Hydrogen Mitigation System Research Articles

Hydrogen hazard mitigation in small modular reactor during SBO severe accident using GASFLOW-MPI

Hydrogen combustion or explosion will lead to over-pressurization of the reactor containment in case of severe accident caused by Station BlackOut (SBO) in an Small Modular Reactor (SMR). In order to mitigate the hydrogen hazard, ignitors as well as recombiners have been typically used in the reactor containment via the hydrogen deliberately burned or recombined catalytically with the oxygen in air. Computational Fluid Dynamics (CFD) tool GASFLOW-MPI, which provides the details of the 3D flow field and hydrogen distribution, is appropriate to analyze hydrogen mitigation system performance. Analysis of mitigation system performance is to evaluate the efficiency of the mitigation of hydrogen hazard in reactor containment. The objective of this paper is to apply the GASFLOW-MPI code to analyze the SBO severe accident in SMR, specifically with regard to analysis of the hydrogen distribution, the risk posed by hydrogen and the mitigation by deliberate ignition and the Passive Autocatalytic Recombiner (PAR). The relevant analysis is presented according to the GASFLOW-MPI features, and indicates that: 1. The hydrogen with/without mitigation distributes well during accident scenario. 2. The ignitor reduces hydrogen accumulation significantly, and 8 units of PARs can mitigate the hydrogen risk in the long-term phase. 3. Both installed mitigation systems (8 PARs and 6 ignitors) can prevent the flame acceleration and local detonation. 4. Utilization of PARs could be better than ignitors in SMR containment, since the passive mitigation with PARs can avoid the failure situations such as the ignitors’ electricity supply cut-off, etc. 5. GASFLOW-MPI provides the possibility of a CFD simulation to a long-time accident scenario in SMR containment.

Passive autocatalitic recombiner characteritics calculation during severe accident on AES-2006 NPP

A modern NPP is equipped by containment to hold radioactive substances and ionizing radiation bounded as design margins prescribe. Hydrogen mitigation system is used to protect containment against hydrogen fire and detonation. The system includes a scope of passive autocatalitic recombiners. Hydrogen is transformed into water, passing through said recombiners. The reaction occurs on catalyst surface. The main catalyst material is a palladium doped platinum. Hydrogen mitigation system parameters during severe accident are of interest. Wise admitted for NPP full scale tests are impossible, so the main analysis are calculations. Recombiner consists of catalytic block and stuck. The stuck provides reagents feeding and products evacuation enhancing natural convection transport. A model for calculations is suggested for recombiner with a plate-type catalyst block. The two free parameters of the model are chemical reaction intensity on catalyst and unit drag. Said parameters are estimated experimental data based on. Passive autocatalitic recombiner characteritics during severe ac- cident on AES-2006 NPP are calculated. The unit capacity is found not less that specification points. Catalyst temperatures, even the mean one, are above 500 °С, exhaust jet temperature exceeds 150 °С. Hydrogen content is high in the jet. Capacity increases linearly with hydrogen concentration at the unit entrance. Atmospheric temperature influence is low. Hydrogen mitigation system overall capacity is constant for uniform or not uniform hydrogen distribution in the containment. The calculated data may be used for recombiner work estimation during accident on AES-2006 NPP.

MELCOR Analysis of a SPARC Experiment for Spray-PAR Interaction during a Hydrogen Release

A series of experiments were performed in the SPARC (spray-aerosol-recombiner-combustion) test facility to simulate a hydrogen mitigation system with the actuation of a PAR (passive auto-catalytic re-combiner) and spray system. In this study, the SPARC-SPRAY-PAR (SSP1) experiment is chosen to benchmark the MELCOR (a lumped-parameter code for severe accident analysis) predictions against test data. For this purpose, firstly we prepared the base input model of the SPARC test vessel, and tested it by a simple verification problem with well-defined boundary conditions. The implementation of a currently used PAR correlation in MELCOR is shown to be appropriate for the simulation of a PAR actuation experiment. In an SSP1 experiment, the PAR is reacting with hydrogen, and the spray actuation starts as soon as hydrogen injection is complete. The MELCOR simulation well predicts the pressure behavior and the gas flow affected by operating both a PAR and spray system. However, the local hydrogen concentration measurement near the inlet nozzle is much higher than the volume average-value by MELCOR, since high jet flow from the nozzle is dispersed in the corresponding cell volume. The experimental reproduction of the phenomena we expect, or, conversely, the identification of phenomena we do not understand, will continue to support the verification of analytical models using experimental data and to analyze the impact of spray on PAR operations in severe accident conditions.

Experimental Study on a Hydrogen Stratification Induced by PARs Installed in a Containment

Hydrogen can be produced in undesired ways such as a high temperature metal oxidation during an accident. In this case, the hydrogen must be carefully managed. A hydrogen mitigation system (HMS) should be installed to protect a containment of a nuclear power plant (NPP) from hazards of hydrogen produced by an oxidation of the fuel cladding during a severe accident in an NPP. Among hydrogen removal devices, passive auto-catalytic recombiners (PARs) are currently applied to many NPPs because of passive characteristics, such as not requiring a power supply nor an operators’ manipulations. However, they offer several disadvantages, resulting in issues related to hydrogen control by PARs. One of the issues is a hydrogen stratification in which hydrogen is not well-mixed in a compartment due to the high temperature exhaust gas of PARs and accumulation in the lower part. Therefore, experimental simulation on hydrogen stratification phenomenon by PARs is required. When the hydrogen stratification by PARs is observed in the experiment, the verification and improvement of a PAR analysis model using the experimental results can be performed, and the hydrogen removal characteristics by PARs installed in an NPP can be evaluated using the improved PAR model.

Superhydrophobic Pd@CeO2/Al2O3 catalyst coating for hydrogen mitigation systems in nuclear power plants

Passive autocatalytic recombination (PAR) was employed as an effective hydrogen mitigation system for nuclear power plants. Despite the great progress achieved in catalysts for PAR, few reports could be found on PAR catalysts, which exhibit strong resistance to poisoning by iodine compounds and the detrimental effect of water, as well as good anti-sintering properties. In this study, a superhydrophobic Pd@CeO2/Al2O3 catalyst coating was prepared through a self-assembly method and the grafting of 1H,1H,2H,2H-perfluorooctyltriethoxysilane. The superhydrophobicity of the catalyst coating effectively alleviated water poisoning. The ceria shell of Pd@CeO2/Al2O3 served as a protective layer against poisoning by iodine compounds because it was difficult for iodine vapor to access the catalyst core of the Pd nanoparticles when the shell pore diameter was below 1.2 nm. Given the interface effect between palladium and ceria, the CeO2 shells also served as an effective barrier to prevent the Pd nanoparticles from aggregating at high temperatures. The superhydrophobic Pd@CeO2/Al2O3 coating showed excellent potential for the mitigation of hydrogen containing various poisons during a nuclear accident.

Analysis of hydrogen risk and its mitigation systems in CPR1000 during shutdown conditions

Analysis of hydrogen risk and the effectiveness of hydrogen mitigation systems during shutdown conditions is of great importance to support the development of the low power and shutdown (LPSD) probabilistic safety assessment (PSA) model and improve the capacity of nuclear power plants (NPPs) to reduce the hydrogen risk during shutdown conditions. This study deals with the identification of hydrogen risk and optimization of current hydrogen mitigation systems in the CPR1000 nuclear power plant during shutdown conditions before vessel head open. A detailed MELCOR input is developed, which includes detailed containment nodalization, accurate configuration of the passive autocatalytic recombiner (PAR) systems, and realistic boundary conditions based on design data and technical specifications. Three cases that represent different coolant inventory and specific hydrogen release paths are selected, and the accident progression and hydrogen risk are analyzed subsequently. The analysis indicates that the hydrogen risk is still an important issue during shutdown conditions, although the accident progression is relatively slow. PAR systems with the current configuration are added to the plant model to verify the effectiveness of the hydrogen mitigation system in the CPR1000 during shutdown conditions. The results indicate that the current configuration of PAR systems is still effective in mitigating the hydrogen risk following accidents during cold shutdown POS with PZR manhole close but is insufficient to mitigate the hydrogen risk during an accident with PZR manhole open. Therefore, two strategies to improve the hydrogen mitigation systems are proposed, and their effectiveness are compared. It is concluded that at least 12 additional PAR systems or an ignitor should be added to the PZR room to avoid severe hydrogen risk during shutdown conditions, and the additional PAR systems is the prior consideration.

Optimization of hydrogen mitigation systems in CPR1000 during a refueling outage

This study deals with the identification of hydrogen risk and optimization of current hydrogen mitigation systems in the CPR1000 nuclear power plant (NPP) during a refueling outage. A detailed MELCOR input is developed, which includes detailed containment nodalization, accurate configuration of the passive autocatalytic recombiner (PAR) systems, and realistic boundary conditions based on design data and technical specifications. Two cases that represent different plant operation states (POS) and specific hydrogen release paths are selected, and the accident progression and hydrogen risk are analyzed subsequently. The analysis indicates that the hydrogen risk is still of great importance during a refueling outage, although the accident progression is relatively slow. PAR systems with the current configuration are added to the plant model to verify the effectiveness of the hydrogen mitigation system in the CPR1000 during a refueling outage. The results indicate that the current configuration of PAR systems is still effective in mitigating the hydrogen risk during an accident in POS E but is insufficient in mitigating the hydrogen risk during an accident in POS H. Therefore, two strategies to improve the hydrogen mitigation systems are proposed, and their effectiveness is compared. It is concluded that mobile ignitors should be employed in the refueling pool region to avoid severe hydrogen risk during a refueling outage. The results of this study can support the development of the low power and shutdown (LPSD) probabilistic safety assessment (PSA) model and improve the capacity of NPPs to reduce the hydrogen risk during a refueling outage.

Numerical method to simulate detonative combustion of hydrogen-air mixture in a containment

During an accident accompanied by core damage in a water-cooled nuclear reactor, a large amount of hydrogen is generated by fuel clad oxidation and released into the reactor containment. Hydrogen mitigation in the containment during a severe accident is important because the integrity of the structures important to safety could be threatened by an explosive combustion of the hydrogen. Even when equipped with a hydrogen mitigation system, if a highly concentrated hydrogen mixture cloud is developed in a compartment of the containment, the compartment integrity with a pressure load from a local detonation must be evaluated. In this study, a reliable numerical method to simulate the detonation of a hydrogen-air mixture was developed for the evaluation of a pressure load from a detonation. The numerical method has the capability to capture shocks robustly as well as resolve combustion phenomena stably. The Euler equations of mass, momentum, energy and gas species are employed as the governing equations of detonation. A central-upwind scheme was adopted as a shock capturing scheme and a reduced chemical kinetic model was applied for combustion. The numerical method was validated by solving experimental cases and applied for understanding detonation shock behaviour in a generic containment.

Detached Eddy Simulation of hydrogen turbulent dispersion in nuclear containment compartment using GASFLOW-MPI

During the severe accident in nuclear power plants (NPPs), hydrogen is generated due to the zirconium-water reaction and released from the breaks in coolant pipe forming a locally high concentration hydrogen cloud in the steam generator (SG) compartment, which plays a key role for hydrogen safety analysis in NPPs. Accurate prediction of the turbulent dispersion process of hydrogen-steam gas mixture is a critical topic for a successful simulation of the flammable cloud distribution in SG compartment. In this study, the high-fidelity temporal evolution of the hydrogen turbulent dispersion in a SG compartment is performed using the Detached Eddy Simulation (DES) based on the parallel CFD code GASFLOW-MPI to capture more detailed unsteady turbulent information. Firstly, the newly developed DES turbulence model is validated using two turbulent benchmarks, a backward-facing step turbulent flow and a hydrogen turbulent jet. The simulation results are consistent well with the experimental data. Then a SG compartment model including one steam generator, two coolant pumps, a hot leg and two cold legs is built using the specialized auto-mesh generation module. There are two modes of turbulent dispersion behavior due to the turbulent driven force in the containment, i.e. jet dominated by initial monument and plume dominated by buoyancy. The simulation result shows that the decay rate for centerline velocity obeys 1/z law as well as hydrogen volume fraction, indicating a turbulent jet during the steam dominated phase. There is also a relatively long potential core region which could impinge on the bottom concrete floor for the downwards jet. While the hydrogen release transfers from a turbulent jet to a turbulent plume outside the region near the inlet during the hydrogen dominated phase. Different from the turbulent jet, the centerline velocity at the plume region decays with the slope 1/z1/3, and the decay rate for the centerline hydrogen volume fraction is 1/z5/3 during this phase. Compared with the jet flow, the potential core region of the plume flow is relatively short, forming a hydrogen cloud near the inlet. The combustibility evaluation shows that the combustion clouds can be generated in the source compartment at the hydrogen dominated phase. However, they will be diluted by the following persistent steam injection from the break. This can provide technical support for the design of hydrogen mitigation system.

Superhydrophobic Pt–Pd/Al2O3 catalyst coating for hydrogen mitigation system of nuclear power plant

Passive auto-catalytic recombiner (PAR) system is an important hydrogen mitigation method which has been applied in most modern light water nuclear reactors. The two challenges for the highly efficient PAR are the detrimental effect of water and poisoning by fission products. In this study, to address the two challenges, superhydrophobic Pt–Pd/Al2O3 catalyst coatings were prepared by wet impregnation method and the grafting of 1H,1H,2H,2H-perfluorooctyltriethoxysilane.The formation of a Pt–Pd intermetallic compound was confirmed by in situ diffuse reflectance infrared Fourier transform infrared spectroscopy for the Pt–Pd/Al2O3 catalyst. The Pt–Pd/Al2O3 catalyst exhibited a superior resistance of water poisoning to the monometallic catalysts. In addition, compared with the monometallic catalysts, the least influence by the iodine poisoning was observed for the Pt–Pd/Al2O3 catalyst, which is attributed to the smallest influence on the bindings of H2 and O2 on the Pt–Pd intermetallic compound by the iodine addition. For the reactor with the superhydrophobic Pt–Pd/Al2O3 catalyst coating, under the conditions simulating the nuclear accident, the reaction was ignited immediately as soon as the hydrogen was introduced at 298 K and the hydrogen conversion kept 100% when the reaction temperature exceeded 398 K. The superhydrophobic Pt–Pd/Al2O3 catalyst coating showed great potential for the mitigation of hydrogen containing various poisons during the nuclear accident.

Investigation of a Hydrogen Mitigation System During Large Break Loss-Of-Coolant Accident for a Two-Loop Pressurized Water Reactor

Hydrogen release during severe accidents poses a serious threat to containment integrity. Mitigating procedures are necessary to prevent global or local explosions, especially in large steel shell containments. The management of hydrogen safety and prevention of over-pressurization could be implemented through a hydrogen reduction system and spray system. During the course of the hypothetical large break loss-of-coolant accident in a nuclear power plant, hydrogen is generated by a reaction between steam and the fuel-cladding inside the reactor pressure vessel and also core concrete interaction after ejection of melt into the cavity. The MELCOR 1.8.6 was used to assess core degradation and containment behavior during the large break loss-of-coolant accident without the actuation of the safety injection system except for accumulators in Beznau nuclear power plant. Also, hydrogen distribution in containment and performance of hydrogen reduction system were investigated.

Proposal and Analysis of Hydrogen Mitigation System Guiding Hydrogen in Containment Building

This study is about a hydrogen mitigation system in a containment building like an offshore or a nuclear plant. A hydrogen explosion is possibly happened after condensation of steam if hydrogen releases with steam in a containment buildings. Passive autocatalytic recombiner is the one of the measures, but the performance of this equipment is not sure because the distribution of hydrogen is very irregular and is not predicted correctly. This study proposes a new approach for improving the hydrogen removing performance with hydrogen-guiding property. The steam is simulated and analysed. The results show that the shallow air containment reduced over 55% of the released hydrogen and the deep air containment type reduces over 80% of released hydrogen.

Analysis of hydrogen risk mitigation with passive autocatalytic recombiner system in CPR1000 NPP during a hypothetical station blackout

Hydrogen safety has attracted extensive concern in severe accident analysis especially after the Fukushima accident. In this study, a similar station blackout as happened in Fukushima accident is simulated for CPR1000 nuclear power plant (NPP) model, with the computational fluid dynamic code GASFLOW. The hydrogen risk is analyzed with the assessment of efficiency of passive autocatalytic recombiner (PAR) system. The numerical results show that the CPR1000 containment may be damaged by global flame acceleration (FA) and local detonation caused by hydrogen combustion if no hydrogen mitigation system (HMS) is applied. A new condensation model is developed and validated in this study for the consideration of natural circulation flow pattern and presence of non-condensable gases. The new condensation model is more conservative in hydrogen risk evaluation than the current model in some compartments, giving earlier starting time of deflagration to detonation transition (DDT). The results also indicate that the PAR system installed in CPR1000 could prevent the occurrence of the FA and DDT. Therefore, HMS such as PAR system is suggested to be applied in NPPs to avoid the radioactive leak caused by containment failure.

Effect of spray on performance of the hydrogen mitigation system during LB-LOCA for CPR1000 NPP

During the course of the hypothetical large break loss-of-coolant accident (LB-LOCA) in a nuclear power plant (NPP), hydrogen is generated by a reaction between steam and the fuel-cladding inside the reactor pressure vessel (RPV). It is then ejected from the break into the containment along with a large amount of steam. Management of hydrogen safety and prevention of over-pressurization could be implemented through a hydrogen mitigation system (HMS) and spray system in CPR1000 NPP. The computational fluid dynamics (CFD) code GASFLOW is utilized in this study to analyze the spray effect on the performance of HMS during LB-LOCA. Results show that as a kind of HMS, deliberate igniter system (DIS) could initiate hydrogen combustion immediately after the flammability limit of the gas mixture has been reached. However, it will increase the temperature and pressure drastically. Operating the DIS under spray condition could result in hydrogen combustion being suppressed by suspended droplets inside the containment. Furthermore, the droplets could also mitigate local the temperature rise. Operation of a PAR system, another kind of HMS, consumes hydrogen steadily with a lower recombination rate which is not affected noticeably by the spray system. Numerical results indicate that the dual concept, namely the integrated application of DIS and PAR systems, is a constructive improvement for hydrogen safety under spray condition during LB-LOCA.

ICONE19-43410 ANALYSIS OF HYDROGEN MITIGATION IN A SEVERE ACCIDENT

Hydrogen deflagration or detonation in a nuclear power plant containment building may threaten the integrity of the containment or cause local damage in severe accidents. Dual igniter-recombiner concept has been applied in AP1000 and provided to other advanced reactors. With the lumped-parameter safety analysis computer code for the light water reactor severe accident analysis, a model based on the plant design characteristics has been built for analyzing hydrogen behavior and the effectiveness of hydrogen mitigation system in severe accidents. Considering the large-dry containment and passive safety characteristics, 2-inch Loss-of-Coolant-Accident (LOCA) is selected as the representative severe accident sequence according to the Probabilistic Risk Assessment (PRA) result. Three different cases are defined as Case 1: Only igniters operational, Case 2: Only PARs operational and Case 3: Igniters and PARs operational. The nature of hydrogen generation and distribution is analyzed and the effectiveness of hydrogen control system is evaluated under the three different scenarios. The risk of hydrogen combustion has been shown with the Shapiro diagram of hydrogen burn limits. The results show that AP1000 hydrogen mitigation system can provide adequate defense under the given severe accident to maintain the containment integrity, but there would be a certain amount of hydrogen deflagration or detonation risk in specific containment compartments.

Three-dimensional behaviors of the hydrogen and steam in the APR1400 containment during a hypothetical loss of feed water accident

During a hypothetical severe accident in a nuclear power plant (NPP), hydrogen is generated by an active reaction of the fuel-cladding and the steam in the reactor pressure vessel and released with the steam into the containment. In order to mitigate hydrogen hazards which could possibly occur in the NPP containment, a hydrogen mitigation system (HMS) is usually adopted. The design of the next generation NPP (APR1400) developed in Korea specifies that 26 passive autocatalytic recombiners and 10 igniters should be installed in the containment for a hydrogen mitigation. In this study, an analysis of the hydrogen and steam behavior during a total loss of feed water (LOFW) accident in the APR1400 containment has been conducted by using the computational fluid dynamics (CFD) code GASFLOW. During the accident, a huge amount of hot water, steam, and hydrogen is released into the in-containment refueling water storage tank (IRWST). The current design of the APR1400 includes flap-type openings at the IRWST vents which operate depending on the pressure difference between the inside and outside of the IRWST. It was found from this study that the flaps strongly affect the flow structure of the steam and hydrogen in the containment. The possibilities of a flame acceleration and a transition from deflagration to detonation (DDT) were evaluated by using the Sigma–Lambda criteria. Numerical results indicate that the DDT possibility was heavily reduced in the IRWST compartment by the effects of the flaps during the LOFW accident.

Hydrogen combustion experiments in a 1/4-scale model of a nuclear power plant containment

Hydrogen combustion experiments were carried out in a Froude-modeled 1/4-scale facility, reproducing the geometry of a nuclear power plant containment building, to evaluate the performance of a hydrogen mitigation system based on the controlled combustion concept. Hydrogen burning was initiated by a distributed ignition system using glow plug igniters. The objective of the study was to obtain data on the combustion phenomenology and on the thermal environment produced by the burning of hydrogen as might occur during a recoverable degraded core accident. The thermal environment was characterized through measurements of three quantities: gas temperatures, gas velocities, and radiant heat fluxes. Several highly-instrumented tests were performed in the test facility with the internal geometry adjusted so as to reproduce four possible plant configurations. Following a preassigned release history, hydrogen was introduced at the bottom of the test volume through spargers and vent holes located below the surface of a body of water modeling the suppression pool existing in the plants. Two hydrogen injection histories were used during these tests, simulating two different accident recovery scenarios. The dominant combustion mode observed during the tests involved diffusion flames anchored at the surface of the pool. Other types of combustion, involving lifted diffusion flames and localized burning, were observed at low-oxygen conditions and low hydrogen injection rates, respectively. Other than mild localized initial lightoff burns, deflagrations did not occur in the tests. For oxygen concentrations down to 8%, the distributed ignition system was successful at maintaining the background hydrogen concentration below 4.5–5% by volume.