NRC High Energy Arc Fault (HEAF) Research

1. What is a high energy arcing fault (HEAF)?

High energy arcing fault, or "HEAF," is a term used to describe a sustained discharge of electric current across a gap between two conductors with different voltages. This type of electrical failure is typically observed in switching equipment with voltages of 440 volt (V) and higher.

When switching equipment is operating normally, electric current flows through the intended circuit path. An insulating medium—usually air—prevents the current from flowing to other conductors. However, when the electric field across the insulating medium grows too large, the medium can start to "break down," or become partially conductive, which allows current to jump across the gap. As the current flows across the gap, the resistance of the medium causes the arc to generate significant heat, light, and pressure, which has the potential to damage surrounding equipment.

2. Is a HEAF the same as an arc flash?

The term "arc flash" is a broad term used by the electrical and fire protection communities to refer to an electrical arc. The National Fire Protection Association (NFPA) and Institute of Electrical and Electronics Engineers (IEEE) both have standards that deal with arc flashes; however, the focus of these standards is personnel protection.

HEAFs and arc flashes are similar, but not identical. Both HEAFs and arc flashes start with an electrical arc. The difference between them is duration, or how long the discharge lasts before being extinguished. Arc flashes are arcs that are extinguished quickly—usually within a few electrical cycles, or fractions of a second. HEAFs are arcs that are sustained for longer times. While there is no precise threshold for a HEAF, the term usually refers to arcing faults that last for two seconds or longer.

Nuclear Power Plants (NPPs) use protective relays to quickly detect and isolate electrical faults like arcs. When protective relays work as designed, they often extinguish the arc quickly and prevent it from becoming a HEAF.

3. What causes a HEAF?

A review of NPP operating experience has identified a number of different causes for HEAFs. These include:

• Contamination/foreign material ingress: When foreign material finds its way into the electrode gap, it can lower the resistance of the gap causing current to flow.
• Poor connections: If an electrical connection is not secure, such as a misaligned breaker or loose bolts, an unexpected gap can occur in the circuit, causing an arc.
• Aged electrical connections: As electrical connections age, they can begin to develop resistance. Over time, the connection can deteriorate to the point where an unexpected gap occurs, causing an arc.
• Human error: Operators and technicians can incorrectly install, or accidentally energize, equipment, leading to an arcing fault.
• Design errors: Electrical distribution systems rely on protective relays to isolate faults. If the parameters of the protective systems are not properly set, arcing faults can persist for longer periods of time.

4. Have we seen HEAFs in U.S. nuclear power plants?

Yes. From 1979 to 2017, twenty HEAF events have been observed in operating U.S. NPPs. These events have occurred in iso-phase bus ducts, non-segregated bus ducts, switchgear, and load centers. The most recent publicly available list of HEAF events is NUREG-2169, "Nuclear Power Plant Fire Ignition Frequency and Non-Suppression Probability Estimation Using the Updated Fire Events Database" (ML15016A069); however, this report only documents events through 2009. As part of the ongoing HEAF research program, a thorough review of U.S. operating experience is being conducted, and a more complete and up-to-date list will be published.

5. What about international NPP operating experience?

The Organisation for Economic Co-Operation and Development (OECD)/Nuclear Energy Agency (NEA) conducted a review of fire events for participating countries and identified 48 HEAF events over 30 years of operating experience. These events account for approximately 10% of the documented fire events in an international database of fire events. The report detailing the analysis of these HEAF events is available on the OECD's public website.

6. Can you provide some examples of U.S. and international HEAF events and explain how they impacted reactor safety?

On February 3, 2001 a HEAF occurred at the San Onofre Nuclear Generating Station, Unit 3 located in the US. The HEAF involved a 4.1 kV switchgear enclosure, which caused a fire damaging five electrical enclosures and igniting cable trays above the fire. The fire lasted for over two hours and the damage was so extensive that the exact cause of the fault could not be determined.

On March 18, 2001 a HEAF occurred at the Maanshan NPP located in Taiwan. The arcing, smoke, ionized gases, and fire released by the energetic electrical fault caused damage to other switchgear enclosures, including additional faults which resulted a loss of offsite power (LOOP) incident. This event was further complicated by the failure of the emergency diesel generators, which caused a full station blackout. The conditional core damage probability (CCDP), a measure of the risk to the plant from this event, was estimated to have been 2.2E-3.

On March 28, 2010 two HEAFs occurred at the H.B. Robinson Steam Electric Plant, Unit 2 located in the US. The plant response was complicated by equipment malfunctions and failure of the operating crew to diagnose plant conditions. The HEAF caused failures of multiple circuit breakers and cables, required pump realignments and caused interruptions in power.

On March 11, 2011 a HEAF occurred at the Onagawa Nuclear Power Plant, Unit 1, located in Japan during the Tohoku earthquake. The fault was caused by the seismic activity and damaged multiple sections of medium-voltage switchgear enclosures. The fire lasted for seven hours due to heavy smoke conditions and plant access issues resulting from the earthquake and tsunami.

7. Are HEAFs unique to Nuclear Power Plants?

No. HEAFs can occur anywhere electrical distribution equipment with voltages greater than 440 V is in operation, such as non-nuclear power plants, electrical switchyards, and industrial settings.

8. Can you provide a couple examples of non-nuclear power plants HEAFs?

Some recent examples of non-nuclear power plant HEAFs are:

• In December of 2017, an arcing fault in an underground electrical tunnel at Atlanta's Hartsfield-Jackson airport damaged a switchgear and ignited surrounding cables, causing a fire and 11-hour power outage.
• In December of 2018, an arcing fault in a piece of 138 kV equipment at a Con Edison substation lit up the New York City skyline with a blue glow and caused area power outages.
• In November of 2018, an employee at the coal-fueled Yallourn power station in Australia was working on a 6.6 kV circuit breaker when an arcing fault occurred, fatally injuring him.

9. Why are HEAFs of special interest to the NRC?

Fire protection plays an important role in the safe operation of nuclear power plants. NRC NPP license holders are required to demonstrate that their facilities are capable of maintaining key safety functions in the event of a fire or other hazard. Due to their energetic nature, HEAFs have the potential to quickly damage multiple systems, structures, or components before the installed fire protection systems can actuate. This can challenge the facility's post-fire safe shutdown strategy.

10. How hot does a HEAF get?

The temperature of the area of current flow between the electrodes (the arc column), depends on a number of factors, including the resistance of the gap and the magnitude of the current. Electrical arcs have been observed to reach temperatures of 35,000 degrees Fahrenheit (19,427 °C).

The temperature of an arc's surroundings depends on the properties of the arc itself, as well as enclosure geometry, surface emissivity, exposure duration, and shielding. Operational experience and experimental results show that arcs are capable of melting and vaporizing copper, aluminum, and steel.

11. How does a HEAF do damage to adjacent equipment?

HEAFs can damage adjacent equipment in a number of ways. The most common mode of damage observed in operational experience and testing is direct thermal damage. HEAF events generate extremely high temperatures and heat fluxes which can melt, vaporize, or ignite surrounding equipment and electrical cables. In some cases, the HEAF can ignite surrounding equipment that continues to burn after the arc has been extinguished. There have also been observed instances of pressure damage; as the air around the arc is rapidly heated, it expands. The resulting pressure front can damage nearby equipment. A HEAF also generates plasma, ionized gas, and particulate matter, which can affect the function of nearby equipment.

12. What is a zone of influence (ZOI) and how is it used?

In fire probabilistic risk assessment (PRA), a zone of influence (ZOI) is the area surrounding a piece of equipment that is expected to experience adverse conditions if a HEAF were to occur. This "zone" allows PRA practitioners to determine what other equipment might be damaged as a result of a HEAF, and determine the potential impact on plant safety.

13. How is a HEAF modeled in a fire PRA?

The current guidance for modeling a HEAF in fire PRA can be found in Appendix M of NUREG/CR-6850, Volume 2, "EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities," and Chapter 7 of Supplement 1 to NUREG/CR-6850. In summary, the ZOI for a HEAF in an electrical cabinet is three feet in the horizontal direction and five feet in the vertical direction (i.e. any unprotected cables or vulnerable targets in this region are candidates for damage by a HEAF). The ZOI for a bus duct is a 1.5 foot sphere at the location of the fault, and a 30-degree cone downwards to a maximum diameter of 20 feet.

14. How was the HEAF model and ZOI in Appendix M of NUREG/CR-6850 developed and what is its technical basis?

The fire PRA methodology documented in NUREG/CR-6850, Volume 2, "EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities,” was developed by a joint working group comprised of NRC staff from the Office of Nuclear Regulatory Research (RES) and the Electric Power Research Institute (EPRI). The model for HEAFs was developed through the working group’s review of operating experience, expert judgement, and knowledge of fire physics. One HEAF event was particularly influential in the working group’s development of the ZOI: the 2001 HEAF at the San Onofre Nuclear Generating Station (SONGS), documented in Information Notice IN 2002-27. At the time of the working group’s compilation of the methodology, this event was the most recent and best documented example of a HEAF. The working group used licensee pictures, reports, and data to evaluate the extent of the damage.  This event, combined with the available operating experience from other HEAFs, informed the model and frequency of HEAF events. No experimental testing was performed during the development of Appendix M of NUREG/CR-6850.

15. How frequently do HEAFs occur?

The frequency values that are currently used in fire PRA are documented in NUREG-2169, "Nuclear Power Plant Fire Ignition Frequency and Non-Suppression Probability Estimation Using the Updated Fire Events Database." The frequency values are reproduced in the table below.

HEAFs are split into four categories to account for differences in equipment types and voltages. The "mean frequency" is reported in units of occurrences per reactor-year. Based on this data, HEAF events have occurred, on average, once every 250 reactor-years. Current research is focused on updating the HEAF frequency estimation and adding additional levels of detail to the PRA framework, which will better reflect plant-specific configurations.

16. Is there any international HEAF research going on?

Yes. The NRC is participating in an international research project conducted under the auspices of the OECD/NEA. Separately, Japanese regulatory authorities are conducting HEAF research, and the NRC is coordinating with relevant parties to remain informed about its status and results.

The international community has also provided their insights to NRC's HEAF research program. In 2017, the NRC hosted a Phenomena Identification and Ranking Table (PIRT) exercise workshop, designed to help identify priorities for future HEAF research (ML18032A318). Several international organizations participated, including German, French, Japanese, and Korean regulatory agencies.

17. What is the relationship between voltage, current and duration as it applies to HEAFs?

There are two types of voltage that are relevant when considering a HEAF: the system voltage (the normal operating voltage of the equipment) and the arc voltage (the difference in electric potential across the conductors while the arc is occurring). The system voltage affects the ability of the electric current to strike and maintain an arc, however it is the arc voltage that plays a role in determining the energy release of the HEAF.

The power of an arc is primarily determined by the product of the arc voltage and the arc current. However, operating experience and testing have shown that even high-power arcs may not be particularly severe if they are quickly extinguished. This is where arc duration comes into play; the total amount of energy released by the HEAF is the product of arc power and arc duration. The longer an arc is sustained, the more time it has to generate damaging conditions, which is why limiting the duration of an arc is a key factor in protecting against HEAFs.

18. Why is the NRC concerned about HEAFs involving aluminum components?

The NRC conducted a series of confirmatory HEAF tests between 2014 and 2016. Two of the tests that involved aluminum conductors had energy releases far greater than anticipated. The experimental data from these tests indicated that the ZOI currently postulated for HEAF events might not be sufficient to accurately assess the extent of their impact on plant equipment, especially when aluminum is involved.

19. What makes aluminum potentially more damaging?

When metals like copper and aluminum are exposed to the high temperatures present in HEAFs, they react with the oxygen in air to form metal oxides. Both copper and aluminum release energy when they undergo oxidation, but aluminum releases over ten times as much energy (per mole) as copper does. This extra energy can increase the severity of a HEAF and cause more damage to surrounding equipment.

20. Has the NRC shared this information with licensees and the public?

Yes. After the confirmatory HEAF tests, where the potential for additional energy release was observed, the NRC conducted a review of U.S. operating experience to identify HEAF events where aluminum may have been present. A comprehensive review of the findings and relevant test results were published in Information Notice 2017-04, "High Energy Arcing Faults in Electrical Equipment Containing Aluminum Components," (ML17058A343). An Information Notice is a type of communication the NRC uses to inform licensees of a particular issue, and requires no action or response.

The NRC has also communicated with the public through informational sessions at the annual Regulatory Information Conference (2017, 2018, 2019, and 2021) and public meetings to inform and solicit input from stakeholders and the public.

21. Are the U.S. nuclear power plants safe to operate today considering this hazard?

Yes. Although the NRC is conducting research to more accurately quantify the risk from HEAFs, U.S. NPPs remain safe to operate.The first step in evaluating an emergent issue is for the appropriate regulatory office to conduct an immediate safety review of the issue. This safety review takes into account the characteristics of the issue, the frequency of occurrence, protective measures already in place, and other relevant information. The NRC's Office of Nuclear Reactor Regulation conducted this review in March of 2016, and concluded that no immediate safety concern exists. More information can be found in the full evaluation.

22. What is the status of the aluminum HEAF pre-Generic Issue?

The NRC's GI program is governed by NRC Management Directive (MD) 6.4, "Generic Issues Program" (ML14245A048). MD 6.4 describes the three stages that a proposed GI can go through, provided certain criteria are met: screening, assessment, and regulatory office implementation. If, at any point, the proposed GI no longer meets the criteria for remaining in the GI program, the proposed GI exits the program and is referred to the appropriate regulatory program.
 
The potential for aluminum to increase the energy released during a HEAF was submitted to the NRC's Generic Issues (GI) program in 2016 and designated pre-GI-018. In August of 2021, NRC staff determined that pre-GI-018 no longer met criterion five: "The issue’s risk or safety significance can be adequately determined in a timely manner (i.e., it does not involve phenomena or other uncertainties that would require long-term study and/or experimental research to establish the risk or safety significance)." Accordingly, the issue has exited the GI program and is being dispositioned through continued research and the LIC-504 process.

23. Is the NRC working with any other groups or agencies on this issue?

Yes. The Office of Nuclear Regulatory Research is working with the Electric Power Research Institute (EPRI) under a Memorandum of Understanding (MOU) to advance the state of knowledge regarding HEAFs. The NRC has also contracted with nationally-recognized experts in the fields of measurement, modeling, and high-power testing, such as Sandia National Laboratories, and the National Institute of Standards and Technology.

24. Has anything been done to reduce the frequency of HEAFs?

The EPRI has published a report titled "Critical Maintenance Insights on Preventing High-Energy Arcing Faults". This report makes a number of recommendations regarding equipment maintenance to reduce the probability of a HEAF, including: periodic circuit breaker maintenance, visual inspection of equipment and connections, connection resistance measurements to identify degraded connections, periodic calibration of protective relays, and others.

25. Can HEAFs occur with, or as a result of, external hazards or events?

Yes, although this combination of events has not occurred in the U.S. NPPs. The most notable example of this is the HEAF that occurred in Japan's Onagawa Nuclear Power Plant. The 2011 Tohoku earthquake, which was also responsible for the Fukushima Daiichi accident, caused a HEAF in a 6.9 kV switchgear. The NRC has worked with Japan's Secretariat of Nuclear Regulatory Authority to understand the sequence of events and consequences of the Onagawa HEAF.

26. What are the steps and the schedules for the HEAF research program?

A project plan that addresses the various components of the HEAF research program, major milestones, and analytical approach is available through this web page in the section below.

27. How will the NRC continue to keep the public informed of progress on this issue?

The NRC will continue to keep the public informed of progress through regular updates to the project plan, which will be posted to this web page. The NRC will also continue to hold public meetings to share updates with, and solicit input from, stakeholders and the public. The HEAF research program also includes a number of deliverables, which will be published and made publicly available when completed.