05000390/LER-2012-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. N/A | |
| Event date: | 06-29-2012 |
|---|---|
| Report date: | 01-28-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3902012002R01 - NRC Website | |
CONTINUATION SHEET
No 2012 002 1
I. PLANT CONDITIONS:
WBN Unit 1 was operating at approximately 100 percent power at the time the Probable Maximum Flood (PMF) calculation was issued.
II. DESCRIPTION OF EVENT:
A. � Event:
On June 29, 2012, Tennessee Valley Authority (WA) approved an updated calculation titled "PMF Determination for Tennessee Valley Watershed" that increased the WBN PMF level to Elevation 739.2 feet (ft) Mean Sea Level (MSL). The current Updated Final Safety Analysis Report (UFSAR) PMF elevation is 734.9 ft MSL. WBN reviewed flood sensitive safety-related systems, structures, and components. Essential Raw Cooling Water (ERCW) [EDS code KP] and High Pressure Fire Protection (HPFP) [EMS code BI] equipment required for flood mode operation located on Elevation 722 ft of the Intake Pumping Station (IPS) and the Thermal Barrier Booster Pump (TBBP) motors are affected by the revised PMF level. The increase in calculated PMF elevation resulted from new rim leakage paths (i.e., leakage that occurs when water contained in a reservoir finds an alternate path around the dam) identified during aerial surveys for the proposed TVA Clinch River Site. No actual flooding has occurred. However, because of the unanalyzed condition, the potential existed for WBN to exceed its design basis PMF and adversely affect plant safety.
B. Inoperable Structures, Components, or Systems that Contributed to the Event None.
C. Dates and Approximate Times of Major Occurrences Date � Event December 30, 2009 � A hydrological analysis for WBN Unit 2, which included updated input information and analysis methodology, revised the PMF elevation for WBN Unit 1 and Unit 2 to 738.8 ft MSL. All flood sensitive safety-related systems, structures, and components were reviewed and determined to be unaffected by the increased PMF level. Due to the small margin (less than one inch) between the surge elevation in flooded structures and the TBBP motors, procedures were updated to install a flood protection barrier around the TBBPs during a PMF event.
C. Dates and Approximate Times of Major Occurrences (continued) Date Event February 1, 2012 While conducting preliminary hydrologic analysis for the proposed WA Clinch River site, topographical areas upstream of the Watts Bar Dam were identified that are potential overflow/rim leakage areas during a PMF event. These areas were not discovered during previous hydrology analyses. No immediate/interim actions for impacted safety related equipment were required.
June 29, 2012 Updated PMF calculation changed the WBN design basis PMF to Elevation 739.2 ft. This would result in a design basis flood level of 741.7 ft MSL exterior of the IPS on the Intake Channel side. Because the interior of the IPS is hydraulically connected to the exterior of the IPS on this side of the structure, the water could enter the interior of the IPS on Elevation 741 ft, travel over concrete door thresholds located at Elevation 741.5 ft and potentially affect safety related ERCW and HPFP equipment located on Elevation 722 ft of the IPS. Earlier preliminary calculations had indicated that the WBN PMF elevation could be increased to 739.2 ft MSL and affect IPS equipment, and sandbags were already staged at the IPS to protect equipment during a PMF event.
July 19, 2012 An application to revise the WBN Unit 1 UFSAR regarding changes to hydrologic analysis was submitted for NRC review.
D. Other Systems or Secondary Functions Affected
On November 8, 2012, WBN identified that several components that were previously identified as having low available physical margin during a design basis PMF event were actually partially submerged during a PMF event. This condition was identified in September 2012 by a contractor performing Fukushima Near-Term Task Force walkdowns, but was not recognized by WBN until November 2012. The components identified include the Chilled Water Circulating pumps for the Main Control Room (MCR) and Shutdown Board Room (SDBR) chillers and ancillary equipment required for MCR and SDBR air conditioning system functionality. A portion of the MCR and SDBR chiller pump motors would be submerged during the design basis PMF event. The MCR chiller is required as a part of the Control Room Emergency Air Temperature Control System (CREATCS), which is a technical specification required system.
D. Other Systems or Secondary Functions Affected (continued) During emergency operation, the CREATCS maintains the control room temperature between 60 degrees Fahrenheit and 104 degrees Fahrenheit. The SDBR air conditioning system is not a TS required system but is attendant equipment that must be functional to support the TS definition of operability of the alternating current shutdown boards. An evaluation was performed and determined that WBN would be able to mitigate a PMF event without the SDBR and MCR chiller equipment operable. As a conservative measure, permanent plant modifications will be installed to provide a flood protection barrier around the MCR/SDBR chillers and to seal required ancillary equipment.
E. Method of Discovery
While conducting preliminary hydrologic analysis for the proposed NA Clinch River site, topographical areas upstream of the Watts Bar Dam were identified that are potential overflow/rim leakage areas during a PMF event. This resulted in issuance of an updated calculation titled "PMF Determination for Tennessee River Watershed.
F. Operator Actions
WBN operations procedures were revised to incorporate contingency measures to protect ERCW and HPFP equipment located on Elevation 722 ft of the IPS.
G. Safety System Responses
No safety system activation occurred.
III.� CAUSE OF EVENT The apparent cause for the PMF flood level increasing to Elevation 739.2 ft is due to the use of geographical information system (GIS) technology to more accurately map topographical areas upstream of Watts Bar dam, as well as the use of updated hydrological modeling software.
These technological advances in geographical mapping and computer modeling provide a more conservative determination of flood levels and were not available during the original licensing of WBN Unit 1.
IV. ANALYSIS OF THE EVENT
On June 29, 2012, TVA approved an updated calculation titled "PMF Determination for Tennessee Valley Watershed" that increased the WBN design basis PMF level to Elevation 739.2 ft MSL.
WBN is required to be able to place and maintain the plant in a safe shutdown condition up to the design basis flood level. Earlier preliminary calculations had indicated an increase in PMF elevation, and contingency measures to protect the TBBP motors, and ERCW and HPFP equipment located in the IPS had already been put in place. No actual flooding occurred; however, because of the unanalyzed condition, the potential existed for WBN to exceed its design basis PMF and adversely affect plant safety.
V. ASSESSMENT OF SAFETY CONSEQUENCES
Based on the above "Analysis of the Event," this event did not adversely affect the health and safety of plant personnel or the general public.
VI. C CORRECTIVE ACTIONS A. Immediate and Interim Actions WBN entered the information into the Corrective Action Program as Problem Evaluation Report 573093. Prior to official issue of the updated PMF calculation, sandbags were staged at the IPS to protect ERCW and HPFP equipment during a PMF event.
Procedures for flood mode preparation were revised to include contingency measures for protection of ERCW and HPFP equipment located on Elevation 722 ft of the IPS.
B. Corrective Actions
The corrective actions are being managed by the WBN Corrective Action Program.
Corrective actions include issuance of a procedure to inspect compensatory measures for flood protection semiannually until permanent plant modifications are installed. The TBBP temporary flood barrier will be installed around the TBBPs. Permanent plant modifications will be installed to provide a flood protection barrier for the TBBPs and affected IPS equipment.
VII. ADDITIONAL INFORMATION
A. Failed Components
No failed components have been identified.
B. Previous LERs on Similar Events A review of reportable events within the last three years did not identify any previous similar events.
C. Additional Information
No additional information available.
D. Safety System Functional Failure This event resulted in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v) and NEI 99-02.
E. Loss of Normal Heat Removal Consideration None
VIII. COMMITMENTS
None.