05000334/LER-2013-002
| Beaver Valley Power Station Unit Number 1 | |
| Event date: | 10-04-2013 |
|---|---|
| Report date: | 02-14-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 49413 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 3342013002R01 - NRC Website | |
Energy Industry Identification System (EllS) codes are identified in the text as [XX].
CONDITIONS PRIOR TO OCCURRENCE
Unit 1: Mode 5, shutdown for a refueling outage.
There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event.
DESCRIPTION OF EVENT
During the Beaver Valley Power Station (BVPS) Unit 1 22nd Refueling Outage (1R22), a pre-planned visual coatings inspection was performed on the interior Reactor Containment Building (RCB) steel liner [NH]. On October 4, 2013, the inspection revealed a paint blister located at the 692 foot elevation of the RCB. After cleaning the area and removal of the corrosion products, an area approximately 0.40 inches by 0.28 inches in size was discovered that penetrated through the containment steel liner plate. The liner plate on the RCB's cylindrical wall has a nominal thickness of 0.375 inches. Thickness measurements using Ultrasonic Testing (UT) were obtained around the area of the through wall liner corrosion to determine the extent of thinning. The measurements indicated that the wall thinning extended below the floor level, requiring excavation of concrete to access the liner for repair activities. The affected sections of the liner plate were removed. A visual inspection of the remaining areas of the accessible portion of the interior RCB steel liner was completed. No additional areas of significant corrosion of the interior RCB steel liner were identified.
The corroded sections of the removed steel liner plate were sent to a laboratory for examination and analysis. The laboratory analysis determined that there were two definite through wall penetrations with a possible third penetration slightly off-set from the second. The second and third areas were from a section of liner plate below the concrete floor level. Conservatively, the third area has been included in safety significance and operability determinations. The total area of the three through wall penetrations was calculated to be 0.395 square inches.
Foreign material was discovered trapped within the concrete directly adjacent to the affected section of the carbon steel liner. Samples were taken and sent to a laboratory where visual and chemical analysis was performed. It was determined that the foreign material was rayon. It was also determined that the foreign material contained significant moisture, which enabled an electrolytic corrosion cell to manifest where the foreign material contacted the steel liner.
An Engineering Change Package was developed and implemented to replace the affected section of the containment liner during 1R22.
CAUSE OF EVENT
The cause of this event was determined to be pitting type corrosion, stemming from an electrolytic corrosion cell, within a localized area originating from foreign material that was in contact with the outside surface of the containment carbon steel liner. The foreign material was introduced as a result of inadequate worker practices and quality control during the original construction of the containment wall.
ANALYSIS OF EVENT
The RCB is a reinforced concrete structure with a cylindrical wall, a flat foundation mat, and a dome roof. The inside surface of the containment is lined with a carbon steel liner to ensure a high degree of leak tightness during operating and accident conditions. The concrete foundation mat, cylinder wall and dome are the structural members of the containment structure. The containment structure does not require the participation of the liner as a structural component. No credit is taken for the presence of the steel liner in designing the containment structure to resist earthquake forces or other design loads.
The concrete reactor building is required for structural integrity of the containment under Design Basis Accident (DBA) conditions. The steel liner and its penetrations establish the leakage limiting boundary of the containment. Maintaining the containment Operable limits the leakage of fission product radioactivity from inside the containment to the environment. The RCB is required to be Operable in plant operating Modes 1 through 4 per Technical Specification (TS) 3.6.1 titled "Containment".
A review of the Containment Leakage Rate Testing Program indicates that the maximum allowable containment leakage rate, La, at Pa (calculated peak containment internal pressure for the design basis Loss Of Coolant Accident of 43.1 psig) shall be 0.10 percent of containment air weight per day as defined by TS 5.5.12.c. The containment total allowable La is equal to 6831 SCFD (standard cubic feet per day). A review of the previous RCB integrated leakage test (Type A Leak Rate Test) performed during 1R17 (April 15, 2006) indicated that the "As-Left" Leakage Rate was 2393 SCFD. This shows that there is margin of 4438 SCFD to the TS allowable limit. In addition, since the last Type A Leak Rate Test in 2006, BVPS Unit 1 has averaged a Type B & C Local Leak Rate of approximately 42 percent (1703 SCFD) of the allowable containment leakage rate of 4098 SCFD. These results are based on the maximum pathway leakage measured during refueling outages, including 1R17 (4/13/2006) through 1R21 (5/7/2012). The past results of the Type A Leak Rate Tests performed for BVPS Unit 1 since 1978 were also reviewed. Based on the low leak rate and stable trend, it is concluded that no previous issues have challenged the design integrity of the liner plate.
According to the information provided by the laboratory that analyzed the removed liner plate, there were two definite through wall penetrations and a probable third penetration, which was not evident until the removed plate was sectioned. This third area has been conservatively included in safety significance and operability determinations. Following BVPS Design Engineering assessment, it was concluded the first penetration had an equivalent area of 0.0143 square inches, the second penetration had an equivalent area of 0.323 square inches, and the third penetration had an equivalent area of 0.058 square inches. Therefore, the three penetrations have a total combined area of 0.395 square inches.
The assumed leakage that could be passed through the combined area of the three penetrations is approximately 169 SCFH, or 4053 SCFD, based on a ratio of the test results discussed in LER 50- 339/99-002-00 and previously discussed in BVPS Unit 1 LER 2009-003-00. Combining this leakage rate with the as-left leakage rate of 2393 SCFD, the new total leakage rate would be 6446 SCFD. This total leakage rate is less than the TS allowable La of 6831 SCFD. Based on the projected leakage rate, there is reasonable assurance that the containment air leakage since the last Type A test would not have exceeded the maximum leakage rate allowed by TS 5.5.12.c.
The evaluation described above concluded that there is reasonable assurance that the containment was operable per TS 3.6.1 during the period of time that the plant was operated with a small area of through wall corrosion on the containment steel liner. Therefore, there was no loss of safety function for the Containment pursuant to 10 CFR 50.73(a)(2)(v).
The plant risk associated with the BVPS Unit 1 containment liner penetrations discovered on October 4, 2013 is considered to be very low. This is based on the total area of the containment liner penetrations not impacting the likelihood of core damage, and being much less than the calculated area required to exceed 100% of the containment volume leakage per day. Therefore, the safety significance of the event would be classified as very low safety significance.
This event was reported as a condition resulting in a principal safety barrier being seriously degraded pursuant to 10 CFR 50.72(b)(3)(ii)(A) at 2324 on October 4, 2013 (EN 49413). This condition is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(A).
CORRECTIVE ACTIONS
1. The RCB steel liner was repaired and tested satisfactory prior to entering Mode 4 during the BVPS 1R22 Refueling Outage.
Refueling Outage (1R23).
3. Eight (8) non-random UT examinations of the containment liner were performed during 1R22. The examinations were located on the lower elevation of the RCB corresponding to the initially discovered through wall penetration and were spaced approximately equidistant around the containment, as access permitted. There was no thickness measured below the nominal thickness, and there was no evidence of wall loss.
4. A containment liner exam (100% visual of accessible liner area) was performed. No additional significant indications of corrosion were identified.
Completion of the above and other corrective actions are being tracked through the BVPS corrective action program.
PREVIOUS SIMILAR EVENTS
A review found two prior BVPS Unit 1 and no Unit 2 similar events within the previous seven years.
BVPS Unit 1 LER 2009-003-00 "Containment Liner Through Wall Defect Due to Corrosion" During the Unit 1 1R17 Refueling Outage in 2006, three areas of general pitting corrosion were found on the outside portion of the containment liner that was removed during construction of a temporary opening in the containment structure for the replacement of the steam generators and reactor vessel head.
CR 2013-15748, 15843, 16951