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At 1341(EST), on 01/07/20, it was discovered all trains of the Chilled Water System were simultaneously Inoperable; therefore, this condition is being reported as an eight-hour, non-emergency notification per 10 CFR 50.72(b)(3)(v)(D). During this event, the Bravo train chiller was operating in a maintenance run and the temporary chiller was available and placed in service promptly to restore the safety function. The control room area cooling safety function was restored at time 1435 (EST) when one required train was declared Operable. There was no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified.

• * * RETRACTION ON 1/23/2020 AT 1718 EST FROM JERRY COLLIER TO THOMAS KENDZIA * * *

The purpose of this notification is to retract a previous report made on January 7, 2020, at 1909 EST (EN#54465). A subsequent evaluation determined that the Bravo train chiller, which was running at the time of the event, would be able to perform its safety function and was operable at the time of the event. Therefore, there was no loss of safety function. The NRC Senior Resident Inspector has been notified. Notified R2DO (Coovert).

On 3/29/2012 Duke Energy identified that unanalyzed conditions exist for SSF mitigated events since associated thermal and hydraulic analyses do not consider all initial operating conditions, especially lower operating modes and lower decay heat. Specifically there are four (4) conditions where the SSF is not currently analyzed: 1. SSF operating at less than 525 degrees F and less than normal operating pressure (approximately 2155 psig), 2. SSF operation before four (4) Effective Full Power Days (EFPDs), 3. SSF reactor coolant make up at low Reactor Coolant System (RCS) pressure. 4. A reactor trip from less than 85 percent power and less than 579 degrees F. On 4/4/2012, an immediate determination of operability concluded that for the first three (3) conditions the SSF was operable but degraded/nonconforming (OBDN). For the 4th issue, there was reasonable assurance that 1% delta k/k shutdown margin would be maintained if T average. remained above 500 degrees F. Based on a lack of analysis and an increased likelihood of reducing T average. below 525 degrees F during a 72 hour event, the SSF was declared OBDN with a separate operability determination required to validate the Unit 3 power coastdown and end of life T average. reduction analysis. Until additional analysis is performed, the SSF is inoperable on any unit where the power level is reduced below 85 percent. A second operability determination for Unit 3 concluded that the SSF will maintain greater than or equal to 525 degrees F with an initial power level of 70 percent and a 570 degree F T average. The SSF will be declared inoperable on Unit 3 if power is reduced to less than 70 percent. Seventy percent was chosen as a conservative value to ensure the unit stayed inside the bounds of existing analyses. Unit 3 is currently at approximately 85 percent power and reducing power at approximately 1 percent per day in preparation for the Unit 2 end of core 26 refueling outage. For Unit 3, the SSF is OBDN based on preliminary calculation results. On 4/5/2012, due to a worsening component cooling water system leak on Unit 2, it was necessary to bring the unit down to Mode 3 to implement repairs. Upon down power, when Unit 2 transitioned below 85 percent power, the ability of the SSF to perform its design function, in consideration of the information above, could not be confirmed and the SSF was declared inoperable for Unit 2. Currently, there is no conclusive information that would support SSF operability while Unit 2 is below 85 percent power. As such, this event is being conservatively reported under 50.72(b)(3)(ii)(B), 'The nuclear plant being in an unanalyzed condition that significantly degrades plant safety.' Due to their current power levels, this condition does not affect Units 1 and 3. Initial Safety Significance: Until confirmed by analysis, the lack of decay heat may result in an initial over cooling of the RCS and potentially an interruption of natural circulation or inadequate shutdown margin. Consequently, the SSF was declared inoperable. Corrective Action(s): Additional analyses are being completed to reestablish SSF operability to bound the unanalyzed entry conditions. The NRC Resident Inspector has been informed.

• * * UPDATE FROM DEAN PORTER TO JOHN KNOKE AT 1403 EDT ON 04/13/12 * * *

Update to ENS Notification Number 47810: ENS Notification number 47810 identified an unanalyzed condition for Oconee Unit 2. This update includes Oconee Units 1, 2 and 3 because the condition of the SSF affects all three units during certain plant conditions. The Standby Shutdown Facility (SSF) serves as a backup for existing safety systems. Currently the events that rely upon the SSF for mitigation are unanalyzed on Units 1 and 2 when the power level is reduced below 85 percent and on Unit 3 when the power level is reduced below 70 percent, or for any unit operating with less than 4 Effective Full Power Days (EFPDs) at 100% full power since its most recent shutdown. Since the SSF events are unanalyzed for these conditions and until further analysis and evaluation can be completed, Duke Energy is conservatively calling the SSF inoperable when in these conditions. For these conditions, this event is being reported for Oconee Units I, 2, and 3 under 50. 72(b )(3)(ii)(B), that is, the nuclear plant being in an unanalyzed condition that significantly degrades plant safety. Based on current analyses, the SSF will be declared inoperable, and action statements entered on Units 1 or 2 if power is reduced below 85% power and on Unit 3 if power is reduced below 70% power, or for any unit operating with less than 4 EFPDs since its most recent shutdown. Initial Safety Significance: Until confirmed by analysis or evaluation, the lack of decay heat may result in an initial over cooling of the RCS and potentially fail the various acceptance criteria of the events required to be augmented by the SSF. Consequently, the SSF will be declared inoperable for the conditions stated above. Unit 3 is being shut down for a routine refueling outage. Units 1 and 2 continue to operate at 100% power with no problems. Corrective Action(s): Additional evaluations are being completed to establish whether the existing analyses are applicable to the conditions outside of which they were performed and, if not, there is a reasonable assurance that successful mitigation can be accomplished with the existing procedure. If not, further licensing action may be required. These evaluations were initiated on April 4, 2012, and are ongoing. The licensee has notified the NRC Resident Inspector. Notified the R2DO (Kathleen O'Donohue).

Event: At 2029 hours, Oconee Unit 3 was in Mode 5 for a refueling outage. The administrative controls which comprise one train of Low Temperature Overpressure Protection (LTOP) were not yet in service, but a dedicated LTOP operator was in place as a Tech Spec required compensatory measure. Instrument Technicians performing a procedure erroneously isolated the low range RCS pressure indication, which removed the Power Operated Relief Valve (PORV) from service while required for LTOP. This placed Unit 3 in a one-hour action statement per Technical Specification 3.4.12, Condition H. Initial Safety Significance: This is potentially a condition which could have prevented the fulfillment of a safety function (LTOP). Both the automatic PORV train and the Admin Control train were inoperable per Technical Specifications. However, a dedicated LTOP operator was in place meeting the compensatory measures requirement for continued operation per TS 3.4.12, Condition F related to the Admin Control requirement. No event occurred while in this condition which would challenge the LTOP function. Corrective Action(s): Operator at the controls recognized the loss of indication and contacted the Instrument Technicians. They verified their error and returned the instrument to service within one hour. The licensee will notify the NRC Resident Inspector.

• • • UPDATE FROM R.P. TODD TO J. KNOKE AT 16:34 ON 06/29/06 ***

At 0215 EDT on 5-2-06, Oconee made an ENS notification to report a condition which could have prevented the fulfillment of a safety function, specifically Low Temperature Overpressure Protection (LTOP). The LTOP Technical Specification (TS) 3.4.12 requires that a) the Power Operated Relief Valve (PORV) be operable, and b) administrative controls be in place to assure greater than 10 minutes are available for operator action to mitigate an LTOP event. As stated in the initial report, during shutdown for a refueling outage instrument technicians erroneously isolated the low range RCS pressure indication, which made the PORV inoperable for automatic operation while required for LTOP. In addition, the administrative controls were not yet fully established. However, if a specific sub-set of administrative controls are in place, the TS allows credit for a dedicated LTOP operator as a compensatory measure. Upon further review, Oconee has confirmed that a) the PORV remained available for manual initiation by the dedicated LTOP operator, and b) the dedicated LTOP operator and associated sub-set of administrative controls were in place. This satisfied the required actions of TS 3.4.12 Condition F and assured that an LTOP event could be mitigated. Therefore, Oconee concludes that this event did not constitute a potential loss of safety function. Actions were taken to restore the instrument alignment to restore automatic actuation capability for the PORV within the required action time per TS 3.4.12. Therefore there was no operation in a condition prohibited by Tech Specs. As a result, the event is not reportable under 50.72 or 50.73 and the ENS notification is hereby retracted. Corrective Action(s): As stated above, the instrument alignment was restored to return the PORV to an operable status for automatic actuation. Subsequently, the full set of administrative controls were established. As shutdown continued, the unit exited the LTOP region. The licensee notified the NRC Resident Inspector.

The Keowee Hydro Units provide the emergency power source for the Oconee Units. At 0357 on 10/19/04, Keowee Unit 1 (and the overhead emergency power path) was declared inoperable in order to perform scheduled maintenance. At 1730, Keowee Unit 2 (and the underground emergency power path) was declared inoperable due to the loss of breaker control power associated with Keowee Unit 2 auxiliaries. At 1811, as required by Technical Specification 3.8.1 condition I, both standby buses were energized from a Lee Combustion Turbine via an isolated power path. At 1828, a Operability Verification of Keowee Unit 2 to the overhead emergency power path was performed with all acceptance criteria met. At 1904, Keowee Unit 2 to the overhead emergency power path was declared operable. Technical Specification 3.8.1 condition I was exited. Initial Safety Significance: Between times 1730 to 1811, both on site emergency power paths were inoperable. During this time period a condition existed that could have prevented the fulfillment of the safety function of systems that are needed to mitigate the consequences of an accident. Corrective Action(s): At 1828, an Operability Verification of Keowee Unit 2 to the overhead emergency power path was performed with all acceptance criteria met. At 1904, Keowee Unit 2 to the overhead emergency power path was declared operable. Technical Specification 3.8.1. condition I was exited. A team is established to investigate the loss of breaker control power associated with Keowee Unit 2 auxiliaries. The licensee will notify the NRC Resident Inspector.

• * * RETRACTION FROM TODD TO HUFFMAN AT 1428 EDT ON 10/26/04 * * *

The Keowee Hydro Units (KHUs) provide the emergency power source for the three Oconee Units. On 10/20/04 at 0135 Oconee made a notification to the NRC of a loss of safety function. Reference Event Number: 41140. The event was due to an unexpected failure on KHU 2. At the time of the event, KHU 1 was out of service for planned maintenance and was in the early stages of testing for return to service. Each KHU can be aligned to either an overhead or an underground power path to supply emergency power to Oconee. At 1730, KHU 2 and the underground emergency power path were declared inoperable due to the loss of breaker control power associated with KHU 2 auxiliaries while aligned to the underground path. By the Oconee Technical Specification 3.8.1 Bases, this breaker (ACB-8) is required to be operable in order to consider KHU 2 operable while aligned to the underground path. Since KHU 2 did not meet the TS configuration, and since KHU 1 testing was not complete, both trains were inoperable and the event was reported as a loss of safety function. At 1811, as required by Technical Specification 3.8.1 condition I, both standby buses were energized from a Lee Combustion Turbine via a dedicated power path. KHU 2 was re-aligned to the overhead path and its associated auxiliary power breaker (ACB-6). The event was terminated at 1904 when KHU 2 was declared operable to the overhead emergency power path after completion of an Operability Verification test. Justification for conclusion that there was no loss of safely function while KHU 2 was declared inoperable (approximately one hour duration): KHUs are capable of 'black start', i.e. starting and operating on battery power alone. Therefore, if there had been an event resulting in an emergency start signal during the one hour interval of vulnerability, KHU 2 would have been able to start and operate for approximately one hour with the loss of control power to ACB-8. Operations procedures include guidance to close breaker ACB-8 manually using a maintenance closure handle. These steps would have been performed approximately 30 minutes into the postulated event. Based on the results of testing and troubleshooting of ACB-8, Oconee has concluded that ACB-8 could have been closed manually and, once closed, would have successfully provided auxiliary power for the duration of any event. Thus Oconee concludes that KHU 2 should be considered available, with reasonable expectation for performing its safety function during this event. Therefore Oconee has concluded that this event was not a reportable loss of safety function and Event Number 41140 is withdrawn. The resident inspector and R2DO (Bonser) have been notified.

The licensee reported that both digital trains of the automatic feedwater isolation system were declared inoperable when one channel's input failed to zero and about the same time the second channel showed a shift in readouts which was fairly significant that they also declared it inoperable. These two channels have a shared neutral bus and it is believed that there may be a loose connection somewhere in the circuit. They are currently troubleshooting the problem. Plant will initiate a unit shutdown if the problem cannot be corrected by 0600 on 03/01/04 (12-hour LCO A/S). The NRC Resident Inspector will be notified.

• * * * UPDATE ON 03/23/04 @ 1348 BY RANDY TODD TO C. GOULD * * * * RETRACTION

Update- FNS 40557, dated 3-1-04, addressed an apparent loss of safety function on Oconee Unit 2. Specifically, the Automatic Feedwater Initiation System (AFIS) was considered inoperable, and therefore unable to perform its required safety functions. During that event, Operations shift personnel conservatively declared AFIS Channel 2 inoperable because it demonstrated an unexpected shift in readout concurrent with the failure of Channel 1. Subsequently Duke power concluded that Channel 2 remained capable of performing its safety function despite the shift in readout, which was in the conservative direction. Therefore, Duke Power has concluded that there was no reportable loss of safety function associated with this event and hereby retracts ENS notification 40557. Duke Power notes that, during the event, both channels were intentionally disabled for a brief period as a planned part of the repair activity. This was done to prevent the possibility of a spurious actuation during the repair. However, per guidance in NUREG 1022, removal of both trains of a safety function as part of a planned activity which results in entry into a Tech Spec action statement is not reportable. Initial Safety Significance: Operations declared TS 3.3.13 Condition B, which allowed 12 hours to come to Mode 3. Repairs were completed prior to initiation of shutdown. Corrective Action(s): A loose wiring connection was repaired. The NRC Resident Inspector was informed.