The Nuclear Regulatory Commission (NRC) has begun a Special Inspection into potential leakage at the bottom of the reactor vessel at South Texas 1.
Unlike the Alloy 600 vessel head problems experienced at Davis Besse and elsewhere, the bottom of the reactor pressure vessel had previously not been thought to present a problem. Such a leak 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 refuelling 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 explanation is that a small intermittent leak developed around Alloy 600 instrument tubes penetrating the reactor pressure vessel. However, to date, an STP project team re-examining the instrument guide tubes has 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 700 litres of coolant over as much as four years.
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