05000269/LER-2013-001
| Oconee Nuclear Station, Unit 1 | |
| Event date: | 02-06-2013 |
|---|---|
| Report date: | 04-08-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2692013001R00 - NRC Website | |
EVALUATION:
BACKGROUND
At the time this condition was identified, Oconee Nuclear Station (ONS) Units 1, 2, and 3 were operating in Mode 1 at approximately 100 percent power. No structures, systems or components were out of service at the time of this event that contributed to this event. This Blockhouse, and 230kV Switchyard Relay House and could render emergency power equipment inoperable during design basis events that may occur coincident with high ambient temperature conditions. This condition is reportable under the following criteria:
Operations Prohibited by Technical Specifications per 10 CFR 50.73(a)(2)(i)(B) Degraded or Unanalyzed Condition per 10 CFR 50.73(a)(2)(ii)(B) Event or Condition that Could Have Prevented Fulfillment of a Safety Function per 10 CFR 50.73(a)(2)(v)(A-D) Common Cause Inoperability of Independent Trains or Channels per 10 CFR 50.73(a)(2)(vii) The AC Power System consists of the offsite power sources and the onsite standby power sources, Keowee Hydro Units (KHU). This system is designed to supply the required engineered safeguards (ES)[EllS:JE] loads of one unit and safe shutdown loads of the other two units and is arranged that no single failure or a loss of offsite power (LOOP) can disable enough power sources to jeopardize plant safety.
The emergency AC electrical power system (i.e., the Keowee Emergency Power System [EllS:EK]) consists of two KHUs (KHU-1, KHU-2) that provide an emergency on-site power source for ONS via separate and independent paths. One path is the Keowee Underground power path through transformer CT-4 [EllS:XFMR] and the Standby Buses [EllS:EB] and the other path is the Keowee Overhead power path through the Keowee Main Step-up Transformer to the 230 KV Switchyard [EllS:FK] and the individual Oconee Unit's Start-up Transformers.
The 230kV Switchyard Relay House is cooled by two Heating Ventilation & Air Conditioning (HVAC) units. The 230kV Switchyard Relay House contains the controls and safety related 125VDC power supply needed for controlling the power circuit breakers (PCBs) in the 230kV switchyard. Control of the PCBs is needed to provide offsite power to the ONS units, as well as to isolate the 230kV switchyard yellow bus for aligning the Keowee Overhead power path.
The Unit 1 and 2 Blockhouse (Enclosure) is comprised of two compartments. One compartment contains the CT-4 transformer for the Underground emergency power path and a pair of 36" exhaust fans [EllS:FAN] for ventilation. The other compartment contains the Unit 1 and 2 main feeder buses (MFB)[EllS:EB], the Keowee Overhead power path bus and breakers for Units 1 and 2, the Standby. Bus for all three ONS Units, and Unit 3 Standby Bus to MFB sections. The Standby Bus and Unit 1 and 2 Normal and Emergency Source Breakers, as well as the High Pressure Service Water (HPSW) [EllS:KP] Pump Breakers, Auxiliary Service Water (ASW) [EllS:BA] Switchgear Feed and Standby Shutdown Facility (SSF) [EllS:NB] supply are contained in the B1T and B2T Switchgear in this room. Additionally, the bus duct [EllS:BDUC] for the Unit 1 and Unit 2 7kV system passes through this room. This compartment is ventilated by a pair of 24" exhaust fans. The two compartments are separated by a fire wall with a normally open fire door.
The CT-4 transformer supplies the redundant 4160V standby power buses via the Keowee Underground emergency power path from Keowee Hydro Station. This Keowee Underground power path and CT-4 transformer are sized to carry full engineered safeguards auxiliaries of one unit plus auxiliaries for safe shutdown of the other two units.
The Unit 3 Blockhouse (Enclosure) is comprised of a single compartment with ventilation provided by a pair of 24" exhaust fans. This enclosure houses the Main Feeder Buses for Unit 3, as well as the bus and breakers for the Keowee Overhead power path for Unit 3. Switchgear (3B1T and 3B2T) for the Unit 3 MFB Connections to the Normal and Emergency Sources are contained in this room. Also one of the two bus ducts from the Standby Bus (located in the Unit 1 and 2 Blockhouse) to the Unit 3 4160V Switchgears (3TC, 3TD, and 3TE) passes through the Unit 3 Blockhouse.
The MFBs provide power to each of the three redundant engineered safeguards switchgear bus sections that serve the engineered safeguards auxiliaries. The MFBs receive their power from one of three power sources, the unit's auxiliary transformer (Normal), the startup transformer (Emergency), or the standby power buses (Standby). The auxiliary transformer is the normal source of power during normal operation. However, upon loss of their normal source of power, the 4160V MFBs are transferred to the unit's startup transformer where power is supplied from either the 230kV transmission system, or from one of the two Keowee Hydro Units via the 230kV switchyard (Keowee Overhead path) or to the standby power buses where power is supplied from one Keowee Hydro Unit via the 13.8kV underground feeder (Keowee Underground path) through CT-4 transformer or from the 100kV transmission line through CT-5 transformer.
The original plant design issues in the Unit 1 and 2 Blockhouse may (outcome varies based upon conditions such as ambient temperatures, single failure assumptions, train availability, etc) affect transmission of power from each emergency power path required to be in service by Technical Specification (TS) 3.8.1, AC Sources Operating, and TS 3.8.2, AC Sources Shutdown.
TS 3.8.1 provides the most limiting requirements for operability as it applies to emergency power system capability to mitigate events during MODES 1, 2, 3, and 4 when electric loads to event mitigation equipment will be highest resulting in the highest, heat loads in the Unit 1 and 2 Switchgear Blockhouse.
TS 3.8.1 requires AC electrical power sources to be operable during normal operation MODES 1, 2, 3, and 4 as follows:
1. Two offsite sources on separate towers connected to the 230 KV switchyard to a unit startup transformer and capable of automatically supplying power to one main feeder bus; and 2. Two KHUs with one capable of automatically providing power through the Keowee Underground emergency power path to both main feeder buses and the other capable of automatically providing power through the Keowee Overhead emergency power path to both main feeder buses.
TS 3.8.2 requires emergency AC electrical power sources to be operable during shutdown operations (MODES 5 and 6, and when moving recently irradiated fuel) as follows:
A. One AC source from the offsite transmission network available or connected to one of the following:
1 230 KV switchyard to a unit startup transformer to one main feeder bus, 2. 230 KV switchyard, or 525 KV switchyard for Unit 3, to the main step-up and unit auxiliary transformer to one main feeder bus, or 3. Central switchyard to one main feeder bus B. One of the following emergency power sources capable of supplying the onsite AC electrical power distribution system:
1. One KHU capable of providing power through the Keowee Underground emergency power path to one main feeder bus, 2. One KHU capable of providing power through the Keowee Overhead emergency power path to one main feeder bus, or 3. One LCT energizing one standby bus via an isolated power path to one main feeder bus.
EVENT DESCRIPTION
The original plant design issues were identified by Duke Energy engineers performing an Extent of Condition review associated with an original design issue identified in LER 269/2011-06, Revision 0, Pressurizer Heater Capacity Non-Compliant with Technical Specification 3.4.9. The Extent of Condition review for the associated cause evaluation was to validate that a sampling of QA-1 breakers were properly rated for their expected environment. The assessment was completed in September 2012 and identified three areas where the original HVAC analyses lacked adequate inputs. Electrical equipment heat loads lacked adequate documentation.
These areas are the:
o Unit 1 and 2 Blockhouse (affected breakers are associated with protection and conduction paths for 4160V Switchgear B1T, B2T and CT-4 Transformer), o Unit 3 Blockhouse (affected breakers are associated with Switchgear 3B1T and 3B2T), and o 230kV Switchyard Relay house (affected breakers are associated with the ability to feed through the Keowee Overhead path).
This issue was placed into the corrective action program and development of new comprehensive heat load analyses was started using both Duke Energy engineers and a contract engineering firm experienced in thermal modeling. During the preparation of these analyses, it was determined that the original electrical equipment heat loads were underestimated and that the design of the power supplies and controls for these HVAC systems left the equipment potentially inoperable in single failure scenarios in high ambient temperature conditions. These three areas were individually evaluated in three phases which were prioritized based on factors including potential safety significance and operating experience. Phase 1 Blockhouse (completed 11/20/2012). Phase 3 evaluated the 230kV Switchyard Relay house (completed 1/21/2013). In each phase electrical equipment heat loads and capabilities were identified, a thermal model of the area was developed, and a Prompt Determination of Operability was performed. Upon completion of the analyses and Prompt Determinations of Operability of each of these three phases a comprehensive reportability evaluation was performed which was completed on 2/6/2013. The conclusion of the reportability evaluation resulted in a determination that this issue was reportable under the following reporting criteria:
Operations Prohibited by Technical Specification per 10 CFR 50.73(a)(2)(i)(B) The inadequacies in the original electrical equipment heat load analysis as well as the HVAC and 2 Blockhouse, the Unit 3 Blockhouse, and/or 230 kV Switchyard Relay House would be operable during events that may occur coincident with single failure and high ambient temperature conditions. An example would be a loss of coolant accident (LOCA)/LOOP. In this scenario the LOCA/LOOP is postulated on one unit with concurrent LOOPs on the other two units combined with an assumed single failure of the Keowee Overhead power path during a time period with high ambient temperatures. By design, the LOCA/LOOP unit would preferentially load to the CT-4 transformer and the single failure of the Keowee Overhead power path would force the two LOOP units' loads onto CT-4 transformer (Keowee Underground power path). The additional loading of the two LOOP units, in combination with postulated high ambient temperatures leads to excessively high temperatures in the Unit 1 and 2 Blockhouse.
This condition could cause CT-4 transformer to exceed vendor limits which is postulated to cause the failure of the redundant emergency power option (Keowee Underground power path), thus creating a condition prohibited by TS 3.8.1.d during these high ambient temperature conditions.
Degraded or Unanalyzed Condition per 10 CFR 50.73(a)(2)(ii)(B) The inadequacies in the original electrical equipment heat load analysis as well as the HVAC system design deficiencies for emergency power equipment in the Unit 1 and 2 Blockhouse, the Unit 3 Blockhouse, and/or 230 kV Switchyard Relay House for events that may occur coincident with single failure and high ambient temperature conditions could result in an unanalyzed condition. An example would be an event consisting of a LOCA/LOOP on one unit with concurrent LOOPs on the other two units and an assumed single failure of the Keowee Overhead power path during a time period with high ambient temperatures. In this scenario CT- 4 is postulated to overheat and fail because of the increased heat generation due to the additional electrical loading from the two LOOP units combined with the high ambient temperatures. This could cause the Keowee Underground power path to be inoperable. Per NUREG 1022 Rev. 2 a failure to be able to mitigate the consequences of an accident that significantly degrades plant safety is reportable as an unanalyzed condition. In addition NUREG 1022 Rev. 2, states that the discovery that a system required to meet the single failure criterion does not is also considered unanalyzed and is reportable under this criterion.
Event or Condition that could have prevented Fulfillment of a Safety Function per 10 CFR 50.73(a)(2)(v)(A-D) Over the past three years there have been several times where ONS has taken the Keowee Overhead power path out of service for planned maintenance. A review of these timeframes revealed that there were certain periods when the Keowee Overhead power path was out of service and the Keowee Underground power path is postulated to not have been able to mitigate the consequences of accident because of the high ambient temperatures. Therefore the Emergency Power system may not have been able to fulfill its safety function with both trains (Keowee Overhead and Underground power paths) considered inoperable. Loss of both paths constitutes loss of the onsite emergency power and is reportable under the following criterion 10 CFR 50.73(a)(2)(v)(A-D).
Common Cause Inoperability of Independent Trains or Channels per 10 CFR 50.73(a)(2)(vii) The original plant design deficiencies associated with the structures listed above created vulnerabilities that could result in failures to mitigate the consequences of a design basis accident which occurred during periods of high ambient temperatures. These design deficiencies impacted two independent trains of emergency power paths (Keowee Overhead and Keowee Underground power paths). Per NUREG 1022 Rev.2 this is reportable for a common cause that resulted in inoperability of two independent trains.
CAUSAL FACTORS
Duke Energy has determined the causal factors are as follows:
1. Use of inadequate and incomplete inputs and methods in design of Unit 1 and 2 Switchgear Blockhouse, Unit 3 Switchgear Blockhouse, and 230kV Relay House HVAC systems during original plant design.
2. The original design of HVAC power sources does not have adequate redundancy and control logic which can prevent HVAC operation when postulating credible single failures in all three locations.
CORRECTIVE ACTIONS
Immediate:
1. Entered Prompt Determination of Operability Process 2. Put the following Compensatory measures into effect:
a. Within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of an event in which CT-4 transformer is energized for emergency power and there is a loss of all Unit 1 and 2 Blockhouse ventilation fans, open both personnel doors to the Unit 1 and 2 Blockhouse.
b. Within 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> of the restoration of load shed power to the 230kV Switchyard Relay House for any event that may require emergency power to be provided by the 230kV Switchyard or the Keowee Overhead Path, determine temperature of the 230kV Relay House central corridor and the Battery Rooms and perform the following:
- Monitor 230kV Relay House temperature on a 3 hour3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> frequency while doors are closed and ensure 230kV Relay House Air Handling Units (AHUs) are operating properly.
- If 230kV Relay House AHUs are not operating properly, open both personnel doors and the roll up door to the 230kV Relay House prior to temperature inside the 230kV Relay House exceeding 110F.
Planned:
Blockhouse, and the 230kV Relay House:
1. Determine the design basis for the thermal environment and document in a QA-1 calculation.
2. Determine the electrical heat loads for use in development of the thermal environment design basis and document in a QA-1 calculation.
3. Modify the HVAC in the structures (see note above) to remove the non-conforming condition.
The corrective actions indicated above are not considered NRC Commitment items. There are no NRC Commitment items contained in this LER.
SAFETY ANALYSIS
Duke Energy used a risk-informed approach to determine the risk significance associated with the deficiencies of the Oconee Units 1 and 2 Blockhouse ventilation and the 230 kV Relay House HVAC.
The risk-significant deficiency for the Unit 1 and 2 Blockhouse is comprised of a failure of the Unit 1 and 2 Blockhouse ventilation fans to provide sufficient room cooling to the CT-4 transformer, Units 1 and 2 main feeder busses and standby busses on a Loss of Offsite Power (LOOP) when the CT-4 transformer is required to supply emergency power loads. The fan failure can occur due to a loss of the common power source or due to an independent failure.
The CT-4 transformer is the dominant heat source for the Unit 1 and 2 Blockhouse. A room heat-up evaluation of the maximum required electrical loads on CT-4 for a three unit LOOP showed that a single CT-4 transformer room fan is sufficient to prevent a failure of the Unit 1 and 2 Blockhouse components for outside ambient temperatures up to 95°F. The Unit 3 Blockhouse accident scenario was not modeled because it is bounded by the Unit 1 and 2 Blockhouse accident scenario.
The risk-significant deficiency for the 230 kV Relay House is comprised of a failure of the relay house HVAC units to provide sufficient room cooling to the 230 kV switchyard power controlled breakers' (PCBs) relaying. For the risk analysis it was conservatively assumed that power to the HVAC units is failed on a loadshed; however, power to the HVAC units would be restored by procedure. The PCB relaying failures lead to a loss of the Keowee overhead path and prevent restoration of offsite power through the 230 kV switchyard. A room heat-up evaluation showed that the HVAC units are not required to provide room cooling for outside ambient temperatures less than 90°F for the risk analysis mission time.
These deficiencies were determined to have a low risk impact when compared to a system without deficiencies (e.g., independent power supply, not constrained by outside temperature).
The low risk impact is due to the reliability of the blockhouse ventilation fans, the capability of the running train of blockhouse fans to automatically restart after an initial loadshed and the small exposure period for the more severe cases involving outside ambient temperatures in excess of 90°F and 95°F.
ADDITIONAL INFORMATION
This event was discovered as an extent of condition assessment from a similar ONS event. This previous event questioned the impact on the qualification of breakers in containment from potential increasing ambient temperatures following an Standby Shutdown Facility (SSF) event.
This event was documented in LER 269/2011-06, Pressurizer Heater Capacity Non-Compliant with Technical Specification 3.4.9. In addition the corrective action program data for the last five years was also reviewed. Other than this previous event documented in LER 269/2011-06, no other similar events were identified.
Energy Industry Identification System (EllS) codes are identified in the text as [XX]. This event is considered not reportable under the Equipment Performance and Information Exchange (EPIX) program. No component was selected because no specific equipment failures occurred.
There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.
References:
1) Duke Energy letter from T. Preston Gillespie to Victor McCree (NRC), ONS Standby Shutdown Facility and Bus Duct Studies, dated January 26, 2012 2) Duke Energy letter from T. Preston Gillespie to Victor McCree (NRC), Standby Shutdown Facility Design Review Project Plan and Schedule, dated February 17, 2012 3) NRC letter from Victor McCree to T. Preston Gillespie (Duke Energy), Confirmatory Action Letter - Oconee Nuclear Station, Units 1, 2, and 3 Commitments to Perform a Comprehensive Standby Shutdown Facility Design Review and an Evaluation of Modifications / Procedure Changes to Reduce the Risk of Bus Duct Faults, dated March 6, 2012 4) NRC letter from Victor McCree to T. Preston Gillespie (Duke Energy)Closure of Confirmatory Action Letter Item — Oconee Nuclear Station, Units 1, 2, and 3, Commitment to Perform a Comprehensive Evaluation of Modifications/Procedure Changes to Reduce the Risk of Bus Duct Faults, Dated August 16, 2012 Dear Mr. McCree:
In References 1 and 2, Duke Energy communicated to the Nuclear Regulatory Commission (NRC) its commitment to perform a comprehensive design, licensing, and operational review of the Oconee Nuclear Station (ONS) Standby Shutdown Facility (SSF) and to evaluate modifications and/or procedure changes to reduce core damage frequency associated with bus duct faults. The NRC confirmed these commitments, identifying them as Item 1 and Item 2 respectively in Confirmatory Action Letter, CAL 2-12-001 (Reference 3). The due date for Item 1 was March 29, 2013 and the due date for Item 2 was July 12, 2012.
www.duke-energy.com U.S. Nuclear Regulatory Commission April 8, 2013 Page 2 Duke energy provided a letter to the NRC, dated July 12, 2012 that described actions taken to satisfy the bus duct evaluation (Item 2 of the CAL). These actions were deemed acceptable to the NRC, and communicated to Duke by letter dated August 16, 2012 (Reference 4).
Duke Energy completed the comprehensive review of the SSF and its associated final report (Item 1 of the CAL) on March 28, 2013, which was prior to the CAL due date of March 29, 2013.
Therefore, as requested in the CAL, this letter provides notification that the SSF Review is complete, and as such, is ready for the NRC closure process to be initiated.
This letter contains no new regulatory commitments.
If there are any questions regarding this submittal, please call David Haile of ONS Regulatory Affairs at (864) 873-4742.
Sincerely, Scott L. Batson Vice President Oconee Nuclear Station U.S. Nuclear Regulatory Commission April 8, 2013 Page 3 cc:
Mr. John P. Boska, Project Manager (By electronic mail only) U. S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation One White Flint North, M/S O-8G9A 11555 Rockville Pike Rockville, MD 20852 Mr. Jonathan Bartley U. S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, Georgia 30303-1257 Mr. Ed Crowe Senior Resident Inspector Oconee Nuclear Site Ms. Susan E. Jenkins, Manager Radioactive & Infectious Waste Management Division of Waste Management South Carolina Department of Health and Environmental Control 2600 Bull St.
Columbia, SC 29201