05000269/LER-2014-001
| Oconee Nuclear Station, Unit 1 | |
| Event date: | 03-28-2014 |
|---|---|
| Report date: | 05-27-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 2692014001R00 - NRC Website | |
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2014 01 00
EVALUATION:
BACKGROUND
Keowee Hydroelectric Station (KHS) The KHS [EIIS: EK] contains two units each rated 87,500kVA which generate 13.8kV. Upon a loss of power from the Oconee Nuclear Station (ONS) generating unit and 230kV switchyard [EIIS: EA], emergency power is supplied from both KHS units through two separate and independent routes, i.e., an overhead and underground path. The underground path consists of an approximate 4000 foot underground 13.8kV cable [EllS: CBL] feeder to 12/16/20 MVA Transformer [EIIS: XFMR] CT4 that supplies the redundant 4160V standby power buses [EIIS: BU]. The underground emergency power path is arranged with double air circuit breakers [EIIS: 52] so that it can be connected to either KHS generator bus.
The underground path is aligned at all times to one hydroelectric generator [EIIS: GEN] on a predetermined basis and is automatically energized along with Transformer CT4 whenever that generator is in service in either emergency or normal mode. The underground path and associated transformer are sized to carry full engineered safeguards auxiliaries of one unit plus auxiliaries for safe shutdown of the other two units. The emergency electric power system [EIIS: EB] provided for each nuclear generating unit possesses certain inherent design features which improve its reliability over limited capacity split-bus arrangements usually provided in nuclear power plants.
The KHS to transformer CT-4 13.8kV circuit consists of six (6) single 750 kcmil conductor cables with 15kV nominal ethylene propylene rubber (EPR) insulation and bronze tape shield/armor with an overall jacket. There are two conductors per phase. The cables were procured as QA-1 and were constructed, tested and qualified to applicable industry standards. The conductor cable manufacturer is The Okonite Company.
The KHS transformer CT-4 13.8kV circuits are installed undergound in trenches/duct banks that are QA-1 concrete steel reinforced structures designed to withstand seismic events and windborne missiles. The undergound cable trench is designed and sealed to preclude water entry. In addition, passive drains [EIIS: DRN] are provided in the trenches as a contingency. Some instrumentation and control cables for Keowee operation are configured in the same trench as the power cables.
The basic design criterion for the electrical portion of the emergency electric power system of a nuclear unit, including the generating sources, distribution system, and controls is that a single failure of any component, passive or active, will not preclude the system from supplying emergency power when required. Special provisions have been employed to accomplish this which include a double bus-double breaker distribution system, redundant circuit breaker trip coils and circuits, diverse protective relaying for each circuit breaker, redundant load shedding and transfer logic equipment, physical separation and other features.
Protected Service Water (PSW) System The PSW system is designed as a standby system for use under emergency conditions. The PSW system provides added "defense in-depth" protection by serving as a backup to existing safety systems and as such, the system is not required to comply with single failure criteria. The PSW system is provided as an alternate means to achieve and maintain safe shutdown conditions for one, two or three units following certain postulated scenarios.
The PSW system reduces fire risk by providing a diverse power supply to power safe shutdown equipment in accordance with the National Fire Protection Association (NFPA) 805 safe shutdown analyses. The PSW system requires manual activation and can be activated if normal emergency systems are unavailable.
The function of the PSW system is to provide a diverse means to achieve and maintain safe shutdown by providing secondary side decay heat removal, RCS [EIIS: AB] pump seal cooling, RCS primary inventory control, and RCS boration for reactivity management following plant scenarios that disable the 4160V essential electrical power distribution system [EIIS: EB]. The PSW electrical system is designed to provide power to PSW mechanical and electrical components as well as other system components needed to establish and maintain a safe shutdown condition. The system is designed with adequate capacity and capability to supply the necessary loads and is electrically independent from the station electrical distribution system. No credit is taken in the safety analyses for PSW system operation following design basis events. The KHS to PSW 13.8kV circuits are installed in the same trench as the KHS to CT-4 cables.
EVENT DESCRIPTION
As part of the 2014 Component Design Basis Inspection (CDBI), the NRC raised issues regarding potential non- compliance with 10 CFR 50.55a(h)(2) and a potential unanalyzed condition. The concern involved the potential impact that faults on the 13.8kV cables from the KHS to CT4 and from KHS to PSW could have on nearby cables, e.g., Instrumentation and Control (I&C) cables that could either, (1) prevent the remaining Keowee unit aligned to the overhead path from a successful emergency start or (2) disable operation of the Keowee unit aligned to the overhead path after an emergency start.
The following power and control cables are routed in Trench 3:
- 13.8kV power cables from KHS to transformer CT-4,
- 13.8kV power cables from KHS to the Protected Service Water (PSW) switchgear,
- 4.16kV cables from switchgear 1TC to transformer CX,
- supervisory cables from ONS to KHS for both KHS units,
- one train of KHS emergency start cables, and
- breaker Keowee Power Feed (KPF) control.
The power cables in Trench 3 include a "Metallic Shield/Armor Tape" consisting of non-magnetic bronze tape. The functions of the bronze tape are to (1) confine the electric field to inside the conductor insulation, (2) provide a grounded path for charging and fault currents and (3) provide mechanical protection. The inspection also questioned whether this bronze tape can be credited as armor when evaluating failures per IEEE-279, Section 4.2 and thus whether credit can be taken for its single failure mitigation properties.
For the power cables from the KHS to transformer CT-4, and from KHS to PSW, Duke Energy considered a three- phase bolted fault at the end connections and a phase-to-ground fault as bounding conditions. This analysis has been challenged in that Duke Energy should have considered additional faults to occur anywhere along the cable route, such as a phase-to-phase fault, a three-phase fault, a high impedance ground fault, a double phase-to- ground fault, and a three phase-to-ground fault.
Although Duke Energy has evaluated the 13.8kV cable design and considers the design to be robust, further cable analysis and/or testing is being conducted. The results of planned power cable analysis and/or testing will provide the information necessary to update the licensing basis documentation. This condition is being conservatively reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
CAUSAL FACTORS
At the time of report submittal, a causal evaluation was not complete. This report will be supplemented, as necesasry, following the conclusion of that evaluation.
The preliminary apparent cause of this event is a missed opportunity to update the licensing basis in 2002, when the underground trench and 13.8kV power cables were modified, and again in 2013 when the PSW power and control cables were placed in service.
CORRECTIVE ACTIONS
Immediate Corrective Action:
1. An Immediate Determination of Operability (IDO) was performed and concluded the existing system design is adequately robust to address circuit faults.
Subsequent Corrective Actions:
1. A Prompt Determination of Operability (PDO) was performed and concluded the existing system design is adequately robust to address postulated circuit faults.
2. Completed a vendor analysis of the 13.8kV cable design.
Planned Corrective Action:
1. Complete the causal evaluation and implement required corrective actions.
SAFETY ANALYSIS
An operability determination concluded that 13.8kV power cables were operable, but due to the deficient licensing basis, this issue constituted a nonconforming condition in that the basis for treatment of bronze tape as armor is not sufficiently documented. The results of planned power cable testing and analysis will provide the necessary information needed to update the licensing basis documentation.
Duke Energy has evaluated the cable design and as-installed configuration and considers the design to be safe.
This is substantiated by: (1) an independent analysis that concluded that underground cable bronze tape is functionally equivalent to the mechanical properties of armored-cable; (2) the conclusion that the current design meets industry standards and is supported by Regulatory Guidance; (3) a review of Operating Experience (OE) and risk insights which support the acceptability of the cable design; and (4) feedback by industry subject matter experts who validated the acceptability of the cable design.
Duke Energy used a risk-informed approach to determine the risk significance associated with the condition. The analysis results concluded that the postulated single failure of the Keowee emergency power supply represents an insignificant impact to plant risk.
ADDITIONAL INFORMATION
A review of the Oconee Nuclear Station LERs over the last five (5) years did not reveal a similar event with the same or similar cause.
An industry OE search was performed to determine if any OE exists for 3-phase cable faults or phase-phase cable faults along a power cable and determine applicability of these failures to the Trench 3 power cable design. This OE search started with the following parameters: Database: INPO ICES; Documents: Search All Document Types; Time Frame: Anytime; Keywords: "medium voltage cable fail." The searches of the INPO and EPRI data identified 390 cable failures. These failures were reviewed against the KHS undergound path design. Based upon the information available no cable failures were identified to be applicable to the KHS underground path design.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX]. This event is not considered 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.