the problem was fixed.

Officials on Friday acknowledged finding the residue after an official with the Nuclear Regulatory Commission was quoted in a published report saying the plant was leaking.

"Their preliminary thinking is they do have a small crack," Brian Sheron of the NRC said in Friday editions of The (Cleveland) Plain Dealer. A call to the NRC Saturday was not immediately returned.

Workers at Ohio's Davis-Besse nuclear plant, which has a similar pressurized water reactor, found evidence last spring that a boric acid leak had bored part way through a 6-inch-thick steel cap on a reactor vessel there. Plant owner FirstEnergy Corp. said it believed the residue washed down during cleaning, but tests are planned next month to be certain.

In the statement released Friday by the South Texas Project, officials said the powdery material was found April 12 on the outside of two instrument guide tubes where the tubes enter the bottom of the reactor. The reactor is encased in a concrete and steel-lined containment building.

No reactor has ever been shown to have cracks or leaks in the instrument-carrying tubes, The Plain Dealer said. Any such disclosure, if confirmed, would be a serious development for the nation's nuclear plants. A large enough leak, undetected, could impede the ability of emergency pumps to cool radioactive fuel.

Test results at the Texas plant, about 70 miles southwest of Houston, indicate the residue came from reactor coolant fluid, plant officials said. Halpin said the residue was boric acid, which is part of the coolant system.

If problems are found, he said, they could be repaired by welding.

The guide tubes where the residue was found are not integral to the reactor's operation, he said. "They are instruments we use to monitor the activities of the plant," he said of the 58 tubes. "It is backup instrumentation essentially."

The plant's other unit continues to operate at full power. The plant's two reactors combine to produce more than 2,500 megawatts of electricity for customers from Houston to Austin to San Antonio to Corpus Christi.