Unlike the leak at Davis Besse, this is the result of a leak from the bottom of the reactor pressure vessel, which had previously not been thought to present a problem. A leak at the bottom of the RPV would be harder to repair.

STP Nuclear Operating Company officials notified the NRC on 13 April that they identified potential leak indications at the bottom of Unit 1’s reactor vessel during a visual inspection. Small deposits of boron crystals were identified at two bottom mounted instrumentation penetrations, indicating a possible leak of reactor coolant. STP 1 has been shut down since March 25 for a refueling outage.

The NRC’s Special Inspection team, comprised of metallurgical and engineering specialists, will monitor the utility’s investigation and evaluation of the potential leaks. The inspection is being conducted to better understand the circumstances surrounding the boron crystal deposits and to evaluate the licensee’s corrective action plans.

One possible explanation is that a small, intermittent leak developed around insulated tubes penetrating the reactor pressure vessel. However, to date, an STP project team have reexamined the instrument guide tubes, and found no additional residue. Plant operators said that the deposits, smaller than an aspirin tablet, could be the result of a leak of up to 700l of coolant from the reactor over as much as four years.