05000390/LER-2002-005
| Watts Bar Nuclear Plant (Wbn) Unit 1 | |
| Event date: | 09-27-2002 |
|---|---|
| Report date: | 11-26-2002 |
| Initial Reporting | |
| 3902002005R00 - NRC Website | |
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 1 05000390
I. PLANT CONDITION(S)
Watts Bar Unit 1 was operating at 100% reactor
II. DESCRIPTION OF EVENT
A. � Event:
power when this event occurred.
Bar Nuclear Plant Unit 1 was operating at 100% (Energy Industry Identification System (EIIS) RLY) actuated. This was a result of the loss of Service Transformer (CSST) C (EIIS code Bar Hydroelectric generating plant (WBH) resulted in the automatic start and loading of (EIIS code GEN). As designed, both B the A train motor-driven auxiliary feedwater auxiliary feedwater pump (EIIS code Specification LCO 3.8.1.A was entered for requires restoration of the offsite source 1B-B load stripping relay actuated. � This line to CSST D. Both the B train EDGs loaded onto the B train 6.9 kV shutdown feedwater pump and B train centrifugal charging Technical Specification LCO 3.8.1.D was inoperable.
of unusual event (NOUE) due to the loss of which resulted from a fire at the Watts Bar Technical Support and the Operational Support at 0926 EDT.
the WBH fire and remained there until the fire at the fire location and callout of additional 10 CFR 50.54(x) was invoked to address this requirement of the WBN Fire Protection EDGs operating throughout the event. Offsite configuration from Sequoyah and Rockwood lines.
to be a Generic Letter (GL) 91-18, Conditions and On Operability," non- At 0824 EDT on September 27, 2002, Watts power when 6.9 kV shutdown logic board panel code PL) 1A-A load stripping relay (EMS code the 161kV offsite power line to Common Station XFMR) which occurred due to a fire at the Watts (EIIS code GEN). The loss of this power line both A train Emergency Diesel Generators (EDGs) train EDGs started in the standby mode, and pump (EIIS code BA/MO/P) and turbine-driven BA/TRB/P) also automatically started. Technical one offsite power circuit being inoperable which within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
At 0842 EDT, the 6.9 kV shutdown logic boardpanel was a result of a loss of the 161 kV offsite power which had been operating in the standby mode, boards, and the B train motor driven auxiliary pump (EIIS code CB/P) automatically started.
entered for both offsite power circuits being At 0852 EDT, WBN Unit declared a notification both offsite 161 kV preferred power sources Hydroelectric Generating Plant (WBH). The Centers were activated and in full operation The WBN fire brigade was dispatched to fight was extinguished. Since the brigade remained brigade staffing took greater than two hours, departure from the minimum fire brigade staffing Report.
WBN remained at 100% power with all four power was restored using an interim offsite TVA evaluated and determined this configuration "Resolution of Degraded and Non-Conforming FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) conforming condition that supports functionality and operability of both 161 kV preferred power sources per GDC 17, "Electrical Power Systems." Condition D of LCO 3.8.1 was exited at 0125 EDT on September 28 when the first qualified offsite source was returned to service.
WBN remained in Condition A of LCO 3.8.1 until the second qualified offsite line was returned to service at 0300 EDT. The NOUE was exited at 0308 EDT on September 28, 2002.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
There were no inoperable plant structures, components, or systems that contributed to the event.
C. Dates and Approximate Times of Major Occurrences:
September 27, 2002 (All times are EDT) 0824 D Loss of the 161kV offsite power line to CSST C occurred due to a fire at the Watts Bar Hydroelectric generating station. The loss of this power line resulted in the automatic start and loading of both A train EDGs. As designed, both B train EDGs started in the standby mode, and the A train motor driven auxiliary feedwater pump and turbine-driven auxiliary feedwater pump also automatically started. D Technical Specification LCO 3.8.1.A was entered for one offsite power circuit being inoperable which requires restoration of the offsite source within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
0840 D Dispatched the Watts Bar Nuclear Plant fire brigade to the WBH to assist with fire fighting efforts.
0842 D Loss of the 161 kV offsite power line to CSST D occurred. Both the B train EDGs, which had been operating in the standby mode, loaded onto the B train 6.9kV shutdown boards, and the B train motor driven auxiliary feedwater pump and B train centrifugal charging pump automatically started. Technical Specification LCO 3.8.1.D was entered for both offsite power circuits being inoperable.
0845 D Requested offsite ambulance and fire department assistance.
0852 D Declared a Notification of Unusual of Event (NOUE) 0911 D Notified NRC of NOUE Notified State of Tennessee 0920 D Initiated recall of additional fire brigade personnel 0926 D Staffed the Technical Support Center FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) 1040 � Invoked 10 CFR 50.54(x) for the WBN-1 fire brigade not meeting minimum manning requirements for over two hours as required by the Fire Protection Report and License Condition 2.f because the fire brigade was offsite at WBH fighting the fire.
1101 � Reported to the NRC the use of 10 CFR 50.54(x) to deviate from the above License Condition because the fire brigade was offsite at WBH fighting the fire. Event number 39221.
1130 � Fire brigade onsite staffing restored. � Compliance with the License Condition restored.
1353 � Incident commander reports fire out at WBH.
1516 � CSSTs A and D re-energized from 161kV WBH switchyard (not loaded).
1526 � CSSTs B and C re-energized from 161kV WBH switchyard (not loaded).
1558 � Notification to NRC under 10 CFR 50.72 - 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> notification due to ESF actuation.
Event number 39224.
1820 � NRC notified that restoration of offsite power would be evaluated under NRC Generic Letter 91-18 as an operable but degraded condition.
September 28, 2002 0012 � CSST C paralleled with the 1A-A EDG on the 1A-A 6.9 kV shutdown board.
0116 � CSST C paralleled with the 2A-A EDG on the 2A-A 6.9 kV shutdown board.
0125 � Exited LCO 3.8.1.D to restore one offsite circuit within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. � Entered LCO 3.8.1.A to restore the remaining offsite circuit to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> still applicable.
0139 � 2A-A EDG secured FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE ( 3 ) Watts Bar Nuclear Plant (WBN) Unit 1 05000390 � II 0200 � CSST D paralleled with the 1 B-B EDG on the 0210 � 1 B-B EDG secured.
0232 � CSST D paralleled with the 2B-B EDG on the 0246 � 2B-B EDG secured.
0300 � All offsite circuits declared operable exited LCO 0308 � Terminated NOUE.
D. Other Systems or Secondary Functions Affected:
1 B-B 6.9 kV shutdown board.
2B-B 6.9 kV shutdown board.
3.8.1.A.
logic board panel 1A-A load of off-site power through use of of Offsite Power." Procedure use was of an unusual event was declared, as Technical Support and Operations Director judgment. A being offsite to fight the fire at the and timely. Conservative Control Room staff during the event.
of the event status and expected manner.
of the diesel generators and the impacted by the diesel generator on Site Emergency See Item G, Safety System Response.
E. Method of Discovery:
kV Shutdown and the loss "Loss notification The onsite Fire Brigade were appropriate and the Main members in an appropriate the operation start. Loads The event was immediately evident when 6.9 stripping relay actuated.
F. Operator Actions:
The Operations personnel responded to the fire procedure Abnormal Operating Instruction (A01)-35, performed correctly and in a timely manner. A required, due to the loss of two offsite sources.
Support Centers were activated as needed, based 10 CFR 50.54(x) was declared due to the WBN WBH. Actions taken by Operations personnel judgment was exhibited by the Shift Manager Frequent crew briefs were held to update all crew actions to be taken. Response to alarms were
G. Safety System Responses:
The principal impact of the event on WBN was normal loads shed in response to a diesel generator FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 1 05000390 start and the loss of offsite power included ice condenser air handling units, spent fuel pool cooling (EIIS code DA), intake pumping station sump pump (EIIS code P), nitrogen skid, 6.9 kV Common Board B loads, and primary makeup water pumps (EIIS code P). Loads not auto sequenced onto the emergency diesels were evaluated and loaded onto the diesel generators as required. The equipment impacted was handled by the operations crew and normal operation continued.
III. CAUSE OF THE EVENT
A. Immediate Cause:
The cause of the loss of offsite power is discussed below in Part B, "Root Cause.
B. Root Cause:
The root cause for the loss of offsite power to the WBN Unit 1 was due to a fire that occurred at the WBH. Since the offsite power comes through the WBH 161kV switchyard, it was disrupted when the fire destroyed some of the cabling in the protective and control circuits associated with these lines.
C. Contributing Factor:
There were no contributing factors for this event.
IV. ANALYSIS OF THE EVENT
As discussed previously, the principal impact of the event on WBN was the operation of the EDGs and the normal loads shed in response to a EDG start. Loads impacted by the EDG start and the loss of offsite power included ice condenser air handling units, spent fuel pool cooling, intake pumping station sump pump, nitrogen skid power, 6.9 kV Common Board B loads, and primary makeup water pumps. Loads not auto sequenced onto the EDGs were evaluated and loaded onto the EDGs as required. The equipment impacted was handled by the operations crew and normal operation continued.
The loss of redundancy in offsite power supply increased the station risk profile until the offsite sources could be restored. A01-40, "Station Blackout," contains provisions for backfeeding the Unit 1 6.9 kV shutdown boards from the 500 kV system in the event the diesels are not available. This capability is credited in the WBN risk profile. The backfeed capability was present but was not used due to EDG availability. Therefore, additional unused power feed capability existed. Recently, WBN also completed four procedures (A01-43.01 though 43.04) to allow cross-connect of diesels to various shutdown boards in the event that the normal shutdown board and diesel tie was not available. These procedures were not needed since all four EDGs started and powered each respective shutdown board. 0 In addition, non-event related activities were suspended in the plant to mitigate any additional risk until each activity was confirmed to not have an impact on plant risk.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) Watts Bar Nuclear Plant (WBN) Unit 1 05000390 NUMBER I
V. ASSESSMENT OF SAFETY CONSEQUENCES
The Updated Final Safety Analysis Report (UFSAR) 15.2.9, "Coincident Loss of Onsite and External (Offsite) AC Power to the Station," documents a complete loss of all offsite power to the plant auxiliaries. This event assumes a coincident turbine generator trip. The reactor coolant system response is bounded by other events evaluated in the UFSAR including 15.2.7, "Loss of External Load and/or Turbine Trip," 15.2.8, "Loss of Normal Feedwater," and 15.3.4, "Loss of Forced Reactor Coolant Flow." These events assume secondary side induced reactor coolant system heatup (loss of normal feedwater) or primary side induced reactor coolant system heatup (loss of forced reactor coolant flow). These events cause an increase in reactor coolant system temperature and subsequent increase in reactor coolant system pressure. The UFSAR analyses demonstrate the events are mitigated with DNB limits.
The event at WBN caused a loss of shutdown board power from offsite sources due to the loss of both preferred 161kV lines. The diesel generators automatically loaded and continued to supply emergency power to the safeguard buses. Upon initiation of the loss of 161 kV power to the C CSST (0824 EDT), all four of the diesel generators started and the A train 6.9kV Shutdown Boards and A train Emergency Diesels were tied as designed. Subsequently the D CSST was lost (0842 EDT) as the WBH fire progressed and the B boards and B diesels were tied as designed. Load shedding and auto-sequencing of loads proceeded normally with no anomalies observed. The plant continued to operate at 100 percent power with backfeed of generation to power station auxiliaries. The reactor coolant pumps continued to operate on generated power and feedwater continued to flow normally. Therefore, no heatup event occurred as described in the UFSAR and the safety significance was well below the analyzed events.
TVA estimated risk of continued operations during the event using conservative assumptions. TVA has since completed a risk assessment of the event in which some of the conservatisms were removed to more realistically assess the actual plant response. Since the required safety-related equipment all started during the event, the fail-to-start probabilities were set to zero for this equipment. The model was also modified to assume a loss of preferred offsite power for all events. The zero maintenance model was used, and risk for the 20 hour2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> exposure was calculated.
The core damage calculation resulted in an increase of 7.6E-7 incremental core damage probability (ICDP) for the period.
In summary, the events described by this LER are bounded by previous analysis provided in the UFSAR with only a very small increase in ICDP.
VI. CORRECTIVE ACTIONS
A. � Immediate Corrective Actions:
Once the fire was extinguished at the WBH, TVA's efforts were focused upon the restoration of the WBN's offsite power sources. Onsite Engineering working with TVA's Transmission Power FACILITY NAME (1) DOCKET LER NUMBER ( 6 ) PAGE (3) Services Group identified a configuration which would restore the offsite power feeds. TVA performed an evaluation of this configuration and determined it to be an operable configuration.
Although operable, TVA considered this configuration to be degraded under the guidance presented in Generic Letter 91-18 due to the protective relaying scheme that was put into place and the temporary loss of the hydro generating capability. This configuration will remain in place until future repairs can be made.
Currently to alleviate the degraded condition, TVA is working to repair the WBH switchyard and to restore the WBH units to operation. Pending the final fire investigative findings, additional corrective action may be considered and this report updated.
B. � Corrective Actions to Prevent Recurrence:
Not Applicable VII. � ADDITIONAL INFORMATION
A. Failed Components:
There were no failed components involved in this LER.
B. Previous LERs on Similar Events:
The only other loss of offsite power that has been reported was Licensee Event Report 390/2002- 004 which coincidentally occurred on September 21, 2002. However, these events are unrelated since the cause for that LER was determined to be a momentary partial loss of offsite power resulting from the inadvertent manual operation of Breaker 934 at the hydroelectric generating plant.
C. Additional Information:
None.
D. Safety System Functional Failure Consideration:
This event is not considered a safety system functional failure in accordance with NEI 99-02.
E. Loss Of Normal Heat Removal Consideration:
Because the event did not involve a reactor trip, it is not considered heat removal in accordance with NEI 99-02.
a scram with loss of normal VIII. � COMMITMENTS None.