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Los Alamos combustion modeling for nuclear systems
Nuclear Engineering and Design 125 (1991) 403-410 North-Holland
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Los Alamos combustion modeling for nuclear systems J.R. Travis, W . S . G r e g o r y a n d F . R . K r a u s e Nuclear Technology and Engineering Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 15 February 1990
The occurrence of severe fire accidents in nuclear systems has resulted in research activities to model combustion phenomena in these systems. A combustion analysis of an entire facility is needed to determine the transient pressure and heating loads on the containment or ventilation structures and safety-related equipment, to plan emergency evacuation, to validate existing fire protection programs or suggest changes, and to integrate fire safety with other safety analysis requirements. The first part of this paper reviews traditional fire analysis methods and identifies capabilities that can be retained for modeling nuclear facilities. Additional capabilities are outlined that will be needed to accomodate the unique architecture and design-basis accidents of nuclear facilities. In later sections, the Los Alamos modeling approach for both nonreactor and reactor systems is presented.
I. Introduction The occurrence of severe fire accidents in nuclear systems has resulted in research activities to model combustion phenomena in these systems. The bestk n o w n accidents are the cable tray fires in the Brown's
Ferry nuclear power plant [1] and the deflagration of hydrogen during the Three-Mile Island Accident [2]. Severe fires involving gloveboxes and high-efficiency particulate air (HEPA) filters have occurred in the Rocky Flats facility [3], which processes nuclear materials. Significant, but less severe, flammable liquid fires
Table 1 Nuclear facility fire hazards Combustion
Fire zone location
Observed fire growth
Design fire hazard
Cable tray fires
Containment building. Cable spreading
Local accumulation of flammable and toxic vapors
Loss of nuclear and/or fire safety systems. Reignition of deep seated fires
Filter fires
Confluence of fire pr .~lucts and fresh air
Ignition of additional filter fires and duct fires
Failure of supply or exhaust fans and HEPA filters
Flammable liquid fires
Diesel generator fuel, coolant pump lubricants
Flame elongation by external feed back of oxygen depleted air; wall temperature rise in sealed containment buildings
Failure of fire barriers, fire suppression systems, and safety related equipment
Hydrogen fires
Tritium production facilities, Nuclear reactor containment buildings, Reactor off gas systems
Accumulation and combustion of flammable gases, Hydrogen release into ventilation system. Reversal of ventilation system flows. Gas temperature below flashover threshold
Deflagrations/detonation~, Rupture of radioactivity containment barriers. Survival of safety related systems. Release of smoke, radioactivity or toxic chemicals
0029-5493/91/$03.50
© 1991 - Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d )
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J.R. Travis et al. / Los Alamos combustion .~,,c~ieiing
have occurred in diesel generator rooms and turbines [4]. Table 1 ~ives a summary of the combustible materials in nuclear facilities. A combustion analysis of an entire facility is needed to determine the transient pressure and heating loads on the containment or ventilation structures and safety-related equipment, to plan emergency evacuation, to validate existing fire protection programs or suggest changes, and to integrate fire safety with other safety analysis requirements. Fire safety requirements are different for reactor and nonreactor systems. Reactor systems are designed to produce power and/or nuclear materials, and nuclear reactor criticality transients may lead to design-basis or severe accidents [5]. For example, 1000 kg of hydrogen was generated during the Three-Mile Island accident by the oxidation of roughly one-half of the zirconium fuel cladding in the core. Additional hydrogen and carbon monoxide could be released by the interaction of degraded or molten core materials with concrete basement structures. Thus, fires in reactor systems may be initiated and then propagate during a transient release of heat, carbon monoxide, and hydrogen. Nonreactor systems involve activities other than power or nuclear material poduction. Typical activities include glovebox operations, fuel fabrication, and spent fuel reprocessing or storage. Industrial hygiene usually requires that such operations be conducted in closed rooms in which a plant ventilation system prevents the release of radioactivity or toxic chemicals by maintaining a negative pressure differential toward contaminated areas during normal operations. These systems have complex ventilation systems that exhaust air from multiple rooms or laboratories. Section 2 revie:vs traditional fire analysis methods [6] and identifies capabilities that can be retained for modeling nuclear facilities. Additional capabilities are outlined that will be needed to accomodate the unique architecture and design-basis accidents of nuclear facilities. In later sections, the Los Alamos modeling approach for both nonreactor and reactor systems is presented.
2. Overview of numerical models Traditional models simulate combustion product release and motion in residential and commercial buildings such as hotels, shopping malls, and warehouses. This traditional approach describes combustion by scaling empirical heat and mass release data with irradiated fuel surface areas [7]. The movement of combustion
products in the building is calculated by tracking the evolution of hot gas layers in the burn room and in adjacent rooms [8]. Building fire models by themselves cannot be used for nuclear facilities because nuclear facilities unique architecture introduces special modes of fire growth. Also, fire hazards exist in nuclear facilities that are absent in commercial buildings. Most of the observed fire-growth phenomena and design accidents listed in table 1 fall in this special category. Traditional fire models must be extended to represent the special characteristics and architecture of nuclear facilities. The associated modeling requirements are derived below by reviewing traditional models for heat release, combustion product release, and motion of combustion products. The desired additional capabilities are derived by comparing traditional models with existing lumpedparameter codes and three-dimensional field models. Our review identifies the lumped-parameter codes and field models that can predict combustion product flows in a burn room and throughout a nuclear facility. 2.1. Traditional fire models
Much research and verification testing have gone into the development of computer models that refiably predict the release and motion of combustion products in buildings [6]. The following review identifies the modeling capabilities that are applicable to both nuclear facilities and buildings. 2.1.1. Release of combu.~ti~n products and heat The rate of combustion product release per irradiated fuel area is calculated in a two-step process. The first step is to estimate the heat flux to the fuel by tracking the exchange of heat radiation among the fuel and surrounding fires, hot layers, and hot walls. The net heat across the radiation-exposed fuel surface then is converted to the volatilization of hydrocarbon fuels by multiplying with an empirical heat of gasification. This heat has been measured in small-scale calorimeter fire tests [7]. The second step is to convert the volatilization mass-burning rate to a combustion heat release rate and a smoke release rate using two additional empirical parameters. The first parameter is an apparent heating value that has been measured in compartment-size [8] calorimeter tests. Multiplying the mass burning rate by this parameter gives the rate of combustion heat release. The second parameter denotes the "theoretical" heating value that has been measured in an oxygen bomb calorimeter. The ratio of these two heating values is the
J.R. Travis et al. / Los Alamos combustion modeling combustion efficiency; deviations of the combustion efficiency from 1 are used to estimate smoke release. Thus, smoke in this approximation denotes volatiles that will not burn in a room fire but that will burn in an oxygen bomb calorimeter. This smoke is made up mostly of soot and may include unburned flammable gases (CO) a n d / o r fuel droplets. Calorimeter tests measure oxygen consumption and then derive the apparent heating value by assuming that 1 g of oxygen consumption releases 13 kJ of heat regardless of fuel configuration, combustion environment, or chemistry [9]. The apparent heat of combustion is equivalent to an entrainment coefficient. Thus, the combustion effic;ency should decrease, and smoke release should increase with oxygen depletion in the combustion environment. This decrease has been observed in under-ventilated calorimeter fire tests [10], where the presence of oxygen-depleted air is indicated by increased gas temperatures. Forced-ventilation compartment fire tests [11] show that oxygen intake by complete combustion increases by a factor of 2 when little or no oxygen is available through entraining the burn-room atmosphere [12]. Therefore, numerical models of combustion product release in nuclear facilities should track all constituents that could be recirculated to the fuel inside and outside the burn room. Inefficient combustion and the associated release of smoke and unburned flammables are to be anticipated when the actual burn-room intake of oxygen falls below the increased oxygen demand of ventilation-controlled fires. The extension of the building fire release rates to nuclear facilities may be unsatisfactory because empirical factors, which are time-invariant during the clean-burning stage of a fire, change with the environment of ventilation-controlled fires (temperature, smoke, flammable gas, and oxygen). Accurate release rates of ventilation-controlled fires may require an extraordinary amount of calorimeter fire tests unless other approaches to entrainment coefficients can be found that are insensitive to the accumulation of combustion products near the fire. One thermodynamic approach is to predict entrainment from wail heat losses and experimental flame temperatures [13]. Wall heat losses are available from lumped-parameter codes or three-dimensional field models or may be estimated from compartment fire tests. They are known to be ,;:,sensitive to the accumulation of combustion products in both open-door and forced-ventilation fire tests. Empirical flame temperatures for ventilation-controlled fires are available from investigations of flames burning near extinction [14]. These temperatures are insensitive to fuel chem-
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istry and the combustion environment. An initial application of the thermodynamic approach generated acceptable pretest values for crib and pool fire mass burning rates [13].
2.1.2. Motion of combustion products Building fire models describe the motion of combustion products through the evolution of hot gas layers in the burn room and adjacent rooms [8]. A set of coupled ordinary differential equations is used to predict the time-dependent composition and temperature of individual layers from the conservation laws for mass, momentum, and energy. Gas-layer locations are selected by the modeler and represent local heat and mass exchange phenomena, which are represented by source terms. Corridors are treated as other rooms. The model is restricted to multiroom architectures and open-door ventilation passages. The following local exchange phenomena are essential for a reliable prediction of gas layers outside the burn room. (a) Unidirectional flow: wall and time variation of wall heat-transfer coefficients, - mass flux in and out of individual layers. (b) Bi-directional flow: - entrainment by fire plumes, - entrainment by door jets, radiation heat exchange among fire plumes, hot layers, and walls. The idealization of three-dimensional flows as hot layers with bi-directional exchange is the key for operating building fire models on very modest computers such as personal computers. This simplification may not be desirable for nuclear facilities because the place of local accumulations of combustion products and heat often cannot be supplied as input and may not take the form of horizontally extended hot layers. The heat- and mass-exchange modules that are used for building fire simulations are independent of the location of local accumulations and are applicable to nuclear facilities as well as to buildings. -
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2.2. Desired nuclear-facility-specific capabilities The building fire models cannot be used directly in nuclear facilities because their architecture differs from standard corridor-connected rooms. The following de-~ sired modeling capabilities are derived from such architectural differences.
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J.R. Travis et al. / Los Alamos combustion modeling
2.2.1. Nonreactor facilities
The architecture of nonreactor facilities is characterized by ventilation systems tl'at are designed to prevent the accumulation of toxic or radioactive products'at room elevations where people are breathing. Thus, the development of horizontally extended hot gas layers is unlikely and may violate industrial hygiene requirements. However, the local accumulation of radioactive trace constituents, combustion products, and unburned flammables must be modeled whether or not the modeler knows their location. The capability to predict local accumulations of heat and combustion products is needed to avoid corrosion and heat loads on safety systems (fire detectors, safety sensors, computer chips, and so on) and the degradation of ventilation control systems (control dampers, fans, and filters). The capability to predict the temperature and concentration of locally accumulated flammable gases or fuel vapors is needed to predict fire spread by fire balls and duct fires. Models for release of radioactive trace constituents are needed to simulate nuclear-facility-specific design accidents. Release might proceed as resuspension by locally high velocities on burn room floors; it also might be produced as radioactive smoke in a combustion of gloveboxes, contaminated storage devices, or clothing. The release of radioactive trace constituents may violate industrial hygiene regulations. The concentration of radioactive trace constituents must be predicted throughout the entire facility to identify such potential violations. Thus, desirable modeling capabilities include three-dimensional flow fields in rooms with gioveboxes or nuclear material testing laboratories and particle motion with gravitational deposition and resuspension. Filter fires are ignited by high temperatures and the accumulation of some flammable liquid in or on the filter; vital filter banks are protected by automatic water discharge systems. The capability to simulate droplet evaporation and wall condensation heat transfer is desired to reliably predict the temperature of filters and the temperature of local combustion product or flammable gas accumulations upstream of filters.
2.2.2. Reactor facilities
The architecture of reactor facilities is characterized by tall, sealed containment buildings. The combustible materials are flammable liquids and cable trays. The combustion of hydrogen and carbon monoxide, which are generated during accident conditions as discussed in the introduction, requires additional modeling capabilities.
Flammable liquid fires produce large amounts of heat and may raise temperatures throughout the containment building by more than 100°C. Strong updrafts initiate .movement of hot air and moderate amounts of smoke throughout the building and control the oxygen supply to the fire location. Fires become ventilation-controlled very quickly, even without a ventilation system. The capabifity to model the feedback of heat and smoke to the fuel is needed because it affects the heat and mass release rates as discussed above. New methods of estimating flame length also are needed because long fames may raise local temperatures to flashover thresholds (600 o C) or spread out of the burn room. Current research shows that flames burning near extinction grow long because they proceed by the reignition of locally fuel-rich pockets of flammable vapors [15]. Raising the inlet gas temperature should prolong flames because reignition requires less heat release by local combustion. Very long flames become possible when oxygen-depleted air is fed back to the burn room. Such feedback may be generated by threedimensional flows in the vicinity of the burn room ("bath tub vortex") that cause bi-directional flows in vertical flow passages. Flames may now reach out of the burn room in search of new oxygen that no longer flows into the burn room. The capability to predict flame elongation by heating of inlet air of recirculation of oxygen-depleted air is very desirable for reliable prediction of fire spread. Simple models of a temperature-dependent and continuous diffusion reaction rate have been successful in predicting flame elongation response to the feedback of oxygen-depleted air. Cable fires burn slowly but produce very large amounts of smoke, toxic combust';or,, products (hydrochloric acid), and flammable vapors. The same cable tray can release combustion products at "~ery different rates, depending on both the temperature and the smoke in the combustion environment [16]. Thus, cable fire hazards are sensitive to ventilation conditions because the slow heat release rate does not break up local and extended accumulations of smoke and corrosive or flammable gases. Therefore, the accurate simulation of combustion product concentrations throughout the facility is required. The same capability is needed in reactor facilities because of health hazards and the degradation of engineered safety or computer systems by acid and warm cable fire smoke. Accumulations of hydrogen and carbon dioxide may burn as continuous diffusion flames, may deflagrate, or may detonate under certain combinations of local temperature and flammable gas concentra'i~,~. ~ome mod-
J.R. Travis et al. / Los Alamos combustion modeling
eling advances for simulating such fires have been made and are discussed below.
3. N o n r e a c t o r s y s t e m s
Nonreactor systems are those nuclear facilities that involve many fuel cycle operations. These operations and processes could involve fuel fabrication, fuel reprocessing, spent fuel storage, and so on. Accidents involving combustion are events considered by the Department of Energy and the U.S. Nuclear Regulatory Commission (USNRC) in their safety analyses. Los Alamos National Laboratory has developed a handbook that is devoted to the analysis of accidents in these types of facilities for the USNRC [17]. The unique features that affect combustion accidents in nonreactor nuclear facilities are the following. - forced ventilation, - network-type flow paths, - radioactive material involvement. Nuclear facilities do not have open doors and windows. Air is brought into, through and out of the facility by forced-ventilation systems. The air passes from less contaminated zones to more contaminated zones. These forced-ventilation systems can consist of hundreds of ducts, many rooms and gloveboxes, multiple fans, and many filtration devices all in an interconnected network. The possibility of fire in these systems, the involvement of radioactive materials, and the fact that the ventilation system provides the primary pathway to the environment has led to the development of numerical models to analyze these systems for combustion accidents. The computer code that has been developed at Los Alamos to simulate combustion analysis in nonreactor systems is called FIRAC [18]. The numerical method in FIRAC for modeling nonreactor systems that involve nuclear facility ventilation systems is the "lumped-parameter" method. This method consists of describing the interconnected ductwork by linkages that are called elements or branches and junction points called nodes. The models used to describe the effects of a fire in a nonreactor system include combustion, gas dynamics, heat transfer, and material transport. The detailed equations and numerical models are desdribed elsewhere [19]. However, the models and methods developed above describe an approach than can be used to analyze the effects of a fire in a nonreactor system. The energy from the combustion process and the material generated, including both nonradioactive and radioactive material, are calculated by a zone-
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type combustion model. This model serves as a source term for distribution of energy and material throughout the nonreactor system network that is usually the facility's ventilation system. Gas-dynamic equations are used to solve for the pressures, densities, m~iss flows, and temperatures. In addition, the heat-transfer models for the network duct walls are described. Finally, the equations describing the transport of the combustion products are outlined. The final result is the distribution of radioactive materials and temperatures throughout the system.
4. N u c l e a r r e a c t o r c o n t a i n m e n t s y s t e m s
4.1. Introduction
The major concern for accidents involving combustion in light-water reactor (LWR) containments is the resulting static or dynamic pressure loads. Combustion may cause a breach of the containment or affect important safety-related equipment that may be damaged because of either pressure loads or high temperatures. After the Three-Mile Island Accident (a severe, or degraded-core~ accident), it was found that significant quantities of hydrogen had been generated from the fuel-cladding/water reaction. When released into the containment, this hydrogen burned by one or more combustion modes and thereby posed a threat to the containment integrity, internal structures, and safety-related equipment. Other combustibles such as electrical cable insulation and hydrocarbon lubricating oils for cooling electric pump bearings also have come under examination as possible fuels. In fact, the 1975 Browns' Ferry nuclear plant fire (electric cable insulation ignited from a candle used for leak detection) caused direct losses of $10 million and indirect losses of $30 million related to business interruption. Modeling containment building geometries is a challenge. The Heiss Dampf Reactor (HDR) containment near Frankfurt, West Germany, is 60 m high and 10 m in diameter. It contains 2 stair wells, an elevator shaft, several vertical open hatchways, and about 60 rooms. This particular containment has roughly 11300 m 3 of free volume, or approximately one-sixth the free volume of a typical US pressurized water reactor (PWR) containment. Experiments simulating cable tray, hydrocarbon lubricating oils, and severe accident combustion phenomena currently are being conducted. The USNRC has supported research at Los Alamos and Sandia National Laboratories to develop combustion models to evaluate threats to the reactor contain-
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ment and safety-related equipment. Current levels of research will coordinate model validation with ongoing experiments at the HDR facility. The detailed field equations and numerical models are presented elsewhere [19]. Using the field equation model coupled with finite-rate global chemical kinetics, we have successfully analyzed hydrogen and hydrocarbon diffusion flames occurring in a nuclear reactor containment under accident conditions. These combustion modes are the easiest to model and analyze when compared with other modes of combustion such as propagating flames in premixed fuel/oxidizer volumes. Deflagations, flame acceleration and transition from deflagation to detonation (DDT), and detonations are all important combustion modes that have not been modeled successfully in complex reactor containment structures. In the next section, we will address some of these issues and recommend approaches for solutions.
terns. In this case, a multidimensional model is needed and can be coupled with the lumped-parameter ventilation network. This capability currently does not exist, and research needs to be devoted to establishing this capability, Research toward developing system safeguard models also need to be pursued. These models would simulate the effect of sprinklers, halon discharge, and demisters. With these models, the adequacy of the fire protection system can be evaluated. Finally, the lumped-parameter codes and models need to be validated through experiments. Large systems of interconnected ductwork, rooms, filters, dampers, and fans need to be included in the experimental system. This experimental system, with proper instrumentation for measuring temperatures, pressures, flows, and aerosol concentrations, would be invaluable for obtaining experimental data. 5.2. Reactor combustion modeling
5. Research directions 5.1. Nonreactor combustion modeling
The basic framework necessary for modeling combustion phenomena associated with nonreactor systems has been established. However, several research directions are needed. These research areas include and nonradioactive combustion characteristics, - material transport model improvement, - lumped-parameter and multidimensional model coupling, development of system safeguard models, and - experimental validation and verification of system (network) modeling codes. Very little is known about how radioactive material burns, particularly how it may combine with nonradioactive burning material. Currently, highly empirical data are being used to determine the airborne fraction of radioactive release from contaminated material. Research needs to be performed to develop better models using a larger experimental data base. The transport of smoke with its radioactive component is perhaps the single most important aspect of nuclear materials fires. Models that accurately simulate the behavior of particulate and gaseous material as it moves through the nuclear facility are needed. In particular, interparticle dynamics along with particle depletion models need to be developed and refined. The lumped-parameter approach is not suitable for large volumes (such as large rooms) in nonreactor sys- r a d i o a c t i v e
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The field equation approach has been applied successfully to diffusion flame combustion modes in complex reactor containment geometries. To evaluate other major concerns involving combustion phenomena during a severe accident in a reactor containment, we must develop modeling capabilities to investigate flame propagation in premixed, complex, multidimensional volumes. This includes laminar and turbulent deflagations, flame acceleration and transition from deflagation to detonation, and detonations. We also wish to model radioactive/nonradioactive aerosol dynamics and the interaction of the nonoxidized and oxidized aerosols with the propagating flame. Accident mitigation concepts such as water sprays and their influence on a steam-air-hydrogen environment must be assessed. All of these phenomena must be evaluated in terms of thermal and mechanical loads on .the containment and on safety-related equipment. Some of these research areas include - adaptive gridding, - detailed chemical kinetics, turbulence, radiation heat transfer, - aerosol dynamics, and - model validation with experimental data. For propagating flames through complex geometries, spatial resolution and accuracy of the flame and the near vicinity of the flame are important. This will require dynamic implementation of adaptive gridding algorithms [20] to sufficiently resolve the flame as it propaTates through the containment. -
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Global finite-rate chemical kinetics, which were used for diffusion flame analysis, will not be adequate for propagating flames. For the wide range of conditions (pressure, temperature, and gas composition) likely to be found in reactor containments, it will be necessary to couple the fluid dynamics with a detailed chemical kinetics reaction set as suggested by Oran et al. [21]. The disadvantage of directly coupling chemical kinetics with 48 reactions and 9 species is that for large time-dependent, three-dimensional problems, the solution algorithm is currently computationally prohibitive. An alternative approach [22,23] could be to solve the detailed chemical kinetics for the expectec', range of pressures, temperatures, and gas compositions. From these conditions and solution.% we can determine 'an induction time, a reaction time, and the amount of chemical energy released. These parameters could be approximated with analytic functions [22] or tabulated in a parameter space table. A modified combustion parameter, P, transport equation [23] can be written as aP
a--i +
"
=
.(o
,P ) +
1
+,(7+, P, x,)'
(1)
where ~-+ and 0 represent the induction time and turbulent transport coefficient, respectively. Initially, P-= 0, so that when P _~ 1, the induction time has elapsed and the chemical energy of combustion is released over a period of time equal to the reaction time, ~'R- In this way, an approximate method for coupling the fluid dynamics and detailed chemical kinetics can be achieved. The disadvantage of this procedure is that fluid dynamics effects like acoustic waves and turbulence intensities would not actually influence the individual chemical reaction rates and therefore the reaction time. It may be possible to add an additional variable to the parameter space table to account for turbulence and the influence turbulence has on the reaction rates, induction time, and reaction time. If the reaction rates could be correlated to the intensity or kinetic energy of the local turbulent conditions, then the first term on the rig.hthand side of eq. (1), the turbulent diffusion of P, coulc~ be eliminated from the equation. For diffusion flame modeling, we have found that turbulent buoyant plumes generally are represented adequately with the two-equation K-c model. However, Zyvoloski [24] has shown that the most accurate plume dynamic predictions are provided with the three-equation ec-.--T '2 model. For this model, the additional transport equation for the average temperature fluctuations squared is solved. Reynold stresses and turbulent energy fluxes then are calculated and used directly in
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the Reynolds-averaged Navier-Stokes equations. At this point, it is not clear whether this three-equation model is needed to describe diffusion flame plume dynamics. tf the survival and proper functioning of safety-related equipment located near diffusion flames remains an unresolved safety issue, then it may be necessary to use a three-equation model. Also, if the diffusion flame plume impinges on the steel shell liner, a higher temperature can be predicted than what actually is observed. Although this result is conservative, there is actually more entrainment of cooler material into the plume than predicted. It may be necessary for "best-estimate" calculations of thermal loads on the containment shell to use the three-equation model. Radiation heat transfer from diffusion and propagating flames can be of major importance. For example, in optically thick regions near hydrocarbon diffusion flames, the coupling between carbon or soot, the fluid field, and the radiation field will be strong, whereas in optically thin regions far from the flame, this coupling will be weak. To include these effects, we consider adopting an extension of a nonequifibrium radiation model originally developed by Alme, Westmoreland and Fry [2.5] and then extended by Daly [26]. In this model, the local energy densities of the radiation field, the fluid, and the particles can be different. Because the speed of light is inherent in these equations, any practical solutions must be obtained from an implicit finitedifference form of the equations. Water sprays and aerosol dynamics are similar problems. For example, in a steam-air-hydrogen environment, the spray water droplets and aerosol particles provide condensation nucleii to reduce the steam concentration in the mixture. Two undesired physical effects occur: (1) hydroger~ concentrations increase as the steam condenses, and (2) turbulence levels increase with condensation and droplet and particle momentum exchange with the continuous gas phase. Nonoxidized aerosols in the presence of hydrogen combustion could increase the energy release as aerosols contribute to the combustion process. On the other hand, water sprays such as mists can be huge energy sinks and provide mitigation methods for suppressing or controlling potentially damaging combustion modes. These tradeoffs must be evaluated and assessed to provide input for severe accident management pol~c,'es. These modeling questions and the coupled phenomena are difficult, and the physical geometry to which they must. be applied is so complex that model and code verification is essential. This can be accomplished only with modelers and experimenters working very closely in a collaborative effort to resolve and understand the
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relevant phenomena. Separate-effect experiments must be performed to validate individual physical and chemical processes wherever possible. Integrated tests at several scales will provide data and the confidence for computer models to be used to extrapolate to full-scale geometries. Only in this way can numerical modeling and simulations address the issues and answer the questions concerning the problems involved in nuclear reactor safety.
References [1] A.J. Pryor, The Browns' Ferry Nuclear Plant Fire, Society of Fire Protection Engineers technical report 77-2 (1977). [2] J.O. Henrie, and A.K. Postma, Analysis of the Three Mile Island (TI'11-2) hydrogen burn, Proceedings 2nd Int. Topical Meeting on Nuclear Reactor Thermal Hydraulics, Santa Barbara, California (January 1983), p. 1157. [3] D.E. Patterson, The Rocky Flats fire, Fire Journal 64 0970) 5. [4] K.N. Dungan and M.S. Lorenz, Nuclear Power Plant Fire Loss Data, Electric Power Research Institute Contractor report EPRI NP-3179 (July 1983). [5] J.P. Church, Safety Analysis of Savannah River Production Reactor Operation, Savannah River Laboratory report DPSTSA-100-1 (September 1983). [6] Society of Fire Protection Engineers, SFPE Handbook of Fire Protection Association (SEFE, Quincy, Massachusetts, September 1988). [7] A. Tewarson, Generation of heat and chemical compounds in fire, in: SFPE Handbook of Fire Protection (SFPE, Quincy, Massachusetts, September 1988), p. 179. [8] W.W. Jones, A multicomponent model for the spread of fire, smoke, and toxic-gases, Fire Safety Journal 9 (1985) 55. [9l V. Babrauskas, Burning rates, in" SFPE Handbook of Fire Protection (SFPE, Quincy, Massachusetts, September 1988), p. 2-1. [10] J. Steciak, A. Tewarson and J.S. Newman, Fire Properties of Combustible Materials Commonly Found in Nuclear Fuel Cycle Facilities, Factory Material Research Corporation report FMRC J.I.0G3RS.RC (February 1983). [11] N.J. Aivares and F.R. Krause, Experimental Simulation of Forced Ventilation Fires, Los Alamos National Laboratory report LA-UR-84-1691 (1984). [12] N. Aivares, K. Foote and P.O. Pagni, Forced ventilation enclosure fires, Combustion Science and Technology 39 (1984) 55-81.
[13] F.R. Krause and W.S. Gregory, A Thermodynamic Fire Zone Model for Tall Containment Buildings, 1987 Status Report on the HDR Safety Program, Kernforschungszentrum, Karlsruhe, West Germany (December 1987). [14] A. Hamins and K. Seshardi, The structure of diffusion flames burning pure, binary and ternary solutibns of methanol, heptane and toluene, Combustion and Flame 68 (1987) 295. [15] A.R. Masri, R.W. Bilger and R.W. Dibble, Conditional probability density functions measured in turbulent nonpremixed flames of methane near exanction, Combustion and Flame 74 (1988) 267. [16] F.R. Krause and W.H. Schrnidt, Burn Mode Analysis of Horizontal Cable Tray Fires, Sandia National Laboratory report NUREG/CR-2431 (February 1982). [17] J.E. Ayer, A.T. Clark, P. Loysen, M.Y. Ballinger, J. Mishima, P.C. Owczarski, W.S. Gregory and B.D. Nichols, Nuclear Fuel Cycle Facility Accident Analysis Handbook, US Nuclear Regulatory Commission report N U R E G / CR-1320 (1988). [18] B.D. Nichols and W.S. Gregory, FIRAC Users Manual: A Computer Code to Simulate Fire Accidents in Nuclear Facilities, Los Alamos National Laboratory report LA10678-m, NUREG/CR-4561 (April 1986). [19] J.R. Travis and T.L. Wilson, HMS: Theory and Computational Model, Los Alamos National Laboratory report, to be published. [20] E.S. Oran and J.P. Boris, Numerical Simulation of Reactive Flow (Elsevier, New York, 1987). [21] E.S. Oran, T.R. Young, J.P. Boris and A. Cohen, Weak and strong ignition - 1. Numerical simulations of shock tube experiments, Combustion and Flame 48 (1982) 135. [22] E.S. Oran and J.P. Boris, Weak and strong ignition - II. Sensitivity of the hydrogen-oxygen system, Combustion and Flame 48 (1982) 149. [23] E.S. Oran, J.P. Boris, T.R. Young, M. Flanigan, M. Picone and T. Burks, Simulations of Gas Phase Detonations: Introduction of an Induction Parameter Model, Naval Research Laboratory memorandum report 4255 (June 1980). [24] G. Zyvoloski, Simulation of Intense Fires: A Comparison of Turbulence Models, Los Alamos National Laboratory report in preparation. [25] M.L. Alme, C. Westmoreland and M.A. Fry, Non-Equilibrium Radiation for the HULL Code, Air Force Weapons Laboratory report AFWL TR-76-244 (1976). [26] B.J. Daly, Modifications of the CONCHAS -PSRAY Code for Entrained Flow Gasification Studies, Final Report 5/1/84-12/31/85, Los Alamos National Laboratory report LA-10754-MS (June 1986).
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Los Alamos combustion modeling for nuclear systems J.R. Travis, W . S . G r e g o r y a n d F . R . K r a u s e Nuclear Technology and Engineering Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 15 February 1990
The occurrence of severe fire accidents in nuclear systems has resulted in research activities to model combustion phenomena in these systems. A combustion analysis of an entire facility is needed to determine the transient pressure and heating loads on the containment or ventilation structures and safety-related equipment, to plan emergency evacuation, to validate existing fire protection programs or suggest changes, and to integrate fire safety with other safety analysis requirements. The first part of this paper reviews traditional fire analysis methods and identifies capabilities that can be retained for modeling nuclear facilities. Additional capabilities are outlined that will be needed to accomodate the unique architecture and design-basis accidents of nuclear facilities. In later sections, the Los Alamos modeling approach for both nonreactor and reactor systems is presented.
I. Introduction The occurrence of severe fire accidents in nuclear systems has resulted in research activities to model combustion phenomena in these systems. The bestk n o w n accidents are the cable tray fires in the Brown's
Ferry nuclear power plant [1] and the deflagration of hydrogen during the Three-Mile Island Accident [2]. Severe fires involving gloveboxes and high-efficiency particulate air (HEPA) filters have occurred in the Rocky Flats facility [3], which processes nuclear materials. Significant, but less severe, flammable liquid fires
Table 1 Nuclear facility fire hazards Combustion
Fire zone location
Observed fire growth
Design fire hazard
Cable tray fires
Containment building. Cable spreading
Local accumulation of flammable and toxic vapors
Loss of nuclear and/or fire safety systems. Reignition of deep seated fires
Filter fires
Confluence of fire pr .~lucts and fresh air
Ignition of additional filter fires and duct fires
Failure of supply or exhaust fans and HEPA filters
Flammable liquid fires
Diesel generator fuel, coolant pump lubricants
Flame elongation by external feed back of oxygen depleted air; wall temperature rise in sealed containment buildings
Failure of fire barriers, fire suppression systems, and safety related equipment
Hydrogen fires
Tritium production facilities, Nuclear reactor containment buildings, Reactor off gas systems
Accumulation and combustion of flammable gases, Hydrogen release into ventilation system. Reversal of ventilation system flows. Gas temperature below flashover threshold
Deflagrations/detonation~, Rupture of radioactivity containment barriers. Survival of safety related systems. Release of smoke, radioactivity or toxic chemicals
0029-5493/91/$03.50
© 1991 - Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d )
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J.R. Travis et al. / Los Alamos combustion .~,,c~ieiing
have occurred in diesel generator rooms and turbines [4]. Table 1 ~ives a summary of the combustible materials in nuclear facilities. A combustion analysis of an entire facility is needed to determine the transient pressure and heating loads on the containment or ventilation structures and safety-related equipment, to plan emergency evacuation, to validate existing fire protection programs or suggest changes, and to integrate fire safety with other safety analysis requirements. Fire safety requirements are different for reactor and nonreactor systems. Reactor systems are designed to produce power and/or nuclear materials, and nuclear reactor criticality transients may lead to design-basis or severe accidents [5]. For example, 1000 kg of hydrogen was generated during the Three-Mile Island accident by the oxidation of roughly one-half of the zirconium fuel cladding in the core. Additional hydrogen and carbon monoxide could be released by the interaction of degraded or molten core materials with concrete basement structures. Thus, fires in reactor systems may be initiated and then propagate during a transient release of heat, carbon monoxide, and hydrogen. Nonreactor systems involve activities other than power or nuclear material poduction. Typical activities include glovebox operations, fuel fabrication, and spent fuel reprocessing or storage. Industrial hygiene usually requires that such operations be conducted in closed rooms in which a plant ventilation system prevents the release of radioactivity or toxic chemicals by maintaining a negative pressure differential toward contaminated areas during normal operations. These systems have complex ventilation systems that exhaust air from multiple rooms or laboratories. Section 2 revie:vs traditional fire analysis methods [6] and identifies capabilities that can be retained for modeling nuclear facilities. Additional capabilities are outlined that will be needed to accomodate the unique architecture and design-basis accidents of nuclear facilities. In later sections, the Los Alamos modeling approach for both nonreactor and reactor systems is presented.
2. Overview of numerical models Traditional models simulate combustion product release and motion in residential and commercial buildings such as hotels, shopping malls, and warehouses. This traditional approach describes combustion by scaling empirical heat and mass release data with irradiated fuel surface areas [7]. The movement of combustion
products in the building is calculated by tracking the evolution of hot gas layers in the burn room and in adjacent rooms [8]. Building fire models by themselves cannot be used for nuclear facilities because nuclear facilities unique architecture introduces special modes of fire growth. Also, fire hazards exist in nuclear facilities that are absent in commercial buildings. Most of the observed fire-growth phenomena and design accidents listed in table 1 fall in this special category. Traditional fire models must be extended to represent the special characteristics and architecture of nuclear facilities. The associated modeling requirements are derived below by reviewing traditional models for heat release, combustion product release, and motion of combustion products. The desired additional capabilities are derived by comparing traditional models with existing lumpedparameter codes and three-dimensional field models. Our review identifies the lumped-parameter codes and field models that can predict combustion product flows in a burn room and throughout a nuclear facility. 2.1. Traditional fire models
Much research and verification testing have gone into the development of computer models that refiably predict the release and motion of combustion products in buildings [6]. The following review identifies the modeling capabilities that are applicable to both nuclear facilities and buildings. 2.1.1. Release of combu.~ti~n products and heat The rate of combustion product release per irradiated fuel area is calculated in a two-step process. The first step is to estimate the heat flux to the fuel by tracking the exchange of heat radiation among the fuel and surrounding fires, hot layers, and hot walls. The net heat across the radiation-exposed fuel surface then is converted to the volatilization of hydrocarbon fuels by multiplying with an empirical heat of gasification. This heat has been measured in small-scale calorimeter fire tests [7]. The second step is to convert the volatilization mass-burning rate to a combustion heat release rate and a smoke release rate using two additional empirical parameters. The first parameter is an apparent heating value that has been measured in compartment-size [8] calorimeter tests. Multiplying the mass burning rate by this parameter gives the rate of combustion heat release. The second parameter denotes the "theoretical" heating value that has been measured in an oxygen bomb calorimeter. The ratio of these two heating values is the
J.R. Travis et al. / Los Alamos combustion modeling combustion efficiency; deviations of the combustion efficiency from 1 are used to estimate smoke release. Thus, smoke in this approximation denotes volatiles that will not burn in a room fire but that will burn in an oxygen bomb calorimeter. This smoke is made up mostly of soot and may include unburned flammable gases (CO) a n d / o r fuel droplets. Calorimeter tests measure oxygen consumption and then derive the apparent heating value by assuming that 1 g of oxygen consumption releases 13 kJ of heat regardless of fuel configuration, combustion environment, or chemistry [9]. The apparent heat of combustion is equivalent to an entrainment coefficient. Thus, the combustion effic;ency should decrease, and smoke release should increase with oxygen depletion in the combustion environment. This decrease has been observed in under-ventilated calorimeter fire tests [10], where the presence of oxygen-depleted air is indicated by increased gas temperatures. Forced-ventilation compartment fire tests [11] show that oxygen intake by complete combustion increases by a factor of 2 when little or no oxygen is available through entraining the burn-room atmosphere [12]. Therefore, numerical models of combustion product release in nuclear facilities should track all constituents that could be recirculated to the fuel inside and outside the burn room. Inefficient combustion and the associated release of smoke and unburned flammables are to be anticipated when the actual burn-room intake of oxygen falls below the increased oxygen demand of ventilation-controlled fires. The extension of the building fire release rates to nuclear facilities may be unsatisfactory because empirical factors, which are time-invariant during the clean-burning stage of a fire, change with the environment of ventilation-controlled fires (temperature, smoke, flammable gas, and oxygen). Accurate release rates of ventilation-controlled fires may require an extraordinary amount of calorimeter fire tests unless other approaches to entrainment coefficients can be found that are insensitive to the accumulation of combustion products near the fire. One thermodynamic approach is to predict entrainment from wail heat losses and experimental flame temperatures [13]. Wall heat losses are available from lumped-parameter codes or three-dimensional field models or may be estimated from compartment fire tests. They are known to be ,;:,sensitive to the accumulation of combustion products in both open-door and forced-ventilation fire tests. Empirical flame temperatures for ventilation-controlled fires are available from investigations of flames burning near extinction [14]. These temperatures are insensitive to fuel chem-
405
istry and the combustion environment. An initial application of the thermodynamic approach generated acceptable pretest values for crib and pool fire mass burning rates [13].
2.1.2. Motion of combustion products Building fire models describe the motion of combustion products through the evolution of hot gas layers in the burn room and adjacent rooms [8]. A set of coupled ordinary differential equations is used to predict the time-dependent composition and temperature of individual layers from the conservation laws for mass, momentum, and energy. Gas-layer locations are selected by the modeler and represent local heat and mass exchange phenomena, which are represented by source terms. Corridors are treated as other rooms. The model is restricted to multiroom architectures and open-door ventilation passages. The following local exchange phenomena are essential for a reliable prediction of gas layers outside the burn room. (a) Unidirectional flow: wall and time variation of wall heat-transfer coefficients, - mass flux in and out of individual layers. (b) Bi-directional flow: - entrainment by fire plumes, - entrainment by door jets, radiation heat exchange among fire plumes, hot layers, and walls. The idealization of three-dimensional flows as hot layers with bi-directional exchange is the key for operating building fire models on very modest computers such as personal computers. This simplification may not be desirable for nuclear facilities because the place of local accumulations of combustion products and heat often cannot be supplied as input and may not take the form of horizontally extended hot layers. The heat- and mass-exchange modules that are used for building fire simulations are independent of the location of local accumulations and are applicable to nuclear facilities as well as to buildings. -
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2.2. Desired nuclear-facility-specific capabilities The building fire models cannot be used directly in nuclear facilities because their architecture differs from standard corridor-connected rooms. The following de-~ sired modeling capabilities are derived from such architectural differences.
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J.R. Travis et al. / Los Alamos combustion modeling
2.2.1. Nonreactor facilities
The architecture of nonreactor facilities is characterized by ventilation systems tl'at are designed to prevent the accumulation of toxic or radioactive products'at room elevations where people are breathing. Thus, the development of horizontally extended hot gas layers is unlikely and may violate industrial hygiene requirements. However, the local accumulation of radioactive trace constituents, combustion products, and unburned flammables must be modeled whether or not the modeler knows their location. The capability to predict local accumulations of heat and combustion products is needed to avoid corrosion and heat loads on safety systems (fire detectors, safety sensors, computer chips, and so on) and the degradation of ventilation control systems (control dampers, fans, and filters). The capability to predict the temperature and concentration of locally accumulated flammable gases or fuel vapors is needed to predict fire spread by fire balls and duct fires. Models for release of radioactive trace constituents are needed to simulate nuclear-facility-specific design accidents. Release might proceed as resuspension by locally high velocities on burn room floors; it also might be produced as radioactive smoke in a combustion of gloveboxes, contaminated storage devices, or clothing. The release of radioactive trace constituents may violate industrial hygiene regulations. The concentration of radioactive trace constituents must be predicted throughout the entire facility to identify such potential violations. Thus, desirable modeling capabilities include three-dimensional flow fields in rooms with gioveboxes or nuclear material testing laboratories and particle motion with gravitational deposition and resuspension. Filter fires are ignited by high temperatures and the accumulation of some flammable liquid in or on the filter; vital filter banks are protected by automatic water discharge systems. The capability to simulate droplet evaporation and wall condensation heat transfer is desired to reliably predict the temperature of filters and the temperature of local combustion product or flammable gas accumulations upstream of filters.
2.2.2. Reactor facilities
The architecture of reactor facilities is characterized by tall, sealed containment buildings. The combustible materials are flammable liquids and cable trays. The combustion of hydrogen and carbon monoxide, which are generated during accident conditions as discussed in the introduction, requires additional modeling capabilities.
Flammable liquid fires produce large amounts of heat and may raise temperatures throughout the containment building by more than 100°C. Strong updrafts initiate .movement of hot air and moderate amounts of smoke throughout the building and control the oxygen supply to the fire location. Fires become ventilation-controlled very quickly, even without a ventilation system. The capabifity to model the feedback of heat and smoke to the fuel is needed because it affects the heat and mass release rates as discussed above. New methods of estimating flame length also are needed because long fames may raise local temperatures to flashover thresholds (600 o C) or spread out of the burn room. Current research shows that flames burning near extinction grow long because they proceed by the reignition of locally fuel-rich pockets of flammable vapors [15]. Raising the inlet gas temperature should prolong flames because reignition requires less heat release by local combustion. Very long flames become possible when oxygen-depleted air is fed back to the burn room. Such feedback may be generated by threedimensional flows in the vicinity of the burn room ("bath tub vortex") that cause bi-directional flows in vertical flow passages. Flames may now reach out of the burn room in search of new oxygen that no longer flows into the burn room. The capability to predict flame elongation by heating of inlet air of recirculation of oxygen-depleted air is very desirable for reliable prediction of fire spread. Simple models of a temperature-dependent and continuous diffusion reaction rate have been successful in predicting flame elongation response to the feedback of oxygen-depleted air. Cable fires burn slowly but produce very large amounts of smoke, toxic combust';or,, products (hydrochloric acid), and flammable vapors. The same cable tray can release combustion products at "~ery different rates, depending on both the temperature and the smoke in the combustion environment [16]. Thus, cable fire hazards are sensitive to ventilation conditions because the slow heat release rate does not break up local and extended accumulations of smoke and corrosive or flammable gases. Therefore, the accurate simulation of combustion product concentrations throughout the facility is required. The same capability is needed in reactor facilities because of health hazards and the degradation of engineered safety or computer systems by acid and warm cable fire smoke. Accumulations of hydrogen and carbon dioxide may burn as continuous diffusion flames, may deflagrate, or may detonate under certain combinations of local temperature and flammable gas concentra'i~,~. ~ome mod-
J.R. Travis et al. / Los Alamos combustion modeling
eling advances for simulating such fires have been made and are discussed below.
3. N o n r e a c t o r s y s t e m s
Nonreactor systems are those nuclear facilities that involve many fuel cycle operations. These operations and processes could involve fuel fabrication, fuel reprocessing, spent fuel storage, and so on. Accidents involving combustion are events considered by the Department of Energy and the U.S. Nuclear Regulatory Commission (USNRC) in their safety analyses. Los Alamos National Laboratory has developed a handbook that is devoted to the analysis of accidents in these types of facilities for the USNRC [17]. The unique features that affect combustion accidents in nonreactor nuclear facilities are the following. - forced ventilation, - network-type flow paths, - radioactive material involvement. Nuclear facilities do not have open doors and windows. Air is brought into, through and out of the facility by forced-ventilation systems. The air passes from less contaminated zones to more contaminated zones. These forced-ventilation systems can consist of hundreds of ducts, many rooms and gloveboxes, multiple fans, and many filtration devices all in an interconnected network. The possibility of fire in these systems, the involvement of radioactive materials, and the fact that the ventilation system provides the primary pathway to the environment has led to the development of numerical models to analyze these systems for combustion accidents. The computer code that has been developed at Los Alamos to simulate combustion analysis in nonreactor systems is called FIRAC [18]. The numerical method in FIRAC for modeling nonreactor systems that involve nuclear facility ventilation systems is the "lumped-parameter" method. This method consists of describing the interconnected ductwork by linkages that are called elements or branches and junction points called nodes. The models used to describe the effects of a fire in a nonreactor system include combustion, gas dynamics, heat transfer, and material transport. The detailed equations and numerical models are desdribed elsewhere [19]. However, the models and methods developed above describe an approach than can be used to analyze the effects of a fire in a nonreactor system. The energy from the combustion process and the material generated, including both nonradioactive and radioactive material, are calculated by a zone-
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type combustion model. This model serves as a source term for distribution of energy and material throughout the nonreactor system network that is usually the facility's ventilation system. Gas-dynamic equations are used to solve for the pressures, densities, m~iss flows, and temperatures. In addition, the heat-transfer models for the network duct walls are described. Finally, the equations describing the transport of the combustion products are outlined. The final result is the distribution of radioactive materials and temperatures throughout the system.
4. N u c l e a r r e a c t o r c o n t a i n m e n t s y s t e m s
4.1. Introduction
The major concern for accidents involving combustion in light-water reactor (LWR) containments is the resulting static or dynamic pressure loads. Combustion may cause a breach of the containment or affect important safety-related equipment that may be damaged because of either pressure loads or high temperatures. After the Three-Mile Island Accident (a severe, or degraded-core~ accident), it was found that significant quantities of hydrogen had been generated from the fuel-cladding/water reaction. When released into the containment, this hydrogen burned by one or more combustion modes and thereby posed a threat to the containment integrity, internal structures, and safety-related equipment. Other combustibles such as electrical cable insulation and hydrocarbon lubricating oils for cooling electric pump bearings also have come under examination as possible fuels. In fact, the 1975 Browns' Ferry nuclear plant fire (electric cable insulation ignited from a candle used for leak detection) caused direct losses of $10 million and indirect losses of $30 million related to business interruption. Modeling containment building geometries is a challenge. The Heiss Dampf Reactor (HDR) containment near Frankfurt, West Germany, is 60 m high and 10 m in diameter. It contains 2 stair wells, an elevator shaft, several vertical open hatchways, and about 60 rooms. This particular containment has roughly 11300 m 3 of free volume, or approximately one-sixth the free volume of a typical US pressurized water reactor (PWR) containment. Experiments simulating cable tray, hydrocarbon lubricating oils, and severe accident combustion phenomena currently are being conducted. The USNRC has supported research at Los Alamos and Sandia National Laboratories to develop combustion models to evaluate threats to the reactor contain-
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J.R. Travis et al. / Los Alamos combustion modeling
ment and safety-related equipment. Current levels of research will coordinate model validation with ongoing experiments at the HDR facility. The detailed field equations and numerical models are presented elsewhere [19]. Using the field equation model coupled with finite-rate global chemical kinetics, we have successfully analyzed hydrogen and hydrocarbon diffusion flames occurring in a nuclear reactor containment under accident conditions. These combustion modes are the easiest to model and analyze when compared with other modes of combustion such as propagating flames in premixed fuel/oxidizer volumes. Deflagations, flame acceleration and transition from deflagation to detonation (DDT), and detonations are all important combustion modes that have not been modeled successfully in complex reactor containment structures. In the next section, we will address some of these issues and recommend approaches for solutions.
terns. In this case, a multidimensional model is needed and can be coupled with the lumped-parameter ventilation network. This capability currently does not exist, and research needs to be devoted to establishing this capability, Research toward developing system safeguard models also need to be pursued. These models would simulate the effect of sprinklers, halon discharge, and demisters. With these models, the adequacy of the fire protection system can be evaluated. Finally, the lumped-parameter codes and models need to be validated through experiments. Large systems of interconnected ductwork, rooms, filters, dampers, and fans need to be included in the experimental system. This experimental system, with proper instrumentation for measuring temperatures, pressures, flows, and aerosol concentrations, would be invaluable for obtaining experimental data. 5.2. Reactor combustion modeling
5. Research directions 5.1. Nonreactor combustion modeling
The basic framework necessary for modeling combustion phenomena associated with nonreactor systems has been established. However, several research directions are needed. These research areas include and nonradioactive combustion characteristics, - material transport model improvement, - lumped-parameter and multidimensional model coupling, development of system safeguard models, and - experimental validation and verification of system (network) modeling codes. Very little is known about how radioactive material burns, particularly how it may combine with nonradioactive burning material. Currently, highly empirical data are being used to determine the airborne fraction of radioactive release from contaminated material. Research needs to be performed to develop better models using a larger experimental data base. The transport of smoke with its radioactive component is perhaps the single most important aspect of nuclear materials fires. Models that accurately simulate the behavior of particulate and gaseous material as it moves through the nuclear facility are needed. In particular, interparticle dynamics along with particle depletion models need to be developed and refined. The lumped-parameter approach is not suitable for large volumes (such as large rooms) in nonreactor sys- r a d i o a c t i v e
-
The field equation approach has been applied successfully to diffusion flame combustion modes in complex reactor containment geometries. To evaluate other major concerns involving combustion phenomena during a severe accident in a reactor containment, we must develop modeling capabilities to investigate flame propagation in premixed, complex, multidimensional volumes. This includes laminar and turbulent deflagations, flame acceleration and transition from deflagation to detonation, and detonations. We also wish to model radioactive/nonradioactive aerosol dynamics and the interaction of the nonoxidized and oxidized aerosols with the propagating flame. Accident mitigation concepts such as water sprays and their influence on a steam-air-hydrogen environment must be assessed. All of these phenomena must be evaluated in terms of thermal and mechanical loads on .the containment and on safety-related equipment. Some of these research areas include - adaptive gridding, - detailed chemical kinetics, turbulence, radiation heat transfer, - aerosol dynamics, and - model validation with experimental data. For propagating flames through complex geometries, spatial resolution and accuracy of the flame and the near vicinity of the flame are important. This will require dynamic implementation of adaptive gridding algorithms [20] to sufficiently resolve the flame as it propaTates through the containment. -
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J.R. Travis et al. / Los Alamos combustion modeling
Global finite-rate chemical kinetics, which were used for diffusion flame analysis, will not be adequate for propagating flames. For the wide range of conditions (pressure, temperature, and gas composition) likely to be found in reactor containments, it will be necessary to couple the fluid dynamics with a detailed chemical kinetics reaction set as suggested by Oran et al. [21]. The disadvantage of directly coupling chemical kinetics with 48 reactions and 9 species is that for large time-dependent, three-dimensional problems, the solution algorithm is currently computationally prohibitive. An alternative approach [22,23] could be to solve the detailed chemical kinetics for the expectec', range of pressures, temperatures, and gas compositions. From these conditions and solution.% we can determine 'an induction time, a reaction time, and the amount of chemical energy released. These parameters could be approximated with analytic functions [22] or tabulated in a parameter space table. A modified combustion parameter, P, transport equation [23] can be written as aP
a--i +
"
=
.(o
,P ) +
1
+,(7+, P, x,)'
(1)
where ~-+ and 0 represent the induction time and turbulent transport coefficient, respectively. Initially, P-= 0, so that when P _~ 1, the induction time has elapsed and the chemical energy of combustion is released over a period of time equal to the reaction time, ~'R- In this way, an approximate method for coupling the fluid dynamics and detailed chemical kinetics can be achieved. The disadvantage of this procedure is that fluid dynamics effects like acoustic waves and turbulence intensities would not actually influence the individual chemical reaction rates and therefore the reaction time. It may be possible to add an additional variable to the parameter space table to account for turbulence and the influence turbulence has on the reaction rates, induction time, and reaction time. If the reaction rates could be correlated to the intensity or kinetic energy of the local turbulent conditions, then the first term on the rig.hthand side of eq. (1), the turbulent diffusion of P, coulc~ be eliminated from the equation. For diffusion flame modeling, we have found that turbulent buoyant plumes generally are represented adequately with the two-equation K-c model. However, Zyvoloski [24] has shown that the most accurate plume dynamic predictions are provided with the three-equation ec-.--T '2 model. For this model, the additional transport equation for the average temperature fluctuations squared is solved. Reynold stresses and turbulent energy fluxes then are calculated and used directly in
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the Reynolds-averaged Navier-Stokes equations. At this point, it is not clear whether this three-equation model is needed to describe diffusion flame plume dynamics. tf the survival and proper functioning of safety-related equipment located near diffusion flames remains an unresolved safety issue, then it may be necessary to use a three-equation model. Also, if the diffusion flame plume impinges on the steel shell liner, a higher temperature can be predicted than what actually is observed. Although this result is conservative, there is actually more entrainment of cooler material into the plume than predicted. It may be necessary for "best-estimate" calculations of thermal loads on the containment shell to use the three-equation model. Radiation heat transfer from diffusion and propagating flames can be of major importance. For example, in optically thick regions near hydrocarbon diffusion flames, the coupling between carbon or soot, the fluid field, and the radiation field will be strong, whereas in optically thin regions far from the flame, this coupling will be weak. To include these effects, we consider adopting an extension of a nonequifibrium radiation model originally developed by Alme, Westmoreland and Fry [2.5] and then extended by Daly [26]. In this model, the local energy densities of the radiation field, the fluid, and the particles can be different. Because the speed of light is inherent in these equations, any practical solutions must be obtained from an implicit finitedifference form of the equations. Water sprays and aerosol dynamics are similar problems. For example, in a steam-air-hydrogen environment, the spray water droplets and aerosol particles provide condensation nucleii to reduce the steam concentration in the mixture. Two undesired physical effects occur: (1) hydroger~ concentrations increase as the steam condenses, and (2) turbulence levels increase with condensation and droplet and particle momentum exchange with the continuous gas phase. Nonoxidized aerosols in the presence of hydrogen combustion could increase the energy release as aerosols contribute to the combustion process. On the other hand, water sprays such as mists can be huge energy sinks and provide mitigation methods for suppressing or controlling potentially damaging combustion modes. These tradeoffs must be evaluated and assessed to provide input for severe accident management pol~c,'es. These modeling questions and the coupled phenomena are difficult, and the physical geometry to which they must. be applied is so complex that model and code verification is essential. This can be accomplished only with modelers and experimenters working very closely in a collaborative effort to resolve and understand the
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J.R. Travis et al. / Los ,41amos combustion modeling
relevant phenomena. Separate-effect experiments must be performed to validate individual physical and chemical processes wherever possible. Integrated tests at several scales will provide data and the confidence for computer models to be used to extrapolate to full-scale geometries. Only in this way can numerical modeling and simulations address the issues and answer the questions concerning the problems involved in nuclear reactor safety.
References [1] A.J. Pryor, The Browns' Ferry Nuclear Plant Fire, Society of Fire Protection Engineers technical report 77-2 (1977). [2] J.O. Henrie, and A.K. Postma, Analysis of the Three Mile Island (TI'11-2) hydrogen burn, Proceedings 2nd Int. Topical Meeting on Nuclear Reactor Thermal Hydraulics, Santa Barbara, California (January 1983), p. 1157. [3] D.E. Patterson, The Rocky Flats fire, Fire Journal 64 0970) 5. [4] K.N. Dungan and M.S. Lorenz, Nuclear Power Plant Fire Loss Data, Electric Power Research Institute Contractor report EPRI NP-3179 (July 1983). [5] J.P. Church, Safety Analysis of Savannah River Production Reactor Operation, Savannah River Laboratory report DPSTSA-100-1 (September 1983). [6] Society of Fire Protection Engineers, SFPE Handbook of Fire Protection Association (SEFE, Quincy, Massachusetts, September 1988). [7] A. Tewarson, Generation of heat and chemical compounds in fire, in: SFPE Handbook of Fire Protection (SFPE, Quincy, Massachusetts, September 1988), p. 179. [8] W.W. Jones, A multicomponent model for the spread of fire, smoke, and toxic-gases, Fire Safety Journal 9 (1985) 55. [9l V. Babrauskas, Burning rates, in" SFPE Handbook of Fire Protection (SFPE, Quincy, Massachusetts, September 1988), p. 2-1. [10] J. Steciak, A. Tewarson and J.S. Newman, Fire Properties of Combustible Materials Commonly Found in Nuclear Fuel Cycle Facilities, Factory Material Research Corporation report FMRC J.I.0G3RS.RC (February 1983). [11] N.J. Aivares and F.R. Krause, Experimental Simulation of Forced Ventilation Fires, Los Alamos National Laboratory report LA-UR-84-1691 (1984). [12] N. Aivares, K. Foote and P.O. Pagni, Forced ventilation enclosure fires, Combustion Science and Technology 39 (1984) 55-81.
[13] F.R. Krause and W.S. Gregory, A Thermodynamic Fire Zone Model for Tall Containment Buildings, 1987 Status Report on the HDR Safety Program, Kernforschungszentrum, Karlsruhe, West Germany (December 1987). [14] A. Hamins and K. Seshardi, The structure of diffusion flames burning pure, binary and ternary solutibns of methanol, heptane and toluene, Combustion and Flame 68 (1987) 295. [15] A.R. Masri, R.W. Bilger and R.W. Dibble, Conditional probability density functions measured in turbulent nonpremixed flames of methane near exanction, Combustion and Flame 74 (1988) 267. [16] F.R. Krause and W.H. Schrnidt, Burn Mode Analysis of Horizontal Cable Tray Fires, Sandia National Laboratory report NUREG/CR-2431 (February 1982). [17] J.E. Ayer, A.T. Clark, P. Loysen, M.Y. Ballinger, J. Mishima, P.C. Owczarski, W.S. Gregory and B.D. Nichols, Nuclear Fuel Cycle Facility Accident Analysis Handbook, US Nuclear Regulatory Commission report N U R E G / CR-1320 (1988). [18] B.D. Nichols and W.S. Gregory, FIRAC Users Manual: A Computer Code to Simulate Fire Accidents in Nuclear Facilities, Los Alamos National Laboratory report LA10678-m, NUREG/CR-4561 (April 1986). [19] J.R. Travis and T.L. Wilson, HMS: Theory and Computational Model, Los Alamos National Laboratory report, to be published. [20] E.S. Oran and J.P. Boris, Numerical Simulation of Reactive Flow (Elsevier, New York, 1987). [21] E.S. Oran, T.R. Young, J.P. Boris and A. Cohen, Weak and strong ignition - 1. Numerical simulations of shock tube experiments, Combustion and Flame 48 (1982) 135. [22] E.S. Oran and J.P. Boris, Weak and strong ignition - II. Sensitivity of the hydrogen-oxygen system, Combustion and Flame 48 (1982) 149. [23] E.S. Oran, J.P. Boris, T.R. Young, M. Flanigan, M. Picone and T. Burks, Simulations of Gas Phase Detonations: Introduction of an Induction Parameter Model, Naval Research Laboratory memorandum report 4255 (June 1980). [24] G. Zyvoloski, Simulation of Intense Fires: A Comparison of Turbulence Models, Los Alamos National Laboratory report in preparation. [25] M.L. Alme, C. Westmoreland and M.A. Fry, Non-Equilibrium Radiation for the HULL Code, Air Force Weapons Laboratory report AFWL TR-76-244 (1976). [26] B.J. Daly, Modifications of the CONCHAS -PSRAY Code for Entrained Flow Gasification Studies, Final Report 5/1/84-12/31/85, Los Alamos National Laboratory report LA-10754-MS (June 1986).