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At approximately 1400 CST all commercial and FTS 2000 telephone communications to and from Byron Station was lost due to a fiber optic cable cut 20 miles North of the plant, near Rockford, Il. Licensee can contact the NRC Headquarters Operation Officer via a Control Room cell phone. NRC Headquarters Operation Officer is unable to call the site using the cell phone because incoming calls are routed through Rockford, Il. Licensee stated that the line problem is being worked on. NRC Resident Inspector has been notified of this event by the licensee.

• * * Update 12/29/04 at 2139 EST by Koeplin to MacKinnon * * *

Commercial and FTS telephone service returned to service. Licensee will notified the NRC Resident Inspector. R3DO (C. Lipa) & NRR EO (Pat Hiland) notified.

At 0836 the control room received a Unit 1B CV/SI pump room fire alarm. An operator reported smoke in the 1B SI pump room at 0840. An unusual event was declared at 0854 due to HU6, fire in the protected area not extinguished within 15 minutes. Sounds of arcing can be heard but flames have not been located. It is suspected that an RWST heater has failed in a pipe tunnel above the 1B SI pump room. The breaker for the RWST heater has been deenergized. The RWST heater pump has been deenergized. Offsite fire department assistance has been requested. The licensee is ventilating the area in order to perform a more thorough inspection of the area and determine the cause of the fire. Both Units1 and 2 remain at 100% reactor power. The NRC Resident Inspector, State and local officials have been notified by the licensee. The NRC has entered the Monitoring Mode at 1036. The NRC Operations Center notified the following organizations: HHS/FDA (V. Howard-King), USDA (Brown), EPA/NRC (Rawls) and DOE (Silva).

• * * UPDATE FROM MARCHIONDA TO KNOKE AT 12:49 EST ON 02/24/06 * * *

At 11:09 CST on 02/24/06, Byron terminated an Unusual Event that was declared at 08:54 CST today in accordance with HU6 due to a fire in the Protected Area not extinguished within 15 minutes. The RWST (Refueling Water Storage Tank) heater has been confirmed as the source of the smoke that caused the fire alarm. The heater has been deenergized and the area is being ventilated. The NRC exited the Monitoring Response Mode at 12:52 EST. The NRC Resident Inspector and State officials have been notified by the licensee. NRC made notifications to R3DO(Hills), IRD Managers (Leach, Blount, Wilson), NRR EO (Jung), NRR ET (Dyer), HQ PAO (Brenner), R4DO(Pick), DHS (Carledge), FEMA (Liggett), HHS/FDA( Miller), USDA (Byzustek), EPA/NRC (Rawls, Ribordy), and DOE (Silva).

Greater than 25% of the emergency sirens have failed due to a severe thunderstorm in the area. The licensee stated that 30 sirens had failed and that restoration activities were currently in progress. The licensee will notify the NRC Resident Inspector.

• * * UPDATE FROM J. KOEPLIN TO M. RIPLEY 0503 ET 07/18/06 * * *

As of 2317 CDT 07/18/06, the number of inoperable sirens fell below 25% of all sirens with 23 failed sirens not restored to service. Restoration activities remain in progress. The licensee will notify the NRC Resident Inspector. Notified R3 DO (Cameron)

At 1835 on March 25, 2008, Unit 2 experienced a Loss of Offsite Power. At 1849, an Unusual Event was declared. Station Auxiliary Transformer (SAT) 242-2, Phase C, experienced a Phase Overcurrent. This resulted in an isolation of the SAT, a loss of offsite power, and an automatic Fire Protection deluge of the SAT. No fire was confirmed. All equipment responded as required. The appropriate abnormal operating procedure was entered for event mitigation and is in progress. The Unit 2 Emergency Diesel Generators are supplying required safety-related power. Unit 2 remains at power. The plant is stable and all equipment is functioning as required. The licensee is assessing a cross-tie to Unit 1 which is unaffected by this event and in the refueling mode. The licensee notified the State and local authorities and the NRC Resident Inspector. Notified DHS (Greg Ray), FEMA (Erwin Casto), DOE (Sal Morrone), USDA (Timmons), HHS (Rick Turner), EPA/NRC (Petty Officer Thompson Report #866038).

• * * UPDATE BY SCOTT PURIN TO NRC AT 2230 EDT ON 3/25/08 * * *

The emergency buses on Unit 2 have been re-energized by offsite power supply sources via Unit 1 SAT. The Unit 2 EDGs have been secured and placed back in standby. The licensee is developing a recovery plan to restore a Unit 2 SAT to supply offsite power to Unit 2 loads. This will require such things as analyzing chemistry samples of the affected SAT transformers and assessing the protective relaying. In addition, some switchyard realignment will be necessary. The licensee is currently in a 72 hour LCO under Tech Spec 3.8.1 for the current electrical alignment. The NRC Resident Inspector is onsite and the NRC Region continues to monitor the situation.

• * * UPDATE FROM STEVE WIDOLFF TO HOWIE CROUCH AT 0022 EDT ON 3/28/08 * * *

At 2309 (hrs. CDT) on 3/27/08, Byron Unit 2 terminated an Unusual Event that was declared at 18:49 on 3/25/08, in accordance with EAL M.U.1 due to a loss of offsite power to transformers 242-1 and 242-2. Transformer 242-1has been energized from offsite power and is supplying Unit 2 ESF busses 241 and 242. The entry conditions for M.U.1 are no longer met. A press release will be issued. Transformer 242-2 is out of service and isolated for repairs. The licensee notified the NRC Resident Inspector. Notified R3DO (Hills), NRR EO (Ross-Lee), IRD (McDermott), DHS SWO (Doyle), FEMA (Liggett), DOE (Morrone), EPA (Allison), USDA (Timmon) and HHS (Dalziel).

On March 19, 2011, during the Byron Station Unit 1 refueling outage, it was determined that the results of planned ultrasonic (UT) examinations performed on two penetrations of the reactor vessel head would not meet the applicable acceptance criteria. Both require repair prior to returning the vessel head to service. These indications are not in the reactor coolant pressure boundary; however they are very near the toe of the J-groove weld. The examinations were being performed to meet the requirements of 10CFR50.55a(g)(6)(ii)(D) and ASME Code Case N-729-1, to ensure the structural integrity of the reactor vessel head pressure boundary. The UT examinations continue for the remaining head penetrations. All of the penetrations will be examined during the current refueling outage. Repairs are currently being planned and will be competed prior to startup. This is reportable pursuant to 10CFR50.72(b)(3)(ii)(A) since the as found indications did not meet the applicable acceptance criteria referenced in ASME Code Case N-729-1 to remain in-service without repair. The NRC Resident Inspector has been notified.

• * * UPDATE FROM BLAINE PETERS TO JOHN SHOEMAKER AT 0910 EDT ON 03/30/11 * * *

Ultrasonic examination (made on March 19, 2011) resulted in indications on two reactor head penetrations. As mentioned in Event Notification 46686, reactor vessel head penetrations (In-service inspection) ISI examinations were still in progress. On Wednesday, March 30, 2011, two additional Unit 1 reactor head penetrations were found to contain indications that will require repair prior to returning the reactor head to service. The indications on these two penetrations are within the reactor vessel head pressure boundary. The NRC Resident Inspector has been notified by the licensee. Notified the R3DO (Peterson).

At 1007 CDT on September 19, 2011, the Unit 2 Plant Process Computer (PPC) was removed from service for a planned replacement in the current Unit 2 Refueling Outage. The Unit 2 PPC feeds the Safety Parameter Display System (SPDS) used in the Main Control Room (MCR) and the Technical Support Center (TSC). The Unit 2 PPC also feeds the Emergency Response Data System (ERDS). The Unit 1 and Unit 2 PPCs also feed the Plant Parameters Display System (PPDS) used in the MCR, TSC and Emergency Operations Facility (EOF). Meteorological data will remain available. The dose assessment program will remain functional as the Unit 1 PPC will be capable of providing the necessary data through PPDS to run the program. The dose assessment program is not affected by the Unit 2 PPC being out of service. As compensatory measures, a proceduralized backup method to fax or communicate via a phone circuit applicable data to the NRC, TSC, and EOF exists. There is no impact on the Emergency Notification System (ENS) or Health Physics Network (HPN) communication systems. The new Unit 2 PPC is scheduled to be functional on September 25, 2011. However, based on the Mode Unit 2 will be in, this will limit the number of points that would provide usable data. The Unit 2 PPC will be tested as Mode changes occur. The Unit 2 PPC is planned to be declared functional by Mode 2. A follow-up ENS call will be made once the Unit 2 PPC is declared functional. The loss of SPDS and ERDS is a 'major loss of assessment capability' and is reportable under 10CFR50.72(b)(3)(xiii). The NRC Senior Resident Inspector and the State of Illinois (through the Illinois Emergency Management Agency Resident Inspector) have been notified of this ENS call.

• * * UPDATE ON 10/08/11 AT 2130 EDT FROM WELT TO HUFFMAN * * *

As of 1800 CDT on October 8, 2011, the Unit Two PPC is considered operational with respect to the Safety Parameter Display System (SPDS), Plant Parameter Display System (PPDS) and Emergency Response Data System (ERDS). Therefore, a major loss of assessment capability no longer exists on Unit 2. The Byron EP manager contacted the NRC ERDS Center yesterday on 10/7/11 to conduct an ERDS test for Unit 2 to ensure the data was being satisfactorily sent to the NRC. Unit-1 and Unit-2 ERDS data to NRC was tested satisfactory IAW EP-AA-124-F-01. The NRC Resident Inspector and the State of Illinois (through the Illinois Emergency Management Agency Resident Inspector) have been notified of this ENS update. R3DO (Phillips) notified.

VOLUNTARY REPORT - DESIGN VULNERABILITY IN 4.16kV BUS UNDER-VOLTAGE SCHEME

On January 30, 2012, a design vulnerability was discovered at Byron and Braidwood stations in the Engineered Safety Feature 4.16kV bus under-voltage protection scheme for Byron Station Units 1 and 2. Specifically a voltage unbalance created by an open circuit of either the A or C phase from the offsite grid to the System Auxiliary Transformers (SAT) is not designed to actuate the protective relays on the 4.16kV safety bus that provides isolation from the offsite grid and the automatic start and loading of the emergency onsite diesel generators.

Two under-voltage relays are provided on each 4.16kV safety bus, which are combined in a two out of two logic to generate a loss of power signal. The relays are sensing voltage between two phases (i.e., A&B and B&C). An open circuit condition on the C phase or the A phase would not satisfy the two out of two logic. This condition results in both 4.16kV safety buses remaining energized with a bus undervoltage situation and results in equipment protective devices actuating from over-current conditions.

This configuration is a non-conforming condition in that the design of the under-voltage relays and logic was intended to identify degraded grid conditions, not loss of a single phase. With an open circuit on the A or C phase from the grid to the SATs, during normal operations, operators have to diagnose the condition and manually isolate safety buses from offsite power which would automatically start and load the emergency diesel generators. During a design basis event concurrent with an open circuit on A or C phase from the grid to the SATs, analysis performed to date indicates that starting of the ECCS loads would have caused the bus voltage to decrease sufficiently to actuate the under-voltage protective relays and restore cooling with emergency onsite power without challenging fuel design limits.

The 4.16kV safety bus under-voltage protection scheme at Byron and Braidwood is believed to be a typical industry design. This design issue is being evaluated at the other Exelon stations. The results of this evaluation will be shared with the NRC. Therefore, this condition is being reported as a voluntary notification due to its potential generic industry applicability."

The licensee notified the NRC Resident Inspector.

Byron Unit 1 declared an Unusual Event due to a loss of offsite power for greater than 15 minutes (EAL MU-1). The 1A and 1B emergency diesel generators auto-started and automatically restored power to ESF (Engineered Safety Feature) Busses 141 and 142. The 1A Auxiliary Feedwater pump auto-started due to an undervoltage signal to ESF bus 141. The 6.9 KV Busses (Non-ESF) and 4.16 KV Busses (Non-ESF) auto transferred to the Unit Auxiliary Transformers and Unit 1 remained online. The ESF reserve feed breakers are available and in the process of transferring ESF loads over to Unit 2 via the ESF cross-ties. This will allow shutdown of the 1A and 1B diesel-generators and allow alignment to standby status. Switchyard repairs are being initiated. The licensee notified the State of Illinois, Ogle County, Oregon and Byron municipalities and the NRC Resident Inspector. The licensee plans on issuing a news release. Unit 2 remained online and was unaffected throughout the event. Notified Nuclear SSA and NICC vie email.

• * * UPDATE FROM CHRISTOPHER COTE TO CHARLES TEAL ON 2/29/12 AT 2222 EST * * *

Termination of Unusual Event. Repairs to switchyard components are complete. Offsite power has been restored to Unit One System Auxiliary Transformers. Unit One 4 KV ESF busses have been re-energized from Unit One System Auxiliary Transformers and are no longer cross tied to Unit Two System Auxiliary Transformers. This restores the normal electrical lineup to the ESF busses. The entry conditions for MU-1 are no longer met. The licensee has notified the NRC Resident Inspector, the State of Illinois, Ogle County, Oregon and Byron municipalities. Notified R3DO (Stone), NRR EO (Lee), IRD (Morris), and other FEDS (DHS SWO, DHS NICC, FEMA, and NuclearSSA via email).

On April 16, 2015, Byron Station will remove part of the Technical Support Center (TSC) emergency ventilation system from service to facilitate necessary surveillance work on the fire protection system. This work is expected to last approximately 4 hours. This maintenance affects the ability of the TSC ventilation to maintain adequate habitability during the duration of an emergency. If an emergency is declared requiring TSC activation during this period, the TSC will be staffed and activated using existing emergency planning procedures. If the TSC becomes uninhabitable, the Station Emergency Director will relocate the TSC staff to an alternate TSC location in accordance with applicable procedures. This notification is being made in accordance with 10 CFR 50.72(b)(3)(xiii) due to the potential loss of an emergency response facility because of the unavailability of the ventilation system. An update will be provided once the TSC ventilation has been restored to normal operation. The NRC Resident Inspector has been notified.

• * * UPDATE FROM SHANE HARVEY TO JOHN SHOEMAKER AT 1646 EDT ON 4/16/15 * * *

Maintenance has been completed and the TSC emergency ventilation system was restored to service at 1300 CDT on 4/16/15. The NRC Resident Inspector will be notified. Notified the R3DO (McCraw).

On 8/20/2015 at 1755 (CDT), a design flaw was discovered with the pressurizer power operated relief valve (PZR PORV) block valve control circuitry. Specifically, the circuit deficiency for which a design basis fire in the Main Control Room (MCR) or cable spreading room could prevent the PZR PORV block valves from being closed from the local control switch at their associated motor control center (MCC). Engineering has reviewed this issue and determined that a potential fire induced ground in the MCR or cable spreading room could clear the associated control power fuses which would prevent the block valves from operating at the local control switch. These valves are considered to form a High/Low pressure interface which requires postulating a proper polarity DC cable to cable fault. Engineering has reviewed the circuit design and cable routing associated with PORVs 1(2)RY455A and 1(2)RY456 and determined that their associated cables are routed with other DC circuit cables in the MCR control board and cable spreading room raceways, such that this postulated fault could potentially cause spurious opening of one of the PORVs even after the control power fuses have been removed as directed by the station abnormal operating procedures for control room inaccessibility. This identified block valve circuit deficiency prevents the credited safe shutdown action of locally closing the block valves to mitigate the spurious operation of a PORV. Hourly fire watches of the affected MCR and cable spreading room fire zones have been implemented. In addition, the MCR is continuously staffed and the affected cable spreading room fire zones are equipped with detection and automatic suppression. This event is being reported under 10CFR50.72(b)(3)(ii)(B) for 'Any event or condition that results in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety. The licensee has notified the NRC Resident Inspector.

• * * UPDATE AT 1816 EDT ON 9/2/2015 FROM BRIAN LEWIN TO MARK ABRAMOVITZ * * *

During the extent of condition review, an additional design deficiency was identified with respect to the PZR PORV and PZR PORV Block valves control circuitry. Specifically, the current mitigating strategy for removing PZR PORV control power fuses does not adequately prevent a PZR PORV from spuriously opening due to fire induced hot short. Furthermore, local actions to close the associated PZR PORV block valve at the motor control center (MCC) may not be effective because the MCC may not have electrical power during the design basis fire. Therefore, the credited safe shutdown action to remove the PZR PORV control power fuses does not prevent the PZR PORV from spuriously opening during design basis fires in some of the upper and lower cable spreading room fire zones. The affected Fire Zones are the same upper and lower spreading rooms previously identified and fire watches of the affected areas remain in place. The NRC Resident Inspector has been notified. Notified the R3DO (Skokowski).