Intrinsic transport and combustion issues of steam-air-hydrogen mixtures in nuclear containments

Understanding local transport behaviour of steam-air-hydrogen mixture inside the containment and associated hydrogen combustion issues are essential to ensure integrity of the nuclear reactor containment in the event of a severe accident. During a severe accident, various mechanisms occurring inside and outside of reactor pressure vessel may lead to generation of steam, and subsequently hydrogen, which eventually gets released in the containment space. Hydrogen may deflagrate in the presence of an ignition source/hot spot depending on local mixture composition of steam-air-hydrogen; this may further lead to flame acceleration, deflagration to detonation transition, and finally to detonation depending upon local conditions and geometric factors, further increasing the internal pressure and temperature of the containment. The fraction of non-condensable gases may also rise due to simultaneous on-going steam condensation, which not only elevates the possibility of hydrogen combustion, but also in turn, affects the eventual local and average steam condensation rates. In this background, this paper reviews the coupled issues between steam condensation, hydrogen transport, hydrogen combustion criteria, location of its sources, and stratification inside reactor containment under plausible severe accident scenarios. Several experiments in the context of containment thermal-hydraulics and hydrogen combustion are elaborated. Looking into the complexity of the problem, necessity to adopt simulation approach is highlighted. Two types of codes, i.e., lumped-parameter (LP) and computational fluid dynamic (CFD), are scrutinized based on their specific applications and limitations. It is inferred that the containment thermal-hydraulics and ensuing safety strategies must address the issue of steam condensation and hydrogen management simultaneously, through a comprehensive and integrated approach.     

An improved method for hydrogen deflagration to detonation transition prediction under severe accidents in nuclear power plants

This work proposed a new method for prediction of hydrogen Deflagration to Detonation Transition (DDT) on the basis of oxygen concentration in the presence of inerting diluents. Whereas previously, the traditional criterion for deflagration to detonation transition hypothesized an unchanged air composition, it now seems appropriate to question the assumption and consider possible situations in which the presence of inerting gas components incapacitates the old criterion for applications. Under some circumstances (severe accidents in nuclear power plants), hydrogen may be massively generated by intense chemical reactions between zirconium cladding and overheated coolant in the nuclear reactor vessel. In order to prevent hydrogen explosions, Passive Autocatalytic Recombiners (PARs) that mitigate hydrogen risk by hydrogen oxidations have been implemented in the nuclear energy industry worldwide. It consumes a large amount of oxygen as the reactant and gives rise to an increased ratio of inert gas nitrogen to oxygen in the air, the product of which, water mist, also alleviates explosion hazards. The new method addressed on the variation of oxidant volume fraction and proposed new parameters: the equivalent air and the equivalent inert gases concentrations in deflagration to detonation transition criterion. The HYDRAGON code, that has been specially developed for hydrogen analysis in nuclear power plants, implemented both new and original criteria and has been applied to assessments. Close agreements between numerical simulations and a large number of experimental data sets: a wide variety of fuel gases and inert diluents, suggested that such new technique was viable and applicable to predict deflagration to detonation transition for various combustible gases. A hydrogen risk analysis of an advanced pressurized water reactor using the new method was also demonstrated in this paper.     

A review on hydrogen generation,explosion, and mitigation during severe accidents in light water nuclear reactors

Progress of severe accident (SA) can be divided into core degradation and post core meltdown. An important phenomena during severe accidents is the hydrogen generation from exothermal reaction between oxidation of core components, and molten core concrete interaction (MCCI). During the severe accidents, a large amounts of hydrogen is produced, deflagrated and consequently the containment integrity is violated. Therefore, the main objectives of this study is to highlight the source of hydrogen production during SA. First, a thorough literature review and main sources of hydrogen production, hydrogen reduction systems are introduced and discussed. Based on the available results, the amount of produced hydrogen in a typical pressurized water reactor (PWR) and a boiling water reactor (BWR) are estimated to be 1000 and 4000 kg, respectively during in-vessel phase. The average rate of hydrogen production is about 1 kg/s during reflooding of a degraded core. Also, about 2000 kg hydrogen is produced during MCCI for a PWR. The lower and upper range of hydrogen required to initiate combustion is 4.1 and 74 vol percent, respectively. In this paper a review is provided of what has been done in the literature with regard to hydrogen generation in severe accidents of nuclear power plants. In addition, the review identifies the literature gaps and underlines the need of developing a systematic hydrogen management strategy. A hydrogen management strategy is proposed in order to maintain the containment integrity against the probable combustion or hydrogen explosion loads.     

Determination of PAR configuration for PWR containment design: A hydrogen mitigation strategy

This paper deals with determination of the minimum number and identification of the best configurations of passive autocatalytic recombiners (PAR) for the effective design of the containment in a pressurized water reactor (PWR). It considers the current design of PAR in the containment of a PWR and for that tries to identify, through a large number of sensitivity analyses, the minimum required number of PARs in different compartments. In this regard, a qualified nodalization has been developed for best estimate modeling by MELCOR integrated code. The developed model includes primary and secondary systems, containment, and related safety systems. A large number of simulations including the plant specific probabilistic safety assessment and success criteria analysis are used to identify the accident scenario with the highest amount of hydrogen production and risk. We first screened postulated accidents based on the PSA results and then based on the deterministic severe accident computations. It is found that the large break loss of coolant accident (LB-LOCA) without emergency core cooling system (ECCS) actuation is the bounding case from the hydrogen hazard point of view. To find the optimal configuration with minimum number of PARs in the containment, 40 different configurations are analyzed for the selected accident for a Westinghouse type PWR. The main finding of this work is identification of the minimum required number of PARs and their best distribution among the associated compartments. The obtained configuration is equally effective for the hydrogen risk mitigation with 36% reduction in the number of PARs in comparison to the base case design. The methodology of the analysis can be used for other plants.     

Dispersion and behavior of hydrogen for the safety design of hydrogen production plant attached with nuclear power plant

Development of nuclear energy and hydrogen energy both as renewable energy open up a vast range of prospects. The scheme for hydrogen generation station in nuclear power plant has been carried out in china. However, Nuclear Energy is expected to encourage a safety culture that prevents serious accidents while dispersion of hydrogen from a container produces a risk of combustion. The dispersion and behavior of hydrogen production plant attached with nuclear power plant are still poorly understood. In this paper, a dispersion of hydrogen model is established and is calculated under two typical condition with corrected ideal gas state equation. The flammability of hydrogen after dispersion is studied. The range of flammability of dispersion of hydrogen production plant with different pressures, positions and temperatures is obtained. This work could contribute to the marginal hydrogen safety design for hydrogen production station and lay the foundation for the establishment of a safe distance standard that it's necessary to prevent hydrogen explosion.     

CFD prediction of hydrogen passive autocatalytic recombiner performance under counter-current flow conditions

Passive Autocatalytic Recombiners (PAR) are frequently used today as safety devices to mitigate hydrogen risk in confined spaces. The present study aims to investigate by CFD tools the PAR performance under potentially adverse counter-current flow conditions. Experimental data obtained from the THAI+ two-compartment facility are used to validate the numerical simulation. Counter-current flow is created by a fan in the larger vessel which produces a downward flow in the second vessel housing the PAR unit. In the simulation, the H₂ reaction rate is computed by a correlation given by the PAR manufacturer, and hence no detailed chemistry is necessary. In agreement with test data, the simulation results show that PAR operation is not hindered by the imposed counter-current flow, although the plume exiting the PAR is somewhat compressed compared to that existing in quiescent atmospheres. It is also found that the computed parameters of interest (reaction rates, mean flow velocities, hydrogen PAR inlet/outlet concentration, temperature, pressure) agree well with the measured data. This confirms the usefulness of using CFD simulations to predict PAR behavior in complex flows and geometries.     

An evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions

Chemical kinetics in hydrogen combustion for elevated pressures have recently become more relevant because of the implementation of hydrogen as a fuel in future gas turbine combustion applications, such as IGCC or IRCC systems. The aim of this study is to identify a reaction mechanism that accurately represents H₂/O₂ kinetics over a large range of conditions, particularly at elevated pressures as present in a gas turbine combustor. Based on a literature review, six mechanisms of different research groups have been selected for further comparisons within this study. Reactor calculations of ignition delay times show that the mechanisms of Li et al. and Ó Conaire et al. yield the best agreement with data from shock tube experiments at pressures up to 33 bar. The investigation of one-dimensional laminar hydrogen flames indicate that these two mechanisms also yield the best agreement with experimental data of laminar flame speed, particularly for elevated pressures. The present study suggests that the Li mechanism is best suited for the prediction of H₂/O₂ chemistry since it includes more up-to date data for the range of interest.     

Performance evaluation of hydrogen production based on off-peak electric energy of the nuclear power plant

The article investigates the efficiency of commercial hydrogen production by water electrolysis on the base of NPP excess energy with its additional purification higher than 99.9999%, considering its transport. The competitive high purity hydrogen release price has been determined as compared to the market price. Besides, the use of high duty electrolysis plants has been suggested. Moreover, the advantages of water electrolysis cyclic operation while consuming electric energy from NPP as compared to the continuous mode have been presented in the paper.     

Reduction of a detailed reaction mechanism for hydrogen combustion under gas turbine conditions

The aim of this study is to find a reduced mechanism that accurately represents chemical kinetics for lean hydrogen combustion at elevated pressures, as present in a typical gas turbine combustor. Calculations of autoignition, extinction, and laminar premixed flames are used to identify the most relevant species and reactions and to compare the results of several reduced mechanisms with those of a detailed reaction mechanism. The investigations show that the species OH and H are generally the radicals with the highest concentrations, followed by the O radical. However, the accumulation of the radical pool in autoignition is dominated by HO₂ for temperatures above, and by H₂O₂ below the crossover temperature. The influence of H₂O₂ reactions is negligible for laminar flames and extinction, but becomes significant for autoignition. At least 11 elementary reactions are necessary for a satisfactory prediction of the processes of ignition, extinction, and laminar flame propagation under gas turbine conditions. A 4-step reduced mechanism using steady-state approximations for HO₂ and H₂O₂ yields good results for laminar flame speed and extinction limits, but fails to predict ignition delay at low temperatures. A further reduction to three steps using a steady-state approximation for O leads to significant errors in the prediction of the laminar flame speed and extinction limit.     

Effect of hydrogen direct injection on natural gas/hydrogen engine performance under high compression-ratio conditions

In traffic transportation, the use of low-carbon fuels is the key to being carbon-neutral. Hydrogen-enhanced natural gas gets more and more attention, but practical engines fueled with it often suffer from low engine power output. In this study, the inner mechanism of hydrogen direct injection on methane combustion was optically studied based on a dual-fuel supply system. Simultaneous pressure acquisition and high-speed direct photography were used to analyze engine performance and flame characteristics. The results show that lean combustion can improve methane engine's thermal efficiency, but is limited by cyclic variations under high excess air coefficient conditions. Hydrogen addition mainly acts as an ignition promoter for methane lean combustion, as a result, the lean combustion limit and thermal efficiency can be improved. As for hydrogen injection timing, late injection can increase the in-cylinder turbulence intensity but also the inhomogeneity, so a suitable injection timing is needed for improving the engine's performance. Besides, late hydrogen injection is more effective under lean conditions because of the reduced mixture inhomogeneity. The current study shall give some insights into the controlling strategies for natural gas/hydrogen engines.     

Optimization of the working cycle for a hydrogen production and power generation plant based on aluminum combustion with water

The working cycle of a novel hydrogen and power generation system based on aluminum combustion with water is analyzed in order to evaluate the best performance in terms of energy conversion efficiency. The system exploits the exothermic reaction between aluminum and steam and produces thermal power for a super-heated steam cycle and hydrogen as a by-product of the reaction.     

Control of methane flame properties by hydrogen fuel addition: Application to power plant combustion chamber

This study presents a numerical investigation of the effects of mixing methane/hydrogen on turbulent combustion processes taking place in a burner similar to that integrated in gas turbine power plants. Thereby, in comparison to the reference case where the burner is fuelled by 100% of methane, the variations of the axial velocity field, temperature field and mass fraction of carbon monoxide field are examined for different percentages of hydrogen fuel injection. The computed results, obtained by using the software Fluent-CFD, are compared and validated against experimental reference data. Results show that the hydrogen addition to the methane has an impact on all physical and chemical parameters of the reactive system.     

Evaluation of a new computational fluid dynamics model for internal combustion engines using hydrogen under motoring conditions

The present work conducts a preliminary evaluation of a new CFD (computational fluid dynamics) model, which is under development at the authors' laboratory. Using this model, it is feasible to understand how the intake manifold and in-cylinder geometry affect the in-cylinder flow field and the mixing processes taking place in an Otto (spark-ignition) engine. The model is applied on a high-swirl, two-valve, four-stroke, transparent combustion chamber engine running under motoring conditions. To investigate the fuel–air mixing process, hydrogen is injected in the intake manifold. To evaluate the model three case studies are examined. First, the model is applied to simulate the external mixing in the intake manifold with a tee-mixer injection system. Secondly, the transient gas flow field in the intake manifold and engine cylinder is examined over the complete engine cycle. Finally, the transient mixing process in the intake manifold and the spatial and temporal distribution of species concentrations inside the cylinder are numerically computed using the developed model. To validate the model, the results obtained through the test cases examined are compared either with available experimental data or with simulated results, which are obtained using a commercially available CFD code applied under the same conditions.     

Effects of hydrogen addition on engine performance in a spark ignition engine with a high compression ratio under lean burn conditions

In this study, the effects of hydrogen addition on the engine performance were investigated using spark ignition engine fueled gasoline with a compression ratio of 15 at an air excess ratio (λ) of 1.8 and above. At λ = 1.8, the indicated thermal efficiency at the spark timing of the knock limit reached the maximum level under the conditions in which the hydrogen fraction was set to 4% of the heating value of the total fuel. Based on a heat balance analysis, the best hydrogen fraction was found as a balance between the improvement in the burning efficiency and the increase in heat loss. The lean limit was extended when the hydrogen fraction was increased from λ = 1.80 to λ = 2.28. The hydrogen addition achieved the maximum indicated thermal efficiency at spark timing of the knock limit was obtained at λ = 2.04, where the hydrogen fraction was 10%.     

Review of Japan's power generation scenarios in light of the Fukushima nuclear accident

Following on from the Fukushima Daiichi nuclear power plant accident, the Japanese government is now in the throes of reviewing its power policy. Under continuing policies of economic revival and greenhouse gas reduction, it is crucial to consider scenarios for the country to realize reliable, low‐carbon, and economic electricity systems in the future. On the other hand, the social acceptance of nuclear power will affect the final political decision significantly. Therefore, in the present study, proposed power generation scenarios in Japan in light of the Fukushima accident were reviewed comprehensively from economic, environmental, technological, resource, security, and social perspectives. The review concludes that in Japan, (i) renewable energy mainly solar and wind needs to be developed as fast as possible subject to various constraints, (ii) more gas power plants will be used to absorb the fluctuations of intermittent renewable energy and supply electricity gap, (iii) nuclear power will be reduced in the future, but a 0% nuclear power scenario by 2030 is unlikely to be a reasonable choice on most measures and (iv) the effective communication with the public is vital important. Copyright © 2014 John Wiley & Sons, Ltd.     

Research and development of hydrogen fuelled engines in China

The present paper introduces the role of vehicles in the context of Chinese economy, Chinese energy security, Chinese environment and the sustainable development of China; expounds that hydrogen is the promising alternative fuel for vehicles in China; and points that developing hydrogen fuelled engine vehicle is inevitable for the further development of Chinese vehicle industry. Then, the paper reviews the research and development of hydrogen fuelled engines in China, and reports the most achievements obtained by Chinese researchers in the field of the hydrogen fuelled engines which involve hydrogen-enriched gasoline engine, hydrogen-enriched diesel engine, hydrogen-(compressed) natural gas dual (HNG/HCNG) fuel engine, and pure hydrogen internal combustion engine (H₂ICE).     

Experimental investigation on the kinetics of catalytic recombination of hydrogen with oxygen in air

Catalytic recombination of hydrogen with oxygen is one of the most attractive options to control the hydrogen concentration in air. The basic pre-requisite for the process design of any catalytic reactor is the knowledge of kinetic data. In the present study, the kinetic data for the catalytic recombination of hydrogen in presence of 0.5% Pd on alumina catalyst were generated using a packed bed reactor with complete recycle. The experiments were conducted using low concentration of hydrogen in air at different temperatures and the apparent rate constants were estimated assuming a first order reaction with respect to hydrogen. The resistances due to internal and external mass transfer were decoupled from the apparent kinetics and estimated separately. The activation energy and frequency factor were found out using the slope and intercept of the Arrhenius plot. The effect of different process parameters such as temperature, superficial velocity and the catalyst particle size on the overall reaction rate was also studied. The knowledge of the intrinsic kinetics along with the mass transfer can be easily extended for the design of catalytic recombination reactors during scale up.     

Affect of recombiner location on its performance in closed containment under dry and steam conditions

Hydrogen has been the major cause of fire in almost all of the biggest nuclear accidents witnessed by world so far. Many approaches have been investigated and developed worldwide to mitigate the consequences of hydrogen buildup inside the containment of Nuclear Power Plants (NPPs) under severe accident scenarios. One such most promising method is to deploy Passive Catalytic Recombiner Devices (PCRDs). They work on the principle of recombining hydrogen with oxygen from ambient air on catalytic surfaces to form steam and release of the exothermic heat of reaction. The present work describes the development, validation and application of a CFD based detailed 3D model for hydrogen recombination inside PCRD using the governing mass, momentum, energy and species conservation equations from first principle. The model has been integrated into the CFD code FLUIDYN-MP to capture the associated multi-physics phenomena. The integrated tool has been used to assess the most suitable location within a closed geometry for placing the PCRD so as to improve its performance and efficiency. Simulations were performed for different PCRD positions within a closed vessel under dry as well as steam laden conditions. The findings reveal that PCRD location in closed geometry plays important role in its performance. Moreover lower PCRD position helps in more natural convective mixing causing better hydrogen transport towards PCRD inlet and hence more hydrogen consumption.     

Study on combustion and emission characteristics of a n-butanol engine with hydrogen direct injection under lean burn conditions

The n-butanol fuel, as a renewable and clean biofuel, could ease the energy crisis and decrease the harmful emissions. As another clean and renewable energy, hydrogen properly offset the high HC emissions and the insufficient of dynamic property of pure n-butanol fuel in SI engines, because of the high diffusion coefficient, high adiabatic flame velocity and low heat value. Hydrogen direct injection not only avoids backfire and lower intake efficiency but also promotes to form in-cylinder stratified mixture, which is helpful to enhance combustion and reduce emissions. This experimental study focused on the combustion and emissions characteristics of a hydrogen direct injection stratified n-butanol engine. Three different hydrogen addition fractions (0%, 2.5%, 5%) were used under five different spark timing (10° ,15° ,20° ,25° ,30° CA BTDC). Engine speed and excess air ratio stabled at 1500 rpm and 1.2 respectively. The direct injection timing of the hydrogen was optimized to form a beter stratified mixture. The obtained results demonstrated that brake power and brake thermal efficiency are increased by addition hydrogen directly injected. The BSFC is decreased with the addition of hydrogen. The peak cylinder pressure and the instantaneous heat release rate raises with the increase of the hydrogen addition fraction. In addition, the HC and CO emissions drop while the NOx emissions sharply rise with the addition of hydrogen. As a whole, with hydrogen direct injection, the power and fuel economy performance of n-butanol engine are markedly improved, harmful emissions are partly decreased.