1.6 FIRE MODELING IN
SUPPORT OF FIRE PRA
An Educational Program to
Improve the Level of Teaching
Risk-Informed, Performancebased Fire Protection Engineering
Assessment Methods
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1.6 Fire Modeling in Support of
Fire PRA
• Fire PRA primarily applies fire modeling in the fire
scenario development and analysis process.
• A fire scenario in a Fire PRA is often modeled as a
progression of damage states over time, which is
initiated by a postulated fire involving an ignition
source.
• Each damage state is characterized by a time and a
set of targets damaged within that time.
• Fire modeling is used to determine the targets affected
in each damage state and the associated time at which
this occurs.
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1.6 Fire Modeling in Support of
Fire PRA
• An example of scenario progression
through five damage states
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1.6 Fire Modeling in Support of
Fire PRA
• The first damage state usually consists of
damage only to the ignition source itself.
• Depending on the characteristics and
configuration of the scenario, the last damage
state may consist of an HGL formation that
leads to a full room burnout.
• Damage states between the first and final
states capture target sets compromised as the
fire propagates through intervening
combustibles.
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1.6 Fire Modeling in Support of
Fire PRA
• The initiating event (ignition) is characterized
by the ignition source frequency.
• The first damage state captures the event in
which damage is limited to the ignition source
itself.
• The second and third damage states capture
additional targets as the fire continues to grow
or propagate through intervening combustibles.
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1.6 Fire Modeling in Support of
Fire PRA
• The time t1 (as well as any subsequent time
milestones in the progression) at which this
damage is postulated can be determined using fire
modeling tools used to determine which targets are
damaged.
• Each scenario progression postulated in a Fire PRA
is quantified to determine its contribution to fire risk.
• The fire risk metrics are Core Damage Frequency
(CDF) and Large Early Release Frequency (LERF).
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Conceptual representation
of the severity factor
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1.7 MSO FIRE MODELING
APPLICATIONS
An Educational Program to
Improve the Level of Teaching
Risk-Informed, Performancebased Fire Protection Engineering
Assessment Methods
8
1.7 MSO Fire Modeling Applications
• Multiple Spurious Operation (MSO) interactions
is another type of fire modeling application
frequently encountered in commercial NPPs.
• MSOs involve one or more fire-induced
component failures that include spurious
operation due to hot shorts as a result of fire
damage to electrical control cables.
• The consideration of MSOs arises from the
post-fire safe shutdown circuit analysis.
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1.7 MSO Fire Modeling Applications
• Key aspects of the MSO analysis process are:
– Ignition sources are characterized by the 98th
percentile severity fire as defined in NUREG/CR-6850
– Transient combustible materials are assumed
anywhere in the plant unless it is physically impossible
– Fire modeling tools should be verified and validated for
the application
– Fire modeling should be performed in a manner
consistent with the methods described in NUREG/CR6850
– The analysis should include an assessment of model
sensitivity to uncertainty
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1.8 Organization of the Guide
• Chapter 2 - a qualitative overview of the process
for conducting fire modeling
• Chapter 3 - guidance on selecting models to
address typical scenarios in commercial nuclear
power plants
• Chapter 4 - information on determining the
sensitivity and uncertainty associated with fire
modeling calculations
• Chapter 5 - Lists Appendices A through H with
example fire modeling analyses
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Appendix A – Cabinet Fire in Main Control Room
Appendix B – Cabinet Fire in Switchgear Room
Appendix C – Lubricating Oil Fire in Pump Compartment
Appendix D – Motor Control Center Fire in Switchgear
Room
Appendix E – Trash Fire in Cable Spreading Room
Appendix F – Lubricating Oil Fire in Turbine Room
Appendix G – Transient Fire in Multi-Compartment
Corridor
Appendix H – Cable Tray Fire
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