RPV Bottom Head Issues
On April 12, 2003, during visual examination of the instrument penetrations located at the reactor pressure vessel bottom head at South Texas Project Unit 1(STP-1), small amounts of white residue were discovered around the circumference of two instrument penetrations. Susbsequent chemical analysis of this residue revealed the presence of boron and lithium, indicating that this residue had originated from the reactor coolant system. The licensee repaired the instrument penetrations and committed to implementing monitoring programs at STP-1. After reviewing the licensee's activities, the NRC staff concluded that the licensee had taken all actions necessary to ensure a safe restart and operation of STP-1.
As a result of the events at STP-1, the NRC staff issued Bulletin 2003-02, "Leakage from reactor pressure vessel lower head penetrations and reactor coolant pressure boundary integrity," on August 21, 2003.
In 2006, the NRC developed NUREG-1863, "Review Responses to NRC Bulletin 2003-02-Leakage from Reactor Pressure Vessel Lower Head Penetrations and Reactor Coolant Pressure Boundary Integrity," which contained licensees' responses to NRC Bulletin 2003-02.
In 2013, during a scheduled visual examination of the BMI nozzles at Palo Verde Nuclear Generating Station Unit 3, the licensee identified white residue around the BMI nozzle. Licensee's engineering personnel determined that the white residue was most likely boric acid resulting from pressure boundary leakage. Non-destructive examinations confirmed that axial cracking at the nozzle weld was responsible for the leakage. Additional information on the event is available on the NRC website under Agencywide Documents Access and Management System Accession No. ML14177A533.
Laboratory analysis of a sample removed from the J-groove weld confirmed that the cause of the pressure boundary leakage was axial cracking in a sub-surface BMI nozzle J-groove weld flaw that extended into the nozzle.
Current requirements in 10 CFR50.55a(g)(6)(ii)(E) "Augmented ISI requirements: Reactor coolant pressure boundary visual inspections," requires that all licensees of pressurized water reactors augment their inservice inspection program with ASME Code Case N-722-1, "Visual Examinations for PWR Pressure Retaining Welds in Class 1 Components Fabricated with Alloy 600/82/182 Materials Section XI, Division 1," subject to the conditions specified in paragraphs (g)(6)(ii)(E)(2) through (4) of [10 CFR 50.55a]. Consequently, licensee with PWR units with Alloy 600/82/182 BMI instrument penetrations are required to perform visual examinations of these components every other refueling outage.