04/18/03

A Texas utility with a pressurized water reactor similar to Ohio's Davis-Besse plant has told federal regulators the bottom of its reactor vessel appears to be leaking.

"Their preliminary thinking is they do have a small crack," Brian Sheron, Nuclear Regulatory Commission associate director for licensing, said yesterday after speaking with managers at the South Texas Project, in Bay City, Texas, south of Houston.

If confirmed by additional tests, the finding would be an unexpected and serious development, with potential consequences for the nation's commercial nuclear fleet. It is also possible, however, that the leak resulted from some unique condition that does not exist at other plants, Sheron said.

For now, the agency is closely following the plant's work to pin down the source of the traces of dried coolant. South Texas workers found the residue at two locations on the reactor's base last weekend, during a refueling shutdown.

No reactor has ever before been shown to have cracks or leaks in the instrument-carrying tubes that pierce the bottom of the heavy steel reactor vessel, although workers found traces of possible leaks at the bottom of the Davis-Besse reactor last summer. Davis-Besse owner FirstEnergy Corp. believes the residue washed down during attempts to clean the reactor's lid rather than leaking from the bottom. But it has scheduled a test in May to be sure.

The NRC is concerned that if an undetected leak in the reactor's base becomes large enough, it could stymie the ability of the plant's emergency pumps to keep the hot, radioactive fuel bathed with coolant.

Regulators and the nuclear industry had thought the reactor bottom was invulnerable to cracks. The NRC does not require inspections there, although some plants do them voluntarily. Temperatures in the reactor's base are lower than in the vessel's upper reaches - 560 degrees or so, compared to nearly 600 around the lid - so it was believed that the material used to weld the instrument tubes in place wouldn't be exposed to the heat stresses that can cause the welds to crack and leak.

An inspection 18 months ago showed no signs of leakage on the reactor bottom, Sheron said the plant's operator told him. However, a lab test indicated the deposits could be several years old, he said.

South Texas officials could not be reached for comment yesterday.

- John Funk and John Mangels