ML18213A083

Joint EPRI/NRC-RES Fire PRA Workshop - Module III - Fire Analysis -Appendix G: Heat Release Rates

ML18213A083
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Issue date:	07/31/2018
From:	Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute
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Module III - Fire Analysis Fire Fundamentals:

Appendix G - Heat Release Rates Joint RES/EPRI Fire PRA Workshop August 6-10, 2018 A Collaboration of the Electric Power Research Institute (EPRI) & U.S. NRC Office of Nuclear Regulatory Research (RES)

Heat Release Rates Objectives The objectives of this module are:

1. Define heat release rate and heat release rate profile

2. Review the recommended peak heat release rate values for typical ignition sources in NPPs

3. Describe the method provided for developing heat release rate profiles for fixed and transient ignition sources in NPPs NOTE: Appendix G recommends values for ignition sources only. Heat release rates associated with fires propagating outside of the ignition source have to be evaluated accordingly.

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Heat Release Rates Definition Definition: Heat generated by a burning object per unit time.

Q&= m

& H c A BTU/sec or KW m is burning rate [kg/s-m2], Hc is heat of comb [kJ/kg], A is area [m2]

Equivalent terms: energy release rate, fire intensity, fire power HRR profile describes fire intensity as a function of time Peak Q& Fully Growth Decay Incipient Developed Time 3

Heat Release Rates Fire Growth in Electrical Cabinets The t2 function is recommended for modeling the growth phase of the fire: 2 t

Q&(t ) = Min Q&peak ,Q&peak Heat Release Rate Peak HRR kW

= time to peak 0 5 10 15 20 Time [min]

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Heat Release Rates HRR Profile The HRR profile can be expressed as a constant or as a function of time:

Incipient stage: Not recommended to be moeled

- Duration and intensity are uncertain Growth: Depends on the fuel and geometry of the scenario

- Based on engineering judgment and/or experimental observations Fully developed: Usually after the fire reaches its peak intensity

- Also known as steady burning

- Starts at ignition if the growth period is not considered

- A constant fire intensity should be the peak heat release rate of the profile Decay: In general, less hazardous conditions than the growth and fully developed stage 5

FAQ 08-0052: Transient Fires Manual Suppression Curve Fire Growth Time:

- Common trash can (refuse in a trash receptacle):

Can be associated with a t2 fire growth that grows from zero to peak in approximately 8 minutes.

- Common trash bag (refuse in plastic bags not in a receptacle):

Can be associated with a t2 fire growth that grows from zero to peak in approximately 2 minutes.

- Flammable or combustible liquid spills:

Negligible growth time (near infinite growth rate)

Assume peak heat release rate for the spill through the entire duration of the fire (ignition through burnout) 6

Heat Release Rates Fixed Ignition Sources The methodology recommends heat release rate values for various fixed ignition sources Vertical cabinets

- Open/closed

- Qualified/unqualified cables Pumps (electrical fires)

Electric motors HRR for flammable liquid fires should be calculated using the equation Q&= m& H c A Separate guidance for cables, pressurized oil, and hydrogen fires 7

Heat Release Rates Recommended Peak HRR Values Recommended peak HRR values were developed based on expert judgment (Table G-1)

Panel included EPRI and NRC representatives with expertise in fire behavior/phenomena and PRA.

Values are expressed as probability distributions. The panel identified the 75th and 98th percentiles of the distribution for peak HRR.

Primary sources of information included NUREG/CR-4527 and VTT publications Gamma distribution selected:

- Only positive values starting at 0 kW

- Values in the same order of magnitude Corresponding PRA Standard SR: FSS-D5, E3 8

Heat Release Rates Recommended Peak HRR Values Example distribution developed Peak HRR Distribution by the expert panel 75th = 232 kW 98th = 464 kW

= 2.6

= 67.8 0 500 1000 HRR 9

Heat Release Rates Recommended Peak HRR Values (Table G-1)

HRR Gamma Ignition Sources kW/ (Btu/s) Distribution 75th 98th Vertical cabinets with qualified cable, fire limited to 691 2112 0.84 59.3 one cable bundle (65) (200) (0.83) (56.6)

Vertical cabinets with qualified cable, fire in more 2112 7023 0.7 216 than one cable bundle (200) (665) (0.7) (204)

Vertical cabinets with unqualified cable, fire limited 904 2112 1.6 41.5 to one cable bundle (85) (200) (1.6) (39.5)

Vertical cabinets with unqualified cable, fire in 2325 4646 2.6 67.8 more than one cable bundle closed doors (220) (440) (2.6) (64.3)

Vertical cabinets with unqualified cable, fire in 2325 10027 0.46 386 more than one cable bundle open doors (220) (950) (0.45) (366) 69 2112 0.84 59.3 Pumps (electrical fires)8 (65) (200) (0.83) (56.6) 32 69 2.0 11.7 Motors8 (30) (65) (2.0) (11.1) 142 317 1.8 57.4 Transient Combustibles9 (134) (300) (1.9) (53.7)

• See report for footnotes 10

Heat Release Rates Fire Growth in Electrical Cabinets The methodology suggests a fire growth rate for electrical cabinet fires The fire grows to its peak HRR in approximately 12 min The fire burns at its peak HRR for approximately 8 min Based on experiments reported in NUREG/CR-4527 11

Heat Release Rates Assigning HRR Values to Electrical Cabinets Visual examination of cabinet interior is recommended Identify openings in the cabinet walls Identify type of cable: qualified/unqualified Identify cable bundles Qualitatively determine if a fire can propagate from one bundle to another Select the appropriate peak HRR probability distribution 12

Heat Release Rates Examples More than one cable bundle Assuming qualified cable, select distribution with percentiles:

- 75th = 211 kW

- 98th = 702 kW 13

Heat Release Rates Examples Only one cable bundle Assuming qualified cable, select distribution with percentiles:

- 75th = 69 kW

- 98th = 211 kW 14

FAQ 08-0042: Fire Propagation From Electrical Cabinets Purpose & Scope

- Provide clarification on conflicting language in NUREG/CR-6850 related to the description of fire propagation from unvented cabinets Guidance in Appendix G is in conflict with the guidance in chapters 6 and 11 of NUREG/CR-6850

- The scope of this FAQ is limited to the clarification of the conflicting guidance provided in NUREG/CR-6850 related to fire propagation outside unvented cabinets.

Reference:

EPRI 1019259, Supplement 1 to NUREG/CR-6850 15

FAQ 08-0042: Solution Chapter 11 of NUREG/CR-6850 provides the consensus position on fire propagation outside of unvented cabinets

- The following, from the second paragraph on section G.3.3 should be disregarded:

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FAQ 08-0042: Solution Modified language includes description of electrical cabinet features that should be present to prevent fire propagation outside the cabinet

- Fire sealed (not fire rated) at cable entry points

- No vents

- Robustly secured 17

FAQ 08-0043: Location of Fires Within Electrical Cabinets Purpose & Scope

- This FAQ provides clarification on the location of fires within an electrical cabinet.

- The scope of this FAQ is limited to describing the location of a fire postulated in an electrical cabinet in a Fire PRA.

Reference:

- EPRI 1019259, Supplement 1 to NUREG/CR-6850 18

FAQ 08-0043: Solution For cabinets with no vents, the fire should be postulated approximately 1 below the top of the cabinet Analysts should inspect cabinets to determine vent location or the possibility of door openings.

- For vented cabinets, fires should be postulated at the location of the vents

- Fire should be postulated at the top of open doors 19

Heat Release Rates Transient Ignition Sources The peak HRR for transient fires is also characterized with a gamma probability distribution Gamma distribution percentiles:

- 75th = 142 kW, 98th = 317 kW

- = 1.9, = 53.7 Applicable only to localized transient combustibles (trash cans, etc.)

Not applicable to flammable liquid transient fires 20

Heat Release Rates Concluding Remarks Peak HRR values are recommended for some typical fixed and transient ignition sources in NPP fire scenarios Values are for localized ignition source (not for fires propagating outside the ignition source)

HRR for flammable liquid fires can be calculated from fundamental equations HRR for solid ignition sources are generally expressed as probability distributions based on experimental data and expert judgment Revised HRRs for electrical cabinets are discussed in a separate presentation 21