**NRC INSPECTION MANUAL**
APHB

INSPECTION MANUAL CHAPTER 0609 APPENDIX F ATTACHMENT 5

CHARACTERIZING FIRE IGNITION SOURCES

0609, App F, Att 5 		Issue Date:  02/28/05

FIXED AND TRANSIENT IGNITION SOURCE FIRES

Heat Release Rate Profile of Fixed Ignition Sources 

The Heat Release Rate (HRR) profile of fixed ignition sources consists of three stages as shown in Figure A5.1:

· First Stage:  t-squared growth stage during which the HRR increases from 0 kW at ignition (t = 0 seconds) to the peak HRR (HRRpeak) at t = tpeak seconds.  The HRR as a function of time during this stage is given by

· Second Stage:  HRR remains equal to HRRpeak for a period of tsteady seconds.

· Third Stage:  Linear decay from HRR = HRRpeak to HRR = 0 kW in tdecay seconds.  

Figure A5.1 - HRR Profile for Fixed and Transient Ignition Sources

Table A5.1 gives the HRR profile parameters for fixed ignition sources assumed in the Phase 2 based on NUREG-2178, Vol. 1.  The HRRpeak values for electrical enclosures are the 98th percentile HRRs taken from Table 7-1.  The 98th percentile HRR is recommended for screening.  According to Table A5.1, the 98th percentile HRR of an electrical enclosure with a specified configuration and default fuel loading is independent of the type of cables (i.e., thermoset or thermoplastic) it contains.

* See Attachment 3 for definition of “open” and “closed” cabinets

HRR Profile of Electrical Cabinet Fires (HEAF) 

Switchgear and load centers (440V and above) are subject to high energy arcing faults (HEAFs)in addition to the possibility of a general or thermal fire.  As a result, two ignition scenarios need to be considered for electrical cabinets  440 V; HEAF and non-HEAF.  For the HEAF scenario, the vertical ZOI is within 5 ft. above the top of the cabinet, and the horizontal ZOI is within 3ft. of all sides of the cabinet.  All unprotected targets within this region are assumed to be damaged instantaneously when the HEAF occurs and all unprotected secondary combustibles within the region are assumed to ignite instantaneously.  The HRR profile for a HEAF fire has no t-squared growth stage.  The HEAF fire reaches HRRpeak instantaneously at ignition (t = 0 seconds), remains at HRRpeak for 1200 seconds, and subsequently decays linearly to 0 kW in 1200 seconds.  The ZOI and HRR profile for non-HEAF scenarios is determined in the same manner as for electrical cabinets < 440 V.  

HRR Profile of Main Control Board Panel Fires 

The HRR of Main Control Board (MCB) panel fires used in an analysis to determine the probability for control room abandonment is identical to that for the electrical enclosures with the same volume.

HRR Profile for Propagating Electrical Cabinet Fires 

Electrical cabinet fires can be assumed not to propagate to adjacent cabinets if at least one of the following conditions are met:

· The cabinets are separated by a double wall with an air gap, or

· Either the exposed or exposing cabinet has an open top, and there is an internal wall, possibly with some openings, and there is no diagonal cable run between the exposing and exposed cabinet.

If neither of these conditions are met, electrical cabinet fires are assumed to propagate from the exposing cabinet to adjacent exposed cabinets.  There can be one or two exposed cabinets, depending on whether the fire originates in a cabinet located at one end of a bank or in a cabinet that has adjacent cabinets on both sides, respectively.  The HRR profile of a propagating cabinet fire is obtained by combining the HRR profiles of the exposing and exposed cabinets, assuming the exposed cabinets ignite 10 or 15 minutes after the exposing cabinet:

· Assume an ignition delay of 10 minutes if the cabinets are not separated by a metal wall, or if the cabinets are separated by a single metal wall and cables in the adjacent cabinet are in direct contact with the wall.

· Assume 15 minutes if the cabinets are separated by a single metal wall, and cables in the adjacent cabinet are not in contact with the wall.

HRR Profile of Transient Combustible  

The HRR profile for transient combustible fires is of the same form as shown in Figure A5.1.    The corresponding HRR profile parameters are provided in Table A5.1.  These fire characteristics bound transient fire sources with the following characteristics:

· A single plastic or metal trash can of up to 55 gallons size loaded with general waste materials such as paper, packing materials, etc.

· Up to three small office-size trash cans with general waste (e.g., on the order of 2-4 gallons each, typically either plastic or fiberglass construction).

· A single wooden pallet.

· A single small packing crate (no more than 24" cube).

· A plastic bucket of up to 7 gallons in size (e.g., a used paint bucket) with cleaning materials (e.g., rags, brushes, no more than a pint of cleaning solvents).

· One or two plastic trash bags containing general waste materials.

· An open grease bucket up to one gallon.

· A single collection bin for protective clothing (e.g., at a step-off / dress-out area).

If, in the judgement of the analyst, the as-found conditions exceed the above examples, the fire intensity may have to be increased to reflect the as-found conditions.  In that case it is recommended that additional guidance be sought from either the Regional or Headquarters fire protection staff.

OIL FIRES

Liquid fuel spills can be confined or unconfined.  For confined liquid fuel pool fires the area is known and the HRR is a function of the size of the containment area (pool) and combustion properties of the fuel.  The area of an unconfined liquid fuel spill is a function of the spill volume, and the HRR is therefore a function of the volume and combustion properties of the spilled fuel. 

Two distinct oil spill fires may need to be considered.  The first scenario assumes a spill of 100% of the amount of oil that can be spilled.  The second scenario considers a 10% spill.  A severity factor of 0.02 is assigned to the first scenario, and 0.98 is used for the second scenario.  For confined liquid pool fires it is not necessary to evaluate the two scenarios separately if the containment area is large enough to hold 100% of the amount of oil that can be spilled.

Confined Liquid Pool Fires

Table A5.2 gives the steady HRR and burning rate of confined liquid pool fires as a function of the pool diameter for the following liquid fuels:  (1) diesel fuel and fuel oil, (2) lube oil and mineral oil, and (3) silicone fluid.

For non-circular fires with an area Af, an equivalent effective diameter is used, which is calculated as follows: 

Liquid pool fires are conservatively assumed to reach the steady HRR instantaneously at ignition (t=0 seconds).  The burning time can be calculated by dividing the spill volume by the burning rate.

Unconfined Liquid Spill Fires

Table A5.3 gives the steady HRR and burning time of unconfined liquid spill fires as a function of spill volume for the same three liquid fuels.

CABLE TRAY FIRES

Fires in Vertical Stacks of Horizontal Cable Trays

Vertical stacks of horizontal cable trays located within the zone of influence (ZOI) of an ignition source may act as secondary combustibles and contribute to the HRR in the area under evaluation.  Tables and plots of the combined HRR as a function of time for various cable types (TS and TP), tray widths (1.5 or 3.0 ft.), and ignition source/cable tray configurations can be found in table/plot set C in Attachment 8.  These tables and plots are applicable for non-HEAF scenarios.  For HEAF scenarios, add HRRpeak of the ignition source to the cable tray HRR from Figures C.01 (for 1.5 ft. wide trays) or C.02 (for 3.0 ft. wide trays) in Attachment 8.

Flame spread and fire propagation characteristics for fires involving stacks of cable trays are discussed in the section for FDS 2 scenarios in Attachment 3 and in Section 06.03.03 of the basis document (IMC 0308 Attachment 3 Appendix F).  It can be assumed that flame spread and fire propagation will not occur under the following conditions:

· All trays within the ZOI are fully enclosed, i.e., they have solid bottom and top covers.  

· All trays within the ZOI have solid bottoms and are tightly covered with at least one in. of ceramic fiber blanket (e.g., Kaowool)

· All trays are within the ZOI are protected with a rated fire barrier or wrap, except for HEAF scenarios, in which case wraps within the ZOI are assumed to be destroyed and ineffective. 

Vertical Cable Tray Fires

The HRR of a vertical cable tray is conservatively estimated as the product of the width of the tray covered with cables, the height of the tray, and the HRR per unit area (HRRPUA) of the cables.  The latter is equal to 150 kW/m2 for TS cables, and 250 kW/m2 for TP and Kerite cables (from NUREG/CR-7010).  Flames are assumed to spread very rapidly in the vertical direction, and the HRR is therefore assumed to be instantaneous at ignition.

Self-Ignited Cable Fires

The HRR profile of vertical stacks of horizontal cable trays involved in a self-ignited cable fire scenario can be obtained from Figures C.01 (for 1.5 ft. wide trays) or C.02 (for 3.0 ft. wide trays) in Attachment 8.  A conservative estimate of the HRR of a vertical self-ignited cable tray can be obtained as discussed in the previous section.

HOT WORK FIRES

For hot work fires, it will be assumed that the hot work leads to ignition of either transient combustibles, exposed cables, or insulation materials depending on the specific situation.  Transient combustibles could include flammable materials used in conjunction with the hot work itself (e.g., plastic sheeting or non-fire retardant scaffold materials).

· If the hot work is assumed to ignite transients, treat the subsequent fire like any other transient fuel fire.  As-found conditions may be reflected in fire characterization.

· If the hot work is assumed to ignite exposed cables, treat the subsequent fire like a self-ignited cable fire.

· If the hot work fire is assumed to ignite insulation materials, seek additional guidance from Regional or Headquarters fire protection staff.

SEVERE FIRES INVOLVING THE MAIN TURBINE GENERATOR SET

For inspections involving the turbine building, a need to address severe fires involving the main turbine generator set may arise.  In this case, additional guidance will be needed to complete the Phase 2 analysis.  Guidance from either Regional or Headquarters fire protection staff should be sought in the treatment of these fires.

HYDROGEN FIRES

If for a given fire area, hydrogen fires might be a significant factor in the risk quantification, additional guidance will be needed to complete the Phase 2 analysis.  Guidance from either Regional or Headquarters fire protection staff should be sought in the treatment of these fires.

Issue Date:  05/02/18	7	0609 App F Att 5

ATTACHMENT 1

Revision History for IMC 0609, Appendix F Attachment 5

Issue Date:  05/02/18	Att1-1	0609 App F Att 5

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