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MODELS AND CRITERIA FOR PREDICTION OF DEFLAGRATION-TO-DETONATION TRANSITION (DDT) IN HYDROGEN-AIR-STEAM SYSTEMS UNDER SEVERE ACCIDENT CONDITIONS R.KLEIN1, W. BREITUNG2, I. COE3, L. HE4, H. OLIVIER5, W. REHM6, E. STUDER7 1Freie Universität Berlin, Germany (FUB) 2Forschungszentrum Karlsruhe, Technik und Umwelt, Germany (FZK) 3 NNC Ltd., Knutsford, United Kingdom (NNC) 4IRPHR-CNRS Marseille, France (CNRS) 5Rheinisch-Westfälische Technische Hochschule Aachen, Germany (RWTH) 6Forschungszentrum Jülich, Germany (FZJ) 7Institut de Protection et Sûreté Nucléaire, Saclay, France (IPSN) SUMMARY The well-known hydrogen-related nuclear reactor safety problem in case of severe accidents is aggravated by uncertainties in the prediction of high-speed combustion and the transition to detonation. The present project improves the situation by addressing open questions regarding experimental evidence, methods of theoretical and computational modelling, computer code validation, efficient usage of computing resources and current safety assessment methods and technologies. A central scientific hypothesis regarding the physical nature of the deflagration-to-detonation transition (DDT) provides guidelines for the scientific work by suggesting specific experimental and theoretical investigations and the development of suitable numerical and assessment techniques. 1. INTRODUCTION Since the Three Mile Island accident there has been increased interest in hydrogen production, distribution and combustion in light water reactors. There is major concern as these events threaten containment integrity. Within the total range of possible premixed combustion events, namely Ignition - Flame Propagation - Deflagration - DDT - Detonation, particular uncertainty exists for the Deflagration-to-Detonation Transition (DDT). Hence, this project aimes at: (1) increased theoretical and experimental knowledge about DDT processes, (2) obtaining detailed numerical DDT modelling capabilities, (3) improving and validating combustion codes for high performance computers, (4) developing advanced DDT mitigation strategies for of nuclear reactor safety. Two prototypical modes of DDT, both of which occur potentially under severe accident conditions, are investigated: “Mode A” involves pressure wave induced autoignition away from the main combustion front, “Mode B” denotes local quenching, turbulent burnt/unburnt remixing and reignition with transition. Separate series of experiments provide both detailed local information on the physical DDT processes and overall measurements serving as a data

base for code validation. “Mode B”, in contrast to “Mode A”, depends crucially on small scale details of turbulence-combustion interactions. Consequently, its mechanisms are far less understood than those of “Mode A” and the results obtained for it are less complete, suggesting further investigation in future projects. For the same reason our theoretical / numerical developments focus on “DDT Mode B”. In the area of code validation we consider the quality and predictive capabilities of existing combustion codes as well as their performance in a high performance computing environment. Advanced mitigation strategies are proposed on the level of both PSA and computer aided risk analysis using “lumped parameter” and field code simulation models. Global DDT criteria developed here and elsewhere compare length and time scales of the considered system with characteristic scales of the combustible at given thermodynamic conditions. For example the “7 λ”-criterion compares the system dimensions with the detonation cell size “λ” of a mixture. The definition of these characteristic scales does not depend on the mixture itself, so that such criteria can be evaluated as soon as the necessary kinetic and/or experimental information is available. This includes H2-air plus steam or CO2, in which cases the admixtures increase the characteristic scales and reduce the likelyhood of DDT. 2. EXPERIMENTAL INVESTIGATIONS Reaction Kinetics and Auto-Ignition: Auto-ignition characteristics of H2-air-steam / CO2 mixtures were investigated in a heated shock tube at SWL/RWTH-Aachen (Refs. [1],[2]). A database of ignition delay times and schlieren records has been set up revealing details on flame development and shock-detonation interactions under the following initial conditions: Temperatures from 900 to 1350 K, pressures from 0.3 to 1.7 MPa, steam concentrations from 0 to 40% and CO2 concentrations from 0 to 40%. Both mild and strong ignition were identified. In most cases, mild ignition with a secondary explosion was recognized by pressure and photoelectric signals as well as by schlieren photographs. The ignition position for a mild ignition is random if the tube is clean and there is no obstacle and hole on the inner tube surface. The ignition delay time is found to strongly depends on steam concentrations and initial temperatures, but only weakly on the initial pressure. For the same initial temperature and H2 volume fraction steam prolongs the ignition delay time more than CO2. The measured ignition delay times in H2-air mixtures are consistent with theoretical predictions in the literature in the high temperature range, but shorter than predicted in the low temperature range, (Ref. [3]). Analyses of pressure signals show that with the increase of temperature the peak pressure first increases, then decreases. Also, the ignition process is very sensitive to the inner surface quality and to general pollution of the tube. The aims of the auto-ignition characteristics study have been (i) to study fluid mechanical ignition mechanisms, (ii) to investigate the processes of DDT Mode A, (iii) to provide crucial missing chemical kinetical data for gas mixtures and initial conditions relevant to severe accidents. All of these goals have been achieved within this project. Flame Quenching and Reignition: The objective of this part of the work package, (RWTH) (Refs. [1],[4]), was to produce DDT events and to generate experimental data on flame quenching and reignition related to Mode B-DDT. DDT within the flame brush was often identified in different mixtures and detailed information has been achieved, but an obvious reproducible flame quenching and reignition process could not be identified. The second part of this work package was to investigate gasdynamic-chemical resonance effects

which can lead to the build-up of pressure spikes sufficient for the establishment of selfsustained detonations. These results related to Mode A DDT have been achieved. About 80 experiments on DDT were performed. Using a 24 frame Cranz-Schardin camera time resolved schlieren photographs have been obtained related to both modes of DDT behind a ring obstacle. Photographic series give information on gasdynamic features, ignition, flame propagation and explosion waves generated by detonation onset and after passage of the precursor shock by the ring obstacle. Pressure transducers, ionization probes and photomultipliers, intensively used behind the obstacle, provide details about the flame acceleration and pressure development. Behind a ring obstacle Mode A DDT occurs in insensitive gas mixtures provided strong turbulence is generated during the early phase of flame acceleration, while Mode B DDT occurs in sensitive mixtures under weak turbulence. Further results concern the influences on DDT of initial pressure in an obstacle-free tube and of obstacle dimensions in a partially obstructed tube and of the influence of ring diameter on the flame propagation regime. Flame Acceleration and DDT in Obstacle Arrays: The objectives of this work package (INR/FZ-Karlsruhe (FZK)), were (i) to provide integral experimental data for development of physical models and code verification, (ii) to investigate the statistical nature of DDT for well defined initial and boundary conditions and (iii) to measure typical DDT loads in lean hydrogen-air mixtures. The different DDT mechanisms were investigated in three specialized test configurations: The “Idealised Mode A”- experiments involve a shock wave generated by rupture of a membrane. The shock is focused in a 3-D reflector leading to autoignition. The “Prototypic Mode A”-experiments feature a strong precursor wave generated by an accelerating flame in an obstacle array. The shock is again focused in a 3-D reflector and leads to autoignition. In the “Prototypic Mode B”-experiments a turbulent flame is accelerated in an obstacle arraycovering the total tube length. Transition now occurs even without focusing in a reflector. For DDT by “Idealised Mode A” critical incident shock Mach numbers were determined for varying H2 concentrations. At the critical Mach number deflagrative combustion gives way to detonation. Individual tests were highly reproducible indicating that the processes in these tests are mechanistic, governed by reaction kinetics. The tests have proven the significance of DDT Mode A in obstructed geometries: a flame that accelerates in one part of an installation generates pressure waves which, when reflected or focused at obstacles and confinements, can trigger DDT in a distant part of the enclosure. Boundaries for the occurence of DDT Mode B in a fully obstructed tube were found to invole thresholds for pressure amplitudes and wave speeds and a criterion on the coupling of pressure waves with the reaction front, (Refs. [1,5,6]). The tests have resulted in new data on DDT mechanisms which can be used for model development and verification. Simplest are the idealised mode A experiments requiring merely the simulation of reactive compressible flow but not of turbulence. Numerical models should be first verified on these tests as reported by FZJ in Ref. [1]. 3. THEORETICAL ANALYSIS AND NUMERICAL TECHNIQUES Modelling, Scaling Relations, Derivation of Criteria: The physical mechanisms controlling detonation formation within a nonuniform hot pocket have been analyzed at

IRPHE/CNRS (CNRS) and a scaling analysis of the critical conditions for DDT has been developed. Critical size for DDT from hot spot ignition: There is widespread agreement, that the initiation of detonation within a hot pocket is related to resonance of a propagating ignition front with sound waves occuring when the propagation velocity of the ignition front matches the local sound speed. We find that at this location a shock is formed, but it may or may not be sufficient to initiate a detonation. Resonance is not even necessary, as the gas expansion from an energetic quasi-constant volume explosion may directly initiate a detonation. The associated characteristic size of the quasi-constant volume explosion region corresponds to the diameter delimited by the resonance criterion. A scaling argument shows: If the characteristic time of chemical energy release after ignition is shorter than the acoustic transit time across the characteristic size, then the chemical power output will generate a strong shock wave, because the local explosion takes place at approximately constant volume. A critical condition is obtained from these considerations by requiring that (i) there is sufficient chemical energy within the locus of acoustic-ignition resonance and (ii) that, after ignition, chemical energy conversion is faster than sound wave propagation within this region. Thus, in contrast to earlier claims, both the ignition and energy conversion time characteristics of a mixture are relevant to DDT. Influence of turbulent mixing on DDT from hot spot ignition: In the study of the initiation of detonations within a non-uniform hot pocket it has always been assumed that a nonuniform hot pocket would be produced instantaneously. Such studies yield only a length scale for successful initiation, which is insufficient to determine the relevant range of turbulence parameters. However, within a turbulent flame brush the temperature and concentration profiles are established by turbulent mixing at finite rates, and this may limit the detonation development. The turbulent mixing rate does, in fact influence the critical length scale. Thus, if the mixing rate is very low, a detonation cannot be initiated, no matter how large the hot pocket. For very small turbulent integral scales the hot spots generated by turbulent mixing always have a sub-critical size w.r.t. the generation of a sufficiently strong shock wave. For low turbulence intensities, mixing is too slow to allow the local accumulation of a sufficient amount of fresh combustible gas within a single hot spot. These considerations lead to a new “DDT-boundary” in the classical Borghi-Diagram (Ref. [7]). Details can be found in Refs. [1,8]. Numerical Techniques: Here we provide a new numerical framework , developed at BUGH Wuppertal and FU Berlin (FUB), that allows the incorporationg of submodules for laminar deflagrations, fast turbulent deflagrations and detonation waves. Supplemented with suitable DDT criteria, such as developed by CNRS, the evolution of a DDT process can be simulated in principle, provided appropriate modules are available. The deflagration tracking scheme from Ref. [9] represents a deflagration as a reactive discontinuity embedded in a compressible surrounding flow. The flame surface is the level set of a dynamically evolving scalar function. Flame-flow coupling is realized by explicitly invoking Rankine-Hugoniot type jump conditions at the front. The original scheme uses the standard jump conditions and an explicit burning rate as a function of the unburnt gas conditions. This approach is limited to situations where such burning rate laws are available and where the standard RankineHugoniot conditions are applicable.

The scheme has been generalized to incorporate temporally evolving internal flame structures that represent dynamical events within the turbulent flame brush. Suitable internal structure modules enable direct exploitation of the analyses of the previous subsection in a simulation code. The key modifications to the original scheme are the introduction of nonhomogeneous Rankine-Hugoniot conditions and a modified determination of the net fuel consumption. Given the time derivative of all conserved quantities in a frame of reference moving with the combustion zone, modified non-homogenous Rankine-Hugoniot jump conditions are obtained. The propagation speed is not, as in the case of a quasi-steady deflagration, derived from an explicit algebraic burning rate law, but rather results from the integral fuel consumption rate across the flame front. A major achievement is the incorporation of a code module, provided by the canadian subcontractor Combustion Dynamics Ltd. (CDL), that models high speed combustion in the “well-stirred reactor regime”, see Ref [1]. This regime is most relevant for DDT Mode B as it describes reaction after the quenching of coherent flame(let) structures. The module uses a Monte Carlo technique to simulate the evolution of the probability density function (pdf) for temperature and species concentrations within the turbulent flame brush The scheme has been tested so far only on one-dimensional problems and the pdf-module is quite computationally expensive. However, once these limitations are overcome, a single code can be used to simulate all modes of combustion relevant to severe accident scenarios. 4. VALIDATION A range of combustion field codes has been established and tested on a high performance super computer cluster (SCC) by FZJ. Our efforts at verification and validation are illustrated by selected examples of multi-dimensional simulations of fast turbulent hydrogen flames. Computations are compared with experiments at Kurchatov Institute’s RUT test facility. Additional tests were based on experimental DDT data from SWL and FZK (see Refs. [11,11]). The collection of a pool of specialized codes is currently necessary as none of the existing codes reliably represents all combustion phenomena relevant to severe accidents. Our code cluster includes research codes (AIXCO, DET) and commercial codes (CFX, IFSAS). The excessive manpower invested in commercial codes, spent on representation of complex geometries, integration with computer aided design (CAD) tools, etc., cannot possibly be redone for research tools on a regular basis. Thus, the long term strategy for research organizations and engineering companies is the merging of new numerical techniques (such as realized in this project) into ready-to-use (commercial) products. The present validation study aids in assessing the current status in both areas. From a generic scientific point of view, it is more desirable to inquire into the detailed mechanisms of combustion processes and to exploit their essential features to avoid DDT, rather than to obtain largely simplified DDT criteria purely on the basis of, admittedly educated, correlations of experimental results without detailed theoretical insight. We are far from such desirable detailed understanding, but a step in the right direction is to support the scientific issue by computational analyses based on state-of-the-art simulation tools.

While a full fledged prediction of distribution and subsequent combustion would be ideal, a coarse-grained distribution assessment followed by detailed combustion simulations is still useful: By considering series of combustion events whose initial data match the mean statistics of the distribution assessment, detailed combustion simulations allow to explore the range of possible combustion scenarios to be expected. Computer Codes for DDT-related Simulations: In the present study, we have gathered several field codes to simulate DDT-related processes as follows: (1) the general purpose field code CFX from AEA-Technology; (2) the flamelet combustion code AIXCO from RWTH-Aachen and (3) the detonation codes DET (FZK) and IFSAS (CDL-Canada). Most of these codes have been ported to FZJ’s CRAY computer complex to verify or validate numerical results against experimental observation. Here is an overview of the essential properties of these codes and the physical situations they are targeted at: (1) Deflagration Mode: Implicit Na-St Solver CFX-F3D single-phase with k-e, RNG turbulence model and EBU with flame quenching, multi-component fluids Differencing scheme: HYBRID, QUICK, CONDIF; Equation Solver: LINE, ICCG, STONE, AMG; Pressure-Velocity Coupling: SIMPLEC; multi-block grids (2) Transition Mode: Explicit Na-St Solver AIXCO-2D; k-e/Flamelet model + level set front tracking; Gasdynamics: explicit Godunov type, Diffusion: explicit Runge-Kutta (3) Detonation Mode: Explicit Euler Solver DET-2D; one-step and reduced kinetics; Gasdynamics: Harten, Lax, Leer Upwinding; Reaction kinetics: Euler-Cauchy, Heun (4) Fluid-Structure Analysis: Semi-implicit Euler/Na-St Solver IFSAS-3D with combustion models for detonations, shock ignition; Numerical scheme: LW/FCT, Godunov, PISO/TVD, static and dynamic grid refinement The FZJ high performance computing infrastructure: The CRAY computer complex at FZJ consists of following supercomputers (with pre- and post-processing on workstations). CRAY-T90

Vector Supercomputer; Shared Memory; Parallel Vector Proc. (PVP) 12 (16) processors; 4 GBytes Memory; Peak Performance: 24 GFlops. CRAY-T3E Scalar Processors; Distributed Memory; Massively Parallel Proc. (MPP) 512 (256) Processors; 64 GBytes Memory; Peak Performance: 300 GFlops. CRAY-J90 File Server; Single or Interface to T90/T3E; Moderately Parallel 20 processors; 10 GBytes Memory; Peak Performance: 4 GFlops. The field codes have been implemented on this supercomputer complex (SCC). A new SCC has been designed recently consisting of five CRAY machines (1x T90, 2xT3E, 2xJ90) with 800 procs, 100 GB memory, and 500 GFlops peak performance. Multi-D simulations of large-scale combustion experiments showed that such computer resources are needed to accomplish an accuracy of post-test calculations sufficient to evaluate existing uncertainties. Vectorization, Parallelization and Code Developments: DET-2D was ported to the CRAY-T90 by auto tasking for moderate parallelism (up to 10 procs.). Speed-up requires vectorization on this machine. The code was restructured and optimized based on Fortran 90. In porting DET-2D to the CRAY-T3E it was optimized using high-performance fortran (HPF) for massively parallel processing (>100 procs.). Linear speedup of about 70% was observed for 10 to 100 procs.. With the subcontractor AEA-Technology, the CFX-3D versions CFX-4

and CFX-5 were ported to the SCC and optimized for vector- and parallel processing. CFX-4 was ported to the CRAY-T90, CFX-5 to the CRAY-T3E using domain decomposition and PVM (as the message passing paradigm) plus automatic load balancing. Both versions scaled well with increasing processor nodes and grid sizes. With ITM/RWTH Aachen, the deflagration code ERCO-2D (now AIXCO-2D) was ported following the domain decomposition strategy. A massively parallel version for the T3E is under development. There is considerable inertia of code developers against parallelization. Quick, reasonable results can be achieved with moderate resources, e.g., by automated parallelization, but proper scaling with the processors number is often limited and/or code structures are affected such that further developments are impeded. Yet, the present results indicate, that a elaborate code restructuring may be worthwhile as it allows full use of modern computational capacities. Validation example: High speed deflagration in the RUT facility: Experiments for obstacle induced flame acceleration from the RUT channel (RUT-23, 11% H2 -air, BR 30%60%) were re-simulated. CFX-2D/3D simulations using different turbulent combustion models (collision and viscous mixing ansatz), were used to fine tune the reaction parameters. Then, results from CFX and AIXCO were found to be in qualitative agreement. Further twodimensional CFX simulations showed good quantitative agreement with measured x-t diagrams (first channel) and pressure-time histories in the cavity at various positions. Numerical Comparison of Small-scale DDT Experiments: In addition, small-scale laboratory tests, by SWL and FZK, were used for code validation. The IFSAS code with mesh refinement was used to simulate shock induced ignition and DDT Mode A. The comparisons, in Ref. [1,10,11] showed acceptable agreement between experimental and analytical results as well as insight into the transition processes in sensitive hydrogen-air mixtures at 15% H2 in air. Note that the validation studies performed are limited in the sense that typical DDT Mode B events cannot by reproduced with available commercial codes at this stage. 5. MITIGATION DDT-Related Mitigation: The aim of this subtask of IPSN, is to study the occurrence of DDT process at reactor relevant scale for “realistic” severe accident scenarios. DDT occurrence is assessed via simplified macroscopic criteria developed by the partners. These are implemented in the TONUS code (Ref. [12]) both for “lumped” parameter and CFD approaches. The two necessary conditions related to mixture composition (Kurchatov Institute “7 λ”-criterion) and flame acceleration (leading shock critical Mach no.) are combined to produce necessary conditions for DDT onset. Acceleration correlations yielding reliable evaluation of the Mach no. criterion have been derived from the RUT experimental program performed at Kurchatov Institute, Moscow, and sponsored by FZK and IPSN. Emphasis is on the “lumped parameter approach”, for which the state of art in turbulent combustion and DDT is still poor, while it is used predominantly in actual safety analyses. In CFD, the main concern is turbulent combustion. TONUS currently includes a low Mach no. flow solver with an “Eddy Break-up model” for turbulent and an one-step Arrhenius model for laminar combustion. Fully compressible combustion modelling is under development. Mixture sensitivity and flame acceleration criteria have been implemented. “Lumped parameter modelling” indicates that for a French 900 Mwe PWR the occurance of DDT

cannot be excluded given the criteria discussed in Ref. [13]. However, flame acceleration is unlikely due to the absence of obstacles and vents that would provoke it. These conclusions must be regarded as preliminary because many uncertainties remain, due, e.g., to 3D effects. The combustion volume geometry is the most important and complex parameter for flame acceleration. Three main parameters characterizing this influences are the size of obstacles, the distance between obstacles and the degree of confinement (all geometrical discontinuities on the combustion path). In actual nuclear power plant geometry, data like blockage ratio or obstacles spacing cannot always be easily defined due to complexity. The main characteristics of such geometry are the very irregular arrangements compared to well defined experimental conditions. Long channels like explosion tubes, RUT or FLAME facilities can be regarded as quasi-one dimensional but real geometries lead to two or three dimensional combustion processes. The main parameter in this case is the expansion flow created in the unburned mixture, which depends strongly on the degree of confinement. At present time, experimental programs were not designed to get an appropriate modelling of this complex phenomenon and this could be for example an interesting challenge for the next framework program. TONUS extensions for PSA: The goal of this work is to implement the DDT criteria in the PSA level 2 with the conceptual idea to use only the “lumped parameter approach” in order to perform fast running estimations and to go through the following evaluation steps: (1) define key times during the progression of the severe accident scenario (max. hydrogen release, end of core uncovery, vessel failure, short-term molten-core concrete interaction etc.); (2) build a database using core degradation and core-concrete interaction calculations for selected severe accident scenarios (with one single node describing homogeneous mixture of hydrogen/air and steam in the containment (CO is added to H2 and CO2 added to H2O); (3) implement possible non-homogeneity in the hydrogen and steam distribution inside the containment by uncertainties around the point of well-mixed conditions; (4) evaluate DDT criteria by the ”lumped parameter model” according to the preceding gases distribution. This simplified model could also give an estimation of the pressure transient during the combustion process and this could be used to study containment and equipment failure. At present time, all these steps are not performed and the final model will be implemented in the global Containment Event Tree. PSA for Accident Scenarios: This section considers the application to Probabilistic Safety Analyses (PSA) of work to understand the threats to plant safety posed by DDT events in nuclear reactor power stations (e.g. PWR). The application of improved understanding of DDT phenomenology to PSA has been performed by NNC and has sought to exploit the progress made in other work packages and other Fourth Framework Projects. The methodology for assessing the threat due to DDT events and the classification of containment geometries that have been applied in probabilistic safety assessment studies for nuclear power plants has been reviewed together with the refinement of the methodology for assessing accident events. The state-of-the-art classification model is still largely provided by a qualitative methodology based on a classification of the geometrical aspects and the mixture conditions. The work from this study has improved the quantitative framework for identification of critical locations within reactor containments that will allow the threat to DDT to be determined more readily in probabilistic safety assessments. It has also contributed

to the identification of the areas of research which are most relevant to practical safety assessment. The PSA Database Project (4thFWP/PSAL2) provides a pilot database of hydrogen related information. Refinements in the DDT assessment methodology are based on experimental and theoretical studies identified from the Database. The qualitative assessment methodology has been developed further by considering the influence of both compartment geometry and gas cloud dimensions and composition in the initiation and propagation of DDT. The ranking of deflagration and DDT processes via decomposition event trees in terms of their impact on the equipment inside a containment has also been considered. The effects of steam or inert gases and geometrical scaling issues of experimental results have been recognised. Comments on mitigation measures: To explore the fact that DDT is connected with spatial scale relations (“7λ”-criterion) one may compartmentalize open spaces so as to reduce detonability to high sensitivity mixtures. Admixtures of, steam, CO2, NO, etc.can reduce or enhance riscs by affecting sensitivities (see Ref. [13]). Notice, however, that compartmentalization increases the risc of turbulent flame acceleration, as walls and openings are the equivalent of an obstacle array. To avoid replacing one danger with another, compartmentalization must be accompanied by measures against flame acceleration, such as H2 concentration reductions through recombiners or igniters. Igniters, however, trigger combustion! The still limited knowledge on turbulence, turbulent combustion and the actual course of accident sequences precludes the conclusion that igniters lead to net risc reductions! Further research is needed and the results of the present project contribute to the scientific tool kit needed to this end. CONCLUSIONS AND LESSONS LEARNT Global DDT criteria, like the “7λ”-one, assess the possibility of DDT given a few global system parameters. Being based on a small number of rules with reproducible input, they are a suitable basis for regulatory decisions. Given the dynamic evolution of this field, those global criteria with the best validation record should regularly be collected and implemented. Here validation means successful comparison with all available experimental data and, increasingly so in the future, with high quality numerical simulations and analytical preditions. In contrast, local DDT criteria assess the likelihood of the birth of a detonation using local information on temperatures, species concentrations, turbulence characteristics etc. within a flow field. They are key ingredients of numerical combustion simulations tools which, in turn, can be used to sharpen the global DDT criteria used in the regulatory context. Within this framework we summarize our achievements: 1. Concrete approaches towards implementation of global DDT criteria in engineering assessment tools have been proposed by IPSN & NNC. 2. New numerical techniques for turbulent combustion and DDT have been developed by BUGH/FUB & CDL. These efforts aim at supporting the validation and extension of global DDT criteria through specific numerical simulations in the near future. 3. Local DDT criteria have been derived through scaling analyses for the structure of turbulent flame brushes by CNRS. Such criteria serve as necessary building blocks of the numerical techniques under point 2.. 4. Lacking exact analytical solutions for reactive flows, both the numerical tools from 2. and the local criteria from 3. must be validated against experiments. RWTH and FZK

have provided a new, rich database for code validation studies on various levels of flow resolution. Of particular importance for reactor safety is the scaling issue: Experiments for realistic scenarios are unfeasible due to the size of real containments. CFD field codes offer remedy by allowing “virtual testing”, provided they are properly validated under spatio-temporal scaling. As long as rigorous turbulence and turbulent combustion closures do not exist, an interplay of theory, numerics and experiments is needed to approach such scale-uniform models. Ultimately, field codes can be coupled to structural response codes, such as IFSAS, to assess the potential timing and modes of containment failure. We note on the side that, except for some of our experiments, the present work carries over to a wide range of non-nuclear applications. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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[hardcopies and/or electronic copies on CD-ROM are available upon request from FZJ ([email protected]) or FUB ([email protected])]