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On June 28, 2016, the 3C Reactor Building Cooling Unit (RBCU) was discovered to be running in reverse when operating in the low speed mode and Technical Specification (TS) 3.6.5, Condition B, was entered (only low speed is applicable to TS operability). The RBCU was restored to operable on June 29, 2016. It was determined that the inoperability was caused by a wiring error (rolled leads) that occurred during the preceding outage. This provides firm evidence that the RBCU was inoperable from the time Mode 2 was entered (May 15, 2016) until RBCU operability was restored on June 29, 2016. This duration exceeds the 7.5 day cumulative Completion Time allowed by TS 3.6.5. During start-up from the outage, the plant entered Modes 2 and 1 with the RBCU inoperable, which is prohibited by TS 3.0.4. These conditions constitute plant operation prohibited by Technical Specifications and are reportable as an LER per 10 CFR 50.73(a)(2)(i)(B).

The cause was determined to be personnel error by the technicians' improper application of the configuration control process. The technicians received remedial training and a procedure change was initiated to enhance configuration control aspects in the maintenance procedure.

The safety function associated with TS 3.6.5 was available during the inoperability of the 3C RBCU.

On January 31, 2015, Oconee Unit 3 was operating at 100% power in MODE 1 when Control Room operators observed that Main Feedwater flow indicators were oscillating outside of normal parameters. The Control Room supervisor made the decision to manually trip Unit 3 at 1431 hours due to erratic feedwater operation and increasing RCS pressure. A subsequent investigation determined the feedwater flow oscillations were caused by a subcomponent failure of the electrical to pneumatic converter (E/P), 3FDW EP 0007, to properly control feedwater flow for Main Feedwater Control Valve (MFCV) 3FDW-32.

This event was reported as a 4-hour notification to the NRC on January 31, 2015, in Event Notification (EN) number 50781 under 10 CFR 50.72(b)(2)(iv)(B) - Reactor Protection System (RPS) Actuation - Critical. The event is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an actuation of the RPS.

On December 12,T 2008 two containment isolation valves in a Post Accident Liquid Sampling line in Unit 3 were declared inoperable.T Technical Specification Limiting Condition for OperationT (LCO)T 3.6.3 Conditions A and B were entered immediately and power was removed from two normally-closed solenoid-operated valves in the same line to meet the required actions of the LCO.T However,T the unit has been operated since the valves were installed in 1996 until discovery of the inoperable. condition.'T Consequently,T the appropriate Technical Specification required actions were not met within the allowed outage time permitted, and the unit was operated in a condition prohibited by Technical Specifications.

The inoperable condition was caused by the use of soft seat materials which are not qualified for the elevated pressure and temperatures they could be exposed to in post-accident sampling service.T The planned corrective action is to replace the soft-seated valves with a hard-seated design.

This event is considered to have no significance with respect to the health . and safety of the public.

Unit 3 entered its scheduled end-of-cycle 20 refueling outage on April 26, 2003. On May 2, 2003, a .

visual inspection of the bare reactor vessel head (RVH) was performed. Results of the visual inspection revealed two (2) control rod drive (CRD) mechanism (CRDM) nozzles that were suspected of leakage.

Of these, CRDM No. 4 was observed to contain a very thin white coating on the nozzle and CRDM No. 7 appeared to have a small accumulation of boron on the head adjacent to the annulus region. In addition, approximately 6 to 8 CRDMs could not be visually inspected as they were masked by deposits from a Component Cooling (CC) system leak above the RV head.

Subsequent evaluation of the prior refueling outage RVH inspection videotape showed that the CRDM No. 7 deposits were not associated with a new leak but rather were remnants from a prior refueling outage leak and repair campaign where the boron residue had not been completely removed from the RVH during the wash down process. The CRDM No. 4 boron deposit appeared fresher, exhibited characteristics similar to prior RVH leaks and as such, was conservatively identified as a leak. The apparent root cause of the nozzle leak is primary water stress corrosion cracking.

This RVH will be retired from service and replaced with a new RVH prior to unit restart. For this event, the overall safety significance of this event was minimal and there was no actual impact on the health and safety of the public.

At 1512 on March 6, 2016, Oconee Nuclear Station (ONS) Unit 1 experienced a Reactor Trip, initiated by an electrical bushing failure on the Unit 1 Main Step-up Transformer that resulted in a transformer fire. The plant response to the trip was evaluated, and was found to be acceptable. The reactor trip is a RPS actuation and requires an LER per 10 CFR 50.73(a)(2)(iv)(A).

Due to the transformer fire a "Notification Of Unusual Event" was declared at 1520. The fire eventually led to a transformer overhead line failure that caused a switchyard bus lockout and the loss of one emergency power path for the site. The second emergency power path and multiple offsite sources remained operable during the event. The emergency classification was upgraded to an "Alert" condition at 1658, upon determining that the fire had an impact on safety related equipment.

The 4-hour notification related to this reactor trip was bounded by the notifications made for the Emergency Classifications described above (See Event Notification 51770).

Post trip conditions and Emergency event conditions were controlled and maintained within the allowances of Technical Specifications with no personnel injuries or challenges to other safety system actuations. The transformer deluge system activation along with the response of onsite and offsite fire teams promptly contained the fire.

For the 13.8kV Keowee Hydroelectric Station (KHS) power cables contained in the underground trench to both transformer CT-4 and to the Protected Service Water (PSW) system, single failure analysis considered a three-phase bolted fault at the end connections and a phase-to-ground fault as bounding conditions. This single failure analysis was questioned in a recent NRC inspection as not being bounding. Duke Energy evaluated the issue and concludes the cables to be operable but nonconforming with the current licensing basis due to a lack of documentation. Additional cable analysis and/or testing is needed. The results of planned power cable analysis and/or testing will provide the necessary information needed to update the licensing basis documentation. This condition is conservatively being 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.

The preliminary apparent cause of this event is a missed opportunity to update the licensing basis in 2002 when the underground trench and cables were modified, and again in 2013, when the PSW system 13.8kV power cables and control cables were placed in service. Duke will complete the causal evaluation and as necessary, supplemented the report following the conclusion of that evaluation. 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.

On September 4, 2013, the Keowee Hydroelectric Station Unit 2 (KHU-2) experienced an emergency lockout that rendered it inoperable. The Keowee Hydroelectric Station is one of Oconee Nuclear Station's (ONS) emergency power sources. Following troubleshooting activities it was determined that from August 17, 2013 until September 4, 2013, KHU-2 had previously been susceptible to this type of emergency lockout resulting in a condition that exceeded the allowed outage time given in the plant's technical specifications.

An investigation of the event determined that the cause of the event was vibration from a Governor Oil pump that caused relay 86E2X to "chatter." The chatter generated a spurious signal that activated the emergency lockout relay.

A completed corrective action included the replacement of relay 86E2X and relocating it to an environment not susceptible to vibration.

This event is reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by the plant's technical specifications. This event is considered to have minimal safety significance and no consequences with respect to the health and safety of the public.

On 2/6/2013, with all three Oconee Units at 100 percent power, Oconee Nuclear Station (ONS) personnel concluded that emergency power equipment could be adversely impacted by a licensee identified original design issue involving inadequate analysis of electrical equipment heat loads and weaknesses in the Heating Ventilation & Air Conditioning (HVAC) system design.

The investigation determined that the principle causes were associated with inadequate and incomplete inputs and methods in development of HVAC systems during original plant design and Blockhouse, and the 230kV Switchyard Relay House susceptible to single failures.

Compensatory measures were put into place. Corrective actions include modifications to resolve the inadequacies with the original plant design issues.

This event is reportable under the following criteria: Operations Prohibited by Technical Specifications, Unanalyzed Condition, Event that could have prevented Fulfillment of a Safety Function, and Common Cause.

On 11-28-97, during a Unit 1 refueling outage, 1MS-156, - "lB MS Line Atmospheric Vent Block Valve" was replaced with a flexible wedge gate valve design. - On 7-8-02, as part of a project to review manual valves for pressure locking (PL) vulnerability, Engineering identified that 1MS-156, was potentially susceptible to PL. - Valves in this application on other trains/units are solid wedge design. - On 7-10-02 a vent line was installed on the valve bonnet, ending the potential for PL.

At 1600 hours on 7-18-02, with Unit 1 operating in Mode 1 at 100 per cent, 1MS-156 was determined to have been inoperable from 11-28-97 until 7-10-02 due to the potential for PL to prevent opening during postulated events. - At 1843 hours on 7-18-02, an 8-hour Non-Emergency Notification was made to the NRC under 10 CFR 50.72(b)(3)(v).

The root cause of this event is deficient design, due to an unanticipated system interaction, which resulted from a lack of awareness of pressure locking as a concern for manual valves. - This event is considered to have no significance with respect to the health and safety of the public.

On 03/07/2002 Site Engineering concluded that ambient heat losses from the Pressurizer (PZR) on all three Oconee units exceeded the capacity of PZR heaters powered from the Standby Shutdown Facility (SSF). 9 Therefore, the ability to maintain a steam bubble in the PZR for system pressure control during SSF events (Fire, Flood, and Security) might be lost and the existing procedural guidance would not fully address some expected conditions. 9 Normal system operation or other emergency scenarios were not affected because additional PZR heaters are available in those conditions.

PZR heaters support operability of the SSF Auxiliary Service Water System (SSF ASW). 9 Per Technical Specification 3.10.1, the SSF ASW was declared inoperable for all three Oconee units. 9 Abnormal Procedures were subsequently revised to provide adequate guidance and all on-coming Operators received simulator training on the new guidance prior to accepting shift duties.

Additional corrective actions are planned.

The apparent cause was a fundamental lack of appreciation for the potential impact of PZR insulation deficiencies. 9 This event is considered to have no significance with respect to the health and safety of the public.