05000454/LER-2012-004

LER-2012-004, Reactor Pressure Vessel Head Control Rod Drive Mechanism Penetration Nozzle Weld Repair Surface Indications

Docket Number
Event date:
Report date:
Initial Reporting
ENS 48311	10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded
4542012004R00 - NRC Website

A. Plant Operating Conditions Before the Event:

Unit 1 was in Mode 6 -Refueling.

The Reactor Coolant System (RC) [AB] was at ambient temperature and de-pressurized. No structures, systems, or components were inoperable at the start of this event that contributed to the event.

B. Description of Event

During the Byron Station Unit 1 spring 2011 Refueling Outage, EGC performed volumetric and surface examinations of the reactor Vessel Head Penetration (VHP) nozzles in accordance with 10 CFR 50.55a(g)(6)(ii)(D). Examination results for the VHPs 31, 43, 64 and 76 did not meet the applicable acceptance criteria and therefore these nozzles required repair prior to return the reactor head to service. The flaws were repaired using the weld overlay method prior to returning the reactor head to service. On September 14, 2012, during the next required examinations of the head penetrations in the fall 2012 refueling outage, multiple rounded indications were found on the weld overlay material of VHP 31 and 43 of which two exceeded acceptance criteria. These two indications required repair prior to returning the reactor head to service. The two unacceptable indications were:

Penetration 31 - A 3/8 inch rounded indication located at approximately 240 degrees and 025 inches from the intersection of the head weld beads and the vertical penetration tube weld beads - in the vertical weld beads.

Penetration 43 - A 1/4 inch rounded indication located at approximately 170 degrees and 3.0 inches from the intersection of the head weld beads and the vertical penetration tube weld beads - in the vertical weld beads.

Because the as found indications did not meet applicable acceptance criteria to remain in service without repair, this condition was reported in Event Notification 48311, on September 14, 2012, pursuant to 10 CFR 50.72 (b)(3)(ii)(A), as a condition that resulted in a principal safety barrier being seriously degraded. Accordingly, this condition is also being reported as a licensee event report to the NRC in accordance with 10 CFR 50.73 (a)(2)(ii)(A).

NRC FOR 66A (10-2010) C. Cause/s of the Event These indications are the result of surface irregularities on the weld material from the initial installation of the repair in spring 2011.

D. Safety Significance

There is no known degradation mechanism applicable to the weld overlay material used on the nozzles. Accordingly, these indications would not be expected to degrade over time and propagate into a leak path through the RCS boundary.

E. Corrective Actions

The indications were reduced to acceptable size by mechanical means. No additional welding was necessary.

The weld overlays for Penetrations 31 and 43 are scheduled to be re-examined with both volumetric and surface examinations during the next Unit 1 refueling outage in spring of 2014.

F. Previous Occurrences

No previous conditions were identified at Byron Station involved unacceptable indications in overlay material in the RC pressure boundary.