On August 10, 2020, at 1246 CDT, while operating at 80% power, NextEra Energy Duane Arnold (DAEC) experienced a generator load reject and subsequent reactor scram as a result of a loss of offsite power (LOOP) due to extremely high winds. An Unusual Event was declared at 1258. All Safety Systems performed as designed. As a result of the LOOP, the 'A' and 'B' Emergency Diesel Generators automatically started and supplied power to the safety related busses. As a result, the High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) automatically initiated and injected into the reactor. In addition, containment isolations occurred, as expected for this event. This resulted in a 4-hour event report to the NRC under 10 CFR 50.72 section 50.72(b)(2)(iv)(A) - ECCS Injection, 50.72(b)(2)(iv)(B) - RPS Actuation - Critical, 50.72(b)(3)(iv)(A) - Valid Specific System Actuation (reference EN#54826). The NOUE by was exited at 1600 on August 11, 2020. The cause of the LOOP was an extremely severe storm (called a Derecho) with heavy rains and very high winds. This event had no impact on public health or safety. There were no systems, structures, or components inoperable at the time of the event that contributed to the event. This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) and 50.73(a)(2)(iii). There were no radiological releases associated with this event.
On August 10, 2020, at 1246 CDT, while operating at 80% power, NextEra Energy Duane Arnold (DAEC) experienced a Generator Load reject and subsequent reactor scram as a result of a loss of offsite power (LOOP) due to extremely high winds. An Unusual Event was declared at 1258. All Safety Systems performed as designed.
As a result of the LOOP, the 'A' and 'B' Emergency Diesel Generators (EDGs) automatically started and supplied power to the safety related busses. In response to the expected Reactor Pressure Vessel (RPV) level transient, the High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) automatically started and injected. In addition, containment isolations occurred, as expected for this event.
This resulted in a 4-hour event report to the NRC under 10 CFR 50.72 section 50.72(b)(2)(iv)(A) - ECCS Injection, 50.72(b)(2)(iv)(B) - RPS Actuation - Critical, 50.72(b)(3)(iv)(A) - Valid Specific System Actuation (reference EN#54826).
The cause of the LOOP was an extremely severe storm (called a Derecho) with heavy rains and very high winds. This event had no impact on public health or safety. There were no systems, structures, or components inoperable at the time of the event that contributed to the event.
The Unusual Event was exited at 1600 on August 11, 2020.
II. Assessment of Safety Consequences:
In response to the LOOP, the plant experienced a Generator Lockout, Turbine Trip and Reactor Scram. The operating crew entered the appropriate emergency procedures. The EDG's quickly re-energized the safety-related electrical busses and restored power. The Reactor Protection System (RPS) power was never lost as the EDG's were able to recover power to the essential buses quickly. This prevented a Main Steam Isolation Valve (MSIV) isolation signal and thus allowed the Main Condenser to remain available post event. As expected for this event, RPV level dropped below L2 setpoint (119.5") initiating the HPCI and RCIC systems. Reactor level recovered quickly. The Low Low Set System functioned as designed initially opening two primary system relief valves to control reactor pressure.
Operators were quickly able to establish pressure control initially via Steam Line Drains and RCIC and establish a cooldown rate.
Both recirculating pumps tripped when power was lost to their associated non-safety related electrical busses. The operators followed procedures for cooldown with no forced circulation in the core. When Shutdown Cooling was placed in service, forced circulation was re-established which resulted in exceeding the bottom head cooldown limits of 100 degrees per hour. This was anticipated and could not be avoided.
This event did not result in a Safety System Functional Failure. Ill.
Cause of Event: The cause of the LOOP was an extremely severe storm (called a Derecho) with heavy rains and very high winds. Wind speeds exceeded 80 mph for over 20 minutes with peaks on site at greater than 100 mph. The National Weather Service later estimated wind speeds were likely near 130 mph. These winds resulted in all 6 off site power sources (4 - 161 kV and 2 - 345 kV) coming into the DAEC switchyard being damaged or downed. A separate offsite 69 kV line which feeds non-essential plant loads was also damaged. IV.
Corrective Actions: REV NO. - 00 The high winds experienced on August 10, 2020 were not considered a beyond design basis event. The systems and components responded as designed and the overall peak wind speeds were within the Design Basis Tornado. The damage observed coincides with the amount of damage expected for the event. Given this event was outside the control of the station staff and that all components worked as designed, no additional corrective actions beyond repair/recovery efforts related to the wind storm are needed. V.
Additional Information: Previous Similar Occurrences: A review of NextEra Energy Duane Arnold LERs from the previous 5 years identified the following event: LER 2018-004 - Automatic Reactor Scram due to Feedwater Regulating Valve Failure LER 2019-001 - Automatic Reactor Scram due to Loss of Feedwater.
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