|Reporting criterion:||10 CFR 50.73(a)(2)(iv)(B), System Actuation|
10 CFR 50.73(a)(2)(iv)(A), System Actuation
|ENS 41292||10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation|
|2512004004R01 - NRC Website|
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Turkey Point Unit 4 05000251
DESCRIPTION OF THE EVENT
On 12/25/04 Unit 4 experienced a sudden loss of condenser [EIIS: SG] vacuum. Annunciator E-5/3, Condenser Lo Vacuum alarmed [EIIS: SG, PI]. Operators noted one pressure indicator (PI) at 27 inches Hg and one PI at 24.5 inches Hg and lowering rapidly. Operations staff noted Gross Generation had lowered 40 MWe and referred to station Alarm Response Procedures and Off-Normal procedures for actions to restore condenser vacuum. The Steam Jet Air Ejector Hogging jet [EIIS:SB, P] was placed in service to restore vacuum and the operators commenced reducing turbine load. However, vacuum could not be recovered and the reactor was manually tripped when vacuum decreased to about 22 inches Hg.
Actions to resolve the cause of the condenser vacuum loss were completed, and Unit 4 was restarted on 12/26/04. In accordance with 10CFR 50.72(b)(2)(iiv)(B), a four-hour Emergency Notification System report was made to the NRC Operations Center on December 25, 2004, at 08:50 (Event Number 41292) for the manual reactor trip.
ANALYSIS OF THE EVENT
At the time of the event, the plant was in Mode 1 at 100% power with automatic Reactor Coolant System [EIIS: AB] (RCS) pressure control operational. A 100% loss of load resulted when the turbine was tripped by the manual reactor trip due to lowering condenser vacuum. The plant response for this trip was comparable to that for a loss of electrical load event due to a turbine trip, analyzed in UFSAR section 14.1.
UFSAR minimum and maximum values were not exceeded during this transient. The Auxiliary Feedwater System [EIIS: BA] auto-initiated as required, due to the expected decrease in steam generator levels below 10% narrow range on a reactor trip from 100% power. The 4C Charging pump [EIIS: CB, 11 was started during the event to maintain RCS inventory. The RCS pressure remained below the setpoint for pressurizer PORV or Code safety valve actuation. No unexplained aspects of this transient, when compared to the UFSAR, were noted.
CAUSE OF THE EVENT
The Unit 4 manual reactor trip was in response to decreasing condenser vacuum. A subsequent inspection determined that the decreasing condenser vacuum was a result of a leak in one of the turbine slop drains.
The slop drain system drains oil and water leaking from the bearing and gland seals on both ends of the LPI and LP2 (low pressure) turbines. The four PA-inch lines are routed through the condenser and join together at a common oil waste drain. The condenser air in-leakage was stopped by temporarily plugging the leaking slop drain at the inlet and outlet. The conditions causing the event are applicable to the remaining seven slop drain pipes for Units 3 and 4. Therefore, these drains were likewise plugged. A failure analysis of the failed weld in the slop drainpipe on the south condenser south wall was performed once the condenser internals could be accessed (during the 2005 Unit 4 Cycle 22 refueling outage). Examination of the failed bi-metallic weld revealed lack of penetration in the weld root and possible lack of fusion. Porosity was also FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Turkey Point Unit 4 05000251 observed in the weld. The areas that were not properly fused acted as an ideal path for a fatigue crack. It is suspected the steam flow from the turbine induced vibration on the line caused the support U-bolts to fail because of inadequate pipe support (not enough angle supports.) Once the pipe had no restraint, vibration cycled the inadequate bi-metallic weld to the point of failure. The piping was also fit up at an angle to the socket, which may have led to undue external stresses from vibration for which the joint was not designed.
A review of the reporting requirements of 10 CFR 50.72 and 10 CFR 50.73 and NRC guidance provided in NUREG-1022, Revision 2, Event Reporting Guidelines 10 CFR 50.72 and 10 CFR 50.73, was performed for the subject condition. As a result of this review, the condition is reportable as described below.
10CFR50.73(a)(2)(iv)(A) states that the licensee ihall report any event or condition that resulted in a manual or automatic actuation of any of the systems listed in 10CFR50.73(a)(2)(iv)(B). Systems to which the requirements of 10CFR50.73(a)(2)(iv)(A) apply include the Reactor Protection System (RPS) actuation resulting in a reactor scram or manual trip (10 CFR 50.73(a)(iv)(B)(1). Unit 4 was manually tripped in response to the lowering condenser vacuum. The event is also reportable in accordance with 10 CFR 50.73 (a)(2)(iv)(A), due to MSIV closure (10 CFR 50.73(a)(2)(iv)(B)(2)), and AFW system actuation (10 CFR 50.73 (a)(2)(iv)(B)(6)).
ANALYSIS OF SAFETY SIGNIFICANCE
This event had no significant effect on the health and safety of the public. A manual reactor trip was initiated in response to the lowering condenser vacuum. All safety systems operated as designed; and no unexplained aspects of this transient, when compared to the UFSAR, were noted. Therefore, there were no actual safety consequences for the event. The condenser continued to function as a heat sink and condenser steam dumps remained operable throughout this event. Post-trip reviews established that plant parameters were within UFSAR analyzed minimum and maximum values for a loss of load event. No radiological release occurred. Therefore, the health and safety of the public were not affected by this event.
Short Term - As an interim repair, the inlet and outlet ends of the leaking slop drain were plugged. The other slop drains on both units were plugged.
Long Term - During the 2005 Unit 4 Cycle 22 refueling outage, the failed slop drain pipe and the rest of the Unit 4 slop drains (total of four) were repaired, utilizing a repair design that had more support and is more resistant to vibration. The repairs will be inspected during the next unit 4 refueling outage to confirm the effectiveness of the repair and the drains opened as appropriate.
DOCKETFACILITY NAME (1) LER NUMBER (6) PAGE (3)NUMBER (2) Engineering will develop a repair package for the Unit 3 slop drains similar to that done for Unit 4.
refueling outage. Engineering will determine if plugging is required for the Unit 3 slop drains following their repair.
Additionally, the weld control manual is to be revised to add augmented inspection requirements for non nuclear safety welds critical to unit operation.
second component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED: - SLOP DRAIN INTEGRITY FAILURE
A review of previous Licensee Event Reports identified LER 2001-003-00, Manual Reactor Trip Due to Low Condenser Vacuum. However, the root cause was different: inadequately designed sensing lines for the control room condenser vacuum instrumentation and the turbine low vacuum alarm.
However, a review of past condition reports revealed that Unit 3 experienced a similar sudden loss of condenser vacuum, with vacuum decreasing to approximately 23 inches and power decreasing to 667 megawatts. The turbine slop drain piping was found to be the source of the air inleakage causing the vacuum loss (CR 2003-2861).