ML20196L822

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR REACTOR REGULATION 

WASHINGTON, DC 20555-0001 

September 15, 2020 

NRC INFORMATION NOTICE 2020-02: FLEX DIESEL GENERATOR OPERATIONAL 

CHALLENGES 

ADDRESSEES 

All holders of an operating license or construction permit for a nuclear power reactor under 

Title 10 of the Code of Federal Regulations (10 CFR) Part 50, “Domestic licensing of production 

and utilization facilities,” except those that have permanently ceased operations and have 

certified that fuel has been permanently removed from the reactor vessel. 

All holders of and applicants for a combined license under 10 CFR Part 52, “Licenses, 

certifications, and approvals for nuclear power plants.” 

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform 

addressees of recent operational challenges involving diverse and flexible coping strategies 

(FLEX) equipment at nuclear power plants. NRC licensees use this equipment to implement 

FLEX capability for long-term core cooling, spent fuel cooling, and containment integrity in a 

beyond-design-basis event scenario. The NRC expects that recipients will review the 

information in this IN for applicability to their facilities and consider actions, as appropriate, to 

ensure continued compliance with NRC Order EA-12-049, “Order Modifying Licenses with 

regard to Requirements for Mitigation Strategies for Beyond-Design-Basis External Events,” 

dated March 12, 2012 (Agencywide Document Access and Management System (ADAMS) 

Accession No. ML12054A735) and 10 CFR 50.155, “Mitigation of beyond-design-basis events.” 

INs may not impose new requirements, and nothing in this IN should be interpreted to require 

specific action. 

DESCRIPTION OF CIRCUMSTANCES 

River Bend Station 

Event #1 

On September 26, 2019, the licensee at River Bend Station (River Bend) began periodic testing 

of FLEX pump P-1, a pump intended to provide core cooling capability during a beyond-designbasis event. FLEX diesel generator EG-5 powers the pump. This was the first time the licensee 

attempted to start EG-5 without the diesel generator vendor on site to support this activity. 

When the licensee attempted to start EG-5, it started but immediately shut down. The licensee 

was not able to determine the cause of the shutdown and contacted the vendor to schedule 

corrective maintenance. 

On October 31, 2019, the vendor arrived on site to troubleshoot EG-5. The licensee also 

decided to check the operation of four other FLEX diesel generators: EG-1, EG-2, EG-3, and 
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EG-4 with the vendor on site for assistance. The licensee was able to start EG-3, EG-4, and 

EG-5, but shortly after starting the engines tripped either on overspeed (EG-3 and EG-4) or 

overvoltage (EG-5) conditions. After several attempts, the vendor was able to successfully run 

EG-3, EG-4, and EG-5. When the licensee tested EG-1 and EG-2, both started but immediately 

tripped on high coolant temperature. The vendor determined that sensor contacts on the 

coolant temperature switches in EG-1 and EG-2 had failed closed, indicating a false high 

coolant temperature condition. The vendor bypassed the sensors and successfully ran both 

diesel generators. After 30 seconds of operation, the sensor contacts opened, and the diesel 

generators subsequently started and operated without issue. 

The NRC performed a follow-up baseline inspection in December 2019. The inspection report 

is available at ADAMS Accession No. ML20106F203. 

Event #2 

On April 1, 2020, the licensee at River Bend again attempted to perform periodic testing of 

FLEX pump P-1, powered by EG-5. The licensee started EG-5, but the generator tripped on an 

undervoltage condition following closure of the disconnect switch connecting it to the uncoupled 

pump P-1 motor (unloaded condition). After the vendor replaced the diesel generator voltage 

regulator, fuel pump, and fuel injectors, the licensee attempted the test on April 16, 2020. 

Again, EG-5 tripped on an undervoltage condition. Licensee troubleshooting revealed the 

cause of the trip as an incorrect configuration of the EG-5 main breaker instantaneous 

overcurrent setpoint and voltage controller time delay setpoints for over/under frequency and 

over/under voltage. The main breaker instantaneous overcurrent setpoint and voltage controller 

time delay setpoints for over/under frequency and over/under voltage were at the factory default 

values of 1.5 (main breaker instantaneous overcurrent) and zero seconds (over/under 

frequency/voltage), respectively. These settings caused the output breaker to trip during the 

temporary undervoltage condition resulting from starting a large inductive load such as FLEX 

pump P-1. 

The licensee’s extent of condition revealed that the main breaker instantaneous overcurrent 

setpoint and voltage controller time delay setpoints for over/under frequency and over/under 

voltage were similarly misconfigured on four other FLEX diesel generators (EG-1, EG-2, EG-3, 

and EG-4). This condition would have prevented EG-3, EG-4, and EG-5 and their associated 

pumps from performing their functions of pumping water from the suppression pool to the 

reactor low-pressure injection point (EG-3 and EG-4), or pumping cooling water to the 

suppression pool cooling heat exchanger (EG-5) in the event of a beyond-design-basis scenario 

that required these components. Based on licensee testing that confirmed the margins in the 

FLEX diesel generator sizing calculations, the NRC confirmed that EG-1 and EG-2 would still 

have been capable of performing their beyond-design-basis function of providing backup power 

for the station battery chargers and other FLEX loads. 

From May 18-22, 2020, the NRC performed a special inspection at River Bend. The inspection 

report is available at ADAMS Accession No. ML20240A258. 

Clinton Power Station 

On July 11, 2019, the licensee at Clinton Power Station (Clinton) discovered that the electrical 

phase rotation of the “A” FLEX diesel generator was opposite to that of the load center for its inplant loads. If the facility had used the “A” FLEX diesel generator to power any in-plant 

equipment, it would have caused phase-dependent loads to rotate backwards, potentially 
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damaging in-plant safety-related equipment. The licensee took corrective action to change the 

“A” FLEX diesel generator output wiring so that its phase rotation matched the load center. The 

licensee also performed phase rotation checks on the “B” FLEX diesel generator, which is the 

alternate power supply in the licensee’s FLEX strategy. The phase rotation of the “B” FLEX 

diesel generator was correct. The “A” FLEX diesel generator is permanently installed in the 

plant, and the “B” FLEX diesel generator is staged in the FLEX equipment building. In the event 

of a failure of the “A” FLEX diesel generator, the licensee would have deployed the “B” FLEX 

diesel generator to power the in-plant loads. 

The NRC included this issue in a quarterly baseline inspection report which is available at 

ADAMS Accession No. ML20132A319. 

DISCUSSION 

In response to the Fukushima Dai-ichi accident in Japan in March 2011, the NRC issued 

Order EA-12-049 to all power reactor licensees in March 2012. The order requires a 

three-phase approach for mitigating beyond-design-basis external events. It requires licensees 

to have installed plant equipment and resources (Phase 1) and portable onsite equipment and 

consumables (Phase 2) to maintain or restore core cooling, containment cooling, and spent fuel 

pool cooling capabilities until resources can be brought in from off site (Phase 3) to mitigate a 

beyond-design-basis external event. The order requires licensees to develop strategies to 

maintain these functions if there is an extended loss of all alternating current power and loss of 

normal access to the ultimate heat sink. 

In August 2012, the Nuclear Energy Institute (NEI) issued NEI 12-06, “Diverse and Flexible 

Coping Strategies (FLEX) Implementation Guide,” which is endorsed by the NRC and provides 

industry guidance on a methodology that licensees may use to meet the requirements of Order 

EA-12-049. The guidance and subsequent NRC-endorsed revisions contain information on the 

development of site-specific strategies, and on the storage, procedures, maintenance, testing, 

and training for FLEX equipment. The licensees at River Bend and Clinton each described how 

they follow the guidance in NEI 12-06 in a final integrated plan submitted to the NRC. 

Section 11 of the NEI 12-06 guidance allows licensees to procure FLEX equipment as 

commercial grade equipment and outlines standards for design, storage, maintenance, testing, 

and configuration control. As such, the equipment associated with FLEX strategies is generally 

not required to be procured, tested, or otherwise maintained in accordance with the quality 

requirements of 10 CFR Part 50. The events at River Bend and Clinton demonstrate that it is 

important for licensees to ensure that design characteristics, maintenance, testing, and 

modifications that could impact the ability of FLEX equipment to perform when needed are 

maintained in a consistent and up-to-date manner. As commercial grade equipment, FLEX 

diesel generators come equipped with components and setpoints intended to protect the 

engine. These components can fail and cause false trip signals, and factory setpoints can result 

in unintended trip signals that prevent the diesel generators from fulfilling their design function. 

A false trip signal caused the EG-1 and EG-2 failures at River Bend in October 2019, and 

factory setpoints on the generator output breaker caused EG-5 to trip in April 2020. 

NEI 12-06 Section 11.5 states that the licensee’s maintenance program should ensure that 

FLEX equipment reliability is being achieved, and that testing of FLEX equipment should be 

done to verify the design requirements and/or basis. Design, maintenance, and testing 

practices contributed to the FLEX diesel generator performance challenges at River Bend. 

Initial acceptance and post-maintenance testing of the FLEX diesel generators was performed 
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by the vendor using step/resistive loading instead of the actual instantaneous loading that would 

be experienced during a beyond-design-basis external event. When EG-5 was tested under 

realistic loading conditions in April 2020, it failed because the previous testing had not 

considered the impact of instantaneous loading on trip setpoints. 

In January 2019, the FLEX diesel generator vendor replaced the digital controllers on EG-1 

through EG-5 at River Bend. This was done under a commercial-grade process, and licensee’s 

oversight of these functions by the vendor was not appropriately controlled. The change 

impacted the operation of the diesel generator coolant temperature switches, which caused EG1 and EG-2 to trip on high coolant temperature during subsequent testing runs in October 2019. 

The vendor-recommended post-installation testing was not adequate because the licensee 

failed to identify and document critical FLEX diesel generator characteristics and ensure that 

changes were properly implemented, verified, and incorporated into site documentation. 

The guidance in NEI 12-06 states that any deviations from vendor recommendations for periodic 

testing and maintenance should be justified. At River Bend, the vendor recommended 

performing load testing every 2 weeks for EG-1 and EG-2, and monthly for EG-3, EG-4 and EG5. Because the vendor recommendations are based on continual use of equipment and not for 

equipment in standby use, the licensee based its preventive maintenance frequency on generic 

Electric Power Research Institute guidance and ran the diesel generators every six months. 

However, the licensee did not document its justification for deviating from the vendorrecommended testing periodicity. Following the operational challenges at River Bend in 

September 2019, the NRC noted that the licensee’s causal analysis cited the 6-month testing 

periodicity as a possible contributor to the failures of the diesel generators to remain running 

after initial start. 

At Clinton, the phase rotation check, which was identified as a critical parameter in the 

engineering change document, was not translated into a work instruction when performing initial 

testing on the “A” FLEX diesel generator. As a result, the licensee did not identify the incorrect 

phase rotation during original installation. When NRC inspectors questioned the phase rotation 

during the post-compliance inspection, the licensee created an action request to follow up on 

the question. The action request was not properly evaluated and resolved by the licensee, 

which thus missed a second opportunity to correct the phase rotation. 

In addition to meeting the requirements of EA-12-049, many licensees also use onsite FLEX 

equipment to mitigate the risk related to taking safety-related systems out of service and to 

reduce the plant’s overall risk profile. Examples include license amendment requests to 

increase the allowed outage times of safety-related equipment, requests for enforcement 

discretion, and modifications to plant probabilistic risk assessment models. While FLEX 

equipment may be used to reduce risk and allow increased operational flexibility, it is also 

important to note that FLEX equipment should be reliable if it is to be credited for use as a 

backup for any safety-related structures, systems, or components. 
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CONTACTS 

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: Huda Akhavannik, NRR/DRO 

301-415-5253 

E-mail: Huda.Akhavannik@nrc.gov 

 Eric Thomas, NRR/DRO 

301-415-6772 

E-mail: Eric.Thomas@nrc.gov 

Note: NRC generic communications may be found on the NRC public Web site, 

https://www.nrc.gov, under “NRC Library,” “Document Collections.” 
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INFORMATION NOTICE 2020-02, “FLEX DIESEL GENERATOR OPERATIONAL 

CHALLENGES,” DATE: SEPTEMBER 15, 2020 

ADAMS Accession No.: ML20196L822 *email concurrence EPIDS No. A11017/L-2020-GEN-0002 

OFFICE NRR/DRO/IOEB QTE NRR/DRO/IOEB/PM NRR/DRO/IOEB/LA NRR/DEX/EENB 

NAME EThomas* JDougherty* BBenney* IBetts* TMartinezNavedo* 

DATE 7/17/20 7/23/20 7/30/20 7/28/20 8/27/20 

OFFICE NRR/DORL/BC OE NRR/DRO/IOEB/BC NRR/DRO/D 

NAME DWrona* RFretz* LRegner* CMiller* 

DATE 8/11/20 8/10/20 9/14/20 9/15/20 

OFFICIAL RECORD COPY