Evaluation of Fire Models for Nuclear

Power Plant Applications: Cable Tray

Fires

International Panel Report

Compiled by Monideep K. Dey, Guest Researcher

NISTIR 6872

 

NISTIR 6872

Evaluation of Fire Models for Nuclear

Power Plant Applications: Cable Tray

Fires

International Panel Report

Compiled by Monideep K. Dey, Guest Researcher

Fire Research Division

Building and Fire Research Laboratory

June 2002

U.S. DEPARTMENT OF COMMERCE

Donald L. Evans, Secretary

TECHNOLOGY ADMINISTRATION

Phillip J. Bond, Under Secretary of Commerce for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Arden L. Bement, Jr., Director

Appendix D: Benchmark Analysis with CFX, Matthias

HEITSCH, GRS, Germany
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CFX Simulations for the Benchmark Exercise #1 

- Cable Tray Fires of Redundant Safety Trains - 

International Collaborative Project to Evaluate Fire Models 

for Nuclear Power Plant Applications 

Dr. Matthias Heitsch 

Technical Note HET 1/2001 

Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH 

Schwertnergasse 1 

D-50667 Koeln 

Tel: +49 221 2068 678 

Fax: +49 221 2068 834 

Email: het@grs.de

Gesellschaft für Anlagenund Reaktorsicherheit 

(GRS) mbH 
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Content 

List of Figures and Tables........................................................................... 4 

Abbreviations .............................................................................................. 6 

1 Introduction ............................................................................................... 7 

2 The CFD Code CFX ................................................................................... 7 

3 Analyses on Part 1 .................................................................................... 8 

3.1 Base Case................................................................................................... 9 

3.2 Case 1....................................................................................................... 10 

3.3 Case 5....................................................................................................... 10 

4 Analyses on Part 2 .................................................................................. 11 

4.1 Base Case................................................................................................. 12 

4.2 Case 6....................................................................................................... 12 

4.3 Case 10..................................................................................................... 13 

5 Summary.................................................................................................. 13 

6 Continuation of Work.............................................................................. 14 

References................................................................................................ 14 

Figures and Tables ................................................................................... 15 
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List of Figures and Tables 

Fig. 1 View of the room to be modeled ............................................................... 15 

Fig. 2 View of the computer model for part 1 ...................................................... 16 

Fig. 3 Overview of given heat release over time (Part 1) .................................... 16 

Fig. 4 Temperature distribution at the time of strongest heat release (base 

case) ......................................................................................................... 17 

Fig. 5 Lateral view of the fire room after 180 s (base case) ................................ 18 

Fig. 6 Temperature distribution after 600 s (base case)...................................... 19 

Fig. 7 Vertical temperature profile from bottom to top after 600 s (base case) ... 20 

Fig. 8 Heat absorbed by the target cable (base case) ........................................ 21 

Fig. 9 Target cable surface temperature variation for the base case.................. 22 

Fig. 10 Flow field and temperature distribution in the room foe case 1................. 23 

Fig. 11 Heat fluxes for case 1 ............................................................................... 24 

Fig. 12 Surface temperature over time for case 1................................................. 25 

Fig. 13 Flow field in the fire room with ventilation system on (case 5) .................. 26 

Fig. 14 Cable surface temperature with case 5..................................................... 27 

Fig. 15 Comparison of surface temperatures for benchmark part 1...................... 28 

Fig. 16 CFX model for benchmark part 2 .............................................................. 28 

Fig. 17 Temperature distribution after 1200 s (base case).................................... 29 

Fig. 18 Given heat release rate over time (part 2, base case) .............................. 30 

Fig. 19 Heat absorbed by the target cable (part 2, base case) ............................. 31 

Fig. 20 Target cable temperatures during the base case of part 2........................ 32 

Fig. 21 Temperature distribution after 700 s for case 6......................................... 33 

Fig. 22 Target cable temperatures for case 6 ....................................................... 34 

Fig. 23 Profile in the center of the room for base case and case 6....................... 35 

Fig. 24 Heat flows to walls and target cable for case 6......................................... 36 

Fig. 25 Temperature stratification after 4200 s for case 10................................... 37 

Fig. 26 Side view of the room at the end of the maximum power release............. 38 

Fig. 27 Target temperatures over time for case 10 ............................................... 39 

Fig. 28 Comparison of cable temperatures for base case, case 6 and case 10 ... 40 
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Tab. 1 Summary of results of simulations ............................................................ 41 
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Abbreviations 

CFD Computational Fluid Dynamics 

NPP Nuclear Power Plant 
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1 Introduction 

A benchmark exercise has been set up to evaluate the capabilities of codes to model 

relevant phenomena with cable tray fires in a NPP. According to the specification of 

this Benchmark Exercise part 1 [DEF 00] out of the large number of numerical cases 

specified a representative selection has been simulated by the help of the generalpurpose CFD code CFX 4.3. The motivation of the application of CFX has been to find 

out how it performs in comparison with other probably more specialised codes. It is 

also of interest under which conditions the specific characteristics of CFX are beneficial 

and can justify the higher computing costs. So far, due to restrictions in the computing 

resources available not the complete suite of specified test cases has been simulated. 

However, the presented selection is believed to provide a good idea of the capabilities 

when applying CFX. Work will be continued based on the experience got from the 

meeting in Palo Alto in January 2001. 

2 The CFD Code CFX 

The code CFX-4.3 [AEA 99] provides numerical approximations of the Navier-Stokes 

equations on a finite volume basis. The program version applied here uses a blockstructured grid with body fitted coordinates. Block-structured means that all blocks of 

the computational domain have to be designed with a hexahedral shape. With complex 

geometries this implies occasionally finer grids than really necessary. 

The code offers a number of physical models to simulate a wide range of flow problems. Among these are: 

− Arbitrary multi-component mixtures, 

− Turbulence models for low and high Reynolds numbers, 

− Multi-phase models including a versatile multi-fluid model and a homogeneous twophase model, 

− Particle transport model, 

− Complex thermal radiation model based on a Monte-Carlo formulation, 
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− Chemical reaction capability, 

− Convective heat transfer and heat conduction, 

− User Interface to modify existing or add own models. 

The given benchmark makes use of the turbulence (k-ε) and multi-component models 

combined with the thermal radiation package. Chemical reactions, although possible, 

are not included in the simulations so far. 

3 Analyses on Part 1 

In part 1 of the benchmark exercise a trash bag fire in the vicinity of a cable tray inside 

an emergency switchgear room (Fig. 1) is to be simulated. The objective “is to determine the maximum horizontal distance between a specified transient fire from the trash 

bag and tray A (Fig. 1) that results in ignition of tray A” [DEF 00]. For simplicity the cable tray represented by a single power cable of 50 mm. The room has ventilation and a 

fire proof door. A base case and five related simulations with variations of the distance 

between cable tray and fire, the door open or closed and the ventilation system on or 

off are to be investigated. The time to be covered is 600 s. The total heat release from 

the trash bag is specified in Fig. 3 and the radiative fraction is fixed to be 30%. This 

specification implies not to simulate the chemical reactions in the trash bag explicitly 

rather than using the heat release curve and study the convective and radiative flows 

induced by the fire in the trash bag. 

A computer model was developed which is composed of 28400 fluid cells (Fig. 2). The 

grid resolution could be refined easily but is left on this rather crude level to comply with 

the number of test cases and the problem time of this exercise. The model contains the 

trash bag and the target cable (representing tray A) inside the room. In order to save 

computing time the outer walls are not modeled. This results in an overestimation of 

the heat losses from the fire room atmosphere because the heat up of inner wall sections is neglected and consequently the temperature difference gas to wall is too large. 

The given convective heat transfer coefficient of 15 W/m2

K is applied. For some of the 

cases the openings of the ventilation system and the fire door can be opened. In all 

other cases a crack of specified size around the door is available. 
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There are several options to implement the heat release from the trash bag. Currently 

the trash bag is modeled as a solid body with the convective fire heat release from the 

nearest cells around it and with radiation from its surface. The trash bag could also be 

a hollow body with the convective heat released from all the internal cells. Because 

radiation can only be emitted from a surface, in this case the top surface could be used 

for the radiation source. The benchmark specification does not further localize the heat 

sources therefore the first option has been implemented. During the simulations it 

turned out that the shape of the trash bag fire changed from time to time. However if 

numerical reasons or inherent instabilities cause this behavior has not been further 

investigated. 

Conduction in the target cable is included. The cable itself is represented as a cylinder 

of appropriate size and can be moved within the grid according to the different test 

cases. 

The atmosphere within the fire room is assumed to be air. Individual gas species are 

not modeled because the fire chemistry is not included. 

3.1 Base Case 

In the base case the target cable has a horizontal distance of 2.2 m from the trash bag. 

The door is closed and the ventilation system is off. It is the first case simulated and is 

discussed in more detail. The moment of the highest heat release from the trash bag is 

depicted in Fig. 4. The plume around the trash bag and the induced upwards directed 

flow is influenced by the option of the heat release chosen and may be different if modeled by the other option (see chapter 3). The target cable is affected by a flow directed 

downwards as indicated in Fig. 4. At the moment of strongest heat release the warmer 

gas is concentrated below the ceiling of the room as shown in Fig. 5. Some flow is directed towards the crack in the fire door. After 600 s the temperature distribution in the 

room is shown in Fig. 6. At this time gas temperatures do not show a remarkable stratification. Close to the walls temperatures are lower due to the heat losses resulting from 

the high heat transfer coefficient given. This behavior is illustrated in Fig. 7. From bottom to top the temperature does not vary much. Underneath the ceiling it increases 

considerably (buoyant flow) before wall cooling is dominating. With a higher gas temperature the heat flux to the wall increases and provokes a higher temperature gradient 

compared with the bottom region. From the temperature profile in Fig. 7 a subdivision 
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of the room into a hot layer above a cold layer appears to be inadequate. Of interest is 

the distribution of heat flows to walls and target cable. All flows reach their maximum at 

the time with the highest heat release. The total heat flux to the cable in comparison 

with the flow to the walls (Fig. 8) is less than the surface ratio. This may be due to the 

lower wall temperatures. In Fig. 8 decreases the radiative fraction to the cable to a very 

small value when the fire heat release decreases after its maximum. The hotter cable 

then loses energy to the cooler walls. The heat captured by the cable does not lead to 

a measurable increase of the centerline temperature. The surface temperature develops as shown in Fig. 9 and has almost no further increase after the maximum heat flow 

from the trash bag is passed. 

3.2 Case 1 

Case 1 differs from the base case only by another location of the trash bag relative to 

the target cable. The trash bag is directly below the target cable. The moment of maximum heat release is depicted in Fig. 10. Compared with the base case the cable is 

now completely inside the hot gas stream from the fire. This results in a higher heat-up 

of the cable surface as shown in Fig. 12. The maximum is now about 550 K. In the 

base case it was only 360 K. After the maximum heat is passed the surface temperature goes down as well. The power to the cable over time shown in Fig. 11 has a 

maximum of about 700 W. This is considerably more than in the base case with 500 W. 

Another difference is the radiative behavior. With this case in the late phase the cable 

radiates energy to the surroundings and is therefore cooled. 

The centerline temperature remains almost unchanged during the simulation time. 

Other cases with larger distances of the trash bag than the actual will not be able to 

create higher cable temperatures with a chance of ignition (643 K). 

3.3 Case 5 

Case 5 is interesting because of the flow patterns influenced by the ventilation system 

now on. The position of the trash bag is identical to the base case. Compared with the 

base case the cable is now in a more upwards flow. This is depicted in Fig. 13. Equally, 

the heat-up of the cable is very similar and remains low (Fig. 14). The ventilation system with a continuous inflow of cold air does not alter things considerably. 
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Chemical reactions including oxygen consumption have not been modelled. However, 

an oxygen depletion which might be avoided by the fresh air entering through the ventilation opening is not realistic because of the short simulation time. 

A comparison of all three simulated cases in terms of the cable surface temperature is 

depicted in Fig. 15. With the given ignition criterion only the location of the fire directly 

below the target cable would have a chance to ignite the cable over a longer time or 

with a higher heat release. 

4 Analyses on Part 2 

This part of the benchmark is to “determine the damage time of the target cable tray B 

for several heat release rates of the tray stack (A,C2, C1), and horizontal distance D. 

The effects of target elevation and ventilation will also be examined.” [DEF 00]. The 

duration of the fully developed fire is fixed to be 3600 s (including transitions 4800 s). 

To perform a reasonable number of simulations in a short time the computational grid 

was set to have less cells than for part 1. It is shown in Fig. 16. The model now has 

11400 cells. This includes the cells to represent the solids of the cable trays and the 

target cable (instrumentation cable of 18 or 50 mm diameter). The simulated fire heat 

from the trays A, C1, C2, which are lumped together, follows the shape shown in Fig. 

18. The peak can be between 1 and 3 MW. The target cable is considered to be damaged when the centerline of the cable reaches 200 C. 

The release of the heat from the assumed fire is implemented similarly to part 1. The 

convective fraction is placed as volumetric source into the cells closest to the cable 

trays. The radiative fraction of 48% is emitted directly from the solid surfaces. 

With the longer simulation time the heat absorbed by the boundary walls and the subsequent rise of the surface temperature should not be neglected. Therefore a one dimensional heat conduction simulation has been added. Compared with an explicit inclusion of the walls (this means by conducting cells) the computing time is negligible. 

Chemical reactions are not treated in the simulations. Hence no check for oxygen depletion has been done in the code. Only a crude hand approximation has been done. 

From the specifications it remains unclear how to proceed with the fire heat release if 

oxygen depletes for a time but then recovers by the ventilation system. 
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4.1 Base Case 

This case is distinguished from other cases by a peak heat release rate of 1 MW and a 

distance of the power cable (diameter 50 mm) of 6.1 m. The door is closed and the 

ventilation system is off. 

With the higher heat release and all openings closed it is likely that the available oxygen is exhausted soon. An approximation indicates a time of about 1200 s. This time 

has been selected for the illustration in Fig. 17. A global circulation can be observed 

and the temperature is rather uniform. 

It is speculative how the case would further develop if oxygen depletes because this is 

not modeled currently. To be conservative the simulation over the full time and the heat 

release according to Fig. 18 has been performed. 

The heat flow to the cable which is at the same elevation like the burning cable trays 

leads to a rapid heat up of its surface (Fig. 19). Therefore radiation from the cable to 

the colder boundary walls is positive which means that the cable loses energy. Consequently the heat-up of the cable is reduced. A look to the cable temperatures gives Fig. 

20. Although at the surface very soon high values are reached, in the central part of the 

cable only about 50 K increase is obtained. Therefore no damage with the given criterion can be detected. This is true either after 1200 s when the available oxygen tends 

to deplete or after 4800 s when following the given heat release curve to full extent. 

4.2 Case 6 

The base case is only capable of producing a relatively low heat-up of the target. 

Among the specified cases case 6 assumes the highest peak heat value (3 MW) in 

combination with the nearest placement of the target cable to the fire source. With 

higher heat output from the fire oxygen will deplete earlier. According to an approximation this may be after 700 s. After this time the flow field and temperature distribution 

calculated by CFX is shown in Fig. 21. A large vortex has developed with a horizontal 

flow along the floor. Fig. 23 compares the temperature in the center of the room of 

case 6 with the base case. For both cases simulations have been extended beyond the 

oxygen depletion point up to the end of the specified fire duration. Case 6 leads to a 

much higher room temperature. However the early oxygen starvation prevents a target 
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damage. The centerline temperature reaches values above the damage threshold of 

423 K only in the late phase of the simulation. This is shown in Fig. 22. A summary of 

the heat flows received by walls and the target is illustrated in Fig. 24. Right from the 

beginning the target becomes that hot that it constantly loses energy to the outer walls. 

However, by gas convection it is heated further. 

4.3 Case 10 

Both cases analysed up to now suffer from early oxygen starvation although the fire 

power might be strong enough to damage the target cable. A fresh air flow through the 

room might change the situation. Case 10 is comparable with the base case but the 

door is open and the ventilation system is working. Oxygen depletion has therefore 

been excluded. The incoming air is cold and forms therefore a stable stratification in 

the room. Fig. 25 and Fig. 26 illustrate this from different perspectives. The flows out of 

the door and the ventilation system can be seen. A cooling effect to the target cable is 

not expected. If oxygen around the burning cable trays is sufficiently available can not 

be answered unless the migration and distribution of the species involved would be 

modeled in detail. Under the assumption of abundant oxygen to feed the fire, the cable 

centerline temperature is calculated as shown in Fig. 27. There is only little heating-up 

in the center of the cable. 

5 Summary 

Following the benchmark specification a selection of six cases out of a total of 20 for 

both parts has been simulated by the CFD code CFX. Despite this reduced number of 

cases they were selected with the intention to preserve the scope of the benchmark 

and to get representative results. 

The analyses carried out demonstrate the capabilities of CFD codes in simulating fire 

situations. They also outline the higher effort with respect to computing resources. On a 

DEC-Alpha Unix machine with about 350 Mflops simulations needed approximately 

64 h and 153 h for part 1 (28400 cells) and part 2 (11400 cells), respectively. 

In order to keep computing times manageable it was decided to use relatively coarse 

grids for both parts of the benchmark. 
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None of the cases analysed leads to a damage of the target cable according to the 

specified damage criterion for part 1 and 2. This is true if depletion of oxygen is included in the simulations. If these are carried out following the heat release curves to 

full extent then case 6 leads to cable damage. 

6 Continuation of Work 

An obvious continuation of the current work is the simulation of other important test 

cases. Among these are for part 2 case 9 with partial activation of the ventilation system and opening of the door in the room. This will enable to investigate whether oxygen depletion will occur later than in previous cases. A realistic chance of cable damage may involve case 13 with a cable diameter of 15 instead of 50 mm. 

It will be necessary to investigate the quality of the grids for both models applied so far. 

With finer grid cells at around source and target it can be proved if grid convergence 

with the solutions found has been achieved. 

A crucial point for many cases is the depletion of oxygen. To provide realistic simulations mixing and diffusion of oxygen in combination with the consumption of the fire 

need to be included into the fire model of CFX. This means that for the relevant species 

additional conservation equations have to be solved. 

References 

AEA 99 CFX4.3, Documentation, CFX International, AEA Technology plc, Oxfordshire, United Kingdom, 1999. 

DEF 00 Benchmark Exercise, Definition of Problem, Revised June 19-20,2000. 
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Figures and Tables 

Fig. 1 View of the room to be modeled 
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Fig. 2 View of the computer model for part 1 

0 200 400 600

Time

0

100000

200000

300000

400000

Heat Release [W]

Total Heat Flow

Radiative Heat Flow

Heat Releases from Trash Bag

Fig. 3 Overview of given heat release over time (Part 1) 
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Fig. 4 Temperature distribution at the time of strongest heat release (base case) 
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Fig. 5 Lateral view of the fire room after 180 s (base case) 
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Fig. 6 Temperature distribution after 600 s (base case) 
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012345

Height [m]

330

340

350

360

370

380

Temperature [K]

Vertical Temperature Profile

(center of room)

Base Case (m07)

Fig. 7 Vertical temperature profile from bottom to top after 600 s (base case) 
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0 200 400 600

Time [s]

-160000

-120000

-80000

-40000

0

Heat Flow to Boundaries[W]

-600

-400

-200

0

Target Heat Flow [W]

Total heat to walls

Total heat to target

Radiative heat to target

Heat absorbed by Target Cable and Walls

Base Case (m07)

Fig. 8 Heat absorbed by the target cable (base case) 
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0 200 400 600

Time [s]

280

300

320

340

360

Temperature [K]

Target Cable Surface Temperature

Base Case (m07)

Fig. 9 Target cable surface temperature variation for the base case 
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Fig. 10 Flow field and temperature distribution in the room foe case 1 
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0 200 400 600

Time [s]

-160000

-120000

-80000

-40000

0

Heat Flow to Boundaries[W]

-800

-400

0

400

Target Heat Flow [W]

Total heat to walls

Total heat to target cable

Radiative heat to target cable

Heat absorbed by Target Cable and Walls

Case 1 (m01)

Fig. 11 Heat fluxes for case 1 
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0 200 400 600

Time [s]

300

400

500

600

700

Temperature [K]

Target Cable Surface Temperature

Case 1 (m01)

Fig. 12 Surface temperature over time for case 1 
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Fig. 13 Flow field in the fire room with ventilation system on (case 5) 
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0 200 400 600

Time [s]

280

300

320

340

360

380

Temperature [K]

Target Cable Surface Temperature

Case 5 (m01)

Fig. 14 Cable surface temperature with case 5 
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0 200 400 600

Time [s]

300

400

500

600

Temperature [K]

Benchmark Part 1:

Base Case

Case 1

Case 5

Comparison of Surface Temperatures

Fig. 15 Comparison of surface temperatures for benchmark part 1 

Fig. 16 CFX model for benchmark part 2 
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Fig. 17 Temperature distribution after 1200 s (base case) 
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0 1000 2000 3000 4000 5000

Time

0

200000

400000

600000

800000

1000000

Heat Release [W]

Total Heat Release

Radiative Heat Release

Heat Releases from Tray Stack

Part 2

Base Case (m01)

Approximate time of

oxygen depletion

Fig. 18 Given heat release rate over time (part 2, base case) 
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0 1000 2000 3000 4000

Time [s]

-800000

-600000

-400000

-200000

0

Heat Flow to Boundaries [W]

-3000

-2000

-1000

0

1000

2000

Target Heat Flow [W]

Total heat to walls

Total heat to target cable

Radiative heat to target cable

Heat absorbed by Target Cable and Walls

Part 2

Base Case (m01)

Oxygen depletion line

Fig. 19 Heat absorbed by the target cable (part 2, base case) 
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0 1000 2000 3000 4000 5000

Time [s]

300

400

500

600

700

800

Temperature [K]

Surface Temperature

Center Temperature

Target Cable Temperatures

Part 2

Base Case (m01)

Oxygen depletion line

Fig. 20 Target cable temperatures during the base case of part 2 
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Fig. 21 Temperature distribution after 700 s for case 6 
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0 1000 2000 3000 4000 5000

Time [s]

400

800

1200

1600

Temperature [K]

Surface Temperature

Center Temperature

Target Cable Temperatures

Part 2

Case 6 (m01)

Oxygen depletion line

Fig. 22 Target cable temperatures for case 6 
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0 1000 2000 3000 4000 5000

Time [s]

400

800

1200

1600

2000

Temperature [K]

Gase Temperature Profiles

(center of room)

Oxygen depletion

Base Case (1MW)

Oxygen depletion

Case 6 (3MW)

Fig. 23 Profile in the center of the room for base case and case 6 
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0 1000 2000 3000 4000

Time [s]

-3000000

-2000000

-1000000

0

Heat Flow to Boundaries [W]

-10000

0

10000

20000

30000

40000

Target Heat Flow [W]

Total heat to walls

Total heat to target cable

Radiative heat to target cable

Heat absorbed by Target Cable and Walls

Part 2

Case 6 (m01)

Oxygen depletion line

Fig. 24 Heat flows to walls and target cable for case 6 
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Fig. 25 Temperature stratification after 4200 s for case 10 
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Fig. 26 Side view of the room at the end of the maximum power release 
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0 1000 2000 3000 4000 5000

Time [s]

320

360

400

440

480

520

Temperature [K]

Surface Temperature

Center Temperature

Target Cable Temperatures

Part 2

Case 10 (m01)

Fig. 27 Target temperatures over time for case 10 
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0 1000 2000 3000 4000 5000

Time [s]

400

800

1200

1600

600

1000

1400

Temperature [K]

Benchmark Part 2:

Base Case, Center 

Case 6, Center

Case 10, Center

Base Case, Surface

Case 6, Surface

Case 10, Surface

Comparison of Surface and Center Temperatures

Oxygen Depletion

Base Case

Oxygen Depletion

Case 6

Damage Criterion

473K

Fig. 28 Comparison of cable temperatures for base case, case 6 and case 10 
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Tab. 1 Summary of results of simulations 

 Max. UL Temp. 

[K] 

Max. Flux on 

Target [W/m2

]

Max. Target 

CL Temp [K] 

Part 1 

Base Case 360 (180s) 210 300 

Case 1 360 (180s) 210 300 

Case 5 350 (180s) 210 300 

Part 2 

Base Case 680 (1200s) 840 301 (368) 

Case 6 1065 (700s) 5800 (700s) 532(4800s) 

Case 10 525 (4200s) 500 335