Information Notice 2021-03, Operating Experience Related to the Duane Arnold Energy Center Derecho Event on August 10, 2020
| ML21139A091 | |
| Person / Time | |
|---|---|
| Issue date: | 08/11/2021 |
| From: | Chris Miller Office of Nuclear Reactor Regulation |
| To: | |
| Benney B | |
| References | |
| IN 2021-03 | |
| Download: ML21139A091 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 August 11, 2021 NRC INFORMATION NOTICE 2021-03: OPERATING EXPERIENCE RELATED TO THE
DUANE ARNOLD ENERGY CENTER DERECHO
EVENT ON AUGUST 10, 2020
ADDRESSEES
All holders of and applicants for an operating license or construction permit for a nuclear power
reactor issued under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic
licensing of production and utilization facilities, including those that have permanently ceased
operations and certified that fuel has been permanently removed from the reactor vessel.
All holders of and applicants for a power reactor combined license, standard design approval, or
manufacturing license under 10 CFR Part 52, Licenses, certifications, and approvals for
nuclear power plants. All applicants for a standard design certification, including such
applicants after initial issuance of a design certification rule.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
the addressees of operating experience following the Duane Arnold Energy Center (DAEC)
derecho event on August 10, 2020.
The NRC expects that recipients will review the information for applicability to their facilities and
consider actions, as appropriate, to avoid similar issues. INs may not impose new
requirements, and nothing in this IN should be interpreted to require specific action.
DESCRIPTION OF CIRCUMSTANCES
On August 10, 2020, the DAEC experienced severe thunderstorms and high winds associated
with a derecho, a widespread, long-lived, straight-line windstorm associated with a band of
rapidly moving thunderstorms. This storm included wind gusts of 80-100 miles per hour (mph),
with the most extreme winds in the area measured at approximately 130 mph.
At 1202 local time, a severe thunderstorm watch (previously issued at 1138) was upgraded to a
warning. The senior responsible manager directed in-progress fuel handling operations at the
facility to be placed in a safe condition and secured. As severe thunderstorms and high winds
associated with the derecho moved through the area, at 1235 (33 minutes after issuance of the
severe thunderstorm warning), a grid perturbation caused the sites two emergency diesel
generators (EDGs) to automatically start and run unloaded. A short time later, while operating
at 82-percent reactor power, the DAEC experienced a loss of offsite power (LOOP), resulting in
a main turbine trip on reverse power and a subsequent automatic reactor scram. Since the
EDGs were already running, the diesel output breakers immediately closed to maintain power to
ML21139A091 the plant's two electrical safety buses. The flywheels on the 120-volt alternating current (AC)
reactor protection system (RPS) motor-generators stabilized RPS voltage and frequency during
the power transfer. The reactor unit did not lose the RPS, and since this system is the power
supply to the main steam isolation valve solenoids, these valves remained open following the
LOOP, which allowed the unit continued access to the normal heat sink for cooldown for the
duration of the event. After the automatic reactor scram, the reactor water level initially lowered
rapidly because of the loss of feedwater. The reactor core isolation cooling and high-pressure
coolant injection systems automatically initiated and were used to restore and maintain the
reactor water level. At 1258, the licensee declared a Notification of Unusual Event due to the
loss of all offsite AC power to both safety buses for more than 15 minutes.
About 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> after the storm, but before the restoration of offsite power, the emergency
service water (ESW) system that provides cooling water to the EDGs showed signs of
degradation as the differential pressure across the strainers in both ESW trains began
increasing. The high winds resulted in increased debris loads at the intake to the ESW system, which caused clogging of the train B strainer and subsequent decrease of ESW flow below the
value at which adequate cooling to the B EDG was assured by Technical Specifications (TS).
Although the DAEC operators declared EDG B inoperable according to the TS, they
successfully bypassed the train B strainer in accordance with operating procedures, and EDG
B did not experience any degradation. The A train of ESW also experienced some
degradation at the strainer, but not to the point of requiring the strainer to be bypassed. The
first of six offsite power lines (Vinton 161-kilovolt line) was restored on August 11, 2020, at
approximately 1200, more than 23 hours2.662037e-4 days <br />0.00639 hours <br />3.80291e-5 weeks <br />8.7515e-6 months <br /> after the LOOP. The licensee was then able to
energize the startup transformer and energize the essential buses. The licensee terminated the
Notification of Unusual Event at 1600 that day. All six offsite power lines were restored by
August 17, 2020.
In addition to the degradation of the ESW system described above, a small cut as a result of
storm damage was discovered on August 12 in the fifth-floor wall of the DAEC reactor building.
A subsequent test of the secondary containment boundary identified that the vacuum of
0.24 inches of water was less that the TS requirement of 0.25 inches of water. At the time of
discovery, the plant was in Mode 4, which does not require secondary containment to be
operable. However, it is very likely that the cut in the reactor building wall existed while the
plant was in Mode 3 after the automatic reactor scram and, therefore, secondary containment
was inoperable during this period. Although the secondary containment was considered
inoperable, the licensee determined that a vacuum of 0.24 inches of water was sufficient to
maintain the safety function of the DAEC secondary containment.
While the high winds also resulted in minor damage to the DAEC reactor, turbine, and diverse
and flexible coping strategies (FLEX) buildings, the FLEX equipment was not impacted and
remained available. High winds, however, caused more severe damage to the
nonsafety-related cooling towers, which collapsed, thus demonstrating a derechos potential to
introduce widespread damage to systems, structures, and components that are not designed to
withstand effects of sustained high winds.
The NRC conducted followup inspection activities to review the facts surrounding the derecho
event as documented in Duane Arnold Energy CenterNRC Integrated Inspection Report 05000331/2020003 and 07200032/2020001, dated November 6, 2020 (Agencywide
Documents Access and Management System (ADAMS) Accession No. ML20314A150). Additional information appears in Duane Arnold Energy Center LIC-504 Team
Recommendations, dated March 30, 2021 (ADAMS Accession No. ML21084A010), and Final
ASP [Accident Sequence Precursor] Analysis]Precursor, dated March 4, 2021 (ADAMS
Accession No. ML21056A382).
DISCUSSION
Main Conclusions of the Accident Sequence Precursor Analysis
The NRC staffs ASP analysis revealed that risk of core damage for this weather-related LOOP
was driven by the potential risk of both EDGs failing, along with both the high-pressure coolant
injection and reactor core isolation cooling systems. Although the mean conditional core
damage probability of 8x10-4 for this event was high, the risk of core damage was mitigated and
plantwide safety margins were maintained.
The overall risk of this event was significantly impacted by the station blackout scenarios. The
risk associated with these scenarios is particularly high for this plant as there were only two
safety-related EDGs. In addition, as the DAEC is a single-unit site, there was no ability to
crosstie safety-related buses from another unit.
FLEX mitigation strategies implemented in accordance with Order EA-12-049, Order Modifying
Licenses with Regard to Requirements for Mitigation Strategies for Beyond-Design-Basis
External Events, dated March 12, 2012, and codified in 10 CFR 50.155, Mitigation of
beyond-design-basis events, were credited in the DAEC ASP analysis and significantly affected
the results. Specifically, without the mitigation capabilities of the FLEX strategies, the
conditional core damage probability would have been approximately a factor of 10 higher for this
event.
Main Conclusions of the LIC-504 Evaluation
In accordance with the Office of Nuclear Reactor Regulation (NRR) Office Instruction LIC-504, Integrated Risk-Informed Decisionmaking Process for Emergent Issues, Revision 5, dated
March 4, 2020 (ADAMS Accession No. ML19253D401), the NRC staff assessed the DAEC
derecho event to evaluate potential safety impacts to other nuclear power plant (NPP)
licensees.
The NRC staff analyzed eight sample NPPs with different design characteristics that estimated
the risk increases due to a similar combined event (i.e., concurrent challenges to offsite power
supplies and the functionality of the ESW system due to a sudden inrush of debris to the intake
structure) and concluded that the safety implications can vary significantly based on site, plant
design, and plant operating characteristics. Risk analyses for this group of eight sample NPPs
confirmed that the potential increases in risk associated with the issue were below the value for
which the NRC would consider taking immediate regulatory action, such as issuing shutdown
orders or imposing compensatory measures to ensure public health and safety.
The NRC staff gleaned additional risk insights from the ASP analysis and LIC-504 evaluation
including the following site design and operating characteristics that would reduce the risk
exposure at the plants analyzed for a similar combined event and that could, when present, influence the magnitude of the risk impact from this type of event. Site and Design Characteristics
Characteristic Impact of Characteristic on Risk
Frequency of the combined event that causes Sites located in areas that have lower likelihood
a LOOP and a concurrent challenge to the of events such as derechos are at reduced risk.
functionality of the ESW and fire protection
water systems due to debris
Susceptibility of the water source for ESW to Sites that have ultimate heat sink sources that
debris accumulation during a derecho are not prone to accumulation of debris have
reduced risk.
Relative location of the intake to redundant Plants with suction sources that are spatially
ESW trains, as well as the location of fire significantly apart are at reduced risk because
pump suction at plants that use fire protection concurrent blockage of redundant and diverse
water as a diverse capability for EDG cooling suction capabilities is reduced.
Availability of additional diesels that do not Plants with additional AC power sources (often
rely on ESW, in addition to availability of not dependent upon ESW for cooling) that have
diesels procured and installed as part of the ability to provide motive power to essential
FLEX mitigation strategies loads are at reduced risk.
Availability of alternative strategies to provide Plants with alternative strategies to provide
cooling water to EDGs (including water from cooling water to EDGs are at reduced risk.
the fire protection system or other sources)
Ability to promptly recognize the increased Plants that have alarms or annunciators to
differential pressure (P) across strainers inform operators of increasing P across the
ESW strainer and intake structure screens are
at reduced risk.
Ability to bypass the ESW strainers and Plants that have the capability to bypass the
ability of the EDGs to successfully operate in ESW strainers decrease risk since the EDGs
the bypass mode may operate successfully in that temporary
configuration. However, bypassing the ESW
strainers can result in increased risk to
downstream components.
Source of AC power to traveling screens Plants whose traveling screens are powered by
emergency AC power are at reduced risk.
Operating Characteristics
Ability to promptly recognize increased P Early detection and procedures that instruct
across strainers operators to monitor P across the ESW
strainer and intake structure screens upon
receipt of warnings for severe weather, may
decrease risk.
Use of FLEX strategies With appropriate procedures, testing, and
training, FLEX strategies reduce potential risk
increases attributed to this event.
Procedures and abnormal operating Severe weather preparedness procedures and
procedures related to severe weather abnormal operating procedures that:
warnings (1) recognize and take action to minimize
the potential for blockage of intake
structures, traveling screens, and
strainers decrease risk
(2) direct risk management actions for
ongoing site activities (e.g., suspension of fuel movement activities) decrease
risk
Results of the risk analysis performed for the eight NPPs analyzed demonstrate that the
availability of the following three mitigation characteristics would significantly reduce plant risk:
(1) the ability of operators to bypass a clogged ESW strainer, if needed
(2) the ability to align an alternate cooling source, such as fire protection water or another
source of water, to provide cooling to diesel generators
(3) having additional diesel generators (not including FLEX diesels) that are not dependent
on service water for cooling
Of particular note from this derecho event, the debris buildup on the DAEC ESW strainers did
not challenge the functionality of ESW until several hours into the event. However, because of
the extended duration of the LOOP, EDGs, and therefore, the ESW system, were still required
to remain functional to provide AC power. When the ESW system and the EDGs are each
dependent on the continuing functionality of the other to remain operable, even as they both
serve multiple safety functions, diverse capabilities to mitigate the consequences of the loss of
one of these systems can reduce the risk of events that challenge both systems simultaneously.
The NRC staffs evaluation concluded that the derecho at the DAEC demonstrated that the
plants design was adequate to withstand the impacts of high winds and resulting
debris-generated missiles. It also demonstrated that there are additional risk insights gained
that could benefit plants impacted by similar severe weather events in the future.
CONTACT
S
Please direct any questions about this matter to the technical contacts listed below,
/RA/
Christopher G. Miller, Director
Division of Reactor Oversight
Office of Nuclear Reactor Regulation
Technical Contacts: Matthew Leech, NRR Rebecca Sigmon, NRR
301-415-8312 301-415-0895 e-mail: Matthew.Leech@nrc.gov e-mail: Rebecca.Sigmon@nrc.gov
ML21139A091 EPIDS No. L-2021-GEN-0003 OFFICE Author QTE NRR/DRA/BC NRR/DRA/DD OE
NAME MLeech JDoughtery AZoulis MFranovich JPeralta
DATE 6/25/21 6/11/21 6/25/21 7/09/21 5/24/21 OFFICE NRR/DRO/IOEB/PM NRR/DRO/LA NRR/DRO/IOEB/BC NRR/DIRS/D
NAME BBenney IBetts LRegner CMiller
DATE 7/29/21 8/3/21 7/29/21 8/11/21