High Energy Arcing Fault Frequency and Consequence Modeling – Final Report (NUREG-2262, EPRI 3002025942)

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Publication Information

Date Published: April 2023

U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research Washington, D.C. 20555-0001	Electric Power Research Institute (EPRI) 3420 Hillview Avenue Washington, D.C. 20555-0001 Palo Alto, CA 94304-1338
U.S. NRC-RES Project Manager MH. Salley	EPRI Project Managers A. Lindeman M. Randelovic

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Abstract

High energy arcing faults (HEAFs) are one type of hazard modeled in fire probabilistic risk assessments. NUREG/CR-6850 and NUREG/CR-6850, Supplement 1, provide the basic methods to analyze the risk associated with HEAFs in power distribution equipment (switchgear and load centers) and bus ducts (including iso-phase bus ducts), respectively. Since the publication of these two reports, the state of knowledge of HEAF phenomena has advanced significantly. A thorough understanding of the nuclear power plant electrical distribution system and its performance during faulted conditions has been achieved, along with a review and categorization of industry events. Additionally, experimentation (including full-scale testing on HEAF-susceptible equipment, small-scale testing, and simulation) has increased the understanding of parameters that affect the dimensions of the zone of influence (ZOI).

This report combines previous HEAF-related research and provides methods and data to more realistically calculate plant risk due to HEAFs. Ignition frequency and non-suppression estimates are updated with the most recently available industry operating experience. Most importantly, the ZOI selection is greatly expanded. Previously, there was one ZOI for switchgear and load centers, one ZOI for bus ducts, and one ZOI for iso-phase bus ducts. The computational fluid dynamics software Fire Dynamics Simulator (FDS) has been benchmarked against full-scale tests and is used to predict the thermal exposure of targets in the vicinity of a HEAF. FDS simulations are performed for three classes of equipment: load centers, switchgear, and non-segregated bus ducts. The simulations varied parameters such as arc power, arc duration, arc location, electrode composition, and type of equipment. The working group reviewed and grouped the ZOI results from the simulation effort to determine consensus ZOIs for the three equipment classes, with varying levels of detail commensurate with potential risk significance.

A key parameter of the ZOI is the time overcurrent (51) relay setting, or fault clearing time, of the auxiliary power transformer. The faster the fault clearing time, the smaller the energy release. The speed of this protection determines whether the updated medium-voltage switchgear ZOIs are smaller or larger than the ZOI in NUREG/CR-6850. For non-segregated bus ducts, the ZOIs are also dependent on the enclosure material of the bus duct (either aluminum or steel). In general, the ZOIs for non-segregated bus ducts are larger, except for fault clearing times of 2 s or less on the station auxiliary transformer (feed from off-site). The load center supply breaker ZOIs are smaller than the ZOI recommended in NUREG/CR-6850.

Keywords
Arcing fault
Fire events
Fire ignition frequency (FIF)
Fire probabilistic risk assessment (Fire PRA)
High energy arcing fault (HEAF)