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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.

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 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 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.

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.

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.