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| Start date | Reporting criterion | Title | Event description | System | LER | |
|---|---|---|---|---|---|---|
| ENS 50370 | 14 August 2014 23:31:00 | 10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown | Unit 2 Technical Specification Required Shutdown Due to Multiple Inoperable Emergency Diesel Generators | While performing scheduled maintenance on Unit 2 Emergency Diesel Generator (EDG) 2-2, Diablo Canyon Power Plant (DCPP) identified a failed capscrew on engine cylinder 1L. As part of subsequent inspections to determine whether a similar condition existed on any of the other Unit 1 or Unit 2 EDGs, a degraded capscrew was identified on EDG 2-3 cylinder 8L at 1631 PDT on 08/14/2014. No capscrew issues were identified on the Unit 1 EDGs or on Unit 2 EDG 2-1. Although all operational tests of the diesels up to the time of discovery were satisfactorily performed with no indication of degraded performance, the EDG 2-3 was declared inoperable. Because two Unit 2 EDGs were inoperable concurrently, this is being reported as a condition that could have prevented fulfillment of a safety function per 10 CFR 50.72(b)(3)(v). Per the requirements of TS 3.8.1, with two EDGs inoperable, a plant shutdown was commenced at 2031 PDT on 08/14/2014. Therefore, this condition is also being reported in accordance with 10 CFR 50.72(b)(2)(i). Offsite power remained available throughout this condition. EDG 2-2 remains out of service as part of its scheduled maintenance window. This condition did not result in any adverse impact on the health and safety of the public. A press release is planned. The licensee informed the NRC Resident Inspector. The licensee also reported event notifications for 10 CFR 50.72(b)(3)(v)(C) - Control of Rad Release and 10 CFR 50.72(b)(3)(v)(D) - Accident Mitigation. The licensee plans to continue the shutdown to Mode 3 and is developing plans to return one of the two inoperable EDGs to operable status by the time the unit reaches Mode 5. | Emergency Diesel Generator | |
| ENS 50334 | 1 August 2014 00:29:00 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material | Unanalyzed Condition | On 07/31/2014 at 1729 PDT, Diablo Canyon Power Plant (DCPP) identified an unanalyzed condition involving the three class 1E 4160 volt buses on Unit 1 and on Unit 2. Following a postulated main steam line High Energy Line Break (HELB) in the Turbine Building, steam could enter the 4160 volt bus rooms through ventilation ducts that exhaust to the turbine deck where the steam lines are located. New analysis identified that the relative humidity (RH) of the 4160 volt bus rooms could reach 100%, which exceeds the design RH used in the analyses for the protective devices in the room. The high-humidity could possibly prevent the protective devices from operating as intended and could therefore result in the unavailability of the 4160 volt buses to supply power to engineered safety feature systems. This concern is being reported as an unanalyzed condition per 10 CFR 50.72(b)(3)(ii)(B) and as a condition that could have prevented fulfillment of a safety function per 10 CFR 50.72(b)(3)(v). Compensatory measures were taken to isolate the ventilation ducts between the 4160 volt bus rooms and the turbine deck, thereby preventing steam entry into the 4160 volt bus rooms, assuring the protective devices would not be subjected to conditions exceeding the design RH. With this compensatory measure all three 4160 buses on each unit are assured of performing their expected design function following the postulated main steam line break. This concern did not result in any adverse affect on the health and safety of the public. The licensee informed the NRC Resident Inspector. In addition to the 10 CFR sections listed in the header of this event notification, the licensee is also reporting this under 10 CFR 50.72(b)(3)(v)(D).
Diablo Canyon Power Plant is retracting this event notification based on the following: Subsequent to the identification of this event, PG&E conducted additional analysis and testing to support a past operability evaluation. Testing and analysis was completed successfully, demonstrating that the full population of associated components would perform their design safety function in 100% relative humidity conditions during a postulated high energy line break. Based on these analysis and testing results, PG&E concludes that the associated equipment remained operable, and did not represent an unanalyzed condition per 10 CFR 50.72(b)(3)(ii)(B) or a condition that could have prevented fulfillment of a safety function per 10 CFR 50.72(b)(3)(v). Therefore, PG&E retracts its notification of 7/31/2014. The licensee has notified the NRC Resident Inspector. Notified the R4DO (Drake). | Main Steam Line | |
| ENS 49879 | 6 March 2014 17:06:00 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition | Unanalyzed Condition Regarding Potential Tornado Missile Damage to Emergency Diesel Exhaust Plenum | The condition described below is being reported as an unanalyzed condition per 10 CFR 50.72(b)(3)(ii)(B) and per the guidance of NUREG-1022, Rev. 3. On 03/06/2014 at 0906 PST, Diablo Canyon Power Plant (DCPP) identified a nonconforming condition involving the Emergency Diesel Generator (EDG) ventilation exhaust plenums installed in Unit 1 and Unit 2. Specifically, the radiator exhaust plenums and exhaust piping need to be re-evaluated to ensure adequate protection against flying debris that could be generated by a tornado. The occurrence of such an event is highly unlikely and there is no imminent concern regarding severe weather involving tornados. The EDGs are located inside the power plant structure and are capable of performing their safety function. Compensatory measures are being developed to address the associated nonconformance. This event does not adversely affect the health and safety of the public. The licensee informed the NRC Resident Inspector."
This condition does not adversely affect the health and safety of the public. Based on an extent of condition review being performed for this event, the issue identified in the original event notification 49879 has also been determined to similarly affect the ventilation systems associated with the Unit 1 and 2 Vital 480 volt AC switchgear and battery/inverter equipment. The condition described in this update is being reported as an unanalyzed condition per 10 CFR 50.72(b)(3)(ii)(B) and as an event or condition that could have prevented the fulfillment of a safety function per 10 CFR 50.72(b)(3)(v)(A). Compensatory measures are being developed to address the associated condition. The licensee informed the NRC Resident Inspector. Notified R4DO (Azua) | Emergency Diesel Generator | |
| ENS 48819 | 13 March 2013 00:06:00 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor | Unit 2 Loss of Shutdown Monitoring Instrumentation System | On March 12, 2013, at 1706 PDT, with the reactor coolant system not intact, plant operators identified (an) unexpected increase in nuclear reactivity count rate as indicated by source range instrumentation channel N-32. This increase was NOT indicated on redundant wide range instrumentation channels N-51 and N-52. Operators identified that technicians working in the area of the N-32 cable routing had bumped the cabling, resulting in the false increase in the count rate. The source range channel N-32 was subsequently declared inoperable. Prior to this event the other source range channel N-31 had been declared inoperable. The above event resulted in loss of control room audible indication required by DCPP (Diablo Canyon Power Plant) technical specification to monitor the core reactivity condition and consequently caused loss of the monitoring system. Operators retained capability of monitoring core reactivity conditions through use of the two wide range nuclear instrumentation channels N-51 and N-52. In response to this concern, plant operators verified all core alterations were suspended, verified boron concentration was within acceptable limits for current conditions, suspended operations that could dilute the boron concentration, verified reactor trip breakers were open and control rods incapable of withdrawal, and initiated actions to restore to service one of the source range instrumentation channels with its associated control room audible count rate indication. NRC Resident Inspectors have been notified.
This is an update to EN #48819 reported on March 12, 2013 at 2158 PDT. This update informs the NRC that PG&E, in accordance with provisions of TS 3.0.4b, will be restoring the source range instrumentation channel N-32 to service in Mode 5, maintaining the availability and monitoring of the wide range instrumentation channels while in Mode 5 until the source range channel is returned to service. The licensee has notified the NRC Resident Inspector. Notified R4DO (Powers). | Reactor Coolant System Control Rod | 05000323/LER-2013-002 |