05000424/LER-2009-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 09-16-2009 |
|---|---|
| Report date: | 11-09-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4242009001R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
The Ultimate Heat Sink (UHS) at Vogtle Electric Generating Plant (VEGP) consists of the Nuclear Service Water System (NSCW) mechanical draft towers. Two 100% capacity redundant NSCW towers are provided for each unit One tower is associated with each train of the NSCW system. Each NSCW tower consists of a basin that contains the. ultimate heat sink water supply and an upper structure that contains four individual fan spray cells where the heat loads are transferred to the atmosphere. During plant Operational Modes i through 4, the UHS is required to be Operable per Technical Specification 3.7.9. The UHS is considered Operable if it contains a sufficient volume of water at or below the maximum temperature that would allow the NSCW system to operate for at least 30 days following the design basis loss of cooling accident (LOCA) without loss of net positive suction head (NPSH), and without exceeding the maximum design temperature of the equipment served by the NSCW system. The combined storage capacity of the two tower basins per unit meets the intent of the short-term storage requirements without makeup in conformance with Nuclear Regulatory Commission (NRC) Regulatory Guide 1.27.
Due to leakage past NSCW B train manual isolation valves, which had been closed to facilitate upcoming refueling outage activities, the 30 day mission time of the UHS without the need for makeup could not be assured. At the time the NSCW B train manual isolation valves were closed and the expansion joints installed, the deleterious affect the leakage could have on the 30 day mission time was not recognized. Subsequently, it was determined that the leakage past the NSCW manual isolation was in excess of analyzed limits. Therefore, this event is reportable per IOCER50.73(a)(2)(i)(B), based on a condition that existed for a time longer than permitted by the Technical Specifications.
B. UNIT STATUS AT TIME OF EVENT
The NSCW manual isolation valves were closed and the expansion joints installed on Wednesday, September 16, 2009 at approximately 1357 hours0.0157 days <br />0.377 hours <br />0.00224 weeks <br />5.163385e-4 months <br /> Eastern Daylight Time (EDT). At the time of installation, Unit 1 was operating in Mode I at approximately 94% power, in a coast down for the planned refueling outage which commenced on Sunday. September 20, 2009. Unit 1 entered Mode 5 as part of the planned refueling outage shutdown on Sunday, September 20, 2009 at approximately 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br /> Eastern Daylight Savings Time (EDT).
C. DESCRIPTION OF EVENT
On Wednesday, September 16 at approximately 1357 hours0.0157 days <br />0.377 hours <br />0.00224 weeks <br />5.163385e-4 months <br /> EDT, with Unit 1 in Mode 1, in preparation to provide cooling in containment for the upcoming refueling outage ( I RI5), which was scheduled to commence on September 20, 2009, NSCW flow to the B train auxiliary and reactor cavity coolers was isolated by closing manual isolation valves in the supply and return lines. The containment auxiliary cooler and reactor cavity cooler are not credited with � performing any accident mitigation function. Once the valves were isolated, blind flanges located downstream of the NSCW manual isolation valves were removed to allow metal bellows type expansion joints to be connected between the safety related and seismically qualified NSCW piping and the non safety related and non seismically qualified normal chilled water piping.
During removal of the blind flanges and installation of the metal bellows expansion joints, maintenance personnel encountered significant seat leakage that was emanating from the manual isolation valves on the NSCW piping. However, the metal bellows type expansion joints were successfully installed. At this time, it was not realized the deleterious affect the NSCW isolation valve seat leakage could have on the capability of the UHS to meet the 30 day mission time.
Once the expansion joints were installed, the auxiliary containment cooler piping was going to be flushed with demineralized water. Due to problems encountered with trying to perform the piping flush, as a result of the seat leakage emanating past the NSCW manual isolation valves, a troubleshooting plan was developed to quantify the leakage. After the leakage was quantified, it was subsequently determined that during a design basis LOCA, coincident with a Loss of Offsite Power (LOSP) and assuming the non-seismically qualified expansion joint(s) fail, that the 30 day mission time of the UHS could not be assured. Therefore on Saturday, September 19, 2009 at 0306 hours0.00354 days <br />0.085 hours <br />5.059524e-4 weeks <br />1.16433e-4 months <br /> EDT, the Unit 1 VHS was declared Inoperable.
Unit 1 was shutdown on Sunday, September 20, 2009 for the scheduled refueling outage (1R15).
However, since there was firm evidence that the NSCW manual isolation valves were leaking excessively when the metal bellows expansion joints were installed on September 16, 2009, and Unit 1 did not enter Mode 5 until approximately 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br /> EDT on September 20, 2009, the unit operated in a condition longer than allowed by Technical Specification 3.7.9.
D. CAUSE OF EVENT
The cause was determined to be that the manual isolation valves that were used to isolate the NSCW system from the B train Auxiliary Containment Cooler and Reactor Cavity Cooler leaked by their seat excessively. Additionally, the procedure used to remove the blind flanges and install the metal bellows expansion joints did not include any requirement to quantify the amount of allowable leakage past these manual isolation valves prior to removing the blind flanges.
E. ANALYSIS OF EVENT
In order to meet the 30 day mission time for UHS basin inventory, the Emergency Operating procedures require one train of NSCW to be shutdown within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of a LOCA coincident with a LOSP or if normal NSCW basin makeup.is not available. The leaking NSCW manual isolation valves are part of the B train NSCW system. Due to the room leak detection alarms that would have annunciated had the non seismically qualified expansion joints failed, the operators would have ascertained that the leakage was from B train NSCW and would have shut that train down. During the time the B train NSCW manual isolation valves were leaking, all components serviced by the A train NSCW system were operable. In addition, the NSCW system has an inter-tie line that provides makeup water from the operating train to the idle train of NSCW. This design feature prevents water hammer by ensuring the idle train remains full of water which could result from minor leakage losses within the system. Once B train of NSCW was shutdown, leakage past the closed manual isolation valves would have resulted in the annunciation of the inter-tie high flow alarm. The control room operators would have responded to the high flow alarm and followed the guidance in the annunciator response procedure and isolated the inter-tie line. This action would have terminated the inventory loss. Therefore, during a design basis LOCA coincident with a LOSP, and assuming the expansion joints failed as a result of a seismic event, there would still have been sufficient inventory in the NSCW basins to meet the 30 day mission time. Within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of the accident, B train NSCW would have been secured and shortly thereafter the inter-tic line would have been isolated. These actions would have terminated the loss of UHS inventory. The A train NSCW system would have continued to operate. There was sufficient margin available to account for the loss of basin inventory as a result of the leaking manual isolation valves for the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period without challenging the 30 day mission time. Therefore, this event did not result in a loss of safety function.
F. CORRECTIVE ACTIONS
The corrective actions to preclude recurrence of this event include:
I. The NSCW manual isolation valves were reworked during IRIS to minimize seat leakage.
2. The system operating procedure will be revised to include requirements to measure leakage past the NSCW manual isolation valves prior to removing the blind flanges to ensure the leakage past the manual isolation valves is within acceptable limits. The estimated completion date for this corrective action is December 15, 2009.
G. ADDITIONAL INFORMATION
I. Failed Components:
Anchor/Darling 8" Wafer Style Butterfly valves 2. Previous Similar Event:
None 3. Energy Industry Identification System Codes:
Ultimate Heat Sink-BS Nuclear Service Cooling Water System-B1