View source for Information Notice 2021-03, Operating Experience Related to the Duane Arnold Energy Center Derecho Event on August 10, 2020

{{Adams
| number = ML21139A091
| issue date = 08/11/2021
| title = Operating Experience Related to the Duane Arnold Energy Center Derecho Event on August 10, 2020
| author name = Miller C
| author affiliation = NRC/NRR
| addressee name = 
| addressee affiliation = 
| docket = 
| license number = 
| contact person = Benney B
| document report number = IN 2021-03
| document type = NRC Information Notice
| page count = 6
}}
{{#Wiki_filter: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 hours 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 hours 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}}

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