ML18213A092
| ML18213A092 | |
| Person / Time | |
|---|---|
| Issue date: | 07/31/2018 |
| From: | Tammie Rivera NRC/RES/DRA/FRB, Electric Power Research Institute |
| To: | |
| Shared Package | |
| ML18213A072 | List: |
| References | |
| Download: ML18213A092 (60) | |
Text
Module III - Fire Analysis Task 11 - Special Models Part 1: Cables Fires, Cabinet Fire Spread, High Energy Arc Faults, Passive Barriers Joint EPRI/NRC-RES 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)
Module III - Fire Analysis Task 11 - Special Models Part 1a: Cables Fires Joint EPRI/NRC-RES 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)
Fire Models Generally computational fire models are developed to estimate extent and timing of fire growth There are fire scenarios critical to NPP applications that are beyond capability of existing computational fire models
- Special models are developed for prediction of consequences of such scenarios, based on a combination of:
Fire experiments, Operating experience, actual fire events Engineering judgment 3
Special Models Cable fires (modified from IPEEE approaches)
- Cable tray stack and fire spread models High energy arcing faults (new)
- Switchgear room Fire propagation to adjacent cabinets (consolidation)
- Relay room Passive fire protection features (consolidation) 4
Special Models (Part 2)
Main control board (new)
Hydrogen fires (new)
Turbine generator fires (new) 5
Cable Fires No generalized analytical theory is available to accurately model cable fires in all possible configurations in commercial nuclear plants.
Most of the information compiled for Appendix R of NUREG/CR-6850 is in the form of flammability parameters derived from experiments or correlations also developed from experimental data.
The amount of experimental evidence and analytical tools available to model cable tray fires is relatively small when compared to the vast number of possible fire scenarios that can be postulated for NPPs Simplification of these scenarios will be needed 6
Cable Fires Scenarios involving cable fires may start as:
Self-ignited cable fires
- Postulate self ignited cable fires in unqualified cables only
- Self ignited cable fires should be characterized by a cable mass ratio (mass of cables in the room / mass of cables in the plant) representative of the scenario.
- Cable mass ratio is equivalent to the severity factor Or as secondary fires caused by fixed or transient fire sources
- Cable fires caused by welding & cutting should be postulated in both qualified and unqualified cables.
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Cable Fires Cable tray ignition: Simplified cases 8
Thermoplastic Cable 1000 CHRISTIFIRE Report, NUREG/CR-7010 Burner Off Multiple Tray Test 8 800 Heat Release Rate (kW)
HRR (O2 cal.)
600 HRR (mass loss) 400 200 Tray 2 Tray 3 Tray 4 0
0 900 1800 2700 3600 4500 Time (s) 9
Thermoset Cable 200 Burner Multiple Tray Test 12 Tray 1 Off Tray 2 Tray 3 Heat Release Rate (kW) 150 HRR (O2 cal.)
HRR (mass loss) 100 50 0
0 300 600 900 1200 1500 1800 Time (s) 10
Example: Thermoset vs. Thermoplastic Comparison of Thermoset and Thermoplastic Cable HRR 1000 800 Heat Release Rate (kW) 600 Thermoplastic Thermoset 400 200 0
0 900 1800 2700 3600 4500 5400 Time (s) 11
Modeling The Easy Way The Hard Way 12
FLASH-CAT Flame Spread over Horizontal Cable Trays Required Data Cable mass/length Non-metal mass fraction Ignition 5-4-3-2-1 minute rule Upward Spread 35° spread angle Burning Rate 250 kW/m² thermoplastics 150 kW/m² thermosets Lateral Spread 3.2 m/h thermoplastics 1.1 m/h thermosets Heat of Combustion 16 MJ/kg for all 13
5 3000 Predicted Energy Release (GJ) 4 Predicted Peak HRR (kW) 2000 3 2
1000 1
0 0 0 1000 2000 3000 0 1 2 3 4 5 Measured Peak HRR (kW) Measured Energy Release (GJ) 14
Cable Fires Modeling cable fires- Appendix R of NUREG/CR-6850 Cable tray heat release rate using bench scale data
- Replaced with the modeling approach in CHRISTIFIRE Report Horizontal Flame spread rates
- Similar to the ones observed in the CHRISTIFIRE fire test series Fire propagation in cable trays
- Similar to the ones observed in the CHRISTIFIRE fire test series 15
Cable Fires Flame spread Cable tray kf is a constant with a value zf of 0.01 m2/kW xp Fire
(
z f = x p k f Q& 1) 16
Cable Fires Flame spread model 4(q&f ) f 2
v=
(kc )(Tig Tamb )2 Horizontal trays
- is assumed to be 2 mm Cable tray
- q is assumed as 70 kW/m2 zf Vertical trays
- is assumed to be zf
- q is assumed as 25 kW/m2 xp Fire 17
Fire Propagation in Cable Tray Stacks With RG 1.75 Separation (1 of 3) 35 o 35 o n=3 Cable tray stack n=2 h
n=1 Ignition Source Characteristic length
( ( ))
Ln +1 = Ln + 2 hn +1Tan 35 o 18
Fire Propagation in Cable Tray Stacks With RG 1.75 Separation (2 of 3)
First tray to second tray: 4 minutes after ignition of first tray Second tray to third tray: 3 minutes after ignition of second first tray Third tray to fourth tray: 2 minutes after ignition of third tray Fourth tray to fifth tray: 1 minute after ignition of fourth tray Balance of trays in stack: 1 minute after ignition of fifth tray 19
Fire Propagation in Cable Tray Stacks With RG 1.75 Separation (3 of 3) (contd)
If there is a second stack of cable trays next to the first stack, spread to the first (lowest) tray in the second stack will be assumed to occur concurrent with spread of fire to the third tray in the original stack .
Subsequent spread of fire in the second stack will mimic the continued growth of fire in the first stack (e.g., the second tray in the second stack will ignite within 2 minutes of the first tray in the second stack - at the same time as the fourth tray in the first stack.)
Fire spread will occur at the same rate to stacks on either or both sides of the original stack 20
FAQ 08-0049: Cable Tray Fire Propagation Purpose & Scope
- Clarify use of the empirical model for fire propagation within a cable tray stack as presented in Appendix R of NUREG/CR-6850 - EPRI 1011989
- The clarifications in the FAQ are limited to the use of the empirical model for fire propagation in a cable tray stack
Reference:
- EPRI 1019259, Supplement 1 to NUREG/CR-6850 21
FAQ 08-0049: Solution The FAQ clarifies that the model for fire propagation among cable trays should be used only for the configurations described in Appendix R of NUREG/CR-6850
- Angle of propagation
- Rate of propagation
- Cable tray stacks within the zone of influence DO NOT extend the model beyond, at most, three cable tray stacks 22
FAQ 08-0049: Ongoing and Future Work NRC has been doing research program to assess cable tray fire behavior (NIST)
- Full scale testing of fire propagation in cable trays
- Test for different cable types
- Measuring both heat release rate and flame propagation rates
- Intent is to develop better models and guidance for predicting cable fire behavior First phase complete
- See CHRISTIFIRE NUREG/CR-7010 vol. 1 Second phase complete
- See CHRISTIFIRE NUREG/CR-7010 vol. 2 23
CHRISTIFIRE 2, Corridor Cable Fires 24
CHRISTIFIRE 2 Vertical Cable Fires 25
Module III - Fire Analysis Task 11 - Special Models Part 1b: High Energy Arc Fault (HEAF) Fires Joint EPRI/NRC-RES 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)
High Energy Arcing Faults (1 of 15)
Definition Rapid release of electrical energy in the form of heat, vaporized copper, and mechanical force.
An arc is a very intense discharge of electrons between two electrodes that are carrying an electric current. The arc is created by the flow of electrons through charged particles of gas ions that exist as a result of vaporization of the conductive material.
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High Energy Arcing Faults (2 of 15)
Scope:
Switchgears Load centers Greater than or equal to 440 V Bus bars Oil filled outdoor transformers are addressed separately Bus ducts are addressed separately (via FAQ 07-0035) 28
High Energy Arcing Faults (3 of 15)
General characteristics of switchgear based HEAF events (from FEDB)
Indications of heavy smoke in the area, which may delay identification of the fire origin and whether the fire is still burning In nearly all of these events, the HEAF initiates in the feed breaker cubicle, because this is where most of the electrical energy in a high-energy cabinet resides HEAFs occurring in 480V switchgears did not report damage beyond the switchgear itself, but some resulted in the cabinet opening 29
High Energy Arcing Faults (4 of 15)
General characteristics of HEAF events (from FEDB)
Initial use of fire extinguishers may be ineffective in severe HEAF events regardless of the extinguishing agent (CO2, Halon, or dry chemical). The fires were eventually suppressed with water by the fire brigade No conclusions can be made regarding the effectiveness of fixed fire suppression systems for the ensuing fire. Only one event was successfully suppressed, with an automatic Halon system Durations of the fires involving HEAF range from minutes to over an hour. The short durations generally reflect events that do not result in large ensuing fire(s), either in the device itself or external fires.
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High Energy Arcing Faults (5 of 15)
General characteristics of HEAF events (from FEDB)
Sustained fires after the initial HEAF involve combustible materials (cable insulation, for the most part) near the cabinet Damage may extend to cables and cabinets in the vicinity of the high-energy electrical cabinet Damage to cabinet internals and nearby equipment (if observed) appears to occur relatively early in the event 31
High Energy Arcing Faults (6 of 15)
The arcing or energetic fault scenario in these electrical devices consists of two distinct phases, each with its own damage characteristics and detection/suppression response and effectiveness.
The first phase is a short, rapid release of electrical energy followed by ensuing fire(s) that may involve the electrical device itself, as well as any external exposed combustibles, such as overhead exposed cable trays or nearby panels, that may be ignited during the energetic phase.
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High Energy Arcing Faults (6 of 15) (contd)
The second phase, i.e., the ensuing fire(s), is treated similar to electrical cabinet fires described here, with one distinction.
Any closed electrical cabinet subject to a HEAF is opened to a fully ventilated fire. In dealing with postulated switchgear and load center fires, both phases should be considered.
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High Energy Arcing Faults (7 of 15)
The zone of influence 34
High Energy Arcing Faults (8 of 15)
High-Energy Phase: The zone of influence The initial arcing fault will cause destructive and unrecoverable failure of the faulting device, e.g., the feeder breaker cubicle, including the control and bus-bar sections.
The next upstream over-current protection device in the power feed circuit leading to the initially faulting device will trip open, causing the loss of all components fed by that electrical bus. This fault may be recoverable if the initial faulting device can be isolated from the feeder circuit.
The release of copper plasma and/or mechanical shock will cause the next directly adjoining/adjacent switchgear or load center cubicles within the same cabinet bank and in all directions (above, below, to the sides) to trip open.
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High Energy Arcing Faults (9 of 15)
High-Energy Phase: The zone of influence Any unprotected cables that drop into the top of the panel in an open air-drop configuration will ignite
- Cables in conduit or in a fire wrap are considered protected in this context. In other words, if cables are protected (i.e., not exposed) by conduit or fire wrap, they are assumed damaged, but not ignited, and they do not contribute to the fire load.
- Armored cables with an exposed plastic covering are considered unprotected in this context.
Exposed cables, or other exposed flammable or combustible materials or transient fuel materials located within this same region (0.9 m (3 )
horizontally) will be ignited 36
High Energy Arcing Faults (10 of 15)
High-Energy Phase: The zone of influence Any unprotected cables in the first overhead cable tray will be ignited concurrent with the initial arcing fault provided that this first tray is within 1.5 m (5 ) vertical distance of the top of the cabinet. The cable tray fire will propagate to additional trays consistent with the approach provided for the treatment of cable tray fires elsewhere in this document, assuming that the time to ignition of the first tray is zero rather than the normal 5 minutes.
- This applies to any cable tray located directly above the panel.
- This applies to any cable tray above the aisle way directly in front of, or behind, the faulting cabinet, provided some part of that tray is within 0.3 m (12") horizontally of the cabinets front or rear face panel.
- Cables in conduit or in a fire wrap are considered protected in this context.
- Armored cables with an exposed plastic covering are considered unprotected in this context 37
High Energy Arcing Faults (11 of 15)
High-Energy Phase: The zone of influence Any vulnerable component or movable/operable structural element located within 0.9 m (3' ) horizontally of either the front or rear panels/doors, and at or below the top of the faulting cabinet section, will suffer physical damage and functional failure.
- This will include mobile/operable structural elements like fire dampers and fire doors.
- This will include potentially vulnerable electrical or electromechanical components such as cables, transformers, ventilation fans, other cabinets, etc.
- This will exclude fixed structural elements such as walls, floors, ceilings, and intact penetration seals.
- This will exclude large components and purely mechanical components such as large pumps, valves, major piping, fire sprinkler piping, or other large piping (1" diameter or greater).
- This may include small oil feed lines, instrument air piping, or other small piping (less than 1" diameter).
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High Energy Arcing Faults (12 of 15)
Detection and Suppression The amount of smoke from any damaging HEAF event is expected to activate any smoke detection system in the area Manual suppression by plant personnel and the fire brigade may be credited to control and prevent damage outside the initial ZOI from ensuing fires Separate suppression curves are developed for these fires documented in Appendix P to the Fire Modeling procedure
- NSP rates updated in NUREG-2169 (EPRI 3002002936)
- Revised HEAF NSP in 17-0013 (ML18075A083) 39
High Energy Arcing Faults (13 of 15)
Modeling HEAF in the Fire PRA Identify the equipment in the room where a HEAF can be generated. As indicated earlier, this equipment includes, for the most part, 4160 V to 440 V switchgear cabinets, load centers, and bus bars Two types of initiating events should be postulated for each identified equipment:
- A HEAF event with an ensuing fire, and
- A regular equipment fire (no HEAF) 40
High Energy Arcing Faults (14 of 15)
Non-Suppression Probability and Severity Factors Assign a generic frequency for HEAFs listed in Task 6, and apportion it with the location and ignition source weighting factors to the equipment under analysis Assume targets in the ZOI are damaged at time zero The probability of no manual suppression for the targets in the ZOI is 1.0 The severity factor for a scenario consisting of targets in the ZOI only is 1.0 Probability of no automatic suppression for targets in the ZOI is 1.0 The probability of no manual suppression for targets outside the ZOI can be calculated using the detection suppression event tree described in Appendix P, with the HEAF manual suppression curve 41
High Energy Arcing Faults (15 of 15)
Example Consider a HEAF scenario consisting of a switchgear cabinet affecting two targets. A stack of three cable trays is above the cabinet. The first tray in the stack is 0.9 m (3) above the cabinet. It has been determined that one of the targets is in the first tray. The other target is in the third tray.
According to the approach provided in Section M.3, the first target is assumed ignited at the time of the HEAF. The second target is damaged at time 7 minutes (4 minutes for fire propagation from the first to the second tray, and 3 minutes for fire propagation from the second to the third tray).
- A scenario involving target in the first tray
- A scenario involving the two targets CDFi = g WL Wis CCDPi CDFi = g WL Wis Pns CCDPi 42
FAQ 07-0035 - Bus Duct HEAF Issue:
- The guidance was silent on bus duct fires Resolution:
- This was an unintended oversight
- Evidence for bus duct HEAF exists Diablo Canyon, May 2000 Columbia, August 2009
- A method for bus duct HEAF was developed
Reference:
- EPRI 1019259, Supplement 1 to NUREG/CR-6850 43
Bus Duct HEAF (1 of 4)
Bus duct physical configurations can influence the HEAF event Four basic types:
- Cable ducts
- Non-segmented or continuous bus ducts
- Segmented bus ducts
- Iso-phase bus ducts HEAF only associated with segmented and iso-phase
- Separate approaches developed for segmented and iso-phase ducts
- No HEAF for cable ducts or non-segmented ducts 44
Bus Duct HEAF (2 of 4)
General characteristics of bus duct HEAF events Rapid release of energy Potential for physical and thermal damage Potential for secondary fires Potential for release of molten metals 45
Bus Duct HEAF (3 of 4)
Zone of influence of HEAF events for segmented bus ducts.
Assume HEAF event at transition points of segmented bus ducts Molten metal to be ejected from bottom of the bus duct in right conical form at 15° angle Molten metal to be ejected outward up to 1.5 feet spherical zone of influence Subsequent fires depend on cables and other combustible materials within the zone of influence 46
Bus Duct HEAF (4 of 4)
Analyzing HEAF events for iso-phase bus ducts.
Assume a 5 foot spherical damage zone centered at the fault point Covers initial fault and hydrogen gas explosion and fire Subsequent fires depend on cables and other combustible materials within the zone of influence If fault is assumed at main transformer termination point, oil fire may need to be considered 47
Module III - Fire Analysis Task 11 - Special Models Part 1c: Cabinet-to-Cabinet Propagation Model Joint EPRI/NRC-RES 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)
Fire Propagation To Adjacent Electrical Cabinets (1 of 3)
Analytical fire models may be used in all types of fire propagation and damage scenarios.
Appendix S of NUREG/CR-6850 discusses empirical approaches for determining:
- Fire propagation to adjacent cabinets
- Fire induced damage in adjacent cabinets Empirical approach based on SNL and VTT experiments 49
Fire Propagation To Adjacent Electrical Cabinets (2 of 3)
The empirical model for fire propagation consists of the following rules:
Assume no fire spread if either:
- 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 fire spread cannot be ruled out, or cabinets are separated by a single metal wall, assume that no significant heat release occurs from the adjacent cabinet for 10 minutes if cables in the adjacent cabinet are in direct contact with the separating wall, and 15 minutes if cables are not in contact with the wall.
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Fire Propagation To Adjacent Electrical Cabinets (3 of 3)
The empirical model for fire damage consists of the following rules:
Assume loss of function in an adjacent cabinet if there is not a double wall with an air gap.
Assume no damage in the second adjacent cabinet occurs until after the fire propagates to the adjacent cabinet. Assume damage can occur earlier if there are large openings in a wall and plenum areas in which a hot gas layer is likely to form.
Assume no damage to an adjacent cabinet if:
- There is a double wall with an air gap, and
- There are no sensitive electronics in the adjacent cabinet (or the sensitive electronics have been qualified above 82oC).
Assume damage to sensitive electronics occurs at 10 minutes if there is a double wall with an air gap.
Assume damage to sensitive electronics can be prevented before 10 minutes if the fire is extinguished and the cabinet is cooled, e.g., by CO2 extinguishers.
Ongoing research to clarify guidance on cabinet to cabinet propagation incorporating observations from large scale testing and operational experience. Research is near completion.
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Module III - Fire Analysis Task 11 - Special Models Part 1d: Passive Fire Protection Features Joint EPRI/NRC-RES 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)
Passive Fire Protection Features (1 of 8)
Most of the fire protection capabilities of passive fire protection features cannot be evaluated using analytical fire modeling tools.
Empirical approaches Limited analytical approaches Probabilistic approaches 53
Passive Fire Protection Features (2 of 8)
Passive fire protection refers to fixed features put in place for reducing or preventing fire propagation. Some examples are:
Coatings Cable tray barriers Empirical approach Fire stops Dampers Penetration seals Probabilistic approach Doors Walls Limited analytical approach 54
Passive Fire Protection Features (3 of 8)
The analytical approach for modeling the response of passive fire protection features to fire generated conditions consists of a heat transfer analysis.
The boundary conditions are the fire generated conditions. In general, these consist of the heat flux exchanges at the surface of the passive feature.
- Thermo-physical properties of the material are necessary. These properties are readily available for some materials like concrete or steel.
Models can be used for estimating the temperature profile throughout the thickness of the barrier Effects of cracks and gaps in doors or walls should be evaluated only with the objective of analyzing smoke migration.
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Passive Fire Protection Features (4 of 8)
Empirical approaches are possible if you can match your conditions to the fire tests that have been performed SNL tests performed in the 1970s on several coatings
- Cable tray configurations included single cable tray and a two-tray stack
- Exposure fires included gas burner or diesel fuel pool fire
- Tests results:
coated nonqualified cables did not ignite for at least 12 minutes coated, nonqualified cables did not fail for at least 3 minutes and in some cases 10 minutes or more.
- Tests are very difficult to extrapolate - high plant-to-plant variability A basis needs to be established for any credit given to coatings 56
Passive Fire Protection Features (5 of 8)
The empirical approaches consist of replicating the thermal response of fire protection features observed in fire tests in the postulated fire scenarios.
- Cable tray barriers and fire stops: SNL tests 1975-1978
- Same configuration as coating tests
- The following systems were tested:
Ceramic wool blanket wrap, solid tray bottom covers, solid tray top cover with no vents, solid tray bottom cover with vented top cover, one-inch insulating barrier between cable trays, and fire stops.
- Propagation of the fire to the second tray was prevented in each case.
Again, a basis needs to be established for any credit taken
- Tests are not definitive for all cases 57
Passive Fire Protection Features (6 of 8)
Barriers seem to substantially delay cable damage for qualified cable.
The barriers did not delay cable damage for nonqualified cable.
Results considered most appropriate to exposure fires with smaller HRR and to cable trays in a stack threatened by fires in lower trays.
- Each barrier prevents cable tray ignition until well after the fire brigade reaches the scene (i.e., greater than 20 minutes),
- Each barrier prevents damage in qualified cable with solid tray bottom covers until well after the fire brigade reaches the scene.
Again: use the test data, but establish a basis for your application!
- NRC is investigation cable coatings but results remain preliminary -
stay tuned for more 58
Passive Fire Protection Features (7 of 8)
Probabilistic modeling of passive fire suppression systems Dampers: Equipment unavailability obtained from inspection results Penetration seals: Equipment unavailability obtained from inspection results 59
Passive Fire Protection Features (8 of 8)
Something new that is being used by some plants that you may see:
- HEAF Shields - a solid barrier (e.g., steel sheet), perhaps with insulation on top of it, installed above a cabinet that is subject to HEAF events The idea is for the solid steel sheet to deflect materials ejected during the initial high-energy phase of the arc fault event and thereby prevent the early ignition (time=0) of overhead cables
- Builds on the concepts of the HEAF model discussed previously This concept remains untested
- There is currently no accepted generic basis for crediting such barriers in fire PRA (e.g., no FAQ)
- No clear design/installation guidelines have been established
- If you encounter such barriers, you will have to develop your own basis for any credit taken 60