05000455/LER-2007-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A | |
| Event date: | 04-09-2007 |
|---|---|
| Report date: | 06-08-2007 |
| 4552007001R00 - NRC Website | |
B. Description of Event:
In accordance with the requirements of NRC order EA-03-009, "Issuance Of Order Establishing Interim reactor head was ranked in the low Primary Water Stress Corrosion Cracking (PWSCC) susceptibility category. This low ranking required the initial Control Rod Drive Mechanism (CRDM) penetrations nozzle volumetric examinations to be completed in the recent spring 2007 refuel outage (B2R13).
During the B2R13 volumetric examination of the Unit 2 CRDM penetration 68, an ultrasonic test (UT) indication was discovered from 13.5 degrees to 22.5 degrees. The 0-degrees reference is located on the downhill side of the penetration where the nozzle extension is shortest. The indication is located 1.95 inches from the end of the penetration to 2.47 inches towards the J-Groove weld. The bottom toe of the J-Groove weld is located 1.99 inches from the bottom of the tube and the top of the J-Groove weld is located 3.71 inches from the bottom of the penetration. The indication was not surface connected below the J-groove and/or fillet weld.
Following the discovery of the UT indication, a liquid penetrant test (PT) examination was performed to determine if the indication was open to the surface. The PT examination revealed two surface indications: one rounded indication 0.05 inches in diameter and one linear axial indication 0.15 inches long. The indications aligned with the general location and orientation of the UT indication.
Since the indication was consistent with PWSCC, a bare-metal visual (BMV) examination was subsequently completed on 100 percent of the reactor head penetrations, including 360 degrees around each penetration in accordance with NRC order EA-03-009. No evidence of boric acid was identified during the BMV.
Prior to the weld repair, a boat sample from CRDM penetration 68 was obtained for metallurgical evaluation.
This condition is reportable to the NRC in accordance with 10 CFR 50.73 (a)(2)(ii), as a condition that resulted in a principal safety barrier being seriously degraded.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Byron Station Unit 2 05000455 2007 (If more space is required, use additional copies of NRC Form 366A)(17)
C. Cause of the Event
A boat sample was removed from the penetration to determine the cause of the indications. A PT exam was performed on the boat sample excavation site, and a subsurface linear defect which intersected the axial indication was revealed. A portion of the subsurface defect was captured in the boat sample and was identified as a lack of fusion between the next-to-last weld pass and the tube surface. Additionally, hot cracking and lack of fusion between weld passes were observed in the boat sample. The metallurgical evaluations indicated the axial indication was the result of PWSCC which initiated at a subsurface location and propagated toward the wetted surface of the weld. The initiation of PWSCC is attributed to wetting of the subsurface lack-of-fusion defects which created an atypical crevice corrosion environment in the high stress J groove weld region.
D. Safety Analysis
This condition had minimal actual safety consequences. The CRDM penetration 68 indications were identified and repaired prior to any through wall leakage occurring.
The CRDM penetration nozzle weld indications were identified as part of a required inspection to look for flaws of this type. Left undetected, the flaw could have propagated through the alloy 600 weld material, and potentially the penetration material, to form a leak path through the RCS pressure boundary. The consequences of this failure would have been high. Although the leak would have been very small, industry experience has shown that boric acid wastage of the carbon steel reactor head is possible.
The flaw was detected as a small indication and no through wall leakage was noted. Verification of no through wall leakage was accomplished through a bare metal head inspection near penetration 68 as well as the remainder of the reactor head penetrations.
Byron Unit 2 will undergo more frequent inspections consistent with the requirements of NRC order EA-03-009.
E. Corrective Actions
The CRDM penetration nozzle number 68 was repaired prior to return to service of the Unit 2 reactor head.
Consistent with NRC Order EA-03-009, Byron Unit 2 reactor vessel head has been reclassified into the high susceptibility category. Consequently, Byron Unit 2 will comply with the Order's requirements for inspections for a high susceptibility reactor vessel head.
F.Previous Occurrences
There have been no previous occurrences of this nature at Byron.