ML14031A088
| ML14031A088 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 01/30/2014 |
| From: | Fili K D Northern States Power Co, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| BL-12-001, LT-MT-14-007 | |
| Download: ML14031A088 (8) | |
Text
Xcel Energy January 30, 2014 US Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-001 Monticello Nuclear Generating Plant Docket 50-263 Renewed Facility Operating License No. DPR-22 Monticello Nuclear Generating Plant 2807 W County Road 75 Monticello, MN 55362 LT-MT-14-007 10 CFR 50.54(f) Response to Request for Additional Information Regarding Response to Bulletin 2012-01, "Design Vulnerability In Electric Power System," (ML 13351A314).
Reference:
- 1) Letter from Northern States Power Company, a Minnesota corporation (NSPM), d/b/a Xcel Energy to NRC Document Control Desk, "Response to Bulletin 2012-01, "Design Vulnerability in Electric Power System" for MNGP dated October 22, 2012. On December 20, 2013, the Nuclear Regulatory Commission (NRC) requested additional information from licensees to verify that they have completed interim corrective actions and compensatory measures and to determine the status of each licensee's long-term corrective actions. The NRC requested that licensee's provide a written response, signed under oath or affirmation, to the enclosed request for additional information by February 3, 2014. If additional information is needed, please contact Mr. Richard Loeffler at 763-295-1247.
Summary of Commitments:
This letter does not contain any new commitments and does not revise any existing commitments.
Letter L T-MT-14-007 Page 2 I declare under penalty of perjury that the foregoing is true and correct. Executed on January 30, 2014. Karen D. Fili Site Vice-President Monticello Nuclear Generating Plant Northern States Power Company-Minnesota
Enclosure:
Response to Request For Additional Information Regarding Response To Bulletin 2012-01, "Design Vulnerability In Electric Power System cc: NRC Regional Administrator, Region Ill, USNRC NRR Project Monticello Nuclear Generating Plant, USNRC NRC Resident Inspector, Monticello Nuclear Generating Plant, USNRC Enclosure Monticello Nuclear Generating Plant Response to Request For Additional Information Regarding Response To Bulletin 2012-01, "Design Vulnerability in Electric Power System" (Five pages follow)
L-MT-14-007 Enclosure Monticello Nuclear Generating Plant Response to Request For Additional Information Regarding Response To Bulletin 2012-01, "Design Vulnerability in Electric Power System" Background Information On January 30, 2012, Byron Station, Unit 2 experienced an automatic reactor trip from full power because the reactor protection scheme detected an undervoltage condition on the 6.9-kV buses that power reactor coolant pumps (RCPs) Band C (one of two phase undervoltage on two of four RCPs initiate a reactor trip). The undervoltage condition was caused by a broken insulator stack of the phase C conductor for the 345-kV power circuit that supplies both system auxiliary transformers (SATs). This insulator failure caused the phase C conductor to break off from the power line disconnect switch, resulting in a phase Copen circuit and a high impedance ground fault. NRC Request For Additional Information NRC letter (ADAMS ML 13351A314) dated December 20, 2013, requested the following:
In order for the NRC staff to complete its review of responses to the bulletin, the following additional information is requested:
- 1. Provide a summary of all interim corrective actions that have been taken since the January 30, 2012, event at Byron Station, Unit 2, to ensure that plant operators can promptly diagnose and respond to open phase conditions on the offsite power circuits for Class-1 E vital buses until permanent corrective actions are completed.
- 2. Provide a status and schedule for completion of plant design changes and modifications to resolve issues with an open phase of electric power. 1 L-MT-14-007 Enclosure Response 1 -Summary of All Interim Corrective Actions Lessons learned from the events at Byron Station were reviewed and various interim corrective actions were evaluated for safety and efficiency at the Monticello Nuclear Generating Plant (MNGP). Based on the plant's offsite power configuration, electrical design details, and lessons learned, actions were taken to ensure plant operators can promptly diagnose and respond to open phase conditions (OPC):
- Operating instructions and training curriculum were reviewed to ensure operators can diagnose and respond to an OPC.
- Transformer yard rounds are performed and include general and detailed inspections of the transformers to ensure parameters are within expected limits.
- Infrared inspections of yard equipment are being completed as part of routine inspections.
- Operations procedures were revised to include recognition of equipment problems caused by OPC. Northern States Power Minnesota (NSPM) placed this issue into the Corrective Action Program (CAP) to track resolution.
An Engineering Change (EC) Technical Evaluation, "MNGP Degraded Voltage Relay Scheme Susceptibility to Loss of Phase Condition Evaluation," evaluated the single phasing condition and determined that MNGP could be susceptible to a loss of a single phase in the MNGP substation or with the loss of one of the feeders to the 2R Transformer.
Details associated with each interim corrective action are as follows: Operating instructions and training curriculum were reviewed to ensure operators can diagnose and respond to an OPC. NSPM issued an Operations Memo on February 16, 2012, to make operators aware of the Byron OE and potential vulnerability to a similar event at Monticello.
The memo includes direction on how to identify a single open phase condition from a control room panel. Primary breakers have ammeters located on Main Control Room Electrical Distribution panel C-08 which provide individual phase currents and may be used to identify unbalanced phase currents.
Primary buses also have volt meters located on panel C-08 which provide indication of abnormal voltages.
Individually or in combination, these meters may be used to identify abnormal power conditions.
2 L-MT-14-007 Enclosure Additionally, abnormal operation of plant equipment may be an indication of abnormal power conditions.
A second Operations Memo was issued in March 2012 to further familiarize the Operations crews with a newly developed abnormal operating procedure for abnormal phase voltage. The memo also included a discussion on actions required to reset tripped 480V overcurrent devices within the plant electrical distribution system. Operations crew training walkthroughs covering use of the new abnormal operating procedure were completed in the simulator.
Transformer yard rounds are performed and include general and detailed inspections of the transformers to ensure parameters are within expected limits. Operations rounds are performed covering transformers within the Offsite Power circuits.
Operations rounds checks at various frequencies verify transformer operating parameters such as local alarm status, winding temperatures and oil temperatures.
Infrared inspections of yard equipment are being completed as part of routine inspections.
Routine thermography surveillances are performed for the switchyard equipment.
The inspections are performed once in the winter months and monthly from May through September.
Operations procedures were revised to include recognition of equipment problems caused by OPC. Plant operating procedures, including operating procedures for off-normal alignments, specifically call for verification of the voltages on all three phases of the Engineered Safety Feature (ESF) buses.
NSPM initiated procedure changes to provide guidance to the operators in diagnosing and responding to a single-phase open circuit condition.
The procedure changes include a new Abnormal Operating Procedure (AOP), C.4-B.09.06.D, "Non-Essential4.16kV Bus Abnormal Phase Voltage," to address identification and response to a single phase condition.
Subsequently this procedure has been retitled as, "Non-Essential Bus Abnormal Phase Voltage." The procedure provides direction for an open bus 3 L-MT-14-007 Enclosure transfer of the plant electrical buses due to off normal electric plant conditions.
Indications which the operators may encounter are:
- Voltage on the plant buses fluctuating.
- One or more 4kV Bus individual phase voltages less than 3800 volts during steady state operation.
- One or more 13.8kV Bus individual phase voltages less than 12600 volts during steady state operation.
- Unbalanced individual phase current on Ammeters, either on C-08 or at the individual breaker cubicles.
- Red lights on Loss of Voltage relays or Degraded Voltage relays located in the upper portion of 4kV cubicles 152-510 or 152-601.
- Various unexplained trips and/or overloads of 13.8kV, 4kV and/or 480V motors. The following operator instructions were revised to direct operators to the new AOP in the event of a motor trip or over-current condition:
C.6-004-C-31 -Recirculation Drive Motor A Trip C.6-004-C-32
-Recirculation Drive Motor B Trip C.6-006-A-06
-RCT Feed Pump P-2A Trip C.6-006-A-07
-RCT Feed Pump P-2B Trip C.6-006-A Condensate Pump P-1A Trip C.6-006-A Condensate Pump P-1 B Trip C.6-006-A-11
-RCT Feed Pump P-2A Overload C.6-006-A RCT Feed Pump P-2B Overload C.6-006-A Condensate Pump P-1A Overload C.6-006-A Condensate Pump P-1 B Overload C.6-006-C Circ Water PP P-100A Trip C.6-006-C-1 0-Circ Water PP P-1 OOB Trip Operations validated the functionality of the AOP in the plant simulator during the development of the procedure.
Plant operating crews have been trained on the AOP to recognize conditions associated with an abnormal single phase condition.
Response 2 -Status and Schedule for Completion of Plant Design Changes
- Status o The holders of operating licenses and combined licenses for nuclear power reactors are investigating options being researched by several vendors (PSC 2000, EPRI, Schweitzer, etc.) to detect OPC faults. 4 L -MT 007 Enclosure Currently, there is no generic, off-the-shelf technology that has been proven to detect all the required open phase fault conditions for all plant and transformer designs. o The holders of operating licenses and combined licenses for nuclear power reactors are fully engaged in the development of the NEI OPC Guidance Document, as well as development of enhancements to software tools being used to analyze OPC faults. o With the goal of ensuring accurate detection without compromising nuclear safety or increasing plant risk, this new OPC technology is being thoroughly evaluated, will be tested, and will be fully analyzed before installation.
o Vulnerability studies of the OPC faults have been started for MNGP.
- Schedule o NSPM committed to the generic schedule provided in the Industry OPC Initiative for Monticello.
o NSPM intends to meet the milestones of this schedule; however, future deviations may be required to accommodate outage schedules, software and hardware availability, manufacturer's delivery capabilities, licensing delays, etc. o Deviations from the industry OPC initiative schedule will be documented through the deviation/exemption process addressed in the NEI OPC Guidance Document.
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