Fire Ignition Frequency (Task 6)

Task Overview

Background

This task describes the approach to develop frequency estimates for fire compartments and scenarios. Significant changes from the EPRI FIVE method have been made in this task. The changes generally relate to use of challenging events, considerations associated with data quality, and increased use of a fully component based ignition frequency model (as opposed to the location/component-based model used, for example, in FIVE).

Purpose

This section describes the procedure for estimating the fire-ignition frequencies associated with fire ignition sources. Generic ignition frequencies that can be specialized to plant conditions in terms of plant characteristics and plant fire event experience are provided. Uncertainties in the generic frequencies are also provided in terms of 5th, 50th, and 95th percentiles.

Scope

This work package addresses the following fire-ignition frequency related issues:

• Plant specific fire event data review and generic fire frequency update using Bayesian approach,
• Equipment (ignition source) count by compartment,
• Apportioning of ignition frequencies according to compartment-specific configurations, and
• Uncertainty considerations in the fire frequencies.

Related Element(s) of ASME/ANS PRA Standard, ASME-RA-Sb-2013

Fire Ignition Frequency (IGN)

Related EPRI 1011989 NUREG/CR-6850 Appendices

Appendix C, Appendix for Chapter 6, Determination of Generic Fire Frequencies

Appendix F, Appendix for Chapter 8, Walkdown Forms

Supplemental Guidance

EPRI 1019259 NUREG/CR-6850 Supplement 1, Chapter 3

Ignition Source Counting Guidance for Electrical Cabinets (FAQ 06-0016)

Relevant Correspondence:

• FAQ 06-0016, Rev. 1 (ADAMS Accession No. ML070580334)
• Closure Memo, FAQ 06-0016, Counting Electrical Panels and Cabinets, dated October 5, 2007 (ADAMS Accession No. ML072700475)

NUREG/CR-6850 (EPRI 1011989) provided guidance to count cabinets in a “typical” or visible vertical section configuration, however additional guidance was necessary for panels with “atypical” configuration where the guidance for vertical segments could be interpreted in different ways. FAQ 06-0016 was proposed to clarify guidance on electrical panel/cabinet counting for fire frequency.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This guidance in FAQ 06-0016 provides clarification on the fire ignition frequency counting application listed in Section 6.5.6 Bin 15, page 6-16.

Examples: Detailed examples of counting electrical cabinets are provided in Section 3.2 of NUREG/CR-6850 Supplement 1. It is important to note that the counting methodology described in Section 3.2 of NUREG/CR-6850 Supplement 1, doesn’t require a volumetric comparison. Nonetheless, the classification of electrical enclosures in NUREG-2178, Vol. 1, although is consistent and does not affect the existing ignition source counting information available in Chapter 6 of NUREG/CR-6850 and applicable fire PRA FAQs, it requires a volumetric size analysis for asigning HRR probabilities distributions.

EPRI 1019259 NUREG/CR-6850 Supplement 1, Chapter 4

Ignition Source Counting Guidance for High-Energy Arcing Faults (FAQ 06-0017)

Relevant Correspondence:

• FAQ 06-0017, Rev. 2 (ADAMS Accession No. ML071570255)
• Closure Memo, FAQ 06-0017, Clarifying/Enhancing Guidance for Counting High Energy Arcing Faults, dated September 26, 2007 (ADAMS Accession No. ML072500300)

NUREG/CR-6850 (EPRI 1011989) provided guidance for treatment of high energy arcing faults, however inconsistency in the application of the treatment was observed. FAQ 06-0017 was proposed to clarify guidance on high energy arcing fault counting for partitioning of the fire ignition frequency task. The FAQ provides guidance for counting arcing faults based on panel voltages. In addition, the FAQ provides guidance to exclude MCCs from arcing fault counting.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This guidance in FAQ 06-0017 provides clarification on the fire ignition frequency counting application listed in Section 6.5.6 Bin 16, page 6-16.

Examples: Let's assume there is a total of fifty (50) vertical sections associated with HEAFs in a plant. Thirty (30) out of the fifty (50) sections are classified as HEAF for Low-Voltage Panels (Bin 16.a) and the remaining twenty (20) vertical sections are classified as HEAF for Medium-Voltage Panels (Bin 16.b). Therefore, per Table 4-6 in NUREG 2169, the generic frequency that should be applied (i.e., partitioned) equally to the vertical sections classified as HEAF for Low-Voltage Panels (counted as thirty in this example) is 1.52E-04 and the generic frequency that should be applied (i.e., partitioned) equally to the vertical sections classified as HEAF for Medium-Voltage Panels (counted as twenty in this example) is 2.13E-03.

EPRI 1019259 NUREG/CR-6850 Supplement 1, Chapter 5

Ignition Source Counting Guidance for Main Control Board (MCB) (FAQ 06-0018)

Relevant Correspondence:

• FAQ 06-0018, Rev. 1 (ADAMS Accession No. ML070800079)
• Closure Memo, FAQ 06-0018, Rev 1 Ignition Source Counting Guidance for Main Control Boards, dated September 26, 2007 (ADAMS Accession No. ML072500273)

NUREG/CR-6850 (EPRI 1011989) provided guidance for the main control board in Appendix L, however the applicability to Task 6, Bin 4 was unclear. FAQ 06-0018 was proposed to clarify guidance on determining the definition of the main control board for fire ignition frequency and Appendix L applications.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This guidance in FAQ 06-0018 provides clarification on the fire ignition frequency counting application listed in Section 6.5.6 Bin 4 (i.e., the main control board), page 6-15. Examples:

MCB (Bin 4) counting includes:

• Horseshoe, and
• Other detached panels like “bench-board” (also referred as “consoles”) which are:
  • Serving as panels that were detached serving as an integral part of the main plant monitoring and control functions;
  • Located in the center of the operators’ main work area; and
  • Manned on a nearly continuous basis.

Other “back panels” and detached panels housing items such as balance-of-plant and off-site power controls and indicators should be excluded from the main control board and treated as general electrical panels (Bin 15). The conditions of the MCB rear side to be treated as part of the MCB are detailed in See Section 8.2.10.

EPRI 1019259 NUREG/CR-6850 Supplement 1, Chapter 6

Miscellaneous Fire Ignition Frequency Binning Issues (FAQ 07-0031)

Relevant Correspondence:

• FAQ 07-0031, Rev. 0 (ADAMS Accession No. ML071380238)
• Closure Memo, FAQ 07-0031, Rev 1 Clarification of Miscellaneous Ignition Source Binning Issues, dated December 17, 2007 (ADAMS Accession No. ML072840658)

NUREG/CR-6850 (EPRI 1011989) provided guidance on electrical equipment counting, however the FAQ was intended to provide a more consistent basis for counting of miscellaneous electrical equipment. FAQ 07-0031 was proposed to clarify guidance for several ignition source bins to include electrical motors (Bin 14), pumps (Bin 21), transformers (Bin 23), and ventilation subsystems (bin 26). The FAQ also provided guidance on eliminating motors and transformers from ignition source counting based on size or function.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This guidance in FAQ 07-0031provides clarification on the fire ignition frequency counting application listed in Section 6.5.6 Bins 14, 21, 23, and 26, pages 6-16 and 6-18.

Examples:

• Electric Motors (Bin 14) counting includes: any electric motor with a rating greater than 5 hp, unless the motor meets at least one of the two following exclusionary provisions:
  • Motors that are attached to equipment already identified and counted in other bins.
  • Motors, including MOV drive motors, which are totally enclosed regardless of the motor size.
• Pumps (Bin 21) counting includes: pumps rated above 5 hp.
• Transformers (Bin 23) counting includes: all dry-type transformers with a rating greater than 45 kVA and all oil-filled transformers.
• Ventilation Subsystems (Bin 26) counting includes: any ventilation subsystem (air conditioning units, chillers, fan motors, air filters, dampers, etc.) with an electric motor greater than 5 hp.

The electric motors, pumps and ventilation subsystems rated 5 hp or less and the dry transformers with a rating greater than 45 kVA should be excluded.

EPRI 1019259 NUREG/CR-6850 Supplement 1, Chapter 7

Bus Duct (Counting) Guidance for High-Energy Arcing Faults (FAQ 07-0035)

Relevant Correspondence:

• FAQ 07-0035, Rev. 2 (ADAMS Accession No. ML071650151)
• Closure Memo, FAQ 07-0035, Rev 1 Bus Duct Counting Guidance for High Energy Arcing Faults, dated July 16, 2009 (ADAMS Accession No. ML091620572)

NUREG/CR-6850 (EPRI 1011989) provided guidance for treatment of high energy arcing faults in switchgear and load centers, however is absent of guidance for bus duct fires. FAQ 07-0035 was proposed to clarify treatment of high energy arcing faults for bus duct failures. The FAQ provides guidance for counting and fire event frequency for bus ducts.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This guidance in FAQ 07-0035 provides clarification on the fire ignition frequency counting application listed in Appendix M of NUREG/CR-6850 (EPRI 1011989).

Examples: Non-segmented (or continuous) bus ducts and cable ducts): There is no transition points other than the terminations at the end device, so no treatment (no counting) of bus duct faults/fires independent from the treatment of fires for the end devices is required.

Iso-phase bus ducts (Bin 16.2): Count the total number of iso-phase buses (an iso-phase bus includes all three phases) per unit. For individual fire scenarios, the plant-wide frequency is applied (i.e., partitioned) equally to each end of each iso-phase duct counted. For example, typically there is one (1) iso-phase bus per unit in a plant, therefore there are 2 ends for the iso-phase duct counted. In this case, the generic frequency that should be applied (i.e., partitioned) equally to the ends of each iso-phase duct (counted as two for the unit in this example) is 5.91E-0, per Table 4-6 in NUREG-2169.

Segmented bus ducts (Bin 16.1):

Counting approach 1: If the transition points along the length of the segmented bus duct can be identified by external visual inspection, or based on plant electrical construction drawings, then count the total number of transition points. The transition point counting excludes the bus end termination points, which are considered a part of the end device for fire frequency purposes. For example, let's assume there are seven (7) transitions point in a segmented bus duct, in addition to the bus end termination points. In this case, the generic frequency that should be applied (i.e., partitioned) equally to the transition points of the segmented bus duct (counted as seven in this example) is 1.10E-03, per Table 4-6 in NUREG 2169.

Counting approach 2: If the transition points cannot be identified based on external visual inspection, or by plant electrical construction drawings, then the partitioning of fire frequency to a specific fire scenario is based on apportioning of the fire frequency equally along the length of the bus duct. A “per linear foot” fire frequency can then be estimated by dividing the plant-wide fire frequency by the total length of segmented bus duct in the plant. That is, the fire frequency for a given fire scenario would be based on the ratio of the length of duct for which identified targets fall within the bus duct arc fault zone of influence, to the total length of bus duct in the plant.

For example, let's assume there is a forty (40) feet long segmented bus duct in a unit that could affect two targets (A and B). Target A falls within the zone of influence of twenty-five (25) m long of the bus duct and target B within fifteen (15) m long. In this case the generic frequency (1.10E-03, per Table 4-6 in NUREG 2169) should be applied (i.e., apportioned) proportionally to the segmented bus duct length related to each target. That is 25/40 times the plant-wide fire frequency for target A and 15/40 times the plant-wide fire frequency for target B.

Finally a lower limit to the assumed fire frequency for any given fire scenario is also applied. That is, if the length of bus duct for which the identified target(s) fall within the zone of influence is less than 12 linear feet, then a minimum length of 12 feet should be assumed. Following the above example, let's assume there is a forty (40) feet long segmented bus duct in a unit that could affect two targets (A and B). Target A falls within a zone of influence of ten (10) m along the length of the bus duct and target B within forty (40) m along the length of the bus duct. In this case the generic frequency (1.10E-03, per Table 4-6 in NUREG 2169) should be applied (i.e., apportioned) proportionally to the segmented bus duct length related to each target, but a minimum length of 12 feet should be assumed for the shortest segment. That is 12/40 times the plant-wide fire frequency for target A and the remaining frequency (28/40) times the plant-wide fire frequency for target B.

Additional Considerations

FAQ 07-0035 provides additional guidance for characterizing the zone of influence, or damage, for bus duct fires as part of NUREG/CR-6850 (EPRI 1011989) Chapter 11 fire modeling guidance.