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)

Current FPRA Counting Guidance and Fire 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 shall be counted as one. Cells may not be counted individually.	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 shall be counted separately.	NUREG/CR-6850	1.37E-03	EPRI 3002002936 (NUREG-2169)
3	Containment (PWR)	Transients and Hotwork	General transient combustibles or activities located in the 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 NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-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, shall be counted separately.	NUREG/CR-6850 FAQ 06-0018 FPRA-FAQ 14-0008	4.91E-03	EPRI 3002002936 (NUREG-2169)
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 is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 16-0010.	NUREG/CR-6850 FPRA-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 is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-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 NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-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. 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.	Each diesel generator shall be counted separately.	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. Note that compressors associated with the ventilation systems are not part of this bin. Small air compressors used for specialized functions are also not part of this bin.	Air compressors are generally well-defined devices (and includes portable units). The air compressor skid, which could include an air receiver, air dryer, and control panel attached to the compressor, shall 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.	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.	NUREG/CR-6850 NUREG-2178	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 is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 16-0010.	NUREG/CR-6850 FPRA-FAQ 16-0010	2.77E-04	EPRI 3002002936 (NUREG-2169)
12	Plant-Wide Components	Cable Run (self-ignited cable fires)	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 compartments should be taken into account.	The ignition source weighting factor of cable 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 NUREG/CR-6850 section 6.5.7 and Fire PRA FAQs 13-0005 and 16-0010.	NUREG/CR-6850 FPRA-FAQ 13-0005 FPRA-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 shall be counted separately.	NUREG/CR-6850	3.66E-03	EPRI 3002002936 (NUREG-2169)
14	Plant-Wide Components	Electric Motors	The electrical motors with 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 with a rating greater than 5 HP shall be counted.	NUREG/CR-6850 FAQ 07-0031	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 switchgears. 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, – Free-standing electrical cabinets should be counted by their vertical segments, and – To expedite the process, an average number of vertical segments may be used for such cabinets as motor control centers and DC distribution panels. 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).			3.00E-02	EPRI 3002002936 (NUREG-2169)
16.a	Plant-Wide Components	High Energy Arcing Faults - Low Voltage Electrical Cabinets (480-1000V)	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 lower voltage load centers) may be included in vertical segment counts of the switchgear.	Each vertical segment of the switchgear and load center for low voltage (480-1000V) cabinets shall be counted separately.	NUREG/CR-6850 FAQ 06-0017	1.52E-04	EPRI 3002002936 (NUREG-2169)
16.b	Plant-Wide Components	High Energy Arcing Faults - Medium Voltage Electrical Cabinets (>1000V)	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 lower voltage 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 1000V) cabinets shall be counted separately.	NUREG/CR-6850 FAQ 06-0017	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 (see discussion below for a definition of the 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)."	NUREG/CR-6850 Supplement 1 (FAQ 07-0035)	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.	NUREG/CR-6850 Supplement 1 (FAQ 07-0035)	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 shall be counted separately. Multitank hydrogen trailers shall be counted separately.	NUREG/CR-6850	4.93E-03	EPRI 3002002936 (NUREG-2169)
18	Plant-Wide Components	Junction Boxes	The junction box must be fully enclosed with metal panels bolted 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.	Each junction box shall be counted separately. For junction boxes that are not identified in the cable and raceway database, the number of junction boxes in a specific PAU can be assumed to be proportional to the ratio of the number of junction boxes to conduits in a representative, comparable PAU and the cable loading associated with the location.	NUREG/CR-6850 FPRA-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 shall 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.	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.	Each recombiner system shall be counted separately.	NUREG/CR-6850	5.81E-03	EPRI 3002002936 (NUREG-2169)
21	Plant-Wide Components	Pumps and large hydraulic valves	For this methodology, it is assumed that above a certain size, fire ignition is the same for all pumps. Pumps below 5 hp are assumed to have little or no significant contribution to risk. Do not count small sampling pumps. The number of pumps in all plant locations defined as “Plant-Wide” should be estimated. 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 shall be counted separately.	NUREG/CR-6850 FAQ 07-0031	2.72E-02	EPRI 3002002936 (NUREG-2169)
22	Plant-Wide Components	RPS MG Sets	In PWRs, the RPS MG sets are well-defined devices. The electrical cabinets associated with the MG sets are not included as part of these items.	Each RPS MG sets shall be counted separately. Electrical cabinets associated with the RPS MG set shall not be counted, as they are considered to be part of the RPS MG set.	NUREG/CR-6850	2.31E-03	EPRI 3002002936 (NUREG-2169)
23a	Plant-Wide Components	Transformers (oil filled)	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 shall be counted separately.	NUREG/CR-6850 FAQ 07-0031	9.56E-03	EPRI 3002002936 (NUREG-2169)
23b	Plant-Wide Components	Transformers (dry)	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. Transformers with a 45V rating or higher will be counted. The large yard transformers are not part of this count.	Each dry transformers with a rating greater than 45 kVa shall be counted separately.	NUREG/CR-6850 FAQ 07-0031	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 is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-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 NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-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 shall be counted separately.	NUREG/CR-6850 FAQ 07-0031	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 shall be counted separately.	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. 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 shall be counted separately.	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) shall be counted separately.	NUREG/CR-6850	3.69E-03	EPRI 3002002936 (NUREG-2169)
30	Turbine Building	Boiler	Boilers are generally well-defined items. 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.	Each boiler shall be counted separately.	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 is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 16-0010.	NUREG/CR-6850 FPRA-FAQ 16-0010	3.47E-04	EPRI 3002002936 (NUREG-2169)
32	Turbine Building	Main Feedwater Pumps	Main feedwater pumps are generally well-defined entities. If there are ancillary components associated with each pump, it is recommended to include those items as part of the pump.	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.	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 shall be counted separately.	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. 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.	A complex of piping, valves, heat exchangers, oil separators, and often skid-mounted devices are associated with turbine generator hydrogen. The entire complex should be considered as one system and shall be counted separately. 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.	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. 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.	A complex of piping, valves, heat exchangers, oil separators, and often skid-mounted devices are associated with turbine generator hydrogen. The entire complex should be considered as one system and shall be counted separately. 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.	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 is estimated using a ranking scheme that takes into account maintenance activities, occupancy level, and storage of flammable materials. These steps are outlined in NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-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 NUREG/CR-6850 section 6.5.7 and Fire PRA FAQ 14-0007.	NUREG/CR-6850 FPRA-FAQ 14-0007	6.71E-03	EPRI 3002002936 (NUREG-2169)

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.