Passive Autocatalytic Recombiners Research Articles

MELCOR simulation of the SBLOCA induced severe accident for the SMR in a floating nuclear power plant

A floating nuclear power plant (FNPP) is a kind of nuclear power plant on a barge moored specifically in an area of the sea. This study focuses on a severe accident induced by cold leg small break loss of coolant accident (SBLOCA) occurring in a small modular reactor (SMR) in a FNPP of 100 MWt 2-loop pressurized water reactor (PWR) type. The severe accident was modeled using MELCOR computer code. A MELCOR model was developed for the primary and secondary systems, the engineered safety features, and the containment of the SMR. Three severe accident mitigation measures were analyzed using the model, namely the pressurizer (PZR) relief extension, the external vessel reactor cooling (EVRC), and the passive autocatalytic recombiners (PARs). The progression of the severe accident and the effects of three mitigation measures against the severe accident were analyzed. The simulation results indicated that the reactor core was exposed and heat up due to the coolant leakage and finally melted down due to high-pressure and lower-pressure injection failure. However, with the mitigation measures against the severe accident, the reactor pressure vessel (RPV) and containment integrity were maintained, thus preventing the large release of radioactive products into other cabins and the environment. Comparative analysis results also showed that all the three measures performed their scheduled functions and were significant in mitigating the accident. The study is useful in gaining insights into the processes of the severe accident induced by cold leg SBLOCA and can be used to identify appropriate mitigation strategies and countermeasures to deal with the severe accident in the FNPP.

On the nuclear safety improvement by post-inerting small modular reactor with stainless steel cladding

After Fukushima Daiichi accident, the replacement of zirconium-based fuel cladding in Light Water Reactors (LWR) became one of the main challenges of the nuclear industry. Austenitic steel–clad presents some safety advantages comparing to zirconium alloys, noticeably, higher activation energy and lower enthalpy of metal-water reaction. Thus, it produces a slower hydrogen release into the containment following a postulated accident. In this study, a Loss-of-Coolant Accident (LOCA) aggravated by the complete failure of the Emergency Core Cooling System (ECCS) is analyzed for a Small Modular Reactor (SMR). Post-accident injection of inert gas into the containment is used as one of the hydrogen control systems, to enhance safety margins during Severe Accidents (SA). The inertization system is successful in complementing Passive Autocatalytic Recombiners (PAR) to perform combustible gas control.

Temperature Profile Mapping over a Catalytic Unit of a Hydrogen Passive Autocatalytic Recombiner: An Experimental and Computational Fluid Dynamics Study

Passive autocatalytic recombiners (PARs) are used for the removal of accidentally released hydrogen inside confined spaces. The high catalyst surface temperature is an important safety issue to be ...

Experimental study of the effect of carbon monoxide on the performance of passive autocatalytic recombiners

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.

In Situ Gas Monitoring by Fiber-Coupled Raman Spectrometry for H₂-Risk Management in Nuclear Containment During a Severe Nuclear Accident

During a severe nuclear accident leading up to core melting, molten corium–concrete interaction (MCCI) and core-coolant water reaction both release large amounts of hydrogen (H2) gas in the containment atmosphere. According to the Shapiro–Moffette ternary diagram and depending on local partial pressures of H2, air, and water vapor, deflagration/detonation may occur with potential deleterious impact over equipment and structures. CO and CO2 are also of interest, as revealing gases for MCCI. Pressurized Water Reactors (PWRs) of French Nuclear Power Plants (NPPs) are equipped with passive autocatalytic recombiners (PARs), partly mitigating the H2 risk. However, the H2-risk management strategy may be significantly improved by performing in situ monitoring of H2, O2, N2, H2O, CO, and CO2 partial pressures at several locations inside the containment, to account for potential local combustion risk. Raman spectrometry (RS) involves only one laser and spectrometer equipped with a 2-D charge-coupled device (CDD). Raman probes are chemically selective and may be radiation-hardened. Custom-made fiber-coupled Raman probes, linked with a readout unit, were qualified in a climatic chamber, a flame-propagation tube, a 60Co irradiation cell, a 3-D shaking table, a steam jet and the MISTRA facility (1/10 reduced-scale containment mock-up dedicated to thermo-hydraulic tests).

Investigation of the iodine species on platinum catalyst used as hydrogen oxidation

Hydrogen can be oxidized by recombination with atmospheric oxygen over a catalyst, such as Pt/Al2O3, at a temperature much lower than that required for thermal oxidation generating heat and water vapor. However, there is potential for the passive autocatalytic recombiner to generate volatile forms of iodine, namely molecular iodine, by catalytic dissociation of various iodine compounds too. The dissociated atomic iodine on the surfaces of the Pt/Al2O3 catalyst is mainly measured at ambient temperature as I 3d peaks in X-ray photoelectron spectroscopy. It seems that dissociation of iodine compounds on the catalyst surfaces and then production of gaseous I2 by recombination of the dissociated iodine occur prior to the oxidation or reduction of iodine compounds under air or H2. Atomic iodine is predominant on the surfaces of the catalyst in H2 and air conditions, and it is stable even with exposure to H2O. Adsorbed iodine prevents the adsorption of H2 or O2 for recombination to H2O, but it can be desorbed completely at a high temperature.

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.

Measurements of the impact of carbon monoxide on the performance of passive autocatalytic recombiners at containment-typical conditions in the THAI facility

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.

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.

전산유체역학 기반 피동형 자동촉매 수소재결합기(PAR)에 의한 경수로 격납건물 내 수소 거동 연구

As the Fukushima Daiichi nuclear disaster revealed, the presence of a hydrogen cloud in the containment can challenge its integrity if uncontrolled combustion occurs. To mitigate the hydrogen risks, a variety of measures have been proposed, for example passive autocatalytic recombiners (PARs). As the name implies, PARs passively combine hydrogen and oxygen on the surface of a catalyst, generating steam and heat. The heat released during the recombination process results in a buoyancy force that drives the flow. In this paper, the interaction of the gas cloud with a buoyant plum issuing from a PAR unit is investigated using the multi-phase (two-fluid, three-field) computational fluid dynamics code, CUPID. Similar to the OECD/SETH-2 PANDA ST5 test series, the PAR unit is modeled as a heater and simulated using the porous media approach in CUPID. The objective of this report is to investigate the capability of CUPID code to simulate the break-up of the initially stratified layer in response to the activation of a heat source.

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.

Humidity-tolerant H2O2 recombination platinum catalyst for mitigating hydrogen using silicalite-1 as support

Abstract Passive autocatalytic recombiner (PAR) applied in the nuclear plant containments is to use catalytic H2 O2 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 (Al2O3 > SiO2 > SiO2-500 » sil > sil-S) were compared and their hydrophobicity is correlated with their stability in H2 O2 recombination. Over hydrophilic Pt/Al2O3 and Pt/SiO2, 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.

APPLICATION OF PASSIVE AUTOCATALYTIC RECOMBINER IN NUCLEAR POWER PLANT UNDER SEVERE ACCIDENTS

Passive Autocatalytic Recombiner (PAR) is a key safety equipment to mitigate the hydrogen risk in containment of Nuclear Power Plant (NPP). As we all know, PARs have been used to deplete hydrogen in Designed Basis Accidents due to the stable hydrogen depletion performance and flexible installation without power supply and manual operation. However, the application of PAR under Severe Accidents (SAs) have been got more and more attention. In some NPPs, PARs have been already required to be able to work in SAs. Therefore, the hydrogen depletion performance of PAR under SAs is very important for the design of hydrogen depletion system. In this work, first of all, the development of PAR research is overviewed briefly. Then the experiments, simulations or calculations under different conditions of SAs are described in detail, including the illustration of testing device or CFD model. The hydrogen depletion catalyst is also described from the perspective of catalyst preparation and characterization experiments. Finally, the future experiments or simulations which should be paid attention to are recommended.

Math hydrogen catalytic recombiner: Engineering model for dynamic full-scale calculations

To protect a hermetic enclosure and the equipment and systems of a reactor installation housed in it from damage caused by the ignition (explosion) of hydrogen, most nuclear power plants with pressurized water reactors are provided with a hydrogen concentration monitoring system and an emergency hydrogen removal system. These systems prevent the formation of explosive mixtures in the accident localization zone by maintaining the concentration volume of hydrogen in the mixture below the safety limits, which ensures the preservation of the density and strength of the hermetic enclosure and the operability of other localizing safety systems. A key component of an emergency hydrogen removal system is a passive autocatalytic hydrogen recombiner that operates on the principle of the catalytic recombination of hydrogen and oxygen.There is an urgent need for a full-scale dynamic calculation of the development of emergency conditions in a nuclear power plant containment accompanied by a large release of hydrogen. In efforts to achieve this, we constructed, and justified, a simple engineering thermohydraulic model of hydrogen removal in the operation of a passive autocatalytic recombiner based on the available experimental data.This paper presents the application results of the model as a part of contour industry codes RELAP, TRACE and CORSAR, intended, among other things, for carrying out multifactor and full-scale calculations of the dynamics of emergency processes with the release of hydrogen into nuclear power plant premises. This model allows us to substantiate the dynamics of local concentrations of gas components of a mixture in a confined space; the temperature of the mixture, the catalyst and the walls of the box; and the pressure when hydrogen or steam is supplied to the box.We have analysed various rates of hydrogen supply to a closed box to numerically substantiate the time at which the concentration reaches the maximum level. Furthermore, we have calculated the performance for several entrance concentrations of hydrogen, and obtained a satisfactory agreement between the dynamics of the concentrations, the temperatures of the catalyst and gas, and the productivity of the passive autocatalytic hydrogen recombiner. These calculations are based on the results of comparisons between calculated and available experimental data.

Integrity of Alumina Catalytic Support Prepared by Anodization in a High Temperature Steam Environment

As a catalytic support for passive autocatalytic recombiner in nuclear power plants, anodic alumina layer was formed on stainless steel 304 by applying either Al or NiAl diffusion coatings on 304SS, followed by anodization and calcination. The anodic alumina had porous and nano-fibrous morphology and showed comparable hydrogen conversion ratio as the conventional wash-coated catalytic support. Integrity of catalytic supports was evaluated by steam oxidation test at 900 °C for 100 h. The wash-coated catalytic support exhibited oxide breakaway and Fe-oxide formation. On the other hand, α-alumina was formed on anodized catalytic support with Al or NiAl diffusion layers, which showed good adherence to the underlying metal substrate.

Multi-step planning calculations with the GOTHIC code used for the design of complex experiments in the PANDA facility

Abstract The evaluation of the hydrogen risk and mitigation measures is increasingly based on the three-dimensional analyses of the distribution of gases and steam in the containment. Currently, simulations are thus carried out with advanced computational tools, including CFD codes, which require extensive validation using detailed measurements in large-scale facilities. However, since these facilities were designed for specific geometries and accident conditions, global scaling criteria are of limited use for designing new experiments addressing 3-D phenomena in different geometries and for different scenarios. Therefore, planning calculations are required to ensure that the tests are representative of expected plant conditions during hypothetical accidental transients. In this paper, a multi-step methodology (from plant to rig) for designing two types of tests in the PANDA facility within the OECD/NEA HYMERES project is illustrated. The first type of experiments addressed the interaction of flows and temperature fields produced by two PARs (Passive Autocatalytic Recombiners) at different elevations in some region of a generic containment in presence of wall condensation. The second type of experiments addressed natural circulation and gas distribution in a generic two-room geometry. The main results of the two series of experiments that were designed using the outcome of this work have been reported in two papers.

Evaluation of the PAR Mitigation System in Swiss PWR Containment Using the GOTHIC Code

The installation of passive autocatalytic recombiners (PARs) in the containment of operating nuclear power plants (NPPs) is increasingly based on three-dimensional studies of severe accidents that accurately predict the hydrogen pathways and local accumulation regions in containment and examine the mitigation effects of the PARs on the hydrogen risk. The GOTHIC (Generation Of Thermal-Hydraulic Information for Containments) code is applied in this paper to study the effectiveness of the PARs installed in the Gösgen NPP in Switzerland. A fast release of a mixture of hydrogen and steam from the hot leg during a total station blackout is chosen as the limiting scenario. The PAR modeling approach is qualified simulating two experiments performed in the frame of the OECD/NEA (Organisation for Economic Co-operation and Development/Nuclear Energy Agency) THAI (Thermal-hydraulics, Hydrogen, Aerosols and Iodine) project. The results of the plant analyses show that the recombiners cannot prevent the formation of a stratified cloud of hydrogen (10% molar concentration), but they can mitigate the hydrogen accumulation once formed. In the case of the analyzed fast release scenario, which is characterized by increasing loads with large initial flow rate and high hydrogen concentration values, it is shown that, when a large number of recombiners are installed, the global outcome in relation to the combustion risk does not depend on the details of the single PAR behavior. The hydrogen ignition risk can be fully mitigated in a timeframe ranging from 15 to 30 min after the fast release, according to the dependence of the PAR efficiency model on the adopted parameters.

Prevention of hydrogen accumulation inside the vacuum vessel pressure suppression system of the ITER facility by means of passive auto-catalytic recombiners

Hydrogen safety is a relevant topic for both nuclear fission and fusion power plants. Hydrogen generated in the course of a severe accident may endanger the integrity of safety barriers and may result in radioactive releases. In the case of the ITER fusion facility, accident scenarios with water ingress consider the release of hydrogen into the suppression tank (ST) of the vacuum vessel pressure suppression system (VVPSS). Under the assumption of additional air ingress, the formation of flammable gas mixtures may lead to explosions and safety component failure.The installation of passive auto-catalytic recombiners (PARs) inside the ST, which are presently used as safety devices inside the containments of nuclear fission reactors, is one option under consideration to mitigate such a scenario. PARs convert hydrogen into water vapor by means of passive mechanisms and have been qualified for operation under the conditions of a nuclear power plant accident since the 1990s.In order to support on-going hydrogen safety considerations, simulations of accident scenarios using the CFD code ANSYS-CFX are foreseen. In this context, the in-house code REKO-DIREKT is coupled to CFX to simulate PAR operation. However, the operational boundary conditions for hydrogen recombination (e.g. temperature, pressure, gas mixture) of a fusion reactor scenario differ significantly from those of a fission reactor. In order to enhance the code towards realistic PAR operation, a series of experiments has been performed in the REKO-4 facility with specific focus on ITER conditions. These specifically include operation under sub-atmospheric pressure (0.2–1.0 bar), gas compositions ranging from lean to rich H2/O2 mixtures, and superposed flow conditions.The paper gives an overview of the experimental program, presents results achieved and gives an outlook on the modeling approach towards accident scenario simulation.

Simulation of start-up behaviour of a passive autocatalytic hydrogen recombiner

Abstract Heterogeneous catalytic recombination of hydrogen with oxygen is one of the methods used to remove hydrogen from the containment of a light-water nuclear reactor (LWR). Inside a passive autocatalytic recombiner (PAR), hydrogen and oxygen molecules are adsorbed at catalyst active spots and they recombine to yield water. Heat released in this exothermic reaction creates natural convection of gas in the spaces between the elements supporting a catalyst. Hot and humid gas flows upwards into the PAR chimney, while fresh, hydrogen-rich gas enters the PAR from below. Catalytic recombination should start spontaneously at room temperature and low hydrogen concentration. Computational fluid dynamics (CFD) has been used to study the dynamic behaviour of a plate-type Areva FR-380 recombiner in a quiescent environment for several test scenarios, including different rates of increase in hydrogen concentration and temporary catalyst deactivation. A method for the determination of pressure boundary conditions at the PAR exits was proposed and implemented into a CFD code. In this way, transient operation of PAR could be simulated without the need to model gas circulation outside the device. It was found that first a slow downward flow of gas is developed, which may persist until the temperature of the catalyst foils rises. As soon as the gas inside the PAR absorbs enough heat to become lighter than the gas outside the PAR, it starts to flow upwards. Criteria for determining the start-up time of PAR were proposed. Model predictions were also compared with experimental data obtained in tests conducted at the THAI facility.

Reviewing H2 Combustion: A Case Study for Non-Fuel-Cell Power Systems and Safety in Passive Autocatalytic Recombiners

This Review looks at past and current research and development (R&D) on H2 combustion in terms of power generation in gas turbines and safety in passive autocatalytic recombiners (PARs). The drive toward reducing greenhouse gas emissions has forced researchers to look at carbon-free alternatives for power generation. Fortunately, H2 is one such fuel source and has been proposed as a fuel in gas turbines for large-scale power production. The effects of H2 on the heating values, flame speed, burning velocity, flammability range, flashback, blow-off, ignition delay, and emissions have been reported, and the trends and gaps in R&D identified. Properties such as flame speed, burning velocities, and flammability limits at typical gas turbine conditions (high pressures, high temperatures, lean equivalence ratios, and turbulent conditions) still need to be determined experimentally for H2 fuel mixtures, especially for mixtures with higher H2 concentrations and fuel mixtures with other fuels (such as biogas) as an...