05000293/LER-2011-001
| Pilgrim Nuclear Power Station | |
| Event date: | 02-20-2011 |
|---|---|
| Report date: | 04-20-2011 |
| 2932011001R00 - NRC Website | |
EVENT DESCRIPTION:
At 0055 hours6.365741e-4 days <br />0.0153 hours <br />9.093915e-5 weeks <br />2.09275e-5 months <br /> on Sunday, February 20, 2011, the Pilgrim Nuclear Power Station (PNPS) commenced a controlled shutdown of the reactor due to the 'B' train of Reactor Building Closed Cooling Water (RBCCW) being declared inoperable and expected to exceed its 72-hour Limiting Condition for Operability (LCO) as required by TS prior to return to operable status.
With the plant operating at 100% power, leakage of Salt Service Water (SSW) was detected in the RBCCW system due to high chloride levels and increased inventory in the system. An investigation into the event determined that the source of the SSW was isolated to the 'B' RBCCW heat exchanger which is designed to cool RBCCW under normal and post-accident conditions. The quantity of the leakage was determined to exceed the design limits established to ensure post-accident operation of the system and the 'B' train of RBCCW was subsequently declared inoperable.
The leak detection and repair activities identified a single tube leak resulting from an improperly modified tube sleeve (shortened and incorrect bevel). The modified sleeve was installed in a 2005 maintenance outage and over time accelerated wear on the parent tube.
BACKGROUND:
The Reactor Building Closed Cooling Water (RBCCW) System provides cooling to the Core Standby Cooling System (CSCS) components and provides a heat sink for the Residual Heat Removal (RHR) System heat exchangers. The system also provides required cooling to the equipment located in the Reactor Building during normal planned station operations, and to provide a barrier between the primary system and the Salt Service Water (SSW) System.
The RBCCW System consists of two independent closed loops for redundancy during accident conditions.
Each loop has three centrifugal pumps and takes suction from the associated RBCCW heat exchanger. A 500 gallon head tank for each loop is located at the highest point in the system and accommodates system volume changes, maintains static pressure in the loop, detects gross leaks in the system, and provides a means for adding makeup water. The two loops can be cross-tied through two 12-inch cross-tie headers using four valves.
The cross-tie valves are normally closed.
During plant operations, the RBCCW system also functions as an intermediate barrier between system equipment and the SSW system. The RBCCW loop pressure is normally higher than the salt service water system pressure preventing salt water contamination of the RBCCW system. Detectors in the RBCCW system continuously monitor radioactivity levels.
The RBCCW heat exchangers were placed in operation in approximately 1971. As a result of the station's eddy current testing program tube sleeves (also called inserts, shields or ferrules) were installed in the mid 1980's in both RBCCW and TBCCW heat exchangers. These sleeves were made of the same material as the tubes, 90- 10 Copper Nickel.
At 0055 hours6.365741e-4 days <br />0.0153 hours <br />9.093915e-5 weeks <br />2.09275e-5 months <br /> on Sunday, February 20, 2011, the Pilgrim Nuclear Power Station (PNPS) commenced a controlled shutdown of the reactor due to the 'B' train of Reactor Building Closed Cooling Water (RBCCW) being declared inoperable and expected to exceed its 72-hour Limiting Condition for Operability (LCO) as required by TS prior to return to operable status.
With the plant operating at 100% power, leakage of Salt Service Water (SSW) was detected in the RBCCW system due to high chloride levels and increased inventory in the system. An investigation into the event determined that the source of the SSW was isolated to the 'B' RBCCW heat exchanger which is designed to cool RBCCW under normal and post-accident conditions. The quantity of the leakage was determined to exceed the design limits established to ensure post-accident operation of the system and the 'B' train of RBCCW was subsequently declared inoperable. A 10 CFR 50.72 report was required because the subsystem would not have been restored to operable status prior to exceeding the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> TS LCO action statement for one RBCCW subsystem inoperable as defined in TS 3.5.B.3.B. As a result, a Licensee Event Report (LER) per 10 CFR 50.73 is required.
The leak detection and repair activities identified a single tube leak resulting from an improperly modified tube sleeve (shortened and incorrect bevel) which accelerated wear on the parent tube. An extent of condition was performed on the affected heat exchanger and no additional repairs were necessary.
CAUSE OF EVENT:
The direct cause of the SSW leak into the "B" loop RBCCW system was a tube leak in the RBCCW 'B' heat exchanger (E-209B) related to the installation of an undocumented and unauthorized field modified (shortened) inlet end sleeve resulting in accelerated wear on the parent tube.
EXTENT OF CONDITION; All remaining sleeves in E-209B were inspected and verified to be the correct length (8"). The other components that may have modified sleeves installed were the "A" RBCCW heat exchanger (E-209A), and the "A" and "B" TBCCW heat exchangers (E-122A & B, respectively). The existing 8" sleeves are planned to be replaced with 10" long sleeves during the next scheduled maintenance for all of these heat exchangers.
1. E-209A was last inspected in RFO 16 (April 2007) and all sleeves were replaced. There were no indication of any leakage concerns and the heat exchanger is currently scheduled to be inspected in RFO 18 (April 2011) and have all the sleeves replaced with 10" long sleeves.
2. E-122A was last inspected in February 2009 and all 8" long sleeves were replaced at that time. There were 2 tube leaks downstream of inlet end sleeves in December 2010 that were attributed to end step erosion due to high flow (velocity) operation. The inlet end sleeves removed from the leaking tubes were the correct design (8" long). This heat exchanger is scheduled to have all the sleeves replaced with 10" long sleeves in 2011.
3. E-122B was last inspected in March 2010 and all of the sleeves were replaced in kind (8") at that time.
The next scheduled inspection for E-122B is March 2014. After completion of sleeve replacement in E- 122A, the scheduled inspection for E-122B will be reevaluated and revised as required.
FAILED COMPONENT IDENTIFICATION:
The following El IS codes are applicable to this report:
COMPONENTS CODES
Heat Exchanger HX
SYSTEMS CODES
Closed / Component Cooling Water Systems (RBCCW) CC
CORRECTIVE ACTIONS:
Completed Actions:
1. Verified that 100% of the remaining sleeves installed in E-209B heat exchanger are the correct length.
2. Performed the following maintenance activities on E-209B heat exchanger:
a. Open, pressure test and identify leaking tube(s) b. Inspect other tubes as required c. Repair (plug) leaking tube(s) d. Restore E-209B to operation 3. Revised and issued procedures 3.M.4-98 (RBCCW Heat Exchanger maintenance/repair) and 3.M.4-99 (TBCCW maintenance/repair) to include language to preclude recurrence. Note: If difficulties are encountered when installing sleeves, notify maintenance management.
Sleeves should not be modified without an approved design change. Ref CR-PNP-2011- 0721.
Open Actions:
1. Create a signature for verification that the heat exchanger maintenance/repair procedure was completed as written and that a Condition Report (CR) would be written if it could not be done as written.
2. In addition to scheduled inspections, perform maintenance on E-209A including the installation of replacement tube sleeves during RFO 18.
This LER is submitted pursuant to the requirements of 50.73(a)(2)(i)(A) because a Technical Specification required shutdown was completed.
The event occurred during normal power operation while at 100% power with the mode switch in the "RUN" position. The reactor vessel pressure was approximately 1030 psig with reactor water temperature at saturation temperature for that pressure.
The Core Standby Cooling Systems (CSCS) consist of the High Pressure Coolant Injection (HPCI) System, Automatic Depressurization System (ADS), Core Spray (CS) System, and the Residual Heat Removal (RHR) System in the Low Pressure Core Coolant Injection (LPCI) mode. Although not part of the CSCS, the Reactor Core Isolation Cooling (RCIC) System is capable of providing water to the reactor vessel for high pressure core cooling, similar to the HPCI System. These systems were operable when the RBCCW Train 'B' was declared inoperable.
The RBCCW System provides cooling to the CSCS System components and provides a heat sink for the RHR System heat exchangers. The system also provides required cooling to the equipment located in the Reactor Building during normal planned station operations, and provides a barrier between the primary system and the Salt Service Water (SSW) System.
The RBCCW System consists of two independent closed loops. Each loop has three centrifugal pumps and takes suction from the RBCCW heat exchanger. A 500 gallon head tank is located at the highest point in the system and accommodates system volume changes, maintains static pressure in the loop, detects gross leaks in the system, and provides a means for adding makeup water. Head tank level is monitored and will alarm in the main control room if a level deviation exists. The system is designed with sufficient redundancy so that no single system component failure can prevent the system from performing its safety objective.
The heat exchanger leakage in the RBCCW 'B' Train was being closely monitored upon discovery of the high chlorides in the system along with the increase in head tank. The 'A' RBCCW heat exchanger was operable prior to and through the event.
The leak posed no threat to public health and safety. On initial identification of a leak, the plant developed an Operational Decision Making Issue (ODMI) "Non-Accident RBCCW Surge Tank T-201B Monitoring" to track and trend leakage. The trigger points identified in the ODMI were subsequently reached and the plant entered an LCO and shutdown. During the shutdown the RBCCW heat exchanger was repaired, determined to be operable and returned to service
SIMILAR EVENTS:
A review was conducted of Pilgrim Station LERs which involved either failure of an RBCCW subsystem or failure of the heat exchanger The following LER addressed similar concerns:
and the condition resulted in declaring the subsystem inoperable. The condition resulted in entry into TS LCO 3.5.B.3.B and subsequent plant shutdown.