In this work, a numerical methodology based on the geometric approach and turbulent flame closure (TFC) has been applied to simulate weak and strong flame acceleration in hydrogen-air mixtures. Flame tracking is based on the transport equation for the progress variable. To minimize the need for tuning/calibration of model parameters, a generalized transport equation for the flame-wrinkling factor is adopted rather than algebraic closure models. Turbulence modeling was based on Reynolds-averaged Navier–Stokes (RANS). For sharp resolution of shocks, a density-based solver has been adopted. The open-source toolbox OpenFOAM has been used to perform all numerical simulations. A relatively coarse spatial discretization has been employed to enable application to larger domains. A detailed validation study has been carried out for the SSEXHY shock tube facility. For the range of hydrogen concentrations considered, a good prediction of the trend in flame speed has been obtained. The model can capture both weak and strong flame acceleration, as well as their transition, without requiring any model switching or parameter tuning.

Catalytic recombiners have been deployed worldwide in containment buildings of water-cooled nuclear power reactors to mitigate the risks posed by hydrogen generated during a postulated accident. In a plate-type recombiner, the plates are coated with platinum/palladium and arranged in parallel at the lower section of the recombiner box. The box is open at the bottom and top to establish natural circulation. Hydrogen recombines with oxygen in the presence of catalyst-coated plates, generating heat. This heat raises the gas and plate temperatures, which is a function of hydrogen concentration and recombiner geometry. For higher concentrations of hydrogen, the recombiner may itself become a source of ignition due to higher plate temperatures, which needs to be investigated. In reported experiments, ignition has been observed to occur at hydrogen concentrations above 5.5

During severe accident in a water-cooled nuclear power plant, hydrogen can be generated, leading to risks of possible hydrogen combustion and may threaten containment integrity. In this context, it is important to know the flame speed in a combustible mixture containing hydrogen. The ENACCEF test setup is a vertical acceleration tube with periodic obstacles and a dome of large volume at the top. It is a uniquely designed setup to study flame propagation under severe accident conditions in containment atmosphere. In this work, numerical validation has been carried out for one of the tests conducted at the ENACCEF facility using experimental data available in the open literature. A mechanistic combustion modelling framework especially suitable to fast flame propagation has been evolved and implemented using OpenFOAM. It has been observed that the numerical simulations are able to accurately capture a gamut of observed combustion phenomena like slow deflagrations, flame–turbulence interaction and fast flame acceleration. Moreover, benchmarking with numerical results obtained from other research groups suggests that the present results are more in line with the experimental data. The combined validation and benchmarking studies thus affirm the high fidelity of the adopted modelling approach and its numerical implementation with OpenFOAM.

In this work, a numerical methodology based on the paradigm of Turbulent Flame Closure (TFC) has been tested to simulate flame acceleration and Deflagration-to-Detonation Transition (DDT) in hydrogen air mixtures. Flame tracking is based on the transport equation for the progress variable. To minimize the need for tuning/calibration of model parameters, a generalized transport equation for the flame-wrinkling factor is adopted rather than algebraic closure models. The progress variable equation has been augmented with a sub-model based on the autoignition delay time parameter, which is crucial for capturing local explosions. Turbulence modeling is based on RANS. For sharp resolution of shocks, the conservative form of governing equations has been adopted. Relatively under-resolved numerical grid has been employed with the objective of establishing its scalability potential for large-scale computations. The TFC approach is strictly valid for fully turbulent flames. In the present work, its applicability for flame acceleration and DDT in obstructed channels has been investigated. Initial testing has been carried out for a closed and circular shock tube fitted with a single concentric obstacle. Further, a detailed study has been carried out for the GraVent explosion channel. Based on the validation of (1) flame speed variation during flame acceleration, (2) final detonation speed and (3) incident and reflected shock pressures, the capabilities and limitations of the approach have been presented. For the range considered in the present study, reasonably good predictions have been obtained with no tuning/calibration of model parameters. The model is able to capture flame acceleration and predict the conditions under which DDT can take place. However, it is further observed that actual DDT mechanism could not be reproduced.

A significant amount of hydrogen may be released inside the containment of water-cooled nuclear power reactor under postulated accident conditions. Its distribution in the multicompartment containment geometry must be known to manage and mitigate the local hydrogen concentration in combustible pockets. An experimental study to characterize the behavior of a lighter gas (helium in place of hydrogen) in a multicompartment containment studies facility (CSF) has been pursued. Helium distribution experiments have been performed in CSF by varying important accident parameters like helium release rate, injection duration, injection area, and injection direction. The experimental studies performed in CSF depict helium stratification in the upper dome region. Stratification in terms of stratification/effective stratification factor has been determined for a range of experiments. The present experimental studies are important for understanding hydrogen distribution characteristics in multicompartment containment geometry and for benchmarking computational fluid dynamics (CFD) codes. Based on these studies some important prevailing practices for recombiner placement were endorsed.

During postulated accident sequences in water-cooled nuclear reactors, steam and hydrogen may be released from the core and form a flammable mixture in the surrounding containment structure. Combustion of such mixtures and the subsequent pressure rise are an imminent threat for reactor containment integrity. Methods for evaluating combustion pressure rise are important for determining the design safety margins in such scenarios. Typically, combustion calculations are based on NS equations and CFD modelling, which are complex and time-consuming. A simpler and much faster approach is to use thermodynamic analysis to compute the final state after combustion for a given initial state. In the present work, thermodynamic modelling based on free energy minimization is presented. Predictions from the thermodynamic model have first been validated with published experimental data for binary hydrogen–air mixture. Then, parametric studies have been carried to compute combustion pressure rise in ternary mixture of hydrogen–steam–air as it represents more realistic mixture during accident. Finally, the model has been applied to a typical nuclear reactor containment to determine design safety margin as well as margin with respect to functional and structural failure of the containment.

For hydrogen management in the containment of Nuclear Power Plants (NPPs), besides the Passive autocatalytic Recombiners (PAR), the passive dilution of lighter gas plays an important role. This could be an attractive option to optimize the containment design and to estimate the extent of dilution. Passive dilution has many other applications in nuclear industry. The experimental studies of air entrainment in the upward rising helium plume and the resulting dilution of helium gas by the Canadians in terms of Volume Flow Magnification Factor (VFMF) have been utilized for Computational Fluid Dynamics (CFD) validation. The CFD based Fire Dynamics Simulator (FDS) predicted values of VFMF found to be in good agreement with the test data. After FDS code validation, parametric study has been carried out to generate a data base of VFMF for range of hydrogen injection, side opening area and opening height. In present study various Machine Learning (ML) models are evaluated based on two-parameter relationship i.e. non dimensional hydrogen injection and VFMF using the CFD code generated database. The trained ML models were used for the predictions of the mass flow rate of gas entrainment (through opening) in the rising buoyant plume in terms of VFMF. The ML predictions were in good agreement with the predictions against test data. Multivariate Adaptive Regression Splines (MARS) based ML model found to performed best and discussed in the paper. The paper highlights details of methodology of numerical simulation, results of the CFD studies and machine learning based predictions.

A significant quantity of hydrogen may be released in the nuclear containment of water cooled nuclear power reactors in case of severe accident condition. Passive Auto Catalytic Recombiners (PARs) are placed in the containment at suitable places to mitigate the hydrogen hazards. Performance of PAR dependents upon sufficient availability of air at recombiner location. However in certain circumstances during accident condition, the availability of the air may be low due to presence of high steam and hydrogen concentration. This may affect the performance of PAR. A detailed 2-D CFD analysis has been carried out by modelling detailed surface reaction mechanism in general purpose CFD code CFD-ACE+. The paper presents results of PAR performance in oxygen starvation condition by modelling detailed surface reaction mechanism and radiation heat transfer.

In water-cooled power reactor, hydrogen is generated in case of steam zirconium reaction during severe accident condition and later on in addition to hydrogen; CO is also generated during molten corium concrete interaction after reactor pressure vessel failure. Passive Autocatalytic Recombiners (PARs) are provided in the containment for hydrogen management. The performance of the PARs in presence of hydrogen and carbon monoxide along with air has been evaluated. Depending on the conditions, CO may either react with oxygen to form carbon dioxide (CO2) or act as catalyst poison, reducing the catalyst activity and hence the hydrogen conversion efficiency. CFD analysis has been carried out to determine the effect of CO on catalyst plate temperature for 2 & 4% v/v H2 and 1–4% v/v CO with air at the recombiner inlet for a reported experiment. The results of CFD simulations have been compared with the reported experimental data for the model validation. The reaction at the recombiner plate is modelled based on diffusion theory. The developed CFD model has been used to predict the maximum catalyst temperature and outlet species concentration for different inlet velocity and temperatures of the mixture gas. The obtained results were used to fit a correlation for obtaining removal rate of carbon monoxide inside PAR as a function of inlet velocity and concentrations.

Abstract A computational study has been carried out for predicting the behaviour of a pool fire source using the field-model based code Fire Dynamics Simulator (FDS). Time dependent velocity and temperature fields are predicted along with the resulting changes in the plume structure and its width. Firstly, a grid study was performed to find out the best grid size for this purpose. Then calculations were done which showed a very good agreement with earlier reported experimental based correlations for the temperature of the plume region. These studies have been extended to use this field-model based tools for modelling particular separate effect phenomena like puffing frequency and to validate against experimental data. There are several applications in nuclear industry like room fires, wildland fires, smoke or ash disposal, hydrogen transport in nuclear reactor containment, natural convection in building flows etc. In this paper the use of FDS with the advanced Large Eddy Simulation (LES) based CFD turbulence model is described for various applications: Fire simulation for Alpha storage, Bhabhatran teletherapy, pool fire for transport casks, fire PSA of a representative NPP, exhaust air fan buildings of a process plant and smoke dispersion in large fires around NPPs.

A framework for estimation of large early release frequency of a nuclear power plant is presented in this paper. The study aims at evaluating the reliability of containment safety features and analysing the physical processes such as timing and magnitude of radioactivity release. The release categories were formulated based on the amount of release of fission products (FPs) and their time of release. Reliability analysis of engineered safety features (ESFs) of containment was carried out using fault tree technique. The end states of containment event trees were formulated qualitatively based on the expert judgement, and the release categories were assigned accordingly. For interfacing with Level 1 probabilistic safety assessment (PSA), the plant damage categories were modified to include aspects such as time and extent of core damage and amount of metal–water reaction. Containment event trees were developed for these plant damage states, and the large early release frequency (LERF) was estimated for the advanced heavy water reactor (AHWR).

The advanced heavy water reactor (AHWR) employs a double containment concept with a large inventory of water within the gravity-driven water pool (GDWP) located at a higher elevation within the primary containment building. GDWP performs several important safety functions in a passive manner, and hence, it is essential to understand the hydrodynamics within this pool during a LOCA. To achieve this objective, several LOCA scenarios involving RIH break sizes ranging from 2 to 50% are postulated, and the AHWR containment system response is obtained. An in-house containment thermal hydraulics code called “CONTRAN” is used for this purpose. The blowdown mass and energy discharge data for each break size, along with the geometrical details of the AHWR containment, form the main input for the analysis. Apart from obtaining the pressure and temperature transients within the containment building, the focus of this work is on simulating the hydrodynamic phenomena of vent clearing and pool swell occurring in the GDWP. The variation of several key parameters such as pressure and temperature within the primary containment volumes V1 and V2, differential pressure, BOP rupture time, vent clearing velocity, effect of pool swell on the V2 air space pressure, GDWP water level, etc., is presented and discussed in detail, and important findings are highlighted. Further, the effect of neglecting the modeling of pool swell phenomenon on the containment transients is also clearly brought out by a comparative study. The numerical studies presented here give insight into AHWR containment transients that would be useful to both the system designer as well as the regulator.

Experiments were conducted using CsI aerosols in a small scale test chamber to simulate behaviour of aerosols in the containment of a nuclear reactor. The primary focus of the study was on submicron particles (14.3 nm-697.8 nm) due to their hazardous effect on human health. Different wall surfaces, viz., plexiglass, concrete and sandpaper were chosen to study the effect of surface roughness on dry deposition velocity under both quiescent and turbulent conditions. An analytical approach to calculate dry deposition velocity of submicron particles for rough surfaces has been proposed with an improvement in the existing parameterization for shift in the velocity boundary layer. The predicted deposition velocity with the improved parameterization was found to have better agreement with published measured data of Lai and Nazaroff (2005) compared to the existing parameterizations (Wood, 1981; Zhao and Wu, 2006b). There was a significant reduction in root mean square error (RMSE) between predicted, using the improved parameterization and measured deposition velocity (upto 100%) compared to earlier ones. The new analytical deposition approach was coupled with volume conserving semi-implicit coagulation model. This aerosol dynamic model was evaluated against explicit particle size distribution for the first time for rough surfaces. Normalized RMSE between simulated and measured particle size distribution varied in the range of 2%-20% at different instances. The model seems to closely predict submicron particle behaviour in indoor environment. (C) 2016 Elsevier Ltd. All rights reserved.