05000324/LER-2007-002
| Brunswick Steam Electric Plant (Bsep), Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3242007002R01 - NRC Website | |
Energy Industry Identification System (HIS) codes are identified in the text as [XX].
Introduction Initial Conditions At the time of this event, Unit 2 was in the process of startup following a refueling outage which started on March 3, 2007. The Reactor Core Isolation Cooling (RCIC) system [BN] was believed to be operable; however, Surveillance Requirements (SRs) 3.5.3.4 and 3.5.3.3 (i.e., the low and high pressure RCIC system testing, respectively) had not yet been completed since the required reactor pressures had not yet been achieved. All other emergency core cooling systems were operable when the Applicability for TS 3.5.3, "RCIC System," was entered.
Reportability Criteria The Limiting Condition for Operation (LCO) for TS 3.5.3 requires that RCIC be operable. The Applicability for TS 3.5.3 is Mode 1 and Modes 2 and 3 with reactor steam dome pressure greater than 150 psig. LCO 3.0.4 prohibits entry into a mode or other specified condition in the applicability unless, subject to certain exceptions, the LCO is met. It has been determined that the RCIC system was not operable when the Applicability for TS 3.5.3 was entered on April 16, 2007. This condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as operation prohibited by the plant's TSs.
Event Description
The Applicability for TS 3.5.3 is Mode 1 and Modes 2 and 3 with reactor steam dome pressure greater than 150 psig. Unit 2 entered Mode 2 at approximately 2250 hours0.026 days <br />0.625 hours <br />0.00372 weeks <br />8.56125e-4 months <br /> Eastern Daylight Time (EDT) on April 15, 2007, and achieved a reactor steam dome pressure of 150 psig at approximately 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br /> on April 16, 2007. At approximately 1350 hours0.0156 days <br />0.375 hours <br />0.00223 weeks <br />5.13675e-4 months <br /> on April 16, 2007, during performance of SR 3.5.3.4 (i.e., the low pressure RCIC surveillance), RCIC was declared inoperable due to oil leaking from the west RCIC lube oil duplex oil strainer. Additionally, the RCIC barometric condenser vacuum pump did not function properly; however, this did not impact the operability of RCIC.
SR 3.5.3.4 was re-performed at approximately 1822 hours0.0211 days <br />0.506 hours <br />0.00301 weeks <br />6.93271e-4 months <br /> on April 16, 2007, and the west RCIC lube oil duplex oil strainer was leaking. The surveillance was completed unsatisfactorily, TSs met, since RCIC was able to develop the required flowrate. Based on an engineering review, it was determined that RCIC was operable provided the duplex strainer was selected to the east strainer. The RCIC system was declared operable at approximately 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> on April 17, 2007.
Unit 2 continued startup and entered Mode 1 at approximately 1813 hours0.021 days <br />0.504 hours <br />0.003 weeks <br />6.898465e-4 months <br /> on April 17, 2007. This required completion of SR 3.5.3.3 (i.e., the high pressure RCIC surveillance) by 1813 hours0.021 days <br />0.504 hours <br />0.003 weeks <br />6.898465e-4 months <br /> on April 18, 2007. At approximately 1217 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.630685e-4 months <br /> on April 18, 2007, RCIC tripped during performance of SR 3.5.3.3. It was Event Description (continued) believed that the trip was due to an overspeed condition caused by air in the lube oil system. RCIC was manually restarted, in accordance with plant procedures, to purge air from the lube oil system. Prior to performing the manual RCIC start, it was noted that the RCIC trip light at the Remote Shutdown Panel was illuminated. This trip light will illuminate in the event of: (1) overspeed, (2) low suction pressure, or (3) high turbine exhaust diaphragm pressure. Following the manual start, at approximately 1559 hours0.018 days <br />0.433 hours <br />0.00258 weeks <br />5.931995e-4 months <br /> on April 18, 2007, SR 3.5.3.3 was completed satisfactorily and RCIC was declared operable.
Based upon further investigations, discussions with operations personnel, and review of computer data, it was determined that the RCIC trip which occurred during performance of SR 3.5.3.3 on April 18, 2007, was not due to overspeed and that the only signal capable of illuminating the trip light under the test circumstances was low suction pressure, because no overspeed trip or exhaust diaphragm rupture occurred.
The low suction pressure trip occurs when the low suction pressure switch 2-E51-PSL-N006 senses 15 inches HG vacuum. As a result of this determination, RCIC was declared inoperable at approximately 1930 hours0.0223 days <br />0.536 hours <br />0.00319 weeks <br />7.34365e-4 months <br /> on April 19, 2007.
Troubleshooting activities were initiated and several RCIC data ,collection runs performed. During these runs it was determined that the April 18, 2007, RCIC trip and a subsequent April 20, 2007, RCIC trip were due to sensed low suction pressure. On April 24, 2007, a subsequent RCIC run showed that the turbine suction pressure was oscillating between the high and low suction pressure setpoints. These oscillations only occurred for a few seconds during RCIC startup. Engineering determined that the oscillations were a result of air voids in the injection piping which collapsed as a result of starting RCIC. The void collapse introduced a pressure wave of sufficient magnitude to actuate the RCIC low suction pressure trip logic but not sufficient to cause a water hammer large enough to result in physical damage to affected components.
This piping has improper sloping and, therefore, venting during RCIC system restoration following work performed during the refueling outage did not completely remove the voids.
A temporary modification installed a pressure snubber upstream of the 2-E51-PSL-N006 low suction pressure switch to dampen suction pressure oscillations. RCIC was returned to operable status at 0250 hours0.00289 days <br />0.0694 hours <br />4.133598e-4 weeks <br />9.5125e-5 months <br /> on April 28, 2007.
Event Cause The root cause of this event was the lack of a time delay relay on the 2-E51-PSL-N006 RCIC low suction pressure switch combined with the improper sloping of the 25-foot section of injection piping which 'contained air voiding introduced during the 2007 refueling outage.
The lack of a time delay relay made the RCIC system more sensitive to small pressure perturbations.
Industry benchmarking indicates that a time delay relay for the RCIC low suction pressure switch is a typical design feature employed to guard against spurious trips.
Event Cause (continued) Injection pipe sloping has not changed since initial plant construction. A review of-historical, Unit 2 test runs has identified similar flow responses, but to a lesser degree. There is-no historical evidence of RCIC turbine trips as a result of void induced presstire waves. -Historical system performance provides assurance that even though voiding due to pipe sloping has, always been present to some extent, it has not affected system operability. A walkdown of the Unit 1 RCIC injection piping was performed and no sloping , inaccuracies were.identified.
Safety Assessment The safety significance ,of this condition is considered minimal.
The RCIC system is not an Engineered Safety Feature system and no credit is taken in the safety analyses for RCIC system operation. RCIC provides adequate core cooling to prevent the reactor fuel from overheating in the event of reactor isolation accompanied by a loss of feedwater flow: The system provides core cooling following a reactor shutdown by maintaining sufficient reactor water inventory until the reactor is depressurized to a level where the shutdown cooling system -can be placed in operation. RCIC's.
function to operate automatically to maintain sufficient coolant in the reactor vessel to prevent overheating of the reactor fuel, in the event of reactor isolation accompanied by loss of feedwater flow, is a transient response function and not a safety-related function.
Although RCIC was inoperable for automatic injection due to the low suction pressure trips, it was available for injection using plant procedure 20P-.16, "Reactor Core Isolation Cooling Operating Procedure." Therefore, RCIC was available to perform its design functions.
The potential magnitude of the pressure wave induced by air void collapse in the RCIC injection piping would not be sufficient to cause a water hammer large enough to result in physical damage to affected components.
Corrective Actions
The following immediate corrective actions were completed.
- A temporary modification installed a pressure snubber upstream of the 2-E51-PSL-N006 low suction pressure switch to dampen suction pressure oscillations.
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- Filling and venting of the Unit 2 RCIC system injection piping, using 65 psig keep fill pressure with the bypass valve throttled open, was performed in accordance with plant procedure 20P-16 to ensure the elimination of air voiding.
Corrective Actions (continued) The following corrective actions to prevent recurrence were established as a result of this event.
- Time delay relays were added to the Unit 1 and Unit 2 E51-PSL-N006 low suction pressure switches to prevent spurious trips due to momentary pressure oscillations.
- The improper Unit 2 RCIC injection piping slope has been corrected.
Previous Similar Events
A review of LERs and corrective action program condition reports for the past three years identified no similar events.
Commitments No regulatory commitments are contained in this report.