05000334/LER-2009-003
| Beaver Valley Power Station Unit Number 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 45015 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 3342009003R00 - NRC Website | |
There were no structures, components, or systems that were inoperable at the start of the event that contributed to the event. Energy Industry Identification System (EllS) codes are identified in the text as [XX].
DESCRIPTION OF EVENT
On April 21, 2009, during the Beaver Valley Power Station (BVPS) Unit 1 19th Refueling Outage (1R19), an ASME XI Section IWE General Visual examination was performed on the interior Reactor Containment Building (RCB) steel liner [NH]. A suspect area was identified during the inspection. This area, located at the 746 feet elevation of containment, was approximately 3 inches in diameter and exhibited a paint blister that enclosed protruding rust product. A work order was written to clean the area to allow further evaluation. The cleaning activity uncovered a rectangular area equivalent to 0.375 square inches of approximately 1 inch (horizontal) x 3/8 inch (vertical) that penetrated through the entire liner plate thickness. This opening is equivalent to a pipe diameter of 0.69 inch. The liner plate on the building's cylindrical wall has a nominal thickness of 0.375 inches. Thickness measurements were obtained around area of the through wall liner corrosion using ultrasonic testing to determine extent of the thinning. The measurements indicated that the thinning was within an approximate area of 10 square inches (i.e., 2 inches x 5 inches), outside of which the liner returned to nominal thickness. The visual examination 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 section of the liner was removed and sent to a laboratory for analysis. A partially decomposed piece of wood was discovered imbedded in the concrete behind the section of the liner, a sample of which was also sent for analysis. The liner opening was enlarged and the piece of wood, which was approximately 2 inches x 4 inches x 6 inches, was removed. An Engineering Change Package was developed and implemented to replace the affected section of the containment liner.
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 liner plate thickness varies with location and is nominally .25 inches on the floor, .375 inches on the cylindrical wall, and .5 inches on the dome.
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 the containment to the environment. The ANALYSIS OF EVENT (Continued) 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. This containment total allowable La is equal to 6831 SCFD (standard cubic feet per day). A review of the previous RBC integrated leakage test (Type A Leak Rate Test) performed during 1R17 (April 15, 2006) indicated that the "As-Left" Leakage Rate was 2395 SCFD. This shows that there is margin of 4436 SCFD to the TS allowable limit. In addition, over the past 10 years, BVPS Unit 1 has averaged a Local Leak Rate (Type B & C) of approximately 37 percent of the allowable containment leakage rate of 0.6 La. This is based on the maximum pathway leakage measured during refueling outages. The past results of the Type A Containment Integrated Leak Rate Test performed for BVPS Unit 1 since 1978 were reviewed. Based on this review, the Type A Containment Integrated Leak Rate was observed to be low and have a stable trend. Therefore, it is concluded that no previous issues have challenged the design integrity of the liner plate.
Plant personnel observed that paint still covered the affected area during the as-found inspection of the area containing the liner deficiency. When the blistered area was touched, the intact paint blister ruptured. This as-found condition indicated that there was reasonable assurance that no leakage was experienced through the containment liner area containing the through wall liner corrosion since the last Type A test.
Further review of Operating Experience (OE) indicated that similar through wall containment liner corrosion occurred at North Anna Unit 2, Brunswick Unit 2, and D. C. Cook Unit 2.
North Anna Unit 2 had an approximately 1/4 inch diameter liner hole. Testing was performed at North Anna Unit 2 following discovery of the liner hole at various pressures, up to and including the designed peak accident pressure of 45 psig. At 45 psig, leakage was measured to be 21 SCFH (504 SCFD). BVPS Unit 1 containment design and atmosphere conditions are similar to North Anna Unit 2. The leakage from the 0.69 inch equivalent diameter liner hole at BVPS Unit 1 is projected to be 160 SCFH (3,840 SCFD) based on extrapolating North Anna's test results. Combining this leakage rate and the As-Left leakage rate of 2,395 SCFD, the new total leakage rate would be 6,235 SCFD. This total leakage rate is less than the TS allowable La of 6,831 SCFD. Based on the projected leakage rate and findings, 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.
ANALYSIS OF EVENT (Continued) The concrete foundation mat, cylinder wall and dome are the structural members of the containment structure. The steel liner acts as an impervious membrane, transmits loads to the concrete and was used during construction as the inside form for the reinforced concrete wall and dome. 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 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 1.0 CFR 50.73(a)(2)(v).
The as-found BVPS Unit 1 containment liner through wall area of approximately 0.375 square inches is not expected to impact containment overpressure and therefore, will not impact the Net Positive Suction Head of Engineered Safety Function Pumps taking suction from the containment sump. Also, with the through wall liner corrosion being located at containment elevation 746 feet (11 feet above grade), most of the containment release through the opening would be a mixture of steam and air. Therefore, no significant containment sump inventory accumulated at elevation 692 feet is expected to be lost through this containment liner opening during a postulated DBA. Furthermore, the presence of this through wall hole in the containment liner did not, nor would it be expected to cause a plant trip, so no initiating event frequencies are increased. Based on the above, the as-found liner area containing the through wall liner corrosion is not expected to increase the core damage frequency.
In order to have an impact on the Large Early Release Frequency (LERF), the leakage from containment must exceed 100% of the containment volume per day, or 1000 times La. This leakage equates to a hole size in the containment liner and concrete shell of 2 inches in diameter (approximately 3.1 square inches area), at the containment design pressure of 45 psig. Based on the 0.69 inch equivalent diameter of the identified liner opening, the as found rectangular through wall area is not expected to impact the LERF. Therefore, this event is considered to have very low safety significance.
This event was reported as a condition of a principal safety barrier (i.e., containment being degraded due to existence of the rectangular area of approximately 1 inch (horizontal) x 3/8 inch (vertical) that penetrated through the entire liner plate thickness) pursuant to 10 CFR 50.72(b)(3)(ii)(A) at 1526 on April 23, 2009 (EN 45015). Similarly, this condition is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(A).
CAUSE OF EVENT
The cause of the through wall liner corrosion was pitting type corrosion (rust) originating from the concrete side caused by foreign organic material (wood) that was in contact with the carbon containment steel liner. The wood was left behind as a result of inadequate housekeeping practices during the original construction of the containment wall.
CORRECTIVE ACTIONS
1. The area of the RBC liner with the through wall corrosion defect was corrected by replacing the affected portion of the liner and tested satisfactorily.
2. Ultrasonic testing (UT) of the repaired area of the containment liner will be performed during the next Unit 1 Refueling Outage (1R20) to confirm that the repair performed in 1R19 has not introduced an unacceptable wall-thickness degradation mechanism.
3. A containment liner exam (100% visual of accessible liner area) will be performed during the next Unit 1 Refueling outage (1R20).
4. A containment liner exam (100% visual of accessible liner area) will be performed during the next Unit 2 Refueling outage (2R14).
Completion of the above and other corrective actions are being tracked through the BVPS corrective action program.
PREVIOUS SIMILAR EVENTS
A review found one prior BVPS Unit 1 occurrence within the last three years for an event involving corrosion of the outside portion of the containment liner.
During Unit 1 Refueling outage (1R17) in 2006, three areas of general pitting corrosion (rust) were found on the outside portion of the section 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. Two of the three degraded areas were removed and replaced with new plate material. The third area was reinstalled upon restoration of the containment construction opening in 2006. The reinstalled third area was inspected by ultrasonic testing (UT) during Unit 1 Refueling outage (1R19) in 2009 to confirm that additional corrosion is not occurring. Wall thickness measurements of the reinstalled third area were within the calibration tolerances of the installed (2006) condition.
CR 09-57762