05000293/LER-2008-006, Regarding Automatic Scram Resulting from Switchyard Breaker Fault During Winter Storm
| ML090570049 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 02/12/2009 |
| From: | Bronson K Entergy Nuclear Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 08-006-00 | |
| Download: ML090570049 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2932008006R00 - NRC Website | |
text
iEt-Entergy Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station 600 Rocky Hill Road Plymouth, MA 02360 Kevin H. Bronson Site Vice President February 12, 2009 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555
SUBJECT:
Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station Docket No.: 50-293 License No.: DPR-35 Licensee Event Report 2008-006-00 LETTER NUMBER: 2.09.010
Dear Sir or Madam:
The enclosed Licensee Event Report (LER) 2008-006-00, "Automatic Scram Resulting from Switchyard Breaker Fault during Winter Storm" is submitted in accordance with 10 CFR 50.73.
This letter contains no commitments.
Please do not hesitate to contact Mr. Joseph R. Lynch, (508) 830-8403, if there are any questions regarding this submittal.
Sincerely,
/
(5 H..
A Brso'1
,fr.Kevin H. Bronson FXM Enclosure cc:
Mr. James S. Kim, Project Manager Plant Licensing Branch I-1 Division of Operator Reactor Licensing Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission One White Flint North O-8C2 11555 Rockville Pike Rockville, MD 20852 INPO Records 700 Galleria Parkway Atlanta, GA 30399-5957 Mr. Samuel J. Collins Regional Administrator, Region 1 U.S. Nuclear Regulator Commission 475 Allendale Road King of Prussia, PA 19406 Senior Resident Inspector Pilgrim Nuclear Power Station
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- 3. PAGE PILGRIM NUCLEAR POWER STATION 05000-293 1 of 6
- 4. TITLE Automatic Scram Resulting From Switchyard Breaker Fault During Winter Storm
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE)
- 8. OTHER FACILITIES INVOLVED SEQUENTIAL NUMBER REVISION FACILITY NAME DOCKET MONTH DAY YEAR YEAR NUMBER MONTH DAY YEAR N/A NUMBER 05000 FACILITY NAME DOCKET 12 19 2008 2008 006 00 02 12 2009 N/A NUMBER I
1 05000
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR: (Check one or more) 20.2201(b) 22.2203(a)(3)(i) 50.73(a)(2)(i)(C) x 50.73(a)(2)(vii)
N 22.2202(d)
,20.2203(a)(3)(ii) 50.73(a)(2)(ii)(A) 50.73(a)(2)(viii)(A) 20.2203(a)(1) 20.2203(a)(4) 50.73(a)(2)(ii)(B) 50.73(a)(2)(viii)(B) 20.2203(a)(2)(i) 50.36(3)(1 )(i)(A) 50.73(a)(2)(iii) 50.73(a)(2)(ix)(A) 20.2203(a)(2)(ii) 50.36(3)(1)(ii)(A) x 50.73(a)(2)(iv)(A) 50.73(a)(2)(x)
- 10. Power Level 20.2203(a)(2)(iii) 50.36(c)(2) 50.73(a)(2)(v)(A) 73.71 (a)(4) 20.2203(a)(2)(iv)
__50.46(a)(3)(ii) 50.73(a)(2)(v)(B) 73.71 (a)(5) 100 20.2203(a)(2)(v) 50.73(a)(2)(i)(A) 50.73(a)(2)(v)(C)
OTHER 20.2203(a)(2)(vi) 50.73(a)(2)(i)(B) 50.73(a)(2)(v)(D)
Specify in Abstract below or in
EVENT DESCRIPTION
On December 19, 2008, at 1831 hours0.0212 days <br />0.509 hours <br />0.00303 weeks <br />6.966955e-4 months <br />, an unplanned automatic reactor protection system scram signal and scram occurred while operating at approximately 100% power. The event occurred with a severe winter storm in progress with predominantly easterly winds and snow depositing at a rate of approximately one (1") inch per hour.
A significant current to ground fault on ACB 105 actuated the main transformer bus differential relay (87B/MT). In accordance with design, this energized the main transformer lockout relay (86X/MT) and automatically opened ACB-104 and ACB-105, automatically opened the main generator field breaker, automatically tripped the main turbine, and automatically fast transferred APDS 4.16 kV power from the UAT to the SUT. The turbine trip caused fast closure of the turbine control valves and automatic reactor scram. Withdrawn controls rods rapidly inserted.
The ground fault on ACB-105 was significant and was also seen by the Line-355 protective relays. In accordance with design, this automatically opened ACB-102 and ACB-105. ACB-102 automatically re-closed after sensing that the Line-355 fault cleared. ACB-103 was not affected and provided offsite power via Line-342 to the SUT throughout the event.
The turbine trip resulted in automatic closing of the turbine control valves and stop valves. Three (3) turbine steam bypass valves opened to divert steam flow to the main condenser. These turbine steam bypass valves have a capacity for diverting 25% of the rated steam flow. In accordance with the analyzed transient analysis for a load reject event, reactor pressure increased and three (3) of the four (4) main steam relief valves (MSRVs) opened when mechanical set pressure was exceeded. The MSRVs reset and long term reactor pressure control was accomplished using the turbine -steam bypass valves.
The initial reactor vessel pressure increase contributed to a decrease in the reactor water void fraction (shrink). The decrease in the void fraction resulted in a decrease in the reactor water level to about -5" (narrow range). This level is less than the low water level setting (+12" narrow range) for automatic actuation of the Primary Containment Isolation Control System (PCIS) Group II (Sampling System); and automatic actuation of PCIS Group VI (Reactor Water Cleanup System) and the Reactor Building Isolation Control System (RBIS). As expected, PCIS and RBIS systems automatically actuated. Reactor water level was restored to the normal level (-29") using the normal condensate and feedwater systems.
All transient parameters were consistent with the existing transient analysis.
The station APDS 4.16 kV ac power buses were fast transferred from the UAT to the SUT in accordance with design. Offsite power via ACB-103 and Line-342 to the SUT was maintained. Due to operational concerns for potential grid instability, the standby EDGs were manually started and aligned to provide power to the A5 and A6 4.16 kV emergency services buses at 1852 hours0.0214 days <br />0.514 hours <br />0.00306 weeks <br />7.04686e-4 months <br />.
When 4.16 kV ac power fast transferred over to the SUT, the 120 V ac safeguards instrumentation panels (Y3/31 and Y4/41) lost power. This loss of power was not in accordance with design. Operator actions defined in site procedures were taken to restore power by 1850 hours0.0214 days <br />0.514 hours <br />0.00306 weeks <br />7.03925e-4 months <br /> (panels were lost for -19 minutes).
The NRC Operations Center was notified of the event via Event # 44735 in accordance with 10 CFR 50.72 at 21:52 hours on December 19, 2008.
CAUSE
The direct cause of the automatic reactor scram was reactor protection system actuation resulting from fast closure of the turbine control valves.
The root cause of the event was ice and snow build up on ACB-105 "A" phase bushing on the main transformer side and flashover which resulted in a significant current to ground fault. ACB-105 is an ac circuit breaker; model number HVB-SF6, manufactured by the General Electric-Hitachi company.
The direct cause of power loss to the safeguards panels was high in-rush current and transformer X55 input breaker trip; and transformer X56 tap control board malfunction. The X55 and X56 transformers are 480 V ac to 120 V ac transformers, model number PWTAB015120E, manufactured by Rapid Power Technologies Incorporated.
CORRECTIVE ACTION
Corrective actions taken included the following:
Performed a post scram walk down of the switchyard and testing to assess ACB, main transformer, and UAT equipment damage.
An elevated monitoring plan was established and implemented on the main transformer.
The "A" phase bushing on ACB-105 (main transformer side) was damaged due to the ground fault.
Switchyard breakers were aligned with ACB-105 open to allow reactor start and plant generation. The plant was synchronized to the power grid on 12/23/08. The damaged bushing on ACB-105 was replaced on 12/29/08, ACB-105 was returned to service on 12/30/2008.
Power was restored to the 120 V ac safeguards panels during the scram event.
The input breaker to the X55 transformer feed to Y3/31 was replaced.
The transformer tap control board was replaced on the X56 transformer feed to Y4/41.
Corrective actions planned include the following:
Review of potential design changes to improve switchyard resistance to weather related flashovers.
Modify and replace the input breakers on the X55 and X56 transformers.
Complete vendor evaluation of transformer tap control board failure.
Results of these corrective actions will be tracked in the Corrective Action Program (see CR-PNP-2008-3962 and 3963).
SAFETY CONSEQUENCES
The event posed no threat to public health and safety.
The turbine trip system is non-safety related. A turbine trip is a transient that the plant is designed to experience without safety consequence. A turbine trip at greater than 25% rated power is expected to result in a reactor scram due to actuation of the RPS logic on a fast closure of the turbine control valves signal.
N
The load rejection and reactor scram experienced during this event is bounded by the transient analysis described in the Updated Final Safety Analysis Report (UFSAR) Section 14.4.3, "Generator Load Reject without Bypass." At the on-set of the event, the initial pressure rise and opening of some or all of the MSRVs is an expected response to fast closure of the turbine control valves when operating at 100%.
During the transient three of the four MSRVs automatically opened on mechanical overpressure (nominal set-point between 1095 and 1115 psig +/- 11 psi) in accordance with design to relieve reactor pressure.
The turbine steam bypass valves also opened per design and relieved reactor pressure to the main condenser. The main steam safety valves (set-point is 1240 psig +/-113 psi) did not open during the event.
Reactor water level was maintained within the limits expected for the event. Reactor water level decreased to approximately - 5" (narrow range). This level is below the setpoint (+12") for actuation of the primary containment isolation system (PCIS) for Group II (sample valves) and for actuation of PCIS Group VI (Reactor Water Cleanup System) and the Reactor Building Isolation System (RBIS). These systems actuated and operated as designed in response to the low water level condition. Reactor water level was recovered using the normal condensate and feedwater systems and maintained at the normal operating level (-29"). Reactor water level was maintained well above the set-point limit for automatic actuation of the Core Standby Cooling Systems and Group I portion of the PCIS (about -46"), and well above the level corresponding to the top of the active fuel (about -127").
Offsite power was maintained to the 4.16 kV ac emergency and normal service buses. The standby EDGs and Station Blackout Diesel Generator were available. In response to grid stability concerns, safety related 4.16 kV buses A5 and A6 were conservatively placed on the EDGs. The Core Standby Cooling Systems (HPCI System, Automatic Depressurization System, Residual Heat Removal System, and Core Spray System) and the RCIC System were available but were not operated during the event.
The loss of the Y3/31 and Y4/41 120 V ac safeguards panels for approximately 19 minutes during the scram event was not expected. However, plant shutdown was not jeopardized by the loss of these panels. The Pilgrim Station electrical design relies on power restoration to the A5 and A6 4.16 kV buses within 13 seconds for a design basis event involving a loss of offsite power and loss of coolant accident.
During the scram event, when the safeguards panels were de-energized, the SSW Loop 'A' and 'B' pumps and RBCCW Loop 'A' and 'B' pumps were not capable of automatically starting as assumed in the design.
The manual start function of the pumps was not affected. The significance of losing power to the safeguards panels during a load reject scram was assessed. The assessment revealed that the loss of the panels is detectable and that actions to re-energize the panels are addressed in plant procedures. In addition, capability to manually start the SSW and RBCCW pumps was maintained. A risk assessment identified that the Incremental Conditional Core Damage Probability (ICCDP) and the Incremental Conditional Large Early Release Probability (ICLERP) resulting from loss of Y3/31 and Y4/41 during the event was insignificant.
No fuel, reactor, or pressure boundary safety limits were challenged by this event.
REPORTABILITY
This report was submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A) and 50.73(a)(2)(vii).
5ION
SIMILARITY TO PREVIOUS EVENTS A review was conducted of Pilgrim Station LERs since 1974. The review identified a number of similar events which involved switchyard electrical faults resulting in load rejection and reactor scram. LERs 1985-025, 1992-016, 1993-004, and 1993-022 identify events where reactor scram occurred due to the effects of severe weather events including lightning strikes and winter storms. LER 2003-003 identifies an event where a fault on the UAT resulted in load rejection and reactor scram.
ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES The ElIS codes for this report are as follows:
COMPONENTS CODES Breaker, AC 52 Relay, Differential, Protective 87 Relay, Lock-Out 86 Transformer (Main, 480 to 120 V ac)
XFMR Tap Changer, Transformer TTC SYSTEMS CODES Switchyard System FK Engineered Safety Features Actuation (RPS, PCIS, RBIS)
JE Containment Isolation Control System (PCIS, RBIS)
JM Main Generator Output Power System EL Medium Voltage Power System - Class 1 E (4 kV)
EB Low Voltage Power - Class 1 E (120 V safeguards)
ED SION