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| Entered date | Event description | |
|---|---|---|
| ENS 51451 | 5 October 2015 10:00:00 | This 60-day telephone notification is being submitted in accordance with 10 CFR 50.73(a)(1) and 10 CFR 50.73(a)(2)(iv)(A) to report an invalid actuation of the Unit 2, Train B Containment Ventilation Isolation (CVI) at Sequoyah Nuclear Plant. At 1919 EDT on August 7, 2015, during planned performance of a Unit 2 containment vent, the Train B CVI actuated due to an invalid Hi Rad signal from 2-RM-90-131, Containment Vent Radiation Monitor. In addition to the Train B CVI alarm, unexpected alarms were received for 2-RM-90-106, Lower Containment Radiation Monitor and 2-RM-90-112, Upper Containment Radiation Monitor instrument malfunctions as they isolated for the CVI and 2-RM-90-131 Hi Rad alarm. Prior to the invalid Hi Rad alarm, all radiation monitors were stable at their normal values. All required automatic actuations occurred as designed. Upon investigation, the cause of the invalid Hi Rad alarm was due to an exposed shield wire at the 2-RM-90-131 detector. Preventative maintenance had been performed the week prior to the CVI and it is believed the damage occurred at that time. Control Room Operators performed Annunciator Response actions and verified by diverse indications that the subject condition was an invalid Hi Rad signal. There were no indications of degraded reactor coolant system parameters or fuel failure. Applicable Technical Specification (TS) Limiting Condition for Operations (LCOs) were entered and the radiation monitors declared inoperable. No Emergency Response criteria were applicable with the subject radiation monitors inoperable. Radiological surveys performed in the vicinity of 2-RM-90-131 verified no abnormal radiological conditions. Radiation Monitor 2-RM-90-131 was removed from service, the shield wire was repaired and returned to service with no issues. Radiation Monitors 2-RM-90-106 and 2-RM-90-112 were tested and returned to service. The applicable TS LCOs were exited. At the time of the event, plant conditions for a Hi Rad alarm did not exist; therefore, the CVI was invalid. The NRC Resident Inspector was notified. |
| ENS 51265 | 27 July 2015 13:44:00 | At 1043 EDT on 7/27/2015, Sequoyah Unit 1 was at 82% power and continuing to perform a startup when the reactor/turbine automatically tripped. Following the reactor trip, all safety-related equipment operated as designed. Auxiliary Feedwater automatically initiated as expected from the Feedwater Isolation Signal. Unit 1 is currently being maintained in Mode 3, at NOT/NOP (normal operating temperature and normal operating pressure), approximately 545 F and 2235 psig, with auxiliary feedwater supplying the steam generators and decay heat removal via the condenser steam dumps. The immediate cause of the trip was an electrically-induced turbine trip. Due to fluctuating voltage the main generator voltage regulator was taken to manual; immediately after this the unit tripped. Current Temperature and Pressure - Reactor Coolant System (RCS) temperature is 545 degrees F and stable and RCS pressure is 2235 psig and stable. There is no indication of any primary/secondary leakage. All rods fully inserted on the reactor trip and remain inserted. The electrical alignment is normal with shutdown power supplied from off-site power. There is no operational impact to Unit 2. Unit 2 continues to operate in Mode 1 at 100%. There was no impact on public health and safety. Post-trip investigation is in progress and planned restart timeline has not yet been determined. The licensee notified the NRC Resident Inspector. |