High Energy Arc Fault Testing for International Nuclear Power Plant Safety at KEMA Laboratories Chalfont
The United States Nuclear Regulatory Commission (NRC) has identified a potential vulnerability where existing analytical models supporting plant specific safety analyses may be non-conservative. This vulnerability exists for electrical equipment that includes components made of aluminum when subjected to high energy arc fault (HEAF) conditions. Recent testing conducted at the KEMA Laboratories Chalfont facility indicates that the area damaged around the equipment or the “zone of influence” (ZOI) may be larger than postulated in the current methodology for HEAF analysis, NUREG/CR – 6850 EPRI 1011989 “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities, Volume 2: Detailed Methodology”. This methodology supports plant specific safety analyses used by licensees using the National Fire Protection Association Standard 805 (NFPA 805) to meet fire protection regulations (10 CFR 50.48). The presence of aluminum can create a more energetic plasma arc, that under some circumstances, can cause larger damage scenarios to electrical enclosures and/or transport high energy gaseous particles and plasma farther than previously assumed. In addition to NFPA 805 analyses, HEAF events involving aluminum components may require licensees and staff to re-analyze how licensees comply with regulatory requirements under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix A, General Design Criteria 3, “Fire Protection”.
High Energy Arc Faults (HEAF) events are defined as energetic or explosive electrical equipment faults characterized by a rapid release of energy in the form of heat, light, vaporized metal and pressure increase due to high current arcs between energized electrical conductors or between energized electrical components and neutral or ground. HEAF events may also result in projectiles being ejected from the electrical component or cabinet of origin and result in fire. The energetic fault scenario consists of two distinct phases. First phase: short, rapid release of electrical energy which may result in projectiles (from damaged electrical components or housing) and/or fire(s) involving the electrical device itself, as well as any external exposed combustibles, such as overhead exposed cable trays or nearby panels, that may be ignited during the energetic phase. Second phase (the ensuing fire(s)): is treated similarly to other postulated fires as specified in NUREG/CR-6850, EPRI 1011989.
Recent international HEAF testing was performed by the NRC Office of Nuclear Regulatory Research (RES) in collaboration with 7 other member countries including; Canada, France, Finland, Germany, Korea, Japan and Spain. The project was initiated after investigating international operating events which indicated HEAF events were a worldwide fire risk driver constituting roughly 10% of the international database. The OECD Fire Project – Topical Report No.1, “Analysis of High Energy Arcing Fault (HEAF) Fire Events,” NEA/CSNI/R (2013) published in June 2013 documents these international events.
The current program was conducted to explore the basic configurations, failure modes, and effects of HEAF events to confirm the zone of influence (ZOI) for HEAF’s that was specified in NUREG/CR-6850, EPRI 1011989. A total of 26 tests were performed, most electrical equipment with copper components, which exhibited similar damage states to those postulated in the current guidance. However, some unexpected results were obtained when equipment that included components made of aluminum were tested:
- One test included a low voltage (480 V) thin wall switchgear cabinet unit utilizing aluminum bus bars that caused substantially more damage to the switchgear enclosure than in similar tests utilizing copper bus bars. The video recording of the event indicated that the ZOI of the HEAF event is much larger than the current guidance postulates for damage in Appendix M “High Energy Arc Faults” of NUREG/CR-6850, EPRI 1011989. In addition to the larger ZOI, portions of the laboratory test cell were coated in a conductive aluminum combustion byproducts. This coating caused damage to the facility and shorted out electrical equipment due to “plating” effects.
- Another test included a section of bus duct removed from the decommissioned Zion nuclear plant that utilized medium voltage (4.16 kV) non-segregated copper bus bars inside an aluminum bus duct enclosure. The HEAF test was set up with the bus duct section closed off with a piece of insulating material referred to as “red board.” During the arc, the red board was not able to withstand the explosive discharge of energy/gases, resulting in a large discharge (jet) of arcing energy and products (~ 30 feet), substantially beyond the expected ZOI in the current analysis method. Coating of the laboratory test cell by aluminum combustion byproducts was also observed during this test.
Based on the observed physical damage to the test specimens and the video recordings of the tests, it appears that the presence of aluminum in either the bus bars or the bus duct housing can cause a more energetic plasma arc and subsequent metal fire. Under some circumstances this may cause a larger amount of cabinet damage and/or cause the transport of gaseous high energy particles/plasma farther than previously assumed.
The recent international tests suggest that HEAF scenarios involving aluminum components may have a zone of influence that is not bounded by the current guidance in NUREG/CR-6850 EPRI 1011989 “EPRI/NRC-RES Fire PRA Methodology for Nuclear Power Facilities, Volume 2: Detailed Methodology”, thereby underestimating the risk from HEAF events. Additionally, mitigation techniques have been used as part of the transition to NFPA 805 including “HEAF shields” which theoretically enable a NPP to mitigate damage conditions and reduce the risk of a particular scenario. “HEAF shields” currently in use are based solely on engineering judgment and have no design basis, no qualification tests, no test standards, no acceptance criteria and minimal regulatory footprint. Subsequent evaluation may be needed to evaluate these HEAF scenarios in order to maintain an acceptable risk profile. Any non-conservatisms discovered as a result of this HEAF issue would mean that the current baseline risk model at plants that have aluminum components would underestimate the risk.
The results described above demonstrate the practical value of independent physical testing vs. “paper testing” or in this case, sophisticated modelling techniques, to provide engineers and decision makers critical information to safeguard, life, property and the environment. The KEMA Laboratories were selected by the US NRC because of the unique capabilities of our people and our facilities with the added benefit of being completely independent and impartial