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Large quantities of hydrogen can be generated during a severe accident in a water-cooled nuclear reactor. When released in the containment, the hydrogen can create a potential deflagration risk. The dynamic pressure loads resulting from hydrogen combustion can be detrimental to the structural integrity of the reactor. Therefore, accurate prediction of these pressure loads is an important safety issue. In previous papers, we validated a Computational Fluid Dynamics (CFD) based method to determine the pressure loads from a fast deflagration. The combustion model applied in the CFD method is based on the Turbulent Flame Speed Closure (TFC). In our last paper, we presented the extension of this combustion model, Extended Turbulent Flame Speed Closure (ETFC), and its validation against hydrogen deflagration experiments in the slow deflagration regime. During a severe accident, cooling water will enter the containment as steam. Therefore, the effect of steam on hydrogen deflagration is important to capture in a CFD model. The primary objectives of the present paper are to further validate the TFC and ETFC combustion models, and investigate their capability to predict the effect of steam. The peak pressures, the trends of the flame velocity, and the pressure rise with an increase in the initial steam dilution are captured reasonably well by both combustion models. In addition, the ETFC model appeared to be more robust to mesh resolution changes. The mean pressure rise is evaluated with 18% under-prediction and the peak pressure is evaluated with 5% accuracy, when steam is involved. [ABSTRACT FROM AUTHOR]

Highlights • Hydrogen deflagration experiments in vented and closed configuration are discussed. • Effects typical for multi-compartment geometries are analysed. • Scale effect on deflagration parameters, such as peak temperature and peak pressure. • Interaction of engineered safety measures like sprays and FCVS with deflagration. • Effect of hydrogen combustion on FPs source term (e.g. CsI decomposition to I 2). Abstract In the course of a severe accident maintaining containment integrity is of fundamental importance to avoid fission product release to the environment as becomes obvious by a comparison of the severe accidents in Three Mile Island (TMI), Chernobyl and Fukushima. Since containment integrity is mainly impaired by hydrogen combustion in the early and intermediate phase of a severe accident with core degradation, research on hydrogen behaviour was and is in the focus of international research. This paper provides an overview about hydrogen combustion experiments with premixed and stratified atmosphere in multi-compartment facilities, both for vented combustion and combustion in closed facilities. The influence of scale is discussed and a threshold for facility size has been identified above which a further increase of peak pressure and peak temperature is not to be expected. Effects typical for multi-compartment deflagration like jet ignition, displacement of unburnt mixture and interacting flame fronts are assessed. The influence of certain engineered safety measures like spray systems and filtered containment venting systems on hydrogen combustion is discussed. Source-term relevant phenomena related to hydrogen combustion like fission product resuspension and fission product decomposition which may occur during a deflagration or in a standing flame are dealt with by discussion of the respective experimental work. Based on the state-of-the-art review, knowledge gaps and open issues on hydrogen combustion are being identified which require further research work using sufficiently large experimental facilities in order to ease application of the results to reactor containments. [ABSTRACT FROM AUTHOR]

Highlights • Slow hydrogen deflagration modeling validation of large-scale experiment is presented. • Large-scale HYKA-A2 UFPE experiment is used for the validation. • Original weighted laminar flame speed modeling approach is introduced. • Simulated and experimental flame front shapes are also compared. Abstract A postulated severe accident in a nuclear power plant (NPP) can lead to generation of large quantities of hydrogen in the NPP containment. Potential hydrogen combustion and ensuing dynamic pressure loads can be detrimental to the integrity of the containment and its structure, systems and components. This paper provides validation results of a computational fluid dynamics (CFD) modeling approach based on two combustion models, that is the Turbulent Flame-speed Closure (TFC) model and the Extended TFC (ETFC) model. The latter was further elaborated in order to investigate the influence of the laminar flame speed on the flame evolution and propagation modeling. As a result, a newly developed approach in form of the augmented ETFC model was introduced and validated as well. The hydrogen slow deflagration results from the Upward Flame Propagation Experiment (UFPE) conducted in the large-scale HYKA-A2 experimental facility have been used for validation. The main results of the performed validation are presented and discussed, with the focus on axial and radial flame front propagation, maximum pressure, pressure increase rate and flame front development. [ABSTRACT FROM AUTHOR]

Hydrogen combustion inside a post-accident containment environment may pose a threat to the integrity of the containment structure and equipment. In CANDU reactors, combustion-generated overpressures in an accident will be relieved by venting to adjacent from one compartment volumes through relief panels or existing openings. To have a better understanding of the fundamental mechanisms that control the overpressures resulting from a vented hydrogen deflagration, a 120 m 3 large scale vented combustion test facility was constructed at Canadian Nuclear Laboratories. A number of experiments have been completed in this facility to investigate hydrogen combustion behavior simulated in a post-accident reactor containment environment. A selected number of experimental results are presented in this paper to demonstrate several parameter effects on vented combustion dynamics by varying the number of chambers (single, two or three interconnected), volume and vent size, initial turbulence (quiescent vs. turbulent), initial temperature and steam concentration, mixture uniformity (well mixed vs. highly stratified), vent locations, and number of igniters. These results have provided useful insights for hydrogen management for the Canadian nuclear industry and CANDU utilities participating in the CANDU Owners Group research program. Most experimental data have been used for validation of combustion models and development of user guidelines. [ABSTRACT FROM AUTHOR]

Abstract: The potential consequences of hydrogen release and combustion during a severe accident in a Light Water Reactor (LWR) have received considerable attention after the Fukushima accident. The pressure loads resulting from hydrogen combustion can be detrimental to the structural integrity of the nuclear reactor safety systems and the reactor containment. Therefore, accurate prediction of these pressure loads is important from a safety point of view. The considered pressure loads are determined by the turbulent flame acceleration. This flame acceleration is determined by the amount of turbulence generated locally during the combustion process. Lumped parameter codes can only assume, but cannot compute this local turbulence generation process due to their inherent limitations. Therefore, a three dimensional CFD based approach is needed. In this paper, it is demonstrated that it is of utmost importance to apply successive mesh and time step refinement systematically in CFD analyses for the considered application. This has been applied in this paper. Within the field of hydrogen safety, we have not found CFD analyses published earlier in the open literature that were based on successive mesh and time step refinement. In the light of the above, the lack of the demonstration that the numerical errors are negligible in the CFD analyses of hydrogen deflagrations must be considered as a serious shortcoming. Therefore, the authors are developing a CFD methodology that can be used for the analyses of hydrogen deflagrations in experimental facilities as well as in full scale reactor containments, with which it can be demonstrated efficiently that the mesh and time step requirements are fulfilled. Such a CFD methodology is deemed to be essential in order to be able to prove containment integrity under severe accident conditions. The authors are well underway with the development of this methodology, and the first part of the development will be presented in this paper. Furthermore, it is demonstrated that the turbulent flame acceleration, and therefore the resulting pressure loads, depend also strongly on the initial turbulence present in the ignition region. Therefore, it is recommended that for future validations, hydrogen deflagration experiments will be performed where this initial turbulence is accurately measured. [Copyright &y& Elsevier]

Abstract: The selection of the maximum allowable mesh size for a fluid dynamic calculation with Computational Fluid Dynamic (CFD) codes is essential for the reliability of the results assuming suitable physical and numerical models are used. Calculations with CFD codes are necessary for the assessment of the consequences of pressure loads on containment structures due to possible hydrogen combustion in nuclear power plants in a severe accident and on piping system due to pressure wave propagation in case of a pipe break accident or fast closing of a valve in a pipe with forced flow. CFD simulations of the transport and distribution of the released hydrogen/steam as well as the possible combustion during the transient in the containment require an appropriate mesh size to resolve the relevant phenomena and loads. The determination of the mesh size has to take into account: [•] adequate delineation of the containment geometry for accurate hydrogen distribution calculations, [•] sufficient conservative resolution of the combustion phenomena for the determination of pressure wave propagation and pressure loads, [•] no loss of pressure wave loads with relevant frequencies for the structural response analysis of the containment during the combustion calculation. In this paper, it is found that the accuracy of the calculated pressure wave associated with its frequency depends on the mesh size and a simple and easily useable analytical formula for the determination of the maximum allowable mesh size is derived. This formula can be used as a criterion to find out easily and simply the appropriate mesh size for accurate determination of the pressure wave load function with sufficient resolved frequencies relevant for structural response analyses. This criterion in form of a relation is deduced in this theoretical treatment from the connection between the structural characteristic associated with the relevant frequency and fluid property associated with mixture quality and gas temperature as well as numerical determination of the pressure wave load function associated with the accuracy of the approximation. This criterion gives the CFD code users the ability to determine the upper limit of the mesh size during the preparing of the geometry simulation specially for performing a combustion calculation. Applying the criterion presupposes that the used CFD code can calculate correctly the turbulent combustion, flame and pressure wave propagation for the predicted maximum allowable mesh size. The mesh size can be calculated from the analytical formula using: [•] the mixture quality and temperature in the cloud surrounded by structures (containment), [•] the highest relevant frequency for the structural analysis of the containment and [•] the number of points for the accurate approximation of the pressure wave function. The methodology can be applied also for the assessment of the frequency content of the determined pressure wave load function for the response analysis of a piping system like main coolant or steam lines in case of a break accident or other accidents with occurrence of pressure waves. [Copyright &y& Elsevier]

Abstract: During a severe accident in a PWR, large quantities of hydrogen can be generated and released into the containment. The generated hydrogen, when mixed with air, can lead to hydrogen combustion. The dynamic pressure loads resulting from hydrogen combustion can be detrimental to the structural integrity of the reactor safety systems and the reactor containment. Therefore, accurate prediction of these pressure loads is an important safety issue. In a previous article, we presented a CFD based method to determine these pressure loads. This CFD method is based on the application of a turbulent flame speed closure combustion model. The validation analyses in our previous paper demonstrated that it is of utmost importance to apply successive mesh and time step refinement in order to get reliable results. In this article, we first determined to what extent the required computational effort required for our CFD approach can be reduced by the application of adaptive mesh refinement, while maintaining the accuracy requirements. Experiments performed within a small fan stirred explosion bomb were used for this purpose. It could be concluded that adaptive grid adaptation is a reliable and efficient method for usage in hydrogen deflagration analyses. For the two-dimensional validation analyses, the application of dynamic grid adaptation resulted in a reduction of the required computational effort by about one order of magnitude. In a second step, the considered CFD approach including adaptive mesh refinement has been further validated against three hydrogen deflagration experiments performed in the ENACCEF facility. For each test, mesh and time step sensitivity analyses have been performed. From the presented validation analyses, it could be concluded that the maximum pressures were predicted within 13% accuracy, while the rate of pressure rise dp/dt was predicted within about 30%. The eigen-frequencies of the residual pressure wave phenomena were predicted within a few %. Therefore, it was overall concluded that the current model predicts the considered ENACCEF experiments very well. [Copyright &y& Elsevier]

Abstract: This paper presents the modeling of low-concentration hydrogen deflagrations performed with the recently developed KYLCOM model specially created to perform calculations in large scale domains. Three experiments carried out in THAI facility (performed in the frames of international OECD THAI experimental program) were selected to be analyzed. The tests allow studying lean mixture hydrogen combustion at normal ambient, elevated temperature and superheated and saturated conditions. The experimental conditions considered together with the facility size and shape grant a high relevance degree to the typical NPP containment conditions. The results of the simulations were thoroughly compared with the experimental data, and the comparison was supplemented by the analysis of the combustion regimes taking place in the considered tests. Results of the analysis demonstrated that despite the comparatively small difference in mixture properties, three different combustion regimes can be definitely identified. The simulations of one of the cases required of the modeling of the acoustic–parametric instability which was carefully undertaken. [Copyright &y& Elsevier]

Abstract: The IRSN and AREVA NC are currently conducting a common interest fire research programme with the aim, among other things, of improving knowledge of clogging of high efficiency particulate air (HEPA) filters and developing an empirical model for clogging of such filters by combustion aerosols. This model must – insofar as possible – be independent of the nature of the fuel and be able to be integrated in a calculation code covering the interaction between the ventilation and the fire. This paper discusses the influence of various “direct” factors such as the filtration velocity, the mass of deposited aerosol per filter area, the diameter and morphology of the combustion particles, the condensate content of the aerosols, and “indirect” factors such as the air flow feeding the fire and its oxygen content, which influence the evolution of the aeraulic resistance of a clogged filter. [Copyright &y& Elsevier]

Abstract: Burning characteristics of electrical cables are one of the key parameters for the fire hazard assessment of nuclear power plants (NPPs) since the cables are the essential sources of fire in the plants. A three-dimensional (3-D) transient computational fluid dynamics (CFD) code_FDS is adopted in this paper to simulate these characteristics related to the cable burning. Being one of the NRC licensing fire codes, the FDS includes the thermal-hydraulic equations, the turbulence model and the chemical combustion model, etc. In order to assess the CFD fire models used in this code, a burning test using the control cable with the outer jacket of polyvinylchloride (PVC) and the inner insulation of cross-linked polyethylene (XLPE) is conducted. The measured parameters associated with the burning characteristics include the heat release rate (HRR), O2 depletion, and CO and CO2 production, etc. Except the amount of O2 consumption, the predicted transient behaviors of other parameters can reproduce the measured data. Based on the chemical combustion model in the FDS code, this discrepancy may be essentially resulted from the default value of hydrogen fraction (H frac) contained in the soot since the soot yield for the burning of PVC material is high enough that the uncertainty in the H frac value has a prominent effect on the amount of O2 consumption. This explanation can be confirmed by a benchmark calculation for simulating a burning test with the polymethylmethacrylate (PMMA) fuel of low-soot yield. The present simulation works can provide the useful information for the plant staff or the researcher as they would perform the fire hazard analysis in the NPPs using the FDS code. [ABSTRACT FROM AUTHOR]

Abstract: A joint research project was carried out in the EU Fifth Framework Programme, concerning hydrogen risk in a nuclear power plant. The goals were: Firstly, to create a new data base of results on hydrogen combustion experiments in the slow to turbulent combustion regimes. Secondly, to validate the partners CFD and lumped parameter codes on the experimental data, and to evaluate suitable parameter sets for application calculations. Thirdly, to conduct a benchmark exercise by applying the codes to the full scale analysis of a postulated hydrogen combustion scenario in a light water reactor containment after a core melt accident. The paper describes the work programme of the project and the partners activities. Significant progress has been made in the experimental area, where test series in medium and large scale facilities have been carried out with the focus on specific effects of scale, multi-compartent geometry, heat losses and venting. The data were used for the validation of the partners CFD and lumped parameter codes, which included blind predictive calculations and pre- and post-test intercomparison exercises. Finally, a benchmark exercise was conducted by applying the codes to the full scale analysis of a hydrogen combustion scenario. The comparison and assessment of the results of the validation phase and of the challenging containment calculation exercise allows a deep insight in the quality, capabilities and limits of the CFD and the lumped parameter tools which are currently in use at various research laboratories. [Copyright &y& Elsevier]

Multi-dimensional sodium combustion behavior has been numerically investigated in the present paper. A new computer code AQUA-SF has been developed for this purpose. The code includes two sodium combustion models (so called ‘spray combustion’ and ‘pool combustion’), a mass and heat transfer model considering a six-flux gas radiation and a coagulation and sedimentation model of sodium oxide and hydroxide aerosols. The sodium spray combustion rate is evaluated by a summation of the combustion rate of each sodium droplet with an individual diameter. A flame sheet model is applied to situations where sodium spreads out on the floor and a pool combustion takes place. The model assumes an infinitely thin flame above the pool surface and is based on a mass and energy balance in the flame. As the results of numerical analyses of a sodium spray combustion test, a location of high-temperature core region and a maximum temperature agrees with the experiment. Good agreements of an overall transient behavior are obtained in a large-scale sodium combustion test analysis. The numerical analyses also demonstrate that the distributions of temperature and chemical species concentration vary with sodium combustion modes. If sodium scatters and the spray combustion is dominant, the distributions vary in space. When a large amount of sodium exists on a floor and the pool area is enlarged, the distributions are more uniform in space. [Copyright &y& Elsevier]