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Nuclear Engineering and Design 239 (2009) 2056–2069

Contents lists available at ScienceDirect

Nuclear Engineering and Design

journa l homepage: www.e lsev ier .com/ locate /nucengdes

DS simulation of the fuel fireball from a hypothetical commercial airlinerrash on a generic nuclear power plant

olfgang Luther, W. Christoph Müller ∗

RS, Forschungsinstitute, Garching, Germany

r t i c l e i n f o

rticle history:eceived 24 June 2008eceived in revised form 9 April 2009ccepted 13 April 2009

a b s t r a c t

In the aftermath of 9/11 events it became clear that the impact of a fast flying commercial airliner hittingthe NPP could no longer be excluded as a potential external hazard threatening the nuclear power plant(NPP) safety. One of the potential consequences of the impact is the occurrence of a fireball, large enoughto engulf the entire NPP. The knowledge about fireballs from air crashes is rather poor since it is onlybased on footage shot by chance. From careful physical and chemical examinations using first principles,it can be concluded that the physics and chemistry of the kerosene fireball are similar to BLEVE fireballs ingas tank accidents which have been studied during the last decades. The knowledge from these analysescan be applied to air crash fireball analysis.

In order to obtain an adequate understanding of the potential hazards to a NPP engulfed by a fireballa detailed analysis of the fireball is necessary. It is only by a detailed analysis that the effect of the NPPstructures on the evolution of the fireball can be derived. Though the safety-relevant parts of the NPP arestrong concrete structures, according to IAEA regulations the hypothesized entry of combustion productsinto ventilation or air supply systems and the entry of fuel into buildings through normal openings haveto be analyzed in detail. This requires local transient values of the safety-relevant fireball parameters.With the NPP being a very large structure an adequately detailed simulation requires large computinggrids and substantial computing power.

With the release of Version 5 of the Fire Dynamic Simulator (FDS) from NIST in 2007 a simulation toolis now available which is capable to perform simulations of large fireballs on sufficiently large computinggrids. These fireball simulations can be performed also by any other CFD code in which the relevantmodels have been implemented.

The FDS fireball simulation capabilities were validated with the help of a well-documented fireballevent, in which 5.9 to of propane were burnt during a BLEVE impact experiment conducted by the German

BAM in 1999.

To demonstrate the applicability of FDS to nuclear safety analysis a simulation of the impact of a 90to fireball on a generic NPP was performed. The results are presented in this paper and show that FDSrelease Version 5 is an adequate tool to analysis the effect of a fireball on a NPP, even if the largest possibleamount of kerosene involved in the crash is assumed.

The work presented in this paper is based on codes, papers, footage and material that are freely availabler doe

on the Internet. The pape

. Introduction

Before 11 September 2001 nuclear installations were designedo withstand the impact of a typical aircraft fighter of that time.

his impact constituted a design basis accident and the applicantor the licensing of a nuclear power plant (NPP) had to demon-trate that the impact does not lead to unacceptable radioactiveeleases.

∗ Corresponding author. Tel.: +49 8932004426; fax: +49 8932004599.E-mail address: mur@grs.de (W.C. Müller).

029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2009.04.018

s not use any information that is not freely available on the Internet.© 2009 Elsevier B.V. All rights reserved.

On the morning of 11 September 2001, each of the twin towers ofthe World Trade Center in New York City was attacked by a hijackedcommercial airplane and was destroyed by a combination of theplane impact and fire ignited by the fuel aboard each plane. Shortlyafter another hijacked commercial airliner flying at high speed hitthe Pentagon at ground level penetrating into the structure. In allthree events a huge fireball occurred on airplane impact.

In the aftermath of 9/11 it became clear that any new NPP mustbe designed sufficiently robust against terrorist attack and that theimpact of a fast flying commercial airliner hitting the NPP can nolonger be excluded as a potential external hazard threatening theNPP safety. The lesson learnt from the WTC and Pentagon impact is

W. Luther, W.C. Müller / Nuclear Engineerin

Nomenclature

BAM Bundesanstalt für Materialprüfung in BerlinBLEVE Boiling Liquid Expanding Vapor ExplosionCFD Computational Fluid DynamicsFDS Fire Dynamic SimulatorHRR Heat Released RateIAEA International Atomic

Energy AgencyNIST National Institute of Standards and TechnologyNPP Nuclear Power Plant

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SANDIA American National LaboratoryWTC World Trade Center

hat the impact and the onboard jet fuel cause structural damage, areball and a consecutive fire. This paper deals with the simulationf the evolution of the fireball and its safety-relevant consequences.

All current knowledge of fireballs and their consequences resultsrom video footage of the WTC and Pentagon fireballs and other fire-alls observed in air crashes. Fortunately this is not the only sourcef knowledge. The fireballs in air crashes are similar to fireballsccurring in Boiling Liquid Expanding Vapor Explosion (BLEVE)ccidents. BLEVE have occurred frequently in the last 50 years andany of them are documented by video footage. The general behav-

or of a BLEVE can be explained and estimated by simple formulashich show that the consequences of a BLEVE can be calculated by

ne single parameter, the fuel mass involved.All BLEVEs have occurred in open space with only small struc-

ures around such that the fireball can be treated as undisturbedemisphere evolving into a spherical ball. Since a NPP is made upf large geometrical structures influencing the spatial evolution ofhe fireball, a 3D simulation program is needed to calculate theffect of the structures on the expansion of the fireball and to deter-ine details of thermal loads on the NPP which may lead to severe

amage.This paper is doing pioneering work on the safety assessment of

NPP of aircraft fuel impact in case of a large commercial airliner.n this paper it will be shown how the computer program FDS cane used to analyze the consequences of the air crash fireball. Thepplicability of the program was validated by the analysis of theell-documented fireball from the BAM 1999 BLEVE experiment

Droste et al., 1999).In this paper FDS is applied to a generic NPP using a typical

mpact scenario. The results of the FDS simulation are analyzed andvaluated in order to demonstrate the safety assessment based ofhe generic plant. From the results it is obvious that the results forhe generic NPP cannot be applied directly without modificationsf the FDS input to any other NPP.

The paper is organized as follows:

the first sections deal with the regulatory approach, the typesof air crafts that have to be considered, the impact scenario andthe estimation of the percentage of the fuel contributing to thefireball,the next sections are devoted to BLEVEs, the findings from thevideo footage and the theoretical explanations,the following section deals FDS and the simulation of the BAM1999 BLEVE,finally a generic impact scenario for a generic NPP is simulatedwith FDS and the FDS results are used for the safety assessmentof the NPP.

. Some fire dynamic basics

As this paper addresses the nuclear engineering community theiscussion of some basic fire dynamic facts here is in order. Solid

g and Design 239 (2009) 2056–2069 2057

and fluid material does not burn unless it evaporates before. Anyburnable gas will burn if its temperature is above the ignition tem-perature and the fuel fraction mixture with air is between the lowerand the upper flammability limit. Heat can only be transportedby conduction, convection or radiation; radiation is the dominantmechanism in a fireball.

Aircraft fuel is a hydrocarbon mixture with the chemical formulaCnH2n+2 and almost identical with Diesel fuel used in cars. All hydro-carbons behave similar in combustion with a heat of evaporation of∼4 MJ/kg and a heat of combustion of about ∼44 MJ/kg. If hydrocar-bon gas is burnt in the optimal (stoichiometric) ratio the resultinggas mixture reaches a maximum temperature of ∼2000 ◦C.

The adverse property of a fuel droplet cloud is the high volumet-ric energy content, which exceeds a simple gas mixture by ordersof magnitude. Once the fireball is started the droplets evaporatedue to heat radiation and the expansion of the hot gas towardsthe cold ambient air atmosphere feeds oxygen to the fireball. Theouter shape of the fireball is dominated by Taylor instabilities whichgreatly enlarge the burning surface and lead to the self-sustainingrapid turbulent combustion.

3. Regulatory approach to safety assessment of aircraftimpact

Since no national guidelines exists for the analysis of the impactof aircraft fuel the starting point for regulatory approach are theIAEA Safety Guides. The basic IAEA guide for safety assessment ofa NPP is the IAEA Safety Guide GS-G-4.1 “Format and Content ofthe Safety Analysis Report for Nuclear Power Plants” which clearlyindicates that “Guidance on the assessment and verification to beconducted by the design and operating organizations in preparingthe SAR” is provided in Safety Guide NS-G-1.2, “Safety Assessmentand Verification for Nuclear Power Plants”. This guide does not dealexplicitly with the problem but recommends the use of IAEA SafetyGuide NS-G-3.1 “External Human Induced Events in Site Evaluationfor Nuclear Power Plants” published in 2004.

The relevant section is section 5.16, page 26 found in Section 5“Aircraft Crashes”, which reads as follows.

Effects caused by aircraft fuel:The following possible consequences of the release of fuel from

a crashing aircraft should be taken into account:

• burning of aircraft fuel outdoors causing damage to exterior plantcomponents important to safety,

• the explosion of part or all of the fuel outside buildings,• entry of combustion products into ventilation or air supply sys-

tems,• entry of fuel into buildings through normal openings, through

holes caused by the crash or as vapor or an aerosol throughair intake ducts, leading to subsequent fires, explosions or sideeffects.

Section 5.18 gives additional details: “The type of fuel andthe maximum amount of fuel potentially involved in an accidentshould always be evaluated in order to quantify the fire interactioneffects and correlate them with the potential structural damage.The amount of fuel should be evaluated for this purpose on thebasis of the type of aircraft and typical flight plans.”

A fireball analysis using the approach presented in this paper isone way to meet the above requirements.

4. Concept for safety assessment approach

In case that no well-defined regulatory guidelines exist a con-cept for the safety assessment has to be developed first. All relevant

2058 W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069

Table 1Comparison of Airbus A380, Boeing 747 and Phantom F-4.

Large commercial airlines compared with the SANDIA Phantom

Type Airbus A380-800 Boeing Jumbo 747-400 SANDIA F-4 Phantom 19-4-1988

Maximum velocity 0.89 Mach 0.9 Mach –Travel velocity (impact velocity) 0.85 Mach 253 m/s 0.85 Mach 253 m/s 215 m/sMaximum fuel capacity [to] 249 174 4.8 (water)Maximum passengers 555 416 1MWF

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aximum take-off weight [kg] 560 000ing span [m] 79.60

uselage diameter [m] 7.14

ssues must be analyzed and data for these issues have to beefined:

. the type of the aircraft,

. the point of impact,

. the impact velocity,

. the amount of fuel involved in the fireball and in the fuel spillfire,

. the damage done to the exterior of the buildings,

. the vents, where the fireball or smoke may enter, and the hazardsinside the buildings,

. the safety objectives that have to be met.

These issues have to be defined before the start of the assess-ent process in order to make conservative choices in the process

o demonstrate the capability of the NPP to cope with the fireballoads.

In case of a fireball the safety objective are “safe shut-down”nd “no substantial radioactive release to the environment” andhe initiating events to be investigated are “entry of fuel or fire in

he air intake vents or air exhaust vents of the reactor building andhe auxiliary reactor building and the diesel buildings”.

In case of a large spill fire the effect of the smoke and the entryf fuel into cable duct and piping systems or sewage systems has toe investigated after the fireball analysis.

Fig. 1. Comparison of Airbus A380, B

396 890 19 00064.44 11.776.49 1.91

5. Aircraft impact scenarios

As the first step reasonable assumptions concerning the aircraft,its fuel load and the impact velocity have to be made. It is goodsafety analysis practice to define a worst-case scenario. Based onthe assumption that the worst case is defined by both maximumair craft impact velocity and maximum fuel load one has to look atthe largest commercial airliners in service. The largest airliners inservice beside the few Antonov An-124 and An-225 which are flownby Russian crews only are Airbus A380 and Boeing 747. Specific datafor these planes can be found in the Internet and are presented inTable 1.

Fig. 1 shows the aircraft structures are large when compared tothe NPP structures Simulations using point models are an inade-quate approach to the problem, as all details of the structure havebe taken into account.

Jet airliners do not have separate tanks like cars. Instead, theaircraft structure, the spaces between the wing spars, parts of thebody, or the fin, is coated with layers of rubbery sealant to forma series of fuel-tight, leak-proof compartments. The different tanks

are linked by pipes and vents at the wing tip or tail keep the pressureconstant.

Fig. 2 shows the location of the tanks. In defining the accidentscenario one has to make sure the assumptions how and how muchfuel is sprayed are consistent with first principles. It is virtually

oeing 747 and NPP structure.

W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069 2059

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Table 2Documentary footage analyzed for this paper.

Location Plane Date

NASA dryden CID collisionimpact demonstration

Boeing 720 1-12-1984

Fairchild air force base B52 24-6-1994Fairford Mig 29 24-7-1993Mulhouse Habsheim Airbus A320 26-6-1988Okinawa Boeing 737 20-8-2007Oostende airshow Small single engine prop plane 26-7-1997Paris—Le Bourget Mig 29 6-9-1989Ramstein airshow 3 small fighter jets 28-8-1988

Fig. 2. Tanks of the largest commercial airliners in service.

mpossible that both wings are smashed at the reactor building.bviously if the two wings are smashed at different locations the

esult may be two spatially and timely independent fireballs or evenore likely one fireball and one pool fire.As a rule of thumb a 747-400 consumes 4 to of kerosene for take-

ff and about 10 t/h of kerosene in cruise. Even assuming a worstase scenario some reduction has to be made for the cruise fromhe airport to the NPP.

The analysis of the WTC impacts, carried out by the authorsives an impact velocity of 230–270 m/s, while the FEMA reportFEMA, 2002) gives 263 m/s. It should be pointed out that the max-mum velocity close to the surface is only limited by the stagnationressure and can be well above the specified admissible veloci-ies.

. Experience from air crashes

A well-documented experiment on hard target impact of an air-lane is available: the SANDIA Phantom experiment performed in988, when a phantom F-4 fighter was projected on a rocket sledgainst a concrete block and literally crushed to pieces.

The data of the experiment have already been presented inable 1. As a result of the impact a debris cloud of substantialize (∼60 m diameter) is formed. The volumetric ratio of structuralaterial and fuel stimulant (water) is 1:1 and a substantial part of

he debris clouds consists of water droplets. The size of the cloud cannly be explained by the fact that small size water droplets experi-nce a reduced drag when traveling in the wake of larger structuralarticles.

Based on this only experiment with hard target and a smallghter plane it is assumed that hard target impact of a large com-ercial aircraft will be similar: a large cloud is expected consisting

f structural debris and a large amount of fuel, as the ratio structuralaterial/fuel is much higher for commercial airliners.

Sioux City DC 10 19-7-1989Ukraina/Krim TU 134 10-7-2006Lviv SU 27 27-7-2002

In an air crash against a structure it is reasonable to expect thatnot all fuel will be consumed in a fireball but only part of it. Therest of the fuel will be distributed in form of a liquid film on thestructures and on the ground and will result in a subsequent poolfire. In this paper the scenario under investigation focuses on thefireball only and leaves the subsequent pool fire as a task for futurework in this field.

With ignition sources available at large it is expected that thedroplet cloud is immediately ignited and results in a large fireball.

In the 9/11 events, large commercial airplanes impacted soft tar-gets. From the video footage it can be seen that the in the aircraftimpacts on the WTC towers (FEMA, 2002) and on the Pentagon(ASCE, 2003) the aircraft literally disappears in the soft buildingstructure and immediately after entry part of the aircraft fuel isignited into large fireballs of 60–100 m diameter. From the videofootage the expansion velocity of the WTC fireballs can be estimatedto be ∼20 m/s, which is also confirmed by Baum and Rehm (2005).

A lot of documentary footage of aircraft crashes is available onthe Internet and has been analyzed for this paper. A list of theaccidents analyzed is presented is given in Table 2.

The footage shows that most of the air crashes whether they hap-pen on hard runways or soft terrain ground result in an immediatelarge fireball. Typically only part of the onboard fuel is involvedin the fireball. The fireballs show all features of BLEVE fireballsdiscussed in the next section.

Only in case of low impact velocity and soft terrain some aircrashes fortunately did not result in fireballs though fuel wasleaking from the broken wings, e.g. British Midland Kegworth airdisaster 1989.

7. Experience from BLEVEs

The word BLEVE is an acronym for “Boiling Liquid ExpandingVapor Explosion”. This is a type of explosion that can occur whena vessel containing a pressurized liquid is ruptured. A BLEVE canoccur in a vessel that stores a substance that is usually a gas atatmospheric pressure but is a liquid when pressurized. If the vesselis ruptured – generally due to loss of strength of the heated part ofthe vessel that is not cooled by the liquid inside – the sudden pres-sure drop causes the liquid to turn into a two phase mixture, whichis blown out forming a rich in fuel droplet cloud which typicallyresults in a spectacular fireball.

In the last 50 years more than 80 major BLEVEs have occurredtaking the lives of several thousand people and leaving more than10 000 injured (Abassi and Abassi, 2007). Many BLEVEs have beencovered by news crew footage and a large amount of documentary

material on BLEVEs is available on the Internet.

The authors’ conclusions, that BLEVE fireballs are having thesame basic features as aircraft impact fireballs, is based on the twofacts:

2060 W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069

Table 3Documentary footage of hot BLEVEs analyzed for this paper.

Location Fuel Date

BAM Berlin Propane 11-9-1998Bucheon, Korea LPG 11-8-2005LMMT

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2

3

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ucio Blanco, Mexico Propane 11-8-2005urdock, USA LPG 3-9-1983iami Ives Dairy substation Transformer refrigerant 2001

anker truck, Korea Styrene ???

1. The similarity of all low order hydrocarbons in combustion,chemical properties and physical behavior.

. Fireball footage shows the same typical evolution of all fire-balls, except for the initiating phase where different sources forthe initial moment can be found: the initial momentum of aBLEVE fireball originates from the tank rupture while in case ofthe aircraft the initial momentum originates from the impactmomentum of the fast flying aircraft.

From BLEVE experience two different types of BLEVE evolutionsnto fireballs have been observed and discussed in the literature:

1. If the initial momentum is to small or oxygen supply is insuffi-cient a “cold” BLEVE will occur, which means that there is only asmall fire flash, followed by a large spill fire.

. If the initial momentum is large and enough oxygen is available, a“hot” BLEVE will occur. The momentum of the initial phase has tobe sufficient to start turbulent combustion. Then the expandingfireball is self-sustaining, absorbing enough fresh air to continueburning at a more or less constant rate. Typically the hemispher-ical expansion is followed by a buoyancy induced upward flowand the fireball forms the well-known mushroom structure.

In the remainder of this paper the focus is on the simulation ofhydrocarbon fireball of the hot BLEVE type.

The documentary footage of hot BLEVE fireballs that has beennalyzed for this paper is given in Table 3.

From the footage and theoretical analysis it can be concludedhat a hot BLEVE fireball consists of three phases:

1. The initial phase, when the liquid–vapor mixture is blown out andwhich are typically light or white in the video information.

. The expansion phase, when the expanding cloud forms a hemi-spherical fireball which sticks to the ground. This phase ismomentum controlled. The dominating phenomenon is turbu-lent combustion of the hot rich in fuel fireball expanding into thecold ambient air forming an instable surface governed by Taylorinstabilities.

. The up-lift phase, when the hot fireball starts to rise in the more orless spherical mushroom-shaped plume with the vortices suck-ing ambient air into the fireball. This phase is controlled by thehot gas buoyancy. In this phase the remaining fuel is burnedalmost completely (Fig. 3).

The same holds for air crash fireballs. A good illustration of thehree phases can be seen in Fig. 4, which shows a B52 bomberrashing into a concrete runway.

As a consequence of the similarity engineering theories onLEVE expansion and uplift can be applied to air crash fireballsnd provide a method for estimation of fireball size, duration andeight as well as plausibility assessment for more sophisticated

imulations.

Though many BLEVEs have occurred in the last decades andany experiments have been conducted mostly limited from 1 kg to

bout 5 t of hydrocarbons, no well-instrumented experiments haveeen carried out on really large BLEVEs with 50 ore more t of hydro-

Fig. 3. B52 bomber crashes on Fairchild airfield runway.

carbons. All information on large BLEVEs has been derived fromextrapolation the findings from the footage on the visual domainand the formulas obtained in the analysis.

The transient behavior of BLEVE fireballs shows two differentvelocities depending on the physical effect dominating the process:

1. During the expansion phase the velocities are momentum-dominated. The fireball is expanding at a more or less constant

W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069 2061

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expansion velocity and no substantial change in height due touplift is observed.

. During the up-lift phase the expansion is slowed down and thefireball rises at a more or less constant velocity.

The difference between the two phases is more explicit inmaller BLEVEs. The same two phases of evolution have also beenbserved in aircraft crash jet fuel fireballs.

Many engineering formulas have been developed to estimate theaximum size, duration, radiation, missile range and safe stand-

ff distance of a hot BLEVE. A good overview is given in (Abassind Abassi, 2007). A simple formula can be derived for the maxi-um hydrocarbon fireball diameter by assuming that the fireball

s formed by an isochoric combustion followed by an isothermalxpansion:

fireball = 5.8 m1/3fuel

nit of Dfirefall is (m) and unit of mfuel is (kg).An in-depth analysis of the various formulas found in the liter-

ture (Abassi and Abassi, 2007) shows that there is no substantialifference between the formulas if the uncertainties of the visualvaluation of fireball footage are taken into account. Most fireballshow a good agreement with the above formula. It should be men-ioned that the formulas given in standard reference books (TNO,992) and (SFPE, 2003) and also the formulas used in NUREG (2004)UREG-1805 are slightly different but give similar values.

The duration of the fireball is harder to determine than the maxi-

um diameter, since the fireball evolution consists of two different

hases. From scaling arguments the exponent is 1/6 if the fire-all growth is dominated by expansion and 1/3 if it is dominatedy buoyancy. From observations it seems that larger fireballs areuoyancy-dominated. So the following formula is recommended

and height used in the analysis.

by many authors:

td = 0.45m1/3fuel

, mfuel < 3 × 104 kg

td = 2.6m1/6fuel

, mfuel > 3 × 104 kg

The formulas for the height of the fireball (the height of the centerof the sphere approximating the fireball in its latest phase abovethe ground) are controversial and range from:

HFB = 34

Dfireball → 32

Dfireball

In fact the problem lies in the uncertainties of the fireball visi-bility and it is often left to the expert to decide whether the fireballhas already extinguished or is still on fire but is obscured by smokeand soot. A review of large fireball footage leads to the conclusionthat lower height values can be found in large hydrocarbon fireballswith a higher probability.

From the evaluation of the fireball footage it can be observedthat the maximum diameter and the duration are independent ofthe details of the transient.

An analysis of the 9/11 aircraft impact fireballs video informa-tion indicates that the expansion velocity is almost constant andwas ∼20 m/s in the WTC fireballs. In the WTC fireballs the center ofthe fireball always rises linearly in the up-lift phase and the up-liftphase starts at 1/3 of the fireball total duration. Based on this obser-vation and using the evaluation technique described in Section 9the authors have derived from the fireball footage a new universal

formula for the start of the up-lift phase: The phase (up-lift phase)starts at ∼1/3 of the total duration time.

Applying the engineering formulas to the BAM fireball of 5.9 toand a hypothetical fireball from an air crash with 90 to gives thefollowing data (Table 4).

2062 W. Luther, W.C. Müller / Nuclear Engineerin

Table 4Engineering formulas for 5.9 and 90 to hydrocarbon fireballs.

Fuel mass mfuel 5900 kg 90 000 kg

Maximum diameter Dmax = 5.8 m1/3fuel

105 m 260 mMD

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aximum center height hmax = 0.75Dmax 79 m 195 muration, large fireball tmax = 2.6 m1/3

fuel17.4 s

uration, small fireball td = 0.45 m1/3fuel

8.1 s

With respect to the length and time scale, the observationsrom video footage fit well with what is expected from the abovengineering formulas and the evaluation of other fireball videonformation, if the evaluation methods used are those from Sec-ion 9. It is not clear from the papers published in the past, how theuthors evaluated the diameter and height of the fireball and thisact makes comparison difficult.

The BLEVE formulas allow a first guess for the size and evolu-ion of a fireball but they do not take into account the mitigatingr reinforcing effect of building structures on the evolution of thereball. The second limitation of the use of engineering formulas

s that they do predict in detail the local impact of the fireball. Inrder to assess the local effects and the effect of buildings, a 3Dimulation is necessary.

. Overview of the capabilities of FDS

The fireball simulation documented in this report has been car-ied out with the code FDS which is freely available from NIST.he program package consists of two parts, the simulation pro-ram FDS (fire dynamics simulator) (McGrattan et al., 2007a,b,c)nd the visualization program Smokeview (Forney, 2007) (Internet:ww.nist.gov/fds).

The NIST Fire Dynamics Simulator FDS predicts smoke and/orir flow caused by fire, wind, ventilation systems, etc. It consists ofve major modules:

A computational fluid dynamics module, which solves the Large-Eddy form of the turbulence equations using the Smagorinskimodel and the low Mach number approximation. This turbulencemodel has been show to be appropriate for low-speed, thermallydriven flows of smoke and hot gases generated in a fire with anemphasis on smoke and heat transport from fires. This module isusing the Eulerian approach.An improved combustion model for the fire dynamics includingpyrolysis of solids and a mixture fraction combustion model.A module for heat transfer by conduction, convection and radia-tion.A special module for devices and controls, e.g. for the simulationof thermocouples.A module for the simulation of sprinkler sprays using a Lagrangianapproach by tracing a large number of droplets. This module hasbeen extended to simulate fuel sprays, but this feature is still inan early phase of development and still experimental.

FDS has been used for many types of problems like sprinklerctivation in warehouse fires, tunnel fires, tenability in residentialres, and smoke concentration in outdoor fires. Lately FDS has beenuccessfully applied to the analysis of fires in nuclear installationsNUREG, 1824).

After 9/11 a first attempt has been made by the authors to use

DS to simulate the WTC fireballs and other problems relating toreballs caused by aircraft fuel in a targeted aircraft crash. Due to the

imited capabilities of the older FDS versions and limited comput-ng power before 2007, these simulations were restricted to smallreballs up to 5 t.

g and Design 239 (2009) 2056–2069

The simulations documented in this report have been performedwith the new FDS Version 5 using the parallel version on a largecomputer cluster. The main objective of FDS simulations is to cap-ture the global characteristics of the fireball and its evolution.

The main features of Smokeview used in the compilation of thisreport are:

1. Animated iso-surfaces of the heat release rate per unit volume(HRRPUV) that gives a visual impression of the flame boundaries.The “fire” -colored surface is generated by an interpolation of theiso-surface HRRPUV = 30 kW/m3, which is thought to representthe effective flame boundaries but is not necessarily the flameboundary as seen by an observer. In video information often alarge part of the fireball is obscured by soot and smoke. Thismust be kept in mind when a direct comparison is performed.

2. Slice files that represent results recorded for a grid plane (e.g.y = 0.0) by colors, e.g. temperatures, velocity, etc.

In addition to these options, the most relevant global data of thesimulation are saved in special data files to produce time historyplots of selected parameters at selected positions.

9. Rationale of the application of CFD and FDS to fireballsimulation

The application of CFD codes like FDS to the simulation of largefireballs for which no experimental data are available is an extrap-olation of small-scale behavior to larger size. This is justified by thefact that the phenomena governing the evolution of small fireballsare the same as those for large fireballs. A rationale for this approachwill be given in this section.

The physical and chemical phenomena governing the evolutionof a typical indoor or outdoor fire like a burning building or fueltank are highly complex and must be simplified so that they canbe effectively solved by computer simulation. It has always been amoot point in fire simulation that neither experiments nor real lifeobservations are replicated in all details. It is therefore in order toquestion the validity of fire simulations.

In contrast to this a fireball is a short-term fire blast and thephysical and chemical phenomena governing the evolution of afireball are quite clear and simple, as can be guessed from the factthat the simple engineering formulas for the BLEVE give a goodapproximation.

The simulation of a fireball in the open atmosphere withoutobstacles in the flow path can be adequately performed with simpleanalytical models, e.g. spark-ignited spherical combustion.

From the evaluation of fireballs it can be derived that three basicphenomena control the evolution of the fireball: the almost com-plete combustion of hydrocarbons, the expansion of hot gas und thebuoyancy of the hot gas ball. The combustion process is almost thesame as the combustion of a premixed gas sphere, but in this casethe premixed gas is replaced by the mixing process at the fire frontdue to local turbulence.

The flow phenomena are adequately simulated by the Smagorin-ski model of FDS, with the sub-grid turbulence being proportionalto the large-scale turbulence. The fireball starts with no turbulenceat all and turbulence is developing during the evolution. Near wallturbulence has only little effect on the evolution as most of theevolution occurs in the open space. Both the expansion and thebuoyancy phase of the fireball are atmospheric flows in the open

space except for the effect of the structures deflecting the expan-sion flow. The substantial contribution of turbulence to the processis a local effect, as turbulence supplies the oxygen at the fire front.

The main effect of turbulence is on the sub-grid scale affectingthe combustion reaction. FDS is using a simple but adequate Z-

neering and Design 239 (2009) 2056–2069 2063

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run-away mechanisms inherent to turbulent combustion:

1. hot gas from combustion generates turbulent eddies,

W. Luther, W.C. Müller / Nuclear Engi

odel for the reaction assuming that all fuel is immediately burnt ifxygen is available. In the fireball and the FDS simulation the burntot gas pushes outside the fuel rich gas which is burning at the

nterface with the atmosphere almost completely. The amount ofuel burnt at the interface controls the heat release rate per unit areat the flame front. This governing parameter is similar in FDS simu-ations, analysis of fireballs and data of experimental hydrocarbonres. This parameter seems to be a global value for hydrocarbonurning under atmospheric flow conditions. This fact strongly sup-orts the assumption that FDS simulations are also valid for largerreballs of sizes beyond today’s experience.

Sensitivity and uncertainty studies on fireballs have been car-ied out with FDS and have shown a low sensitivity of all inputata except for hydrogen mass and initial expansion velocity. Thetudies demonstrate that the hydrocarbon mass and the choice ofn adequate starting velocity on the grid level are the dominantarameters.

In FDS many sophisticated models have been implemented forhenomena important to typical fires like specific types of combus-ion or all kinds of heat transport including radiation, but all these

odels have only little effect on the result of the calculations andn this way on the evolution of the fireball. The important effect ofadiation is the load on structures outside the fireball. The contribu-ion of all theses models to the FDS fireball simulation is negligible.he radiation effect is not relevant for the NPP safety as most of itstructures are made of concrete.

The big advantage of using CFD codes like FDS, over engineer-ng formulas is the possibility to simulate the effect of structureseflecting the expansion of the fireball and to calculate detailedemperature distributions.

In the paper (Baum and Rehm, 2005) a simple model for fire-all dynamics is presented which has only implemented the basiceatures discussed above and it was shown that this model can ade-uately simulate the observed fireballs. This successful simulationan be taken as another proof for the fact that basic physics andhemistry determine the evolution of a fireball. Out of all modelsmplemented in FDS only these relevant robust features contributeo the simulation results.

In summary, the evolution of a fireball can be simulated usingfew simple models and only these features of FDS are involved

n the fireball simulations. The main advantage of FDS is that hasncorporated all the necessary models and simulates the effect oftructures on the atmospheric flow. Any other CFD code, in whichhe relevant models have been implemented, could be used for areball simulation just as well. This concludes the reasons for usingDS for simulation of fireballs.

0. Validation of FDS by simulation of BAM BLEVE

In 1999 a BLEVE experiment was carried out by BAM (Drostet al., 1999), for which a large amount of documentary material,eport, photographs and video footage are available. Therefore thisxperiment was selected for FDS validation. In this experiment aank wagon with 5.9 t (10 m3) propane was exposed to a fire whiched to vessel failure. The resulting BLEVE fireball had a maximumiameter ∼100 m, a fireball center height ∼80 m and a duration timef ∼7 s, until the fireball extinguished.

Fig. 4 illustrates the method by which this data have beenerived from the video footage.

A frame by frame evaluation to the BAM fireball footage giveshe time plots of diameter and height shown in Figs. 5 and 6.

The figures show that in the first two seconds the fireballxpands to a hemisphere with the diameter equal to the maxi-um diameter Dmax of the rising sphere. At 2 s the up-lift starts

nd the fireball rises within 5 s to its maximum height of 80 m. Theoint is that using the above evaluation technique the diameter

Fig. 5. Results from the BAM BLEVE: fireball diameter.

does not change but the volume increases substantially since thefireball shape changes from hemispherical to spherical and finallyto a mushroom. During the first 2 s the fireball sticks on the groundand after 2 s it starts to rise with a constant upward velocity of∼16 m/s.

In the parallel FDS version the calculation domain is divided intoa set of rectangular volumes called meshes. Each mesh is dividedinto rectangular grid cells. Best simulation results are obtainedusing a uniform grid with equal grid spacing in each direction forall meshes.

The problem that the boundaries of the buildings and otherstructures do not generally coincide with the grid is solved withthe help of a sophisticated method. FDS decides on the basis of per-centage of the grid cell coinciding with objects whether the gridcell is blocked or not.

The boundaries of the computational domain are either closed(no flow across the boundary) or open. In case open boundariesflow across the boundary to the outer external atmosphere andalso back-flow from this external atmosphere to the computationaldomain is allowed, but – of course – turbulence eddies are lost andthere is no realistic feedback. This means that when part of the fire-ball leaves the computational domain, the simulation results can nolonger be considered as reliable.

When performing the FDS simulations it was found that the sim-ulation depends sensitively on assumptions concerning the initialphase of the fireball, which is partly based on expert judgment,

Fig. 6. Results from the BAM BLEVE: fireball height.

2064 W. Luther, W.C. Müller / Nuclear Engineerin

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. turbulent eddies supply fuel-lean regions below flammabilitylimit with oxygen,

. burning regions heat up the surrounding fuel-lean regions whichturn flammable since the flammability limit is lowered due totemperature increase.

Run-away of turbulent combustion has to be avoided and it cane done by carefully feeding the fire load to the system in a physicalay in an adequate time span and within an adequate region.

By numerical experiments it was established that a grid of 1 mength scale is adequate to model phase 2 (expansion phase) andhase 3 (up-lift phase) of the BAM fireball. This grid size is the max-

mum grid size possible in view of the available computing poweror the subsequent simulation of the impact of a 90 to kerosene on aeneric nuclear island, which will be presented in the final section.y showing that this grid is adequate for the small BAM fireball itan be concluded that it is also adequate for a larger fireball.

The gas flow is simulated in FDS using the large eddy approxima-ion. With respect to a grid size of 1 m this means that only eddiesnd other phenomena of a scale size of 1 m are directly representednd computed whereas effects of the smaller scale motion and sizere modeled using the simplifying assumptions of the Smagorin-ki model. This also means that turbulence at scales below 1 m isssumed to be isotropic and homogeneous (Fig. 7).

Only phase 1 (initial phase) of the fireball when it is very smallompared to grid size cannot be modeled adequately. Using a 1 mrid simply means that the initial phase is below the resolution scalef the grid. This phase has to be covered by engineering assump-ions and a simulation model for the initial phase using the meansrovided by FDS. Three different methods have developed to modelhe initial phase of the BAM fireball:

1. Prescribed HRR (heat released rate): a prescribed fire load and anaccompanying air stream are introduced into the grid by a “vent”.The prescribed fire load is modeled internally in FDS by a hotpropane gas flow which starts to burn immediately dependingon the availability of oxygen. The ratio (gas flow + air flow)/(ventarea) is the key factor for the initial momentum input. Since notall parameters used in this method are known sufficiently well,they have to be quantified by expert judgment. These parame-ters include the size of impact area or mass and velocity of airentrained by the aircraft. By numerical experiments it was foundthat the prescribed HRR method shows a low sensitivity on theseparameters when varied within a reasonable range. Using theprescribed HRR the gas flow from a blower introduces a momen-

tum into the system, which has to be used to model the initialmomentum on the initially resting atmosphere from the failingvessel in a BLEVE or from the crashing plane.

. Initial fuel droplet cloud: the aircraft fuel is introduced into thesystem as a burning static droplet cloud at the start of the sim-

g and Design 239 (2009) 2056–2069

ulation. In order to simulate the droplet flow and the aircraftmomentum, this cloud is blown by a vent towards the point ofimpact similar to the prescribed HRR method.

3. Dynamic fuel spray: this technique is the most sophisticated andmost promising approach. The impact of the aircraft fuel is sim-ulated dynamically by fuel droplet sprays. These sprays can bemodeled in detail by a set of parameters including droplet flowrate and droplet velocity. The droplet spray introduces both fireload and momentum into the system.

The use of the dynamic fuel spray model allows calculating theaccumulated droplet mass per unit area (kerosene spill) that is notburnt in the fireball and this fuel mass can be used as input forthe subsequent pool fire simulation. Unfortunately the fuel spraymodel in FDS did not perform as expected due to the early state ofdevelopment of the fuel spray model. Since it has a high potentialfor best-estimate simulation it is highly desirable that future workshould be directed to improve the model performance.

Fig. 8 compares the global parameter “heat released” for thethree methods. The parameters used in the FDS calculations havenot been optimized but were selected in a “best-estimate” fashion.

Fig. 8 shows the global heat released calculated with methods 1and 2 are very similar and are in good agreement with the engineer-ing formulas. Method 1 requires less computing time. When usingmethod 3, however, the fireball burns very fast and the durationtime is much shorter than observed in the experiment or estimatedfrom the engineering formula.

A good agreement between calculated and experimental datafor the BAM fireball has be also been achieved using methods 1and 2 for fireball diameter and height as shown in Figs. 5 and 6.The comparison of footage and flame front simulation gives a goodvisual impression of the quality of the simulation. Fig. 7 shows theiso-surface of the fireball during the up-lift phase.

All three methods are using input parameters that are wellknown and clearly defined but also some input parameters thatare poorly known and in this way are left to the expert judgmentof the analyst—at least to some extent. This is specific for fireballsor FDS simulation but also true for most simulation computer pro-grams. Test calculations revealed that the “prescribed HRR” methodrequires less parameters both well known and those left to thejudgment of the analyst than the other two methods. It showsless sensitivity to the choice of these parameters than the othermethods. It proved more robust than the other methods and rarelyproduced numerical instabilities. It also required less computingtime. This is why method 1 was used in the next section.

11. The FDS analysis model of a generic nuclear island

In this section several scenarios of a hypothesized air crash ofa large commercial airliner on a generic NPP are analyzed usingthe worst case assumption of 90 to of kerosene consumed in thefireball. The available computer power limits the grid size of thesimulation. The required grid size can be easily estimated from theBLEVE formulas. A 90 to fireball has a maximum scale length of260 m. The computer power available was limited to a maximumcomputational domain of 240 m × 240 m × 390 m, when using thegrid length of 1 m. In the simulation the computational domain wasdivided into up to 30 meshes of 240 × 240 × 13 grid and solved withthe parallel version of FDS using ∼18 millions grid cells on a LINUXcluster of 30 CPUs with 2 Gb memory for each CPU. The computationdomain is not large enough to simulation the complete evolution of

the fireball, but it is adequate to simulate the safety-relevant phaseof fireball expansion close to the NPP buildings.

The input for the generic NPP was done on a 10 cm scale. Basedon the choice of the grid length of 1 m FDS converts the detailed NPPgeometry into an assembly of 1 m cubes. Curved surfaces or surfaces

W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069 2065

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hat do not coincide with the main axis are modeled by saw-toothtructures. Fig. 9 shows the resulting NPP geometry used in the FDSimulations and how the curved containment shell is converted inn assembly of rectangular cubes.

As can be seen from Fig. 9, the 1 m grid is fine enough to modelhe geometrical details which have a substantial influence on theomputational results, e.g. the gaps between the buildings.

1.1. Load assumptions

The following load assumptions have been used:

1. It is assumed that only one wing contributes to the fireball andthe total fuel mass of this wing is consumed in the fireball.

. The maximum fuel mass of one wing is 90 t kerosene equivalentto 42.6 GJ/t assuming the typical value for hydrocarbon heat ofcombustion of 42.6 MJ/kg. The resulting total fuel energy addsup to 3834 GJ.

. The impact velocity of the aircraft is 230 m/s.

. Three points and directions of impact have been selected: (seeFig. 9):

Load case 1 (LC 1) is the horizontal impact on the containmentdome from the direction of the turbine hall above the roof.

Fig. 9. FDS model of the generic NPP island.

imate simulation with the three different methods.

• Load case 2 (LC 2) is the horizontal impact on the containmentdome from the direction of the diesel building above the room ofthe control building.

• Load case 3 (LC 3) is an inclined impact into the gap between thecontainment dome and the control building from the direction ofthe diesel building.

The largest unknown in fireball simulations is the assumptionsconcerning the airplane and its impact when crashing on the NPP.A large variety of scenarios can be hypothesized and have to becovered by engineering assumptions.

The FDS calculations presented in this section were carried outusing the prescribed HRR method to model the initial phase. Arti-ficial vents are used to blow the hot gas corresponding to theprescribed HRR into the grid at some distance from the impact pointas indicated by the arrows in Fig. 9. The parameters to choose in thiscase are the size of the injection area (vent) and their distance fromthe structures. Since part of the momentum introduced by the gasflux is consumed by the structures blocking its way an additionalair vent was used to adjust the initial phase of the fireball to a real-istic first expansion which is known from the experience of 9/11fireballs and other fireballs. These parameters were determined bynumerical tests to produce an initial expansion velocity of ∼20 m/s.The vents operate for 1 s and are removed from the grid afterwards.

11.2. Results

This paper focuses on the first seconds of the fireball before itrises above the NPP and the potential hazard of the flame front onthe safety of the NPP. All other effects going along with the fireballlike missiles and heat radiation leading to loss of lives and propertyare not taken into consideration. Secondary or domino effects, too,are not investigated.

With the safety related buildings being protected against pres-sure waves and heat radiation, the questions to be answered by theFDS simulation are:

1. How does the presence of the NPP buildings affect the evolutionof the fireball?

2. Which safety related openings of the buildings are reached bythe fireball?

2066 W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069

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Fig. 10. Load case LC 1: fireball at 2 s.

. Which temperature levels and duration, velocities and fuel mix-tures are reached at these openings?

The global evolution of the fireball in LC 1 is given in Figs. 10–13t a time interval of 2 s. The expansion phase close to the groundasts about 5 s. During this phase the fireball flame front expandsver the NPP structures but its temperature is still low since only amall part of the fuel is burnt and the produced heat is used to heatp the fuel. At ∼5 s the lift-up phase starts and the fireball movespwards and away from the structures and burns away above thePP. During this phase only radiation constitutes a potential hazard.t 10 s the fireball starts to leave the computation domain, whicheans that the FDS results are no longer adequate, but Fig. 13 shows

hat the fireball is not longer in contact with the NPP structures. Thateans that the safety-relevant phase is over.A large-scale turbulent motion can be derived from this synop-

is, which consists of a quasi-hemispherical expansion phase, whichs followed by an up-lift phase. The synopsis shows that until ∼5 s

Fig. 11. Load case LC 1: fireball at 4 s.

Fig. 12. Load case LC 1: fireball at 6 s.

the fireball is in close contact with the NPP buildings. The start ofthe up-lift phase conforms well to the 1/3-rule derived from theevaluation of fireball footage.

Before ∼1.5 s its evolution is controlled by the model for theinitial phase but after this time due to turbulent dissipation thetransient shows the typical self-sustained expansion of the hot gasfrom turbulent combustion. The combustion is controlled and lim-ited by lack of oxygen supply. The fireball size can be approximatedby a hemisphere which is growing in time.

At ∼5 s the fireball starts to rise due to buoyancy and its form

can be approximated by an upward stretched ellipsoid. At this timethe typical vortex due to a rising bulk of hot gas starts to form butdue to grid restrictions the well-known mushroom structure of athermal plume cannot form.

Fig. 13. Load case LC 1: fireball at 8 s.

W. Luther, W.C. Müller / Nuclear Engineering and Design 239 (2009) 2056–2069 2067

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Fig. 14. Load case LC 2: temperature slice plot of the fireball.

Due to the NPP geometry the expansion hemisphere is not sym-etric. Fresh air from the sides of the building and also with some

dditional vorticity is fed to the fireball sitting on top of the build-ng structures. This effect can also be seen in the up-lift phase whenhe fireball takes the ellipsoidal shape.

In LC 1 due to the impact height of 33 m most of the upper sur-aces of the NPP structure come into contact with the flame front. In

Fig. 15. Load case LC 2: the hot fireball high above the NPP.

Fig. 16. Load case LC 3: the fireball is enclosing the NPP.

LC 2 with an impact height of 45 m the flame front comes in contactonly with the upper part of the containment dome as can be seenfrom Fig. 14.

For this case the fireball flame front does not constitute anypotential hazard to the NPP. The bulk of the fuel is burning highabove the NPP, as can be seen from Fig. 15, which shows the endphase of the fireball. It is only in this final state that maximumtemperatures of ∼2000 ◦C are reached.

LC 3 simulates an impact close to the ground. The fireball reachesa large extension at the ground and the flame front comes into con-tact with all structures and all openings. Fig. 16 demonstrates thisadverse effect of low impact height.

From Fig. 14, it can be seen that the fireball is not very hot duringthe first seconds. This statement does not hold in general: when theflame front is passing around obstacles the increase in turbulence

may lead to a substantial increase in burning rate and consequentlyresult in higher local temperatures. In LC 3 this is the case when theflame front runs over the top of the diesel building. Fig. 17 showsthe hot swirl generated at the edge of the diesel building.

Fig. 17. Load case LC 3: hot swirl generated by turbulence at the edge of a structure.

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An even more impressive swirl is found when the flame frontuns over the edge of the emergency feed-water building, as shownn Fig. 18. A consequence from this observation is that in the pres-nce of complex NPP structures with non-smooth surfaces thehenomenon of increased turbulence leading to higher tempera-ures has to be taken into account.

The FDS simulation gives details for the key parameters at theafety-relevant openings. FDS allows calculating local values forany relevant parameters:

gas velocity,temperatures,burning rate,fuel/air ratio,heat flux,radiation flux.

that may be used to assess potential hazard, e.g. at vents andpenings of the safety buildings.

To demonstrate these FDS features, some key parameters at thective air intake in LC 1 will be discussed in detail: normal velocityowards the opening, the fuel mixture fraction and the tempera-ure of the gas flow which are shown in Fig. 19. The air intakes about 50 m away from the point of impact and is reached by

Fig. 19. Time history of safety-relevant parameters at a fictive opening.

g and Design 239 (2009) 2056–2069

the flame front at ∼2 s. It experiences a short blow of gas flowwith ∼30 m/s and after this only smaller atmospheric turbulenceis observed. From 2 to 4 s the gas flow is rich in fuel but at low tem-peratures. It transports a high fuel load and has the capability totransport this fuel load through the opening. The FDS simulationdoes not answer the question what will happen inside the build-ing but it provides the boundary condition for an additional detailanalysis.

After the first heavy blow the normal velocities directed towardsthe opening return to low levels but the temperatures of the gasflow reach a maximum of ∼1000 ◦C and temperatures remain at anelevated level for several seconds. After the first blow the temper-atures stay above 500 ◦C for about 5 s, but at this period the gas islean in fuel.

12. Conclusions

From the real life experience with air crashes it is reasonable toassume that as a consequence of a hypothetical commercial airlinercrash on a generic NPP a gigantic fireball occurs. This fireball mayconstitute a substantial hazard to the safety of installations whichhas to be analyzed in the safety assessment process.

In this paper for the first time a method has been presentedthat is adequate to simulate large fireballs at NPPs and to quantifylocal values for the safety-relevant key parameter and to give thenecessary input data for potential hazard assessment.

The new element in the FDS simulation is that buildings andother objects are included in the simulation and that time historiesof local values for many parameters are calculated and provided forfurther detailed analysis.

FDS was validated with the help of the BAM BLEVE experi-ment with 5.9 to propane. The FDS results show a good agreementbetween calculated and experimental data for both the globalparameters and the visual comparison of footage and flame frontsimulation.

The simulations using the recent parallel version of FDS wereperformed on a large computer cluster allowing a sufficiently highgrid resolution. The FDS results gave new insight in the details ofthe problem:

• Building structures have a substantial impact of the fireball evo-lution.

• In the early phase the fireball is cool, rich in fuel and lean inoxygen.

• Local turbulence caused by obstacles and corners in the flow pathmay result in “hot spots”.

• The highest temperature is reached after the fireball has risenabove the NPP.

In this paper it was demonstrated that FDS is an adequate tool tosimulate the effects fireball caused by the crash of a commercialairliner and by this is a useful tool to assess potential hazard andto derive mitigation measures.Future developments of the FDS arerecommended to make the droplet spray model more stable androbust, to improve the ease of using the program and to reducecomputing times.

Acknowledgements

The authors would like to thank the FDS developers and themembers of the FDS Discussion Group for helpful advice and sug-gestions.

The authors would like to thank the BAM Federal Institute forMaterials Research and Testing, Department Containment Systemsfor Dangerous Goods.

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ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.nucengdes.2009.04.018.

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