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 of ASME/ANS PRA Standard

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

Current FPRA Counting Guidance and Fire Ignition Frequencies

Table 6-1 presents relevant information such as bin, ignition source, counting guidance, and mean fire ignition frequency for ignition sources counted in the Fire PRA. In many cases, the counting guidance provided in NUREG/CR-6850 has been supplemented by additional guidance including formally published in NUREG/CR-6850 Supplement 1 and through the frequently asked questioned (FAQ) process. Links to the counting guidance is provided in the "Counting Reference" column. Similarly, the fire ignition frequencies published in NUREG/CR-6850 are no longer the most current. A second set of fire ignition frequencies was published in NUREG/CR-6850 Supplement 1 (Chapter 10, FAQ 08-0048). A memo on May 14, 2015 from the NRC, clarified that the fire ignition frequencies in NUREG/CR-6850 Supplement 1 (FAQ 08-0048) should be replaced with the most current guidance in EPRI 3002002936 (NUREG-2169).

Table 6-1: Fire Ignition Sources, Counting Guidance, and Ignition Frequencies

Bin	Plant Location	Ignition Source	Description	Count (how)	Counting Reference	Fire Ignition Frequency (Mean)	Fire Ignition Frequency Reference
1	Battery Room	Batteries	Each bank of interconnected sets of batteries located in one place (often referred to as Battery Room).	Interconnected sets of batteries is counted as one. Cells may not be counted individually.	EPRI 1011989 / NUREG/CR-6850	1.96E-04	EPRI 3002002936 (NUREG-2169)
2	Containment (PWR)	Reactor Coolant Pump	The reactor coolant pumps (RCPs) are distinct devices in PWRs that vary between two and four, depending on primary loop design.	Each reactor coolant pump is counted separately.	EPRI 1011989 / NUREG/CR-6850	1.37E-03	EPRI 3002002936 (NUREG-2169)
3	Containment (PWR)	Transients and Hotwork	General transient combustibles and hotwork activities located in Containment (PWR).	The ignition source weighting factor of transient fires is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in FAQ 12-0064 Section 6.5.7.2. The introduction of developing transient influence factors for smaller spaces than fire compartments is discussed in FAQ 14-0007.	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	4.21E-04	EPRI 3002002936 (NUREG-2169)
4	Control Room	Main Control Board	A control room typically consists of one or two (depending on the number of units) main control boards as the central element of the room.	Each main control board, typically consisting of the main horseshoe and nothing else, is counted separately. This bin may also include "benchboard" panels that are detached from, but directly in front of, the main horseshoe (at some plants such panels are referred to as "consoles"). FAQ-14-0008 also clarified that the rear side of the MCB may be treated as part of the MCB if both the rear and front sides are connected together as a single enclosure (including a continuous overhead, or by an overhead with penetrations or vents along it longitudinally, cabinet ceiling, or cables connecting the front and back sides of the MCB).	EPRI 1011989 / NUREG/CR-6850 FAQ 06-0018, Section 5 of Supplement 1 FAQ 14-0008	2.05E-3	NUREG-2178 Vol 2 / EPRI 3002016052
5	Control/Aux/Reactor Building	Cable fires caused by welding and cutting	For this bin, it is assumed that all exposed cables (i.e., cables that are not in conduits or wrapped by noncombustible materials) have an equal likelihood of experiencing a fire caused by welding and cutting across the entire location (Control Building, Auxiliary Building, or Reactor Building).	The ignition source weighting factor of cable fires caused by welding and cutting is estimated using the hot work factor and cable quantity in the fire compartment. The hot work ranking factors are described in Table 6-2 (as updated in FAQ 12-0064). Guidance for this bin is updated in FAQ 12-0064 Section 6.5.7.2 and Fire PRA FAQ 16-0010. The hot work factor is then weighed in combination with a relative numerical estimate of the quantity of cables in the location to the total quantity of cables in the entire location set to generate the final location weighting factor. The cable quantity (either total weight or total combustible load) is typically reported in the Fire Hazards Analysis (FHA).	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 16-0010	7.83E-04	EPRI 3002002936 (NUREG-2169)
6	Control/Aux/Reactor Building	Transient fires caused by welding and cutting	Transient fires due to hotwork activities located in the Control Building, Auxiliary Building, or Reactor Building.	The ignition source weighting factor of transient fires caused by welding and cutting is estimated using a ranking scheme that takes into account the hot work factor. The hot work ranking factors are described in Table 6-2 (as updated in FAQ 12-0064). Guidance for this bin is updated in FAQ 12-0064 Section 6.5.7.2 and Fire PRA FAQ 14-0007 (distributing transient influence factors for smaller spaces than fire compartments).	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	4.44E-03	EPRI 3002002936 (NUREG-2169)
7	Control/Aux/Reactor Building	Transients	General transient combustibles or activities located in the Control Building, Auxiliary Building, or Reactor Building.	The ignition source weighting factor of transient fires is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in FAQ 12-0064 Section 6.5.7.2. Developing transient influence factors for smaller spaces than fire compartments is discussed in FAQ 14-0007.	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	3.33E-03	EPRI 3002002936 (NUREG-2169)
8	Diesel Generator Room	Diesel Generators	Diesel generators are generally well-defined items that include a set of auxiliary subsystems associated with each engine. All diesel generators that are included in the electric power recovery model should be counted here. In addition to the normal safety related diesel generators, this may include the Technical Support Center diesel generators, Security diesel generators, etc. It is recommended that each diesel generator and its subsystems be counted as one unit. The subsystems may include diesel generator air start compressors, air receiver, batteries and fuel storage, and delivery system.	Each diesel generator should be counted separately. It is recommended that the electrical cabinets for engine and generator control that stand separate from the diesel generator be included as part of “Plant-Wide Components - Electrical Cabinets.” Control panels that are attached to engine may be counted as part of the engine.	EPRI 1011989 / NUREG/CR-6850	7.81E-03	EPRI 3002002936 (NUREG-2169)
9	Plant-Wide Components	Air Compressors	This bin covers the large air compressors that provide plant instrument air included in the Internal Events PRA Model. These compressors are generally well-defined devices. They may include an air receiver, air dryer, and control panel attached to the compressor. These items should be considered part of the air compressor. If portable compressors are part of the model, those compressors should also be included in the equipment count for this bin.	Air compressors are generally well-defined devices (and includes portable units credited in the PRA model). The air compressor skid, which could include an air receiver, air dryer, and control panel attached to the compressor, should be counted as one, as they are considered to be part of the air compressor. NOTE: Compressors associated with the ventilation systems and small air compressors used for specialized functions are NOT part of this bin.	EPRI 1011989 / NUREG/CR-6850	4.69E-03	EPRI 3002002936 (NUREG-2169)
10	Plant-Wide Components	Battery Chargers	These are generally well defined items associated with DC buses.	Each battery charger should be counted separately.	EPRI 1011989 / NUREG/CR-6850	1.12E-03	EPRI 3002002936 (NUREG-2169)
11	Plant-Wide Components	Cable fires caused by welding and cutting	For this bin, it is assumed that all exposed cables (i.e., cables that are not in conduits or wrapped by noncombustible materials) have an equal likelihood of experiencing a fire caused by welding and cutting across the entire location (located in the Power Block, but not in the Control Building, Auxiliary Building, Reactor Building, Turbine Building, or Containment (PWR)).	The ignition source weighting factor of cable fires caused by welding and cutting is estimated using the hot work factor and cable quantity in the fire compartment. The hot work ranking factors are described in Table 6-2 (as updated in FAQ 12-0064). Guidance for this bin is updated in FAQ 12-0064 Section 6.5.7.2 and Fire PRA FAQ 16-0010. The hot work factor is then weighed in combination with a relative numerical estimate of the quantity of cables in the location to the total quantity of cables in the entire location set to generate the final location weighting factor. The cable quantity (either total weight or total combustible load) is typically reported in the Fire Hazards Analysis (FHA).	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 16-0010	2.77E-04	EPRI 3002002936 (NUREG-2169)
12	Plant-Wide Components	Cable Run (self-ignited cable fires)	Self-ignited cables fires postulated in fire compartments with unqualified cables only or a mix of qualified cables and unqualified cables.	The cable loading of each compartment should be established using the same approach as that for Bin 5, except that, in this case, all plant fire compartments should be taken into account. The cable quantity (either total weight or total combustible load) is typically reported in the Fire Hazards Analysis (FHA). For rooms where detailed fire modeling is necessary FAQ 13-0005 provides guidance on how to calculate a scenario level ignition frequency (by dividing the quantity of cables in the tray on fire by the total quantity of cable in the room). Consistent with Appendix R of EPRI 1011989, self-ignited cable fire only need to be postulated in compartments which contain unqualified cable types.	EPRI 1011989 / NUREG/CR-6850 FAQ 13-0005 FAQ 16-0010	7.02E-04	EPRI 3002002936 (NUREG-2169)
13	Plant-Wide Components	Dryers	Clothes dryers are generally well-defined units.	Each clothes dryer is counted separately.	EPRI 1011989 / NUREG/CR-6850	3.66E-03	EPRI 3002002936 (NUREG-2169)
14	Plant-Wide Components	Electric Motors	Electrical motors with a power rating greater than 5hp associated with various devices, not including those counted in other bins, are included in this bin. This may include elevator motors, valve motors, etc.	Motors (not included those counted in other bins) with a rating greater than 5 HP are counted. Totally enclosed motors should be excluded from the count because the motor housing would prevent the extension of flames outside the motor casing. See FAQ 07-0031 for the additional guidance.	EPRI 1011989 / NUREG/CR-6850 FAQ 07-0031, Section 6 of Supplement 1	5.43E-03	EPRI 3002002936 (NUREG-2169)
15	Plant-Wide Components	Electrical Cabinets	Electrical cabinets represent such items as switchgears, motor control centers, DC distribution panels, relay cabinets, control and switch panels (excluding panels that are part of machinery), fire protection panels, etc.	Electrical cabinets in a nuclear power plant vary significantly in size, configuration, and voltage. Size variation range from small-wall mounted units to large walk-through vertical control cabinets, which can be 20’ to 30’ long. The configuration can vary based on number of components that contribute to ignition, such as relays and circuit cards, and combustible loading, which also affects the fire frequency. Voltages in electrical cabinets vary from low voltage (120 V) panels to 6.9 kV switchgear. Even though it is expected that these features affect the likelihood of fire ignition, from a simple analysis of the event data involving the electrical cabinets, it was determined that the variation by cabinet type did not warrant separate frequency evaluation. Therefore, one fire frequency was estimated for the electrical cabinets. The following rules should be used for counting electrical cabinets: – Simple wall-mounted panels housing less than four switches may be excluded from the counting process, – Well-sealed electrical cabinets that have robustly secured doors (and/or access panels) and that house only circuits below 440V should be excluded from the counting process, (In this context, the term “well-sealed” means there are no open or unsealed penetrations, there are no ventilation openings, and potential warping of the sides/walls of the panel would not open gaps that might allow an internal fire to escape. “Robustly secured” means that any doors and/or access panels are all fully and mechanically secured and will not create openings or gaps due to warping during an internal fire. For example, a panel constructed of sheet metal sides “tack welded” to a metal frame would not be considered well-sealed because internal heating would warp the side panels allowing fire to escape through the resulting gaps between weld points. A panel with a simple twist-handle latch mechanism would not be considered robustly secured because the twist handle would not prevent warping of the door under fire conditions. In contrast, a water-tight panel whose door/access panel is bolted in place or secured by mechanical bolt-on clamps around its perimeter would be considered both well-sealed and robustly secured. Also note that panels that house circuit voltages of 440V or greater are counted because an arcing fault could compromise panel integrity (an arcing fault could burn through the panel sides, but this should not be confused with the high energy arcing fault type fires)). – Free-standing electrical cabinets should be counted by their vertical segments. 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.	EPRI 1011989 / NUREG/CR-6850 FAQ 06-0016, Section 3 of Supplement 1	3.43E-02	NUREG-2230 / EPRI 3002016051
16.a	Plant-Wide Components	High Energy Arcing Faults - Low Voltage Electrical Cabinets (480-1000 V)	High-energy arcing faults are associated with switchgear and load centers operating between 480 and 1000 Volts. For this bin, similar to electrical cabinets, the vertical segments of the switchgear and load centers should be counted.	Each vertical segment of the switchgear and load center for low voltage (480-1000 V) electrical cabinets is counted separately. MCCs are not included, unless the MCC is associated with switchgear that is used directly to operate equipment such as load centers.	EPRI 1011989 / NUREG/CR-6850 FAQ 06-0017, Section 4 of Supplement 1	1.52E-04	EPRI 3002002936 (NUREG-2169)
16.b	Plant-Wide Components	High Energy Arcing Faults - Medium Voltage Electrical Cabinets (>1000 V)	High-energy arcing faults are associated with switchgear and load centers. Switchyard transformers and isolation phase buses are not part of this bin. For this bin, similar to electrical cabinets, the vertical segments of the switchgear and load centers should be counted. Additionally, to cover potential explosive failure of oil filled transformers (those transformers that are associated with 4.16 or 6.9kV switchgear and load centers) may be included in vertical segment counts of the switchgear.	Each vertical segment of the switchgear and load center for medium voltage (above 1000 V) electrical cabinets is counted separately.	EPRI 1011989 / NUREG/CR-6850 FAQ 06-0017, Section 4 of Supplement 1	2.13E-03	EPRI 3002002936 (NUREG-2169)
16.1	Plant-Wide Components	HEAF for segmented bus ducts	A bus duct where the bus bars are made up of multiple sections bolted together at regular intervals (transition points). Here, the bus bars are contained within open-ended sections of metal covers that are bolted together to form a continuous grounded enclosure running the full distance between termination points. Segmented bus ducts are able to accommodate tap connections to supply multiple equipment termination points. – Segmented bus ducts tend to be longer in comparison to the nonsegmented bus ducts. Segmented bus ducts are used in cases where the required lengths and/or geometries make the use of nonsegmented bus ducts impractical. – The length of each segment may vary depending on supplier and installation details. – Segmented bus ducts tend to connect end devices that are remote from each other. Example: A segmented bus duct might be used to connect an oil-filled transformer located in an outdoor area to equipment (e.g., switchgear) located inside the plant buildings. Note: This bin does not cover nonsegmented or continuous bus ducts or cable ducts. The arc faults for these two categories are inherently included in the treatment of the end device, and no further treatment is needed.	The analyst will need to choose between one of two recommended practices for counting segmented bus ducts as a fire ignition source. The choice will be dependent on whether or not the transition points can be identified based on an external visual inspection of the bus duct. 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. Note that transition point counting excludes the bus end termination points, which are considered a part of the end device for fire frequency purposes. Transition points may be identifiable based on visual observation or review of design drawings. Transition points for the bus bars may, or may not, correspond to junctions in the outer ducting that surrounds the bus bars. It is not intended that the protective duct be removed to identify transition points. However, industry feedback indicates that the joints or junctions in the outer ducting surrounding a bus duct cannot be assumed to correspond to junctions in the bus bars themselves without confirmation. A representative sample of plant applications should be inspected to ensure that the internal bus bar transition points and external duct junctions do in fact align with each other. Once the total count of transition points has been obtained, the plant-wide fire frequency is then partitioned to a specific location based on the number of transition points in the location of interest divided by the total number of transition points for the entire plant. 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. Hence, the analysis must estimate the total length of segmented bus duct present in the plant under analysis. 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. 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. This lower bound is based on the assumption that, lacking specific information on segment lengths, a nominal segment length of 12 feet should be assumed. Any single scenario is then assigned a fire frequency equivalent to that associated with one bus bar segment 12 feet in length (i.e., equivalent to one nominal transition point).	FAQ 07-0035, Section 7 of Supplement 1	1.10E-03	EPRI 3002002936 (NUREG-2169)
16.2	Plant-Wide Components	HEAF for iso-phase bus ducts	A bus duct where the bus bars for each phase are separately enclosed in their own protective housing. The use of iso-phase buses is generally limited to the bus work connecting the main generator to the main transformer.	There should generally be one iso-phase bus per unit (an iso-phase bus includes all three phases). If there is more than one iso-phase bus, simply count the total number of iso-phases buses per unit. For individual fire scenarios, the plant-wide frequency is applied (i.e. partitioned) equally to each end of each iso-phase bus duct counted.	FAQ 07-0035, Section 7 of Supplement 1	5.91E-04	EPRI 3002002936 (NUREG-2169)
17	Plant-Wide Components	Hydrogen Tanks	Hydrogen storage tanks are generally well-defined items. Multitank hydrogen trailers, because they are interconnected, should be counted as one unit.	Each hydrogen tank should be counted separately. Multitank hydrogen trailers should be counted separately.	EPRI 1011989 / NUREG/CR-6850	4.93E-03	EPRI 3002002936 (NUREG-2169)
18	Plant-Wide Components	Junction Boxes	Generally, a junction box is defined as a fully enclosed metal box containing terminals for joining or splicing cables. The box must be fully enclosed with metal panels or welded together but not necessarily well sealed. Cables entering or exiting the junction box should be in metal conduits and have mechanical connections to the metal box. The junction box should include only terminals for joining and splicing cables. For a full definition, refer to FAQ 13-0006.	The number of junction boxes may be difficult to determine. The frequency can be apportioned based on ratio of cables in the area to the total cable in the plant. Therefore, the ignition source-weighting factor of the cables may be used for this bin as well. As an alternative (described in FAQ 13-0006), the frequency of junction box fires in each fire compartment can be apportioned based on the number of junction boxes in the fire compartment divided by the total number of junction boxes in the plant as determined by the cable and raceway database system or when the cable and raceway database cannot provide this information, the number of junction boxes may be estimated in each fire compartment. See FAQ 13-0006 for full guidance.	EPRI 1011989 / NUREG/CR-6850 FAQ 13-0006	3.61E-03	EPRI 3002002936 (NUREG-2169)
19	Plant-Wide Components	Miscellaneous Hydrogen Fires	This bin includes hydrogen fires in miscellaneous systems other than hydrogen cylinder storage, generator cooling, and battery rooms. It is not necessary to count the ignition sources related to this bin.	Each system found in miscellaneous hydrogen systems should be counted separately. This does not include hydrogen cylinder storage, generator cooling, and battery rooms. An alternative is to not count the ignition sources related to this bin and to establish an ignition frequency associated with the components of this bin for a specific compartment or a pipe segment. NOTE: It is important to have a clear definition of system boundaries to ensure that, between this bin and Bin 34, all hydrogen-carrying items of the plant are properly accounted for.	EPRI 1011989 / NUREG/CR-6850	4.82E-03	EPRI 3002002936 (NUREG-2169)
20	Plant-Wide Components	Off-gas/H2 Recombiner (BWR)	Generally there are at least two recombiner systems per BWR.	Each recombiner system should be counted as one unit.	EPRI 1011989 / NUREG/CR-6850	5.81E-03	EPRI 3002002936 (NUREG-2169)
21	Plant-Wide Components	Pumps and large hydraulic valves	This bin includes pumps and large hydraulic valves. Due to a lack of sufficient statistical data, a separate bin was not defined for large valves that include hydraulic fluid powered mechanisms. It is recommended such valves (e.g. Main Steam Isolation Valves, and Turbine Stop Valves) be counted and included in the pump bin.	Each pump with a rating greater than 5 hp should be counted separately (do not count pumps with a horsepower rating of 5 hp or below). NUREG‑2178 Volume 2 / EPRI 3002016052 extends the guidance in FAQ 07‑0031 to Bin 21. Totally enclosed motors should be excluded from the count because the motor housing would prevent the extension of flames outside the motor casing.	EPRI 1011989 / NUREG/CR-6850 FAQ 07-0031, Section 6 of Supplement 1 Description of Treatment for Pump Oil Fires (NRC Recent Fire PRA Methods Review Panel Decisions - Attachment 4) Fire PRA Methods Review Panel Membership (NRC Recent Fire PRA Methods Review Panel Decisions - Attachment 5) Panel Decision (NRC Recent Fire PRA Methods Review Panel Decisions - Attachment 6) NUREG-2178 Vol 2 / EPRI 3002016052	2.72E-02	EPRI 3002002936 (NUREG-2169)
22	Plant-Wide Components	RPS MG Sets	In PWRs, the RPS MG sets are well-defined devices.	Each RPS MG set is counted separately. Electrical cabinets associated with the RPS MG set should not be counted, as they are considered to be part of the RPS MG set.	EPRI 1011989 / NUREG/CR-6850	2.31E-03	EPRI 3002002936 (NUREG-2169)
23a	Plant-Wide Components	Transformers (oil filled)	This bin includes all indoor transformers that are not an integral part of larger components. Control power transformers and other small transformers, which are subcomponents in electrical equipment, should be ignored. Examples include 4160V/480V transformers attached to AC load centers, low-voltage regulators, and essential service lighting transformers. The large yard transformers are not part of this count.	Each indoor oil filled transformers should be counted separately.	EPRI 1011989 / NUREG/CR-6850 FAQ 07-0031, Section 6 of Supplement 1	9.56E-03	EPRI 3002002936 (NUREG-2169)
23b	Plant-Wide Components	Transformers (dry)	This bin includes all indoor transformers that are not an integral part of larger components. Control power transformers and other small transformers, which are subcomponents in electrical equipment, should be ignored. Examples include 4160V/480V transformers attached to AC load centers, low-voltage regulators, and essential service lighting transformers. The large yard transformers are not part of this count.	Each dry transformer with a rating greater than 45 kVa should be counted separately.	EPRI 1011989 / NUREG/CR-6850 FAQ 07-0031, Section 6 of Supplement 1	9.56E-03	EPRI 3002002936 (NUREG-2169)
24	Plant-Wide Components	Transient fires caused by welding and cutting	Transient fires due to hotwork activities located in the Power Block, but not in the Control Building, Auxiliary Building, Reactor Building, Turbine Building, or Containment (PWR).	The ignition source weighting factor of transient fires caused by welding and cutting is estimated using a ranking scheme that takes into account the hot work factor. The hot work ranking factors are described in Table 6-2 (as updated in FAQ 12-0064). Guidance for this bin is updated in FAQ 12-0064 Section 6.5.7.2 and Fire PRA FAQ 14-0007 (distributing transient influence factors for smaller spaces than fire compartments).	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	4.79E-03	EPRI 3002002936 (NUREG-2169)
25	Plant-Wide Components	Transients	General transient combustibles or activities located in the Power Block, but not in the Control Building, Auxiliary Building, Reactor Building, Turbine Building, or Containment (PWR).	The ignition source weighting factor of transient fires is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in FAQ 12-0064 Section 6.5.7.2. The introduction of developing transient influence factors for smaller spaces than fire compartments is discussed in FAQ 14-0007.	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	8.54E-03	EPRI 3002002936 (NUREG-2169)
26	Plant-Wide Components	Ventilation Subsystems	This category includes components such as air conditioning units, chillers, fan motors, air filters, dampers, etc. A fan motor and compressor housed in the same component are counted as one component. Do not count ventilation fans if the drive motor is 5 hp or less.	Each component with a rating greater than 5 HP should be counted separately. NUREG‑2178 Volume 2 / EPRI 3002016052 extends the guidance in FAQ 07‑0031 to Bin 26. Totally enclosed motors should be excluded from the count because the motor housing would prevent the extension of flames outside the motor casing.	EPRI 1011989 / NUREG/CR-6850 FAQ 07-0031, Section 6 of Supplement 1 NUREG-2178 Vol 2 / EPRI 3002016052	1.64E-02	EPRI 3002002936 (NUREG-2169)
27	Transformer Yard	Transformer - Catastrophic	The high-voltage power transformers typically installed in the yard belong to this bin. They include plant output power transformers, auxiliary-shutdown transformers, and startup transformers, etc. Isolation phase bus ducts are also included in this bin to simplify fire frequency analysis.	Each high-voltage power transformer installed in the yard is counted separately.	EPRI 1011989 / NUREG/CR-6850	6.61E-03	EPRI 3002002936 (NUREG-2169)
28	Transformer Yard	Transformer - Non Catastrophic	Similar to Bin 27 this bin includes the high-voltage power transformers typically installed in the yard. However, isolation phase bus ducts are not included in this bin. In a non-catastrophic transformer failure oil does not spill outside transformer tank and the fire does not necessarily propagate beyond the fire source transformer. The analyst can use all the frequency and assume total loss of the “Transformer/Switch Yard” or may split this frequency equally among the large transformers of the area and assume loss of each transformer separately. Loss of offsite power should be determined based on the function of the affected transformer(s).	Each high-voltage power transformer installed in the yard is counted separately.	EPRI 1011989 / NUREG/CR-6850	6.53E-03	EPRI 3002002936 (NUREG-2169)
29	Transformer Yard	Yard Transformers (Others)	Items associated with yard transformers but not the transformers themselves (e.g., oil power output cables) are part of this bin. In the screening phase of the project, the analyst may conservatively assign the same frequency to all the items in this group. If the scenario would not screen out, the frequency may then be divided among the various items in this group. A relative ranking scheme may be used for this purpose. The ranking may be based on the relative characteristics of the items and analysts’ judgment.	Items associated with yard transformers but not the transformers themselves (e.g., oil power output cables) are counted separately.	EPRI 1011989 / NUREG/CR-6850	3.69E-03	EPRI 3002002936 (NUREG-2169)
30	Turbine Building	Boiler	Boilers are generally well-defined items.	Each boiler should be counted separately. All ancillary items associated with each boiler may be included as part of the boiler. Control panels that are installed separate from a boiler may be included in the “Electrical Cabinets (Plant-Wide Components)” bin.	EPRI 1011989 / NUREG/CR-6850	1.09E-03	EPRI 3002002936 (NUREG-2169)
31	Turbine Building	Cable fires caused by welding and cutting	For this bin, it is assumed that all exposed cables (i.e., cables that are not in conduits or wrapped by noncombustible materials) have an equal likelihood of experiencing a fire caused by welding and cutting across the entire location (Turbine Building).	The ignition source weighting factor of cable fires caused by welding and cutting is estimated using the hot work factor and cable quantity in the fire compartment. The hot work ranking factors are described in Table 6-2 (as updated in FAQ 12-0064). Guidance for this bin is updated in FAQ 12-0064 Section 6.5.7.2 and Fire PRA FAQ 16-0010. The hot work factor is then weighed in combination with a relative numerical estimate of the quantity of cables in the location to the total quantity of cables in the entire location set to generate the final location weighting factor. The cable quantity (either total weight or total combustible load) is typically reported in the Fire Hazards Analysis (FHA).	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 16-0010	3.47E-04	EPRI 3002002936 (NUREG-2169)
32	Turbine Building	Main Feedwater Pumps	Main feedwater pumps are generally well-defined entities.	Main feedwater pumps are generally well-defined entities. Ancillary components associated with each pump are considered a part of the pump and should not be counted separately.	EPRI 1011989 / NUREG/CR-6850	4.38E-03	EPRI 3002002936 (NUREG-2169)
33	Turbine Building	Turbine Generator Excitor	The turbine generator excitor is a well-defined item. Generally, there is only one excitor per unit.	Each turbine generator excitor should be counted separately.	EPRI 1011989 / NUREG/CR-6850	8.36-04	EPRI 3002002936 (NUREG-2169)
34	Turbine Building	Turbine Generator Hydrogen	A complex of piping, valves, heat exchangers, oil separators, and often skid-mounted devices are associated with turbine generator hydrogen.	A complex of piping, valves, heat exchangers, oil separators, and often skid-mounted devices are associated with turbine generator hydrogen. Consider the entire complex as one system and assign the ignition frequency of this bin to that system. It is important to have a clear definition of system boundaries to ensure that, between this bin and Bin 19, all hydrogen-carrying items of the plant are properly accounted for. Similar to Bin 29, in the screening phase of the project, the analyst may conservatively assign the same frequency to all the items in this bin. If the scenario would not screen out, the frequency may then be divided among the various items using a relative ranking scheme. The ranking may be based on the relative characteristics of the items and the analysts’ judgment. NOTE: It is important to have a clear definition of system boundaries to ensure that, between this bin and Bin 19, all hydrogen-carrying items of the plant are properly accounted for.	EPRI 1011989 / NUREG/CR-6850	4.12E-03	EPRI 3002002936 (NUREG-2169)
35	Turbine Building	Turbine Generator Oil	Similar to hydrogen, a complex of oil storage tanks, pumps, heat exchangers, valves, and control devices belong to this bin.	A complex of piping, valves, heat exchangers, oil separators, and often skid-mounted devices are associated with turbine generator hydrogen. It is recommended to treat the entire complex as one system and assign the ignition frequency of this bin to that system. Similar to the preceding bin and Bin 29, in the screening phase of the project, the analyst may conservatively assign the same frequency to all the items in this bin. If the scenario would not screen out, the frequency may then be divided among the various items using a relative ranking scheme. The ranking may be based on the relative characteristics of the items and analysts’ judgment.	EPRI 1011989 / NUREG/CR-6850	5.49E-03	EPRI 3002002936 (NUREG-2169)
36	Turbine Building	Transient fires caused by welding and cutting	Transient fires due to hotwork activities located in the Turbine Building.	The ignition source weighting factor of transient fires caused by welding and cutting is estimated using a ranking scheme that takes into account the hot work factor. The hot work ranking factors are described in Table 6-2 (as updated in FAQ 12-0064). Guidance for this bin is updated in FAQ 12-0064 Section 6.5.7.2 and Fire PRA FAQ 14-0007 (distributing transient influence factors for smaller spaces than fire compartments).	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	4.67E-03	EPRI 3002002936 (NUREG-2169)
37	Turbine Building	Transients	General transient combustibles or activities located in the Turbine Building.	The ignition source weighting factor of transient fires is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in FAQ 12-0064 Section 6.5.7.2. The introduction of developing transient influence factors for smaller spaces than fire compartments is discussed in FAQ 14-0007.	EPRI 1011989 / NUREG/CR-6850 FAQ 12-0064 FAQ 14-0007	6.71E-03	EPRI 3002002936 (NUREG-2169)

Table 6-2: Summary Description of Transient Fire Influencing Factors (as updated in FAQ 12-0064)

Influencing Factor	Ranking Value (Note 1)	Where applicable
General Electro-Mechanical (E/M) Maintenance (excluding hot work)	No (0)	Applicable for locations where maintenance activities during power operation are precluded by design and/or operation. (Note 2)
	Very Low (0.3)	Applicable for locations where: (1) access is strictly controlled (not just simple key-card type access) (Note 3), and (2) areas with NO equipment subject to frequent maintenance (Note 4), and (3) location contains no plant equipment or components other than cables, fire detectors, junction boxes, and other minor plant support equipment. Requirement: No violations in administrative controls (Note 5). This rating may not be applied to the MCR but may be applied to the Cable Spreading Room (CSR) devoid of other equipment, and cable vault and tunnel areas meeting the criteria. Other plant locations may also be assigned the "very low" (0.3) ranking factor provided all of the defined criteria are met.
	Low (1)	Applicable for areas with small number of (preventative maintenance/corrective maintenance) PM/CM work orders compared to the average number of work orders for a typical compartment (Note 6), or Applicable for general plant locations where strict permitting procedures are enforced, but do not meet the requirements for a “0.3” (very low) rating factor. Requirement: No violations in administrative controls (Note 5) OR performance monitoring program is in place (Note 7)
	Medium (3)	Applicable for areas with average number of PM/CM work orders (Note 6).
	High (10)	Applicable for areas with large number of PM/CM work orders compared to the average number of work orders for a typical compartment (Note 6).
	Very High (50)	Applicable for areas with significantly more PM/CM work orders compared to the average number of work orders for a typical compartment (Note 6).
Hotwork	No (0)	Applicable for areas in which hot work activities during power operation are precluded by design and/or operation (Note 2).
	Extremely Low (0.1)	Applicable for MCR, if: (1) plant procedures prohibit hot work in the MCR during power operations, and (2) no violations in MCR hot work restrictions (Note 8).
	Very Low (0.3)	Applicable for CSR and cable vault and tunnel areas, provided that: (1) access to the location is strictly controlled (Note 3), (2) the location contains no plant equipment or components other than cables, fire detectors, and junction boxes, (3) hot work during power operations is prohibited by plant procedures, and (4) no violations in administrative controls (Note 5) Applicable for MCR, if extremely low ranking of 0.1 is not applicable
	Low (1)	Applicable for: (1) Small number of hot work related PM/CM work orders associated with hot work compared to the average number of work orders for a typical compartment (Note 6). (2) General plant locations where plant procedures generally preclude hot work activities with exceptions subject to the strictest of permitting requirements. Requirement: No violations in administrative controls (Note 5) OR performance monitoring program is in place (Note 7)
	Medium or Average (3)	Applicable for average number of hot work related PM/CM work orders (Note 6)
	High (10)	Applicable for large number of hot work related PM/CM work orders compared to the average number of work orders for a typical compartment (Note 6).
	Very High (50)	Applicable for plant areas that may experience significantly more PM/CM work orders compared to the average number of work orders for a typical compartment (Note 6).
Occupancy	No (0)	Applicable for compartments where entrance is not possible during plant operation (Note 2).
	Very Low (0.3)	Applicable for: (1) compartments bounded on all sides by controlled physical barriers and normally unoccupied during plant operations. (2) compartments not used as an access pathway for any other plant location. (3) location with access strictly controlled (Note 3).
	Low (1)	Applicable for compartments with low foot traffic or out of general traffic path.
	Medium or Average (3)	Applicable for compartments not continuously occupied, but with regular foot traffic.
	High (10)	Applicable for continuously occupied compartments.
Storage	No (0)	Applicable for compartments where entrance is not possible during plant operation (Note 2).
	Very Low (0.3)	Applicable for: (1) entire fire areas designated “combustible free zones”, and (2) areas with no temporary structures built, stored or moved into the vicinity, comprised at least in part of combustible materials (e.g. wooden scaffolding). Requirement: No violations of administrative controls (Note 5).
	Low (1)	Applicable for: (1) compartments where no combustible/flammable materials are stored by practice but where combustibles may be introduced subject to a permitting process, or (2) compartments where all combustible/flammable material are stored in closed containers and/or placed in dedicated fire-safe cabinets. Requirement: No violations in administrative controls (Note 5) OR performance monitoring program is in place (Note 7)
	Medium or Average (3)	Applicable for areas that contain: (1) small quantities of low-combustibility materials (e.g., solid flame retardant materials) in open storage, or (2) flammable gasses or liquids stored in approved containers and/or flammable combustible storage cabinets.
	High (10)	Applicable for compartments where: (1) combustible/flammable materials are sometimes brought in and left in either open containers for a short time or in a closed container, but outside a dedicated fire-safe cabinet for an extended time. (2) larger quantities of flammable materials (e.g., radiation protection clothing, packing boxes or materials, paints, flammable liquids, oils) are stored.

Notes regarding Table 6-2

Note 1: Intent of Ranking: The overall intent of the weighting factor method is to reflect real differences in the relative likelihood of transient fires in various locations while at the same time preserving the overall plant-wide fire frequency for each ignition source bin. In application the analyst should consider the following points relative to the intent of the transient location factor ranking method: (1) The ranking factor numerical values assigned to each location should reflect relative weighting values within each applicable frequency bin location set. The relative rankings should not look across location sets. For example, when addressing bins 36 and 37 the analyst should not compare locations of the turbine building (the location set for these two bins) to other non-turbine building locations (e.g., to areas of the control building which is covered by bin 25 and 26). (2) The full range of the numerical ranking values is available to the analyst and should, at least nominally, be exercised for each location set. If the full range of the ranking factor values is not exercised, then fire frequency will be distributed more evenly to the applicable fire compartments. If the analyst concludes that a relatively even distribution is the correct answer for the plant and location set, then it is recommended that an explanation should be provided in the PRA documentation.

Note 2: Access precluded by design and/or operation: Examples of areas where maintenance and hot work activities are precluded by design and/or operation, include the following: (1) inerted locations such as inside an inerted BWR containment during power operation, (2) very high radiation areas such as a traversing in-core probe (TIP) room (or equivalent) for a BWR, (3) permanently sealed cable tunnels such as poured concrete cable ways without access or cable tunnels where access ways have been closed by mortared block, (4) cable tunnels with manhole or hatch access where the manholes/hatches are welded shut; BUT, if an urgent situation could occur that would require cutting into the areas to avoid a shutdown, then value of 0.3 should be assigned (5) areas physically too small to allow personnel access under any conditions (e.g., an underground cable chase), (6) areas with extreme thermal environment beyond human tolerance such as the main steam tunnel in a BWR, and (7) locations where the equipment present occupies all the available space such that the storage or placement of transient materials would be physically impossible. The existence of administrative controls in and of itself is not a compelling basis for a rating of 0.

Note 3: Access strictly controlled: Examples of locations where access is strictly controlled (not just simple key-card type access) are as follows: (1) special entry permitting procedures are in place (e.g., access into containment during power operations would be an involved process), (2) confined space access controls are imposed (i.e., per OSHA requirements), (3) limited personnel access lists are established, (4) extra security controls such as locked doors with limited access keys, (5) verbal notification of entry and exit to security or operations personnel is required in a specific location, (6) entry is prohibited without health physics or radiation protection technician present, (7) entry is prohibited without a fire watch, and/or (8) personnel safety tag-outs are required to lock out an automatic suppression system (e.g., Halon or CO₂) prior to entry or prior to conducting a maintenance activity.

Note 4: Equipment requiring maintenance: Examples of equipment that do not require frequent maintenance are the following: cables, fire detectors, junction boxes and other minor plant support equipment such as normal and emergency lighting, access control panels, plant paging or communications equipment, alarms or alarm panels, and security monitoring or support equipment. In general, the presence of any piece of equipment that was counted as a fire ignition source during Step 6 would preclude assignment of “very low” for this factor. Conversely, it cannot be assumed that the lack of countable fire ignition sources implies that the very low ranking factor applies. If equipment items are present that may require maintenance but do not meet the counting criteria (e.g., smaller pumps, motors or ventilation subsystems) then the very low ranking factor would not apply. A rating of 0.3 is applicable to cable spreading rooms (CSR) devoid of other equipment, and cable vault and tunnel areas that have access strictly controlled A rating of 0.3 is not applicable for the Main Control Room.

Note 5: No violations in administrative controls: A rating of 0.3 requires a verification that no violations of the administrative controls related to the influence factor that is being rated (maintenance, hot work, or storage) have occurred over a reasonable prior time period (i.e., five years).

Note 6: Work Orders: The analyst should use engineering judgment to determine the maintenance factor of compartments with no work orders in the selected period of time. The judgment can be based on the characteristics of the compartment relative to compartments with work orders. If the work orders cannot be collected easily, the analyst may use engineering judgment based on personal experience or information gathered from the maintenance personnel of the plant. In this case, the analyst may ask the maintenance personnel to assign a rating number between 0 and 10 in terms of frequency of maintenance at a compartment and to identify the two or three most typical maintenance activities undertaken (e.g., pump overhaul or electrical device replacement).

Note 7: Performance monitoring program: A performance monitoring program is in place and demonstrates that the administrative control programs are meeting expectations and objectives.

Note 8: MCR hot work: The ranking of 0.1 for the MCR requires that a review of plant records confirms that no violations of, or exceptions to, the MCR hot work restrictions while at power have been recorded over some reasonable prior time period (i.e., five years).

Supplemental Guidance

See Wiki Tables 6-1 and 6-2 for the most recent ignition source bins, counting guidance, and fire ignition frequencies.