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Hydrogen (H 2) accumulation in confined areas can pose a threat to the structure's integrity in case of an undesired ignition. Therefore, mitigation measures are critical in preventing such devastating accidents. Passive Autocatalytic Recombiners (PARs) are defined as safety devices developed for preventing H 2 accumulation in confined environments. During PARs operation, the ambient oxygen (O 2) concentration decreases due to the H 2 reaction on the catalyst surface, possibly reaching oxygen-limited conditions. In this work, a 2D transient numerical model was validated against experiments conducted at low O 2 dilution ratios. After validation, the model was also verified by comparing the results with numerical simulations from literature. It was found that oxygen-limited conditions occurred due to low O 2 concentrations at the catalyst surface. At these conditions, H 2 was not entirely recombined, decreasing the PAR efficiency. This indicates that in case of a H 2 release in a confined space, oxygen-limited conditions would cause H 2 accumulation due to limited PAR operation, increasing the risk of an undesired ignition. • Hydrogen recombination under low O 2 conditions can affect the PAR performance. • Oxygen-limited conditions occurred due to low O 2 concentrations at the catalyst surface. • High air flow resulted in a low O 2 residence time at the catalyst surface. • A single step mechanism produced reliable results at low O 2 concentrations. [ABSTRACT FROM AUTHOR]

Featured Application: Thermal hydraulics related to hydrogen safety in a nuclear reactor containment can be simulated by the 3D CFD code with validated steam condensation and hydrogen recombination models. In a pressurized water reactor (PWR) during a loss of coolant accident (LOCA) or a station blackout (SBO) accident, water and steam are released into the containment building. The water vapor mixes with the atmosphere, partially condensing into droplets or condensing on the containment walls. Although a significant amount of water vapor condenses, it coexists with hydrogen generated by the reactor core oxidation. As water vapor condenses, the volume fraction of hydrogen increases, raising the risk of explosion or flame acceleration. As such, water vapor's behavior directly affects hydrogen distribution. To conservatively evaluate hydrogen safety in a PWR during a severe accident, lumped-parameter codes have been heavily used. As a best-estimate approach for hydrogen safety analysis in a PWR containment, a turbulence-resolved CFD code called contain3D has been developed. This paper presents the validation results of the code and simulation results of hydrogen behavior affected by water vapor condensation and hydrogen removal by passive autocatalytic recombiners (PARs) in the APR1400 containment. The results provide insight into the three-dimensional behaviors of the hydrogen in the containment. [ABSTRACT FROM AUTHOR]

Passive auto-catalytic recombiners (PARs) are widely used to mitigate a hydrogen hazard. The first step to evaluate the hydrogen safety by PARs is to obtain qualified test data of the PARs for validation of their analytical model. SPARC PAR tests SP8 and SP9 were conducted to evaluate the hydrogen recombination characteristics of a honeycomb-shaped catalyst PAR. To obtain the hydrogen recombination rate from the PAR test data, two methods, Method-1 and Method-2, introduced by the THAI project, were applied. Since a large gradient of hydrogen concentration developed during hydrogen injection can cause a large error in the hydrogen mass obtained by integrating the measured hydrogen concentrations, a gate was installed at the PAR inlet to homogenize hydrogen in the test vessel before the PAR operation in the tests. A computational fluid dynamics (CFD) code with a PAR model was also applied to evaluate the characteristics of the PAR recombination according to the PAR inlet conditions, and the results were compared with those from Method-1 and Method-2. It was confirmed that the recombination rates from Method-1 require a correction factor to be compatible with results from Method-2 and the CFD simulation in the case of the SPARC-PAR tests.

In a pressurized water reactor (PWR) during a loss of coolant accident (LOCA) or a station blackout (SBO) accident, water and steam are released into the containment building. The water vapor mixes with the atmosphere, partially condensing into droplets or condensing on the containment walls. Although a significant amount of water vapor condenses, it coexists with hydrogen generated by the reactor core oxidation. As water vapor condenses, the volume fraction of hydrogen increases, raising the risk of explosion or flame acceleration. As such, water vapor’s behavior directly affects hydrogen distribution. To conservatively evaluate hydrogen safety in a PWR during a severe accident, lumped-parameter codes have been heavily used. As a best-estimate approach for hydrogen safety analysis in a PWR containment, a turbulence-resolved CFD code called contain3D has been developed. This paper presents the validation results of the code and simulation results of hydrogen behavior affected by water vapor condensation and hydrogen removal by passive autocatalytic recombiners (PARs) in the APR1400 containment. The results provide insight into the three-dimensional behaviors of the hydrogen in the containment.

Hydrogen is a flammable gas that can generate thermal and mechanical loads which could jeopardise the containment integrity upon combustion inside nuclear power plants containment. Hydrogen can be generated from various sources and disperses into the containment atmosphere, mixing with steam and air following a loss of coolant accident and its progression. Therefore, the volumetric hydrogen concentration should be examined within the containment to determine whether a flammable mixture is formed or not. Codes with 3D capabilities could serve this examination by providing detailed contours/maps of the hydrogen distribution inside containment in view of the local stratification phenomenon. In this study, a 3D VVER-1000 as-built containment model was sketched in AutoCAD and then processed into GOTHIC nuclear containment analysis code for hydrogen evaluation. The model was modified to a great extent by installing 80 passive autocatalytic recombiners and locating hydrogen sources to evaluate the performance of the hydrogen removal system inside the containment on maintaining the hydrogen concentration below the flammability limit during a large break loss of coolant accident. 2D profiles and 3D contours of volumetric hydrogen concentration with and without PARs are presented as the simulation outcome of this study. The results were validated against the results of the Final Safety Analysis Report, which also demonstrates the effectiveness of the hydrogen removal system as an engineered safety feature to keep the containment within a safe margin. Detailed 3D contours of hydrogen distribution inside containment can be employed to evaluate the local hot spots of hydrogen, rearranging and optimising the number and location of PARs to avoid the hydrogen explosion inside containment. [ABSTRACT FROM AUTHOR]

Hydrogen mitigation using Passive Autocatalytic Recombiners (PARs) has been widely accepted methodology inside reactor containment of accident struck Nuclear Power Plants. They reduce hydrogen concentration inside reactor containment by recombining it with oxygen from containment air on catalyst surfaces at ambient temperatures. Exothermic heat of reaction drives the product steam upwards, establishing natural convection around PAR, thus invoking homogenisation inside containment. CFD models resolving individual catalyst plate channels of PAR provide good insight about temperature and hydrogen recombination. But very thin catalyst plates compared to large dimensions of the enclosures involved result in intensive calculations. Hence, empirical correlations specific to PARs being modelled are often used in integral containment studies. In this work, an experimentally validated CFD model of PAR has been employed for developing an empirical correlation for Indian PAR. For this purpose, detailed parametric study involving different gas mixture variables at PAR inlet has been performed. For each case, respective values of gas mixture variables at recombiner outlet have been tabulated. The obtained data matrix has then been processed using regression analysis to obtain a set of correlations between inlet and outlet variables. The empirical correlation thus developed, can be easily plugged into commercially available CFD software.

This paper describes a numerical study of the influence of thermal diffusion (the Soret effect) on the operational behaviour of a passive autocatalytic recombiner (PAR). The study proposes a detailed three-dimensional computational fluid dynamics (CFD) model of hydrogen oxidation along a cylindrical-type PAR catalyst section (RVK-500, RET, Russia) inside a small-scale vertical channel. The CFD model was developed in STAR-CCM+ and uses multi-step chemical kinetics with a conjugated approach (surface and gas-phase included). The catalyst temperature and hydrogen conversion with and without the Soret effect in the model were determined numerically and compared against experimental measurements. The experiments reported here have previously been conducted on the cylindrical-type catalyst section, for 5–7 vol % inlet H 2 concentration. Numerical simulations demonstrate that local hydrogen concentration can be increased due to thermal diffusion at the lower side of the stainless-steel frame. Results identified that the catalyst temperature can be underpredicted by 10–20 °C without thermal diffusion included in the model. • Numerical simulations were performed to analyze the thermal diffusion (Soret effect) in a PAR. • CFD model includes a conjugated approach for chemical kinetics of catalytic hydrogen oxidation. • Increase of 0.4–0.6 vol % in local hydrogen concentration due to the thermal diffusion was identified numerically. • Neglected thermal diffusion in CFD model can cause underprediction in catalyst temperature by 10–20 °C. • Results are valuable for PAR validation and relevant safety research. [ABSTRACT FROM AUTHOR]

During an accident, hydrogen distribution in a containment building of a nuclear power plant (NPP) and characteristics of hydrogen depletion by passive autocatalytic recombiners (PARs) differ depending on the thermal-hydraulic behaviors occurring in the containment. A spray system installed in the NPP containment to control the pressure in accident conditions may interact with PAR operations. This study intended to experimentally evaluate the hydrogen removal characteristics of a grid-type PAR when a spray was operating. For the experimental simulation of hydrogen recombination characteristics of the PAR affected by a spray operation, we used the SPARC experimental facility, which was equipped with a pressure vessel capable of controlling the wall temperature with a volume of 82 m3. To measure gas species concentrations, 14 probes each for hydrogen, oxygen, and water vapor were installed. Two tests were designed depending on the spray initiation time. The SSP3 test was an experiment to simulate an accident in which the PAR operated as hydrogen was released after the spray is activated, and the SSP4 test was an experiment to simulate an accident in which the spray began after the operation of the PAR was initiated by the hydrogen release. In the experiment, two contradictory results were obtained, which were an increased start-up delay time of the PAR by early initiation of the spray and a negligible impact on the PAR’s performance by the delayed initiation of the spray.

• A multiphase CFD model for containment analyses was implemented in FLUENT. • A thermal-hydraulics theoretical zero-dimensional model is proposed for pseudo-verification. • FLUENT user-defined functions used to model condensation released. • Strategies to avoid numerical instability from condensation source terms. • Both hydrogen risk and overpressure risk sensitivity to wall condensation model assessed. Marine nuclear reactors, especially when powering submarines, take advantage of most of the benefits which nuclear technology can provide. Due to nuclear fuel high power density, small storage volume, and the long or even dismissed interval of refueling, marine reactors provide good applicability of nuclear power. However, the cladding of the reactor fuel elements employs materials that can generate large amounts of hydrogen posing combustion risks and threats to the containment integrity. Space is limited in a marine reactor containment, and a small amount of hydrogen released can become a potential combustion source. In this study, a computational simulation using the commercial multi-purpose code ANSYS Fluent was performed to provide local distributions of temperature, pressure, hydrogen, and steam concentrations in a reactor containment during an accident. These parameters were used to assess hydrogen combustion risk during an accidental scenario. The utilization of a multipurpose code presented several inconveniences as the necessity of an accurate discretization of the fluid–structure interface, the absences of in-built phase change models for condensation and of a library of hydrogen recombiners to model the functioning of the hydrogen mitigation system. All these capabilities are typically provided by specialized codes used in nuclear industry, and were overcome by the proposed simulation methodology through the implementation of external subroutines to model condensation, heat transfer and the hydrogen recombiners. For wall condensation, the subroutines were calculated based on empirical correlations and heat and mass transfer analogy for steam condensation in the presence of non-condensable gases. Special emphasis was given to steam phase change, as, during the progression of an accident, condensation plays an important role in limiting pressure increase and removing heat in the containment structures. Moreover, condensation also affects the hydrogen combustion risk as it reduces the fraction of steam in the containment atmosphere and allows the expansion of the flammable cloud. Hydrogen combustion risk and the possibility of slow deflagration, flame acceleration, and deflagration to detonation transition were assessed from the utilization of flammability limits, Sigma and Lambda criteria. A theoretical zero-dimensional model has been proposed for the comparison of the simulation results, due to the absence of experimental data. [ABSTRACT FROM AUTHOR]

In this paper, we simulated the change in hydrogen behavior according to a Passive Autocatalytic Recombiner (PAR) under oxygen depletion conditions and compared the results with the THAI project experimental results. In order to reduce calculation cost, we calculated the hydrogen-oxygen bonding reaction in the catalyst region through correlation equations of the hydrogen removal rate according to the PAR type, and we did not consider the shape of the catalyst with a relatively small size and the detailed hydrogen-oxygen bonding reaction. The hydrogen concentration at the PAR inlet and outlet, mixed gas temperature, flow rate at PAR inlet, pressure and total hydrogen removal rate were similar to the experimental results and analysis values in all cases and the reduction in the hydrogen removal rate agreed well under oxygen depletion conditions. However, after the hydrogen injection was stopped, errors of the hydrogen concentration and flow rate at the PAR inlet increased as the buoyancy decreased because the high temperature of the catalyst was not taken into account. • Hydrogen removal rate correlation was applied to 3D simulation of hydrogen behavior for reduction calculation cost. • Results of simulation (hydrogen distribution and hydrogen removal rate etc.) were similar to the experimental value. • The effect of buoyancy in the vessel decreased due to no consideration of the catalyst shape. [ABSTRACT FROM AUTHOR]

Hydrogen is a flammable gas that can generate thermal and mechanical loads which could jeopardise the containment integrity upon combustion inside nuclear power plants containment. Hydrogen can be generated from various sources and disperses into the containment atmosphere, mixing with steam and air following a loss of coolant accident and its progression. Therefore, the volumetric hydrogen concentration should be examined within the containment to determine whether a flammable mixture is formed or not. Codes with 3D capabilities could serve this examination by providing detailed contours/maps of the hydrogen distribution inside containment in view of the local stratification phenomenon. In this study, a 3D VVER-1000 as-built containment model was sketched in AutoCAD and then processed into GOTHIC nuclear containment analysis code for hydrogen evaluation. The model was modified to a great extent by installing 80 passive autocatalytic recombiners and locating hydrogen sources to evaluate the performance of the hydrogen removal system inside the containment on maintaining the hydrogen concentration below the flammability limit during a large break loss of coolant accident. 2D profiles and 3D contours of volumetric hydrogen concentration with and without PARs are presented as the simulation outcome of this study. The results were validated against the results of the Final Safety Analysis Report, which also demonstrates the effectiveness of the hydrogen removal system as an engineered safety feature to keep the containment within a safe margin. Detailed 3D contours of hydrogen distribution inside containment can be employed to evaluate the local hot spots of hydrogen, rearranging and optimising the number and location of PARs to avoid the hydrogen explosion inside containment.

A nanoparticle Pt/Ce–Zr–Y (Pt/CZY) mixed oxide/AAO washcoat catalyst was prepared and evaluated for passive autocatalytic hydrogen recombination (AAO, anodized aluminum oxide). Detailed characterization of the catalyst surface using scanning electron microscopy showed that Pt/CZY was deposited as aggregates, partly covering the AAO surface. The Pt/CZY/AAO catalyst was, therefore, tested for catalytic hydrogen combustion (1–10 vol % hydrogen in the air) at different gas flow velocities (4, 6, 8, 10, and 12 m/s) in a recombiner section testing station. The results showed that the combustion temperature and hydrogen conversion are significantly affected by the inlet conditions, such as hydrogen concentration and gas flow velocity. An accelerated durability test was performed by aging the catalyst in an oxidative atmosphere at 550, 800, and 1100 °C. During the catalytic test, the catalysts aged at 550 and 800 °C retained their catalytic activity, while the catalyst aged at 1100 °C showed no activity toward the hydrogen combustion reaction. X-ray photoelectron spectroscopy results revealed that the Pt concentration increased with increasing aging temperature while Pt oxides concentration decreased. Therefore, it was concluded that the deactivation of the catalyst aged at 1100 °C was rather caused by Pt sintering. To detect Pt particles and to confirm that Pt sintering occurred after the aging treatment at 1100 °C, transmission electron microscopy was carried out. The thermal distribution throughout the catalytic surface was investigated using an infrared camera. The Pt/CZY/AAO washcoat catalyst had high thermal conductivityat 10 vol % hydrogen, it exhibited a temperature gradient of 22.7 °C throughout the catalyst surface. [ABSTRACT FROM AUTHOR]

During an accident, hydrogen distribution in a containment building of a nuclear power plant (NPP) and characteristics of hydrogen depletion by passive autocatalytic recombiners (PARs) differ depending on the thermal-hydraulic behaviors occurring in the containment. A spray system installed in the NPP containment to control the pressure in accident conditions may interact with PAR operations. This study intended to experimentally evaluate the hydrogen removal characteristics of a grid-type PAR when a spray was operating. For the experimental simulation of hydrogen recombination characteristics of the PAR affected by a spray operation, we used the SPARC experimental facility, which was equipped with a pressure vessel capable of controlling the wall temperature with a volume of 82 m3. To measure gas species concentrations, 14 probes each for hydrogen, oxygen, and water vapor were installed. Two tests were designed depending on the spray initiation time. The SSP3 test was an experiment to simulate an accident in which the PAR operated as hydrogen was released after the spray is activated, and the SSP4 test was an experiment to simulate an accident in which the spray began after the operation of the PAR was initiated by the hydrogen release. In the experiment, two contradictory results were obtained, which were an increased start-up delay time of the PAR by early initiation of the spray and a negligible impact on the PAR's performance by the delayed initiation of the spray. [ABSTRACT FROM AUTHOR]

The use of a Pt-based catalyst was evaluated for autocatalytic hydrogen recombination. The Pt was supported on a mixture of Ce-, Zr- and Y-oxides (CZY) to yield nanosized Pt particles. The Pt/CZY/AAO catalyst was then prepared by the spray-deposition of the Pt/CZY intermediate onto an anodized aluminium oxide (AAO) layer on a metallic aluminum core. The Pt/CZY/AAO catalyst (3 × 1 cm) was evaluated for hydrogen combustion (1–8 vol% hydrogen in the air) in a recombiner section testing station. The thermal distribution throughout the catalyst surface was investigated using an infrared camera. The maximum temperature gradient (ΔT) for the examined hydrogen concentrations did not exceed 36 °C. The Pt/CZY/AAO catalyst was also evaluated for prolonged hydrogen combustion duration to assess its durability. An average combustion temperature of 239.0 ± 10.0 °C was maintained for 53 days of catalytic hydrogen combustion, suggesting that there was limited, or no, catalyst deactivation. Finally, a Pt/CZY/AAO catalytic plate (14.0 × 4.5 cm) was prepared to investigate the thermal distribution. An average surface temperature of 212.5 °C and a maximum ΔT of 5.4 °C was obtained throughout the catalyst surface at a 3 vol% hydrogen concentration. [Display omitted] • Reactive Pt/CZY/AAO catalysts were prepared for catalytic hydrogen combustion. • Catalysts contained an aluminum core to rapidly dissipate reaction heat. • Thermal conductivity ensured temperature gradients of <36 °C at 1–8 vol% hydrogen. • No decrease in catalytic activity after 53 days of hydrogen combustion at 239 °C. • Catalysts suitable for safe PAR applications. [ABSTRACT FROM AUTHOR]

To assess the risk of a hydrogen recombination catalyst, its ignition characteristics were experimentally investigated. A layer of catalysts was installed perpendicular to a mixture flow of H2 and air with controlled flow rates. The temperature on the catalyst and that of the mixture upstream of the catalyst were measured using thermocouples. The density variation of the mixture caused by combustion was observed using shadowgraph visualization. When both the flow velocity of the mixture and the fraction of H2 were larger than the ignition thresholds, deflagration started and the temperature of the mixture upstream of the catalyst rapidly increased owing to the propagation of the flame front. The catalyst had the potential to ignite the mixture when the temperature of the catalyst, which increased with the amount of H2 reacting on the catalyst, was sufficiently high. Therefore, it is necessary to maintain the flow velocity of the mixture and the fraction of H2 below the thresholds while monitoring the surface temperature of the catalyst. [ABSTRACT FROM AUTHOR]

• Experimental and modelling results of catalyst performance are analysed and compared. • Temperature and efficiency of cylindrical-type PAR catalyst are evaluated. • Start-up characteristics of a single catalyst section are investigated. • Schmidt number and kinetic theory effect on temperature predictions is compared. • A functional dependence to estimate the efficiency of entire PAR is proposed. Passive autocatalytic recombiners (PARs) are essential safety systems used in nuclear power plants (NPPs) to prevent hydrogen explosions during severe accidents. This study investigates the operational behaviour of cylindrical-type catalysts used in a PAR. The study employs experimental and computational fluid dynamics (CFD) analyses to evaluate the catalyst temperature distribution and hydrogen conversion inside the PAR channel. Experimental analysis measures the hydrogen conversion of the catalyst and temperature distribution by means of hydrogen sensors and an infrared camera. The CFD analysis uses a three-dimensional (3D) model developed in STAR-CCM+ code to simulate the flow and recombination reaction in a cylindrical-type PAR catalyst section. Results indicated that the catalyst has decent conversion efficiency. Furthermore, the temperature over the catalyst section is evenly distributed and it does not exceed the lower hydrogen ignition limit. CFD analysis of the Schmidt number demonstrates that the flow inside the PAR is highly turbulent and Sc t values is in the range of 0.2–0.28. It was found that the recombination reaction occurs in the diffusion regime. The recombination reaction primarily occurs in the catalyst's lower and central parts. Finally, this paper proposes the functional dependence to estimate the efficiency of the entire PAR based on the Sherwood equation and mechanistic transport approach. The results of this study provide crucial insight into the cylindrical-type catalyst operational behaviour in PARs and the CFD model design of cylindrical-type catalysts for the safety analysis of NPPs. [ABSTRACT FROM AUTHOR]

Abstract Passive autocatalytic recombiner (PAR) applied in the nuclear plant containments is to use catalytic H 2 O 2 recombination to lower the explosion risk associated with hydrogen release. Deactivation by water vapor must be taken into account and overcome for the recombination catalyst. For supported metal catalyst, the nature of support plays an important role in such water-poisoning effect. In this work, the pure silica MFI zeolite, silicalite-1, was used as the support for Pt to enhance the humidity tolerance (Pt/sil). Steaming of silicalite-1 is an effective way to further enhance its hydrophobicity (sil-S) due to the healing of silanol defects. A serial of supports with different water adsorption ability (Al 2 O 3 > SiO 2 > SiO 2 -500 » sil > sil-S) were compared and their hydrophobicity is correlated with their stability in H 2 O 2 recombination. Over hydrophilic Pt/Al 2 O 3 and Pt/SiO 2 , the activity declines continuously especially under wet conditions. Hydrophobic Pt/sil and Pt/sil-S are both stable in wet conditions. Because Pt nanoparticles are located on the external surface of zeolite crystals and silanol defects are mainly in the inner part of zeolite crystals, Pt/sil and Pt/sil-S exhibit minor difference in resistance to water vapor in spite of their different hydrophobicity. Graphical abstract Image 1 Highlights • Pure silica zeolite, silicalite-1, was used as the support for Pt to enhance humidity tolerance. • A serial of support (Al 2 O 3 > SiO 2 > SiO 2 -500 » sil > sil-S) were compared and their hydrophobicity is correlated with their stability in H 2 O 2 recombination. • Hydrophilic Pt/Al 2 O 3 and Pt/SiO 2 deactivate but hydrophobic Pt/sil and Pt/sil-S are stable in wet conditions. • Steaming of silicalite-1 is an effective way to further enhance its hydrophobicity due to the healing of silanol defects. [ABSTRACT FROM AUTHOR]

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.

During the past 10 years, the Korea Atomic Energy Research Institute (KAERI) has performed a study to control hydrogen gas in the containment of the nuclear power plants. Before the Fukushima accident, analytical activities for gas distribution analysis in experiments and plants were primarily conducted using a multidimensional code: the GASFLOW. After the Fukushima accident, the COM3D code, which can simulate a multidimensional hydrogen explosion, was introduced in 2013 to complete the multidimensional hydrogen analysis system. The code validation efforts of the multidimensional codes of the GASFLOW and the COM3D have continued to increase confidence in the use of codes using several international experimental data. The OpenFOAM has been preliminarily evaluated for APR1400 containment, based on experience from coded validation and the analysis of hydrogen distribution and explosion using the multidimensional codes, the GASFLOW and the COM3D. Hydrogen safety in nuclear power has become a much more important issue after the Fukushima event in which hydrogen explosions occurred. The KAERI is preparing a large-scale test that can be used to validate the performance of domestic passive autocatalytic recombiners (PARs) and can provide data for the validation of the severe accident code being developed in Korea.

• Passive Autocatalytic recombiner was investigated in the THAI containment test facility within the OECD/NEA THAI-2 Joint Project. • Investigations focused on late phase severe accidents including oxygen starvation. • Hdrogen mitigation by PAR was demonstrated under realistic boundary conditions. • An international code benchmark was performed with code applications using CFD and Lumped Parameter models with detailed chemistry and correlations for the prediction of the passive autocatalytic recombiners. In many reactor containments Passive Autocatalytic Recombiners (PAR) are installed to mitigate the hydrogen threat in case of a severe nuclear reactor accident. PARs oxidize hydrogen catalytically into water vapor. The heat of catalytic reaction drives flow through the PAR unit by natural convection, which continuously brings "fresh" hydrogen-air mixtures to the PAR unit, thereby developing a self-sustained process until either hydrogen or oxygen falls below a minimum concentration. The test series (HR-33 to HR-35) of the OECD/NEA THAI-2 program investigated the influence of temperature, pressure and steam on the onset of hydrogen recombination in case of low or extremely low oxygen concentrations. These conditions with oxygen concentrations below 1 vol–% can occur in accident scenarios (e.g. late accident phase with molten core concrete interaction). A better understanding of the PAR performance in oxygen-limited conditions is important for hydrogen management in severe accidents. The test data of HR-35 was used for a blind benchmark exercise within the OECD/NEA joint project. Lumped parameter codes (MELCOR and COCOSYS) have been used as well as 3D/CFD codes (GASFLOW, GOTHIC and CFX). A variety of PAR models were used, including models based on engineering correlations, models with a detailed nodalization of the PAR unit, and a model with detailed heat and mass transfer process (REKO-DIREKT). Consequently, the benchmark provided a good opportunity to assess the PAR models that are deployed in the state of the art safety analysis tools, especially under conditions representative for the late phase of a severe accident. [ABSTRACT FROM AUTHOR]

• Code validation against the THAI hydrogen recombination experiment shows good agreement. • PARs implementation methodology is improved according to hydrogen risk criteria and combustion. • Hydrogen 3D distribution and hazard mitigation by PARs in SMR containment are investigated. Hazard of hydrogen fast deflagration or explosion could highly lead to an over-pressurization of nuclear reactor containment in case of severe accidents. In order to mitigate the hydrogen hazard, Passive Autocatalytic Recombiners (PARs) are wildly used in water-cooled reactor containment via the hydrogen recombination with the oxygen. The Computational Fluid Dynamics (CFD) code GASFLOW-MPI, which provides the hydrogen concentration details for calculating the recombination rate, is appropriate to analyze PARs performance aiming at the prevention or mitigation of hydrogen risk. The validation of the PARs model implemented in GASFLOW-MPI against THAI-HR experiment is comprehensively carried out. The comparisons indicate good agreements between the predictions and the experimental data, which increase the confidence of using GASFLOW-MPI in a full-scale rector containment. Thereafter, the validated code is used to analyze the severe accident in a water-cooled Small Modular Reactor (SMR), specifically with regard to the hydrogen distribution, the risk posed by hydrogen, the mitigation by PARs and the H 2 combustion under accidental ignitions. The analysis methodology is improved according to the GASFLOW-MPI features, and the simulation results indicate that: 1. Hydrogen distribution is fairly uniform that reduces the limit for the PARs installation since the hydrogen recombination efficiencies of the installed PARs are similar, 2. PARs (8 units) mitigate the hydrogen risk well in the long-term phase, but a supplementary combustion analysis is needed, since the potential of flame acceleration exists in the intermediate short time period. [ABSTRACT FROM AUTHOR]

This work presents the innovator design of a system for the reduction of non-condensable gases in the containment during hypothetical design base accidents in an advanced ESBWR. The system is composed of a steam injector and a passive autocatalytic recombiner, which is called, in this work, SIPAR. The driven flow of the steam injector is steam flow at low pressure of the passive autocatalytic recombiner. In the throat of the steam injector, the hydrodynamic phenomena occur in two-phase flows due to condensation steam when the water is suctioned by the steam injector. The steam injector can be used to pump cold water and produce an outlet water pressure, which is higher than the steam inlet pressure. The steam injector and passive autocatalytic recombiner were modeled with TRAC-BF1 to study the performance of the steam injector at low pressure. Preliminary results show that the SIPAR can be installed into reactor containment for the reduction of non-condensable gases during hypothetical design base accidents in an advanced ESBWR. [ABSTRACT FROM PUBLISHER]

A mathematical model of heat and mass transfer in a passive autocatalytic recombiner (PAR) is developed. Three-dimensional calculations of convection of a hydrogen-containing medium in a hydrogen recombiner of the RVK-315 model series are performed using the ANSYS Fluent commercial code. Due to the periodic structure of the catalytic block, calculations were performed for one rod arranged in its middle. The results of calculations are correlated with the experimental data acquired using the OAO VTI stand. The mismatch between the calculated and experimental data does not exceed 30% in the range of bulk hydrogen concentrations of 1.5–6.5%. [ABSTRACT FROM AUTHOR]

The structure of the catalyst used in a passive autocatalytic recombiner (PA R) is crucial for making the PAR reliably functional in environments of high humidity and for concentrations of hydrogen above 8 to 10 vol %. The temperature of the catalyst has to be kept below 500°C to avoid the autoignition of hydrogen. A new type of catalyst for the PAR, a hydrophobic catalyst on a low porous metal carrier with a screen [HCm(screen)], was designed by Russian Energy Technologies. It consists of a porous Ti plate with the adsorbtion metal Pt. The surface of the catalyst was completely covered by a metal grid. In a series of tests with different small-scale PARs, the HCm(screen) catalyst was found to function under concentrations of hydrogen up to at least 20 vol %. The effects of mass and heat transfer processes (Fick diffusion, Knudsen diffusion, anti Stefan flow) on the thermal regime and characteristics of the working catalyst are discussed. Metal grids of dense weaving appear to be the most suitable for a screen because they have a double function: removing the heat and acting as a gas separation membrane enriching with hydrogen the gas mix in the zone of the catalytic reaction. [ABSTRACT FROM AUTHOR]

A detailed model is proposed for numerical simulation of hydrogen ignition inside box-type passive autocatalytic recombiners (PARs). The model is .focused on the reactive channel flow between two catalytic sheets of a recombiner. It includes complex chemistry and multicomponent transport for homogeneous hydrogen combustion and complex surface chemistry for heterogeneous hydrogen recombination. First calculations are dedicated to H2/air mixtures without steam at atmospheric pressure and room temperature. The analysis of the total homogeneous and heterogeneous heat release rates according to the inlet hydrogen molar fraction reveals three possible operation regimes for the recombiners from pure catalytic conversion to pure gaseous combustion. A physical criterion is then proposed for the ignition of H2/air mixtures inside the recombiners. The numerical ignition threshold at 5.4% of hydrogen without steam is in good agreement with experimental data. The criterion is then applied to the ternary diagram including all representative H2/air/H20 mixtures for severe accident conditions in pressurized water reactors. It shows a sharper transition from the catalytic regime to the gaseous one for high hydrogen concentrations. A specific strategy finally allows defining an extended PAR hydrogen ignition limit in the entire ternary diagram, which is well corroborated by the available experimental database. [ABSTRACT FROM AUTHOR]

Abstract: Passive autocatalytic recombiners (PAR) are widely being used as hydrogen control device in the current and advanced light water reactors (ALWRs). The PARs lend themselves to very effective means of circumventing buildup of combustible or detonable hydrogen gas mixtures in the reactor containment. Korea Nuclear Technology Inc. has recently developed a new PAR system with high porous catalyst material in the shape of honeycomb. The honeycomb PAR catalyst has a design characteristic of improved hydrogen removal performance by increasing the surface area and enhancing the flow rate through the catalyst at the same time, without increasing PAR size compared to the conventional PARs. The experimental study was focused on the development of the hydrogen depletion rate correlation of the honeycomb PAR. Two different sizes of PARs, KPAR-40 and KPAR-T2, have been employed in the tailor-made Integral Test Facility and Performance Test Facility. Multiple tests were conducted in various conditions of pressure, temperature, and hydrogen concentration. The hydrogen depletion rate correlation and the PAR performance constant were determined from the experimental results, which can be applied to the honeycomb PAR system. Also determined was the scale effect due to the PAR size, i.e., the number of catalysts in a PAR. [Copyright &y& Elsevier]

• A 250 L spherical test facility was commissioned to study PAR behaviour. • Scaled H 2 recombination rate results are compared with large-scale H 2 facility data. • Tests were conducted at room conditions, elevated temperatures and pressures. • With a scaling factor, HSTF H 2 recombination rate fit the large-scale data well. After many years of passive autocatalytic recombiner (PAR) experimentation and analytical investigations, there are still gaps in the understanding of PAR behavior. For instance, PAR performance under severe accident conditions including issues related to low oxygen content, presence of carbon monoxide, and reactive or non-reactive aerosols still require examination. Following the closure of the large-scale hydrogen test facilities at AECL's Whiteshell Laboratories (WL), a new Hydrogen Safety Test Facility (HSTF) has been commissioned at Chalk River Laboratories (CRL) to continue the efforts to understand PARs and their operation in various accident scenarios. The HSTF is a spherical pressure vessel with an internal volume of 0.25 m3. This paper presents the experiments performed at the HSTF, investigating the scaling of a PAR under varying concentrations of H 2 , and steam as well as the effects of initial gas temperature and pressure. The experimental data from the HSTF (0.25 m3) will be compared with the data obtained in the large-scale facilities (6.6 m3 and 60 m3) to establish a scaling factor for the HSTF with a reasonable fit over the range of conditions tested. [ABSTRACT FROM AUTHOR]

In this study, a Pt/anodized aluminum oxide (AAO) catalyst was prepared by the anodization of an Al alloy (Al6082, 97.5% Al), followed by the incorporation of Pt via an incipient wet impregnation method. Then, the Pt/AAO catalyst was evaluated for autocatalytic hydrogen recombination. The Pt/AAO catalyst's morphological characteristics were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average Pt particle size was determined to be 3.0 ± 0.6 nm. This Pt/AAO catalyst was tested for the combustion of lean hydrogen (0.5–4 vol% H2 in the air) in a recombiner section testing station. The thermal distribution throughout the catalytic surface was investigated at 3 vol% hydrogen (H2) using an infrared camera. The Al/AAO system had a high thermal conductivity, which prevents the formation of hotspots (areas where localized surface temperature is higher than an average temperature across the entire catalyst surface). In turn, the Pt stability was enhanced during catalytic hydrogen combustion (CHC). A temperature gradient over 70 mm of the Pt/AAO catalyst was 23 °C and 42 °C for catalysts with uniform and nonuniform (worst-case scenario) Pt distributions. The commercial computational fluid dynamics (CFD) code STAR-CCM+ was used to compare the experimentally observed and numerically simulated thermal distribution of the Pt/AAO catalyst. The effect of the initial H2 volume fraction on the combustion temperature and conversion of H2 was investigated. The activation energy for CHC on the Pt/AAO catalyst was 19.2 kJ/mol. Prolonged CHC was performed to assess the durability (reactive metal stability and catalytic activity) of the Pt/AAO catalyst. A stable combustion temperature of 162.8 ± 8.0 °C was maintained over 530 h of CHC. To confirm that Pt aggregation was avoided, the Pt particle size and distribution were determined by TEM before and after prolonged CHC. [ABSTRACT FROM AUTHOR]

• Large-scale experiments on PAR performance in the presence of CO. • Effect of gas temperature and CO concentration on PAR activity. • PAR recombination efficiency and PAR induced ignition with H 2 and CO mixtures. Passive autocatalytic recombiners (PARs) have been implemented in water-cooled nuclear reactors to prevent hydrogen explosion during severe accidents. Carbon monoxide (CO) can also be produced in severe accidents from ex-vessel molten core–concrete interaction. Assessment of scenarios involving ex-vessel interactions requires additional attention to the contribution of CO to combustion loads in containments. The risk of CO combustion can be mitigated by catalytic oxidation of CO in parallel with hydrogen recombination; however, the PAR performance may be affected by catalyst poisoning by CO. Tests were performed to examine the PAR self-start behaviour, recombination rate and efficiency, and ignition induced by hot catalyst plates in the presence of hydrogen and CO mixtures. The experiments were conducted in a 120 m3 test facility with a full-sized PAR and in a lab-scale apparatus with catalyst coupons. These measurements demonstrate that the CO poisoning effect strongly depends on the temperature and gas compositions. CO appears to have no adverse effect on PAR recombination rate during operation. This paper presents select experimental results with a focus on the effect of CO on PAR self-start behaviour. These results will help to refine severe accident management guidelines. [ABSTRACT FROM AUTHOR]

• Passive autocatalytic recombiner tests performed in technical scale containment facility. • Evaluation of influence of carbon monoxide on PAR performance. • PAR exposed to oxygen starvation impacts hydrogen but not carbon monoxide conversion. • PAR ignition limit is shifted to lower hydrogen concentration if carbon monoxide is present. • Catalytic recombination of carbon monoxide in absence of hydrogen under late phase temperature. Passive autocatalytic recombiners (PARs) have become one of the primary measures to mitigate the hydrogen risk for severe accidents in current and advanced water-cooled nuclear power plants (NPPs). The pgm (platinum group metals) coating of the catalyst inside the PAR is capable to oxidize hydrogen into water vapor as well as to convert carbon monoxide into carbon dioxide. The optimal efficiency of conversion requires a significant surplus of oxygen compared to the stoichiometric oxidization. A test series under containment typical boundary conditions was performed in the THAI facility (Thermal-hydraulics, Hydrogen, Aerosols and Iodine) to investigate the PAR performance under the presence of carbon monoxide. Ratios of injection mass flow rates of hydrogen and carbon monoxide into the test facility are investigated between 4:1 and 7:1, based on hypothetical release of CO by the molten core concrete interaction (MCCI). The findings are related to database on PAR performance carried out in previous OECD/NEA projects, THAI (2007–2009), THAI-2 (2011–2014), and THAI-3 (2016–2019). Special emphasis is placed on the investigation of the PAR under conditions of oxygen starvation, taking also into account a potential PAR poisoning by carbon monoxide, and under potential PAR induced ignitions at elevated hydrogen and carbon monoxide concentrations. [ABSTRACT FROM AUTHOR]

• A multi-step model is used to study surface kinetics of H 2 in presence of Pt. • Efficient Correlations for hydrogen recombination have been generated. • Importance of radiation heat transfer in this study is highlighted. • Model have been validated with the data obtained in REKO-3 test facility. • Parametric study has been done for general recombiner geometry and flow conditions. In this study, an accurate and efficient method is proposed to simulate hydrogen mitigation using catalytic recombiner in a nuclear reactor containment. Using a point model and a detailed set of reaction kinetics equations, the variation of hydrogen reaction rates with surface temperature for various hydrogen and water vapor concentration are found. The entire reactant domain is divided into two zones, Zone 1 and Zone 2. In Zone 1, when the mixture is heated, there is a sudden onset of surface reactions at a threshold temperature (catalytic auto ignition temperature) beyond which the reaction rate falls. In Zone 2, the reaction rate gradually builds up beyond a threshold temperature, reaching maxima (maximum reaction temperature) and then falls. Correlations for catalytic ignition/maximum reaction rate temperature and reaction rates have been developed for both the zones. Using the developed correlations, a recombiner CFD model is evolved to simulate the REKO-3 experiments performed in REKO test facility to validate the recombiner model. The present investigation reveal the importance of radiation heat transfer in correctly predicting the catalyst plate temperature. From numerical computations, the best suited radiation model is identified. A parametric study is then carried out to predict the highest catalyst plate temperature under diverse operating conditions. From the obtained results, an engineering correlation is developed which successfully predicts the highest plate temperature for the entire range of REKO-3 data. [ABSTRACT FROM AUTHOR]