Passive Autocatalytic Recombiners Research Articles

Analysis of hydrogen control in a Small Modular Reactor during TLOFW severe accident

During the Fukushima Daiichi nuclear accident in 2011, hydrogen explosions occurred in all units from Unit 1 to Unit 3. Consequently, one of the lessons learned from the Fukushima Daiichi accident is the necessity of implementing hydrogen control and mitigation strategies for most Nuclear Power Plants (NPPs). This paper focuses on the incorporation of Passive Autocatalytic Recombiners (PARs) during the design phase of a small modular Pressurized Water Reactor (SMR-PWR) project. The numerical analyses are conducted using the MELCOR v. 2.2 code. Two scenarios are compared: the Total Loss of Feed Water (TLOFW) severe accident with and without PARs. Saphiro’s diagram is utilized to investigate whether the mixture's composition (hydrogen, air, steam) is flammable for both scenarios. It has been observed that the inclusion of PARs leads to a reduction in hydrogen risk (detonative or deflagrative) as the final hydrogen concentration values fall below the flammability limit. This study is preliminary, and further research is required.

Experimental results for RVK-500 recombiner tested in conditions typical for pressurized water NPP severe accidents

Severe accidents at NPPs with light water coolant can lead to situations when large quantities of hydrogen are released due to zirconium-steam reaction. To avoid explosion consequences, they started to install hydrogen passive autocatalytic recombiners (PARs) at NPPs. Calculating simulation based on PAR models is used to predict and justify the needed quantity of PARs and the chosen place of their location inside the NPP. To validate PAR computational models, experimental data are need. Within the scope of this work, the integrated tests of the RVK-500 type recombiner, which included 20 experiments, were conducted to obtain large body of data on PAR operation in different modes and conditions that can be realized in case of hypothetical accident at NPP. For this purpose, a facility equipped with a measuring set able to work at high pressures, temperatures, and humidity was constructed. The tests were conducted in accord with thepredevopled PAR testing procedure and program to describe PAR characteristics, such as its start-up, H2 recombination rate, efficiency, and ignition limit. In addition, a small-scale facility was used to study the potential of particles separated from the catalytic elements to induce combustion.

Modeling of the influence of local heat sources on a light gas stratification formation and erosion in a large-scale experimental facility using eddy resolving numerical approach

The main objective of this work is to evaluate the results of application of Implicit Large Eddy Simulation (ILES) turbulence modeling approach to simulate the complex hydrogen safety problem. The paper presents the results of numerical simulation of the OECD/NEA HYMERES (HYdrogen Mitigation Experiments for Reactor Safety) HP2 series tests in the PANDA facility, aimed at studying the thermal effect of passive autocatalytic recombiner (PAR) operation on mixing the containment atmosphere, using CABARET-SC1 code. The code is based on the eddy resolving CABARET technique, which allows implicit modeling of the subgrid turbulence scales without using tuning parameters (ILES approximation). The natural convection flow inside the PAR model was calculated explicitly using a simplified model of the heating section and the housing. Good agreement of calculated transient with experimental data was obtained, which indicates the effectiveness of such approach.

Application of passive autocatalytic recombiners for hydrogen mitigation: 2D numerical modeling and experimental validation

The widespread production and use of hydrogen (H2) requires safe handling due to its wide range of flammability and low ignition energy. In confined and semi-confined areas, a H2 leak creates potential for accumulation of flammable gases. A Passive Autocatalytic Recombiner (PAR) is a safety device for preventing H2 accumulation. PAR is equipped with catalyst plates that assist the activation of the H2 recombination with oxygen (O2) at low concentrations. The heat released from the reaction decreases the gas density, creating a self-sustained natural convective flow, continuously supplying the catalyst surface with H2 present in the air surrounding the PAR. In this study, a 2D numerical model is developed to simulate PAR operation. The model is used to analyze surface reactions, forced versus natural convective flow, plate spacing, heat losses, hydrogen recombination rates, carbon monoxide (CO) poisoning and their influence on the catalyst performance. Experimental data are used for model calibration and validation, showing good agreement for different conditions. The model shows that H2 is almost entirely recombined whereas CO is only partially recombined. Natural and forced convection flows result in similar velocity profiles and large plate spacing produces higher temperatures, higher velocities but lower H2 conversion. Moreover, the model shows a new feature for simulating CO poisoning via surface reactions. Overall, the model provided novel insights into PAR operation, showing the importance of surface reactions and it will be useful to assess design aspects and the effectiveness of PARs for H2 mitigation.

Numerical and experimental analysis of cylindrical-type PAR catalyst behaviour

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 Sct 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.

Analytical model development for investigation on hydrogen management studies in the containment of a nuclear reactor

Containment is the ultimate physical barrier to prevent the spread of active air-borne fission products as well as radiation shielding in normal operation as well as accident condition and is designed to enclose whole reactor systems. The design of containment system for Fleet mode of 700 MWe Indian Pressurized Heavy Water Reactors (IPHWRs) incorporates a Primary Containment (PC) of pre-stressed concrete enveloped by a Secondary Containment (SC) of reinforced concrete. For the specification of the design pressure of the containment system, analysis is performed for various postulated accidents such as Loss of Coolant Accident (LOCA) and Main Steam Line Break Accident (MLSB). Further Studies on hydrogen generation and distribution for various accident scenarios in reactor containment of Indian Pressurized Heavy Water Reactors (IPHWR) indicate that during severe accident with core damage, the possibility of global deflagration in absence of mitigating features is high and may challenge the integrity of containment. Therefore, it is essential to incorporate mitigating provisions inside the containment to prevent the hydrogen reaching local detonation and global deflagration during such a scenario.In order to arrive at the design basis of containment system and simulation of large numbers of accident sequence a computer code is required, where mitigating features such as Passive Auto-Catalytic Recombiners, Containment Spray System & Containment Filtered Venting System can be modeled. A system thermal hydraulic computer code has been developed for containment response calculation during normal operation as well as accident conditions including, severe accident to include the new models for mitigating features. This article presents the details of various models of computer code PACSR-SI-2.0, its validation with data observed for hydrogen distribution & removal in Recombiner Test Facility,Battelle Spray Test for containment system & Spray test at IIT Mumbai.

CFD and machine learning based hybrid model for passive dilution of helium in a top ventilated compartment

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

The technology of suppression containment for the SMR in floating nuclear power plants against hydrogen risk

In the event of loss of coolant accident (LOCA) in small modular reactors (SMRs) in floating nuclear power plants (FNPP), appropriate pressure suppression and hydrogen elimination measures must be taken to ensure small modular reactors' safety. This paper utilizes GASFLOW to analyze the environmental conditions and gas behavior following the implementation of pressure suppression and hydrogen elimination measures in SMRs. The results indicate that: 1) The suppression effect is optimal when the capacity of the suppression pool is 200 m3 and the gas–water ratio is 2.33. 2) When passive autocatalytic recombiners (PARs) are used alone, hydrogen can’t be eliminated quickly, and there is still a hydrogen explosion risk in the containment. 3) Using igniters alone can effectively reduce the hydrogen risk. 4) The combination of PARs and igniters can safely and efficiently eliminate the hydrogen risk, and the temperature inside the containment is lower than when the igniter is used alone.

Numerical evaluation of passive autocatalytic recombiner performance under post-accident conditions

In the accident conditions, the operational behavior of passive autocatalytic recombiners (PARs) has garnered significant attention, particularly with regards to the complicated atmosphere in the containment. This paper aims to systematically study and analyze the PAR performance under post-accident conditions. CFD simulation resolving individual catalytic channels of PARs provide valuable insights into hydrogen consumption performance and temperature distribution. A simplified model of PARs, consisting of a gas channel and two catalyst plates, was developed using the CFD software STAR-CCM+. One-step reaction mechanism was used in this model to define the hydrogen consumption rate on the catalytic surface. To study the variation in gas concentration and temperature distribution, physical values of interest were extracted from designated locations. An analysis of catalyst section was conducted to provide a comprehensive understanding of the reaction process occurring within PARs. Subsequent tests were conducted using the developed simplified model to examine the effect of inlet gas components variation and operating conditions on PAR performance under post-accident conditions. Our findings show that the variation of steam concentration primarily alters the heat absorption capacity of the gas mixture, rather than the catalytic reaction rate. Furthermore, inlet velocity and operating pressure play a role in controlling the inlet mass flow rate, thereby influencing the reaction process of the gas channel by introducing varying amounts of hydrogen.

3D Analysis of Hydrogen Distribution and Its Mitigation Using Passive Autocatalytic Recombiners (PARs) Inside VVER-1000 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.

Investigating the effect of spray system on the containment condition of VVER1000/V446 NPP during LBLOCA and TLOFW accidents

The containment system serves as the last barrier against the release of fission products. However, over pressurization or hydrogen detonation can compromise its effectiveness, as evidenced by the Fukushima nuclear power plant (NPP) disaster. This study describes the efficacy of the spray system in reducingcontainment pressure andtemperature, as well as,itsimpacton hydrogen mitigation during large break loss of coolant accident (LBLOCA) and total loss of feed water (TLOFW) accident in the VVER-1000/V446NPP. The evaluation is conductedusing theMELCOR 1.8.6code.The results of the LBLOCA analysis demonstrate that the spray system significantly reduces the pressure and temperature of the containment. It also enhances hydrogen mitigation by increasing the amount of recombined hydrogen by around 4% and reducing the onset time of hydrogen recombination by 90,000 s. In the TLOFW accident scenario, the spray system is effective in reducing the containment pressure from 0.41 MPa to 0.27 MPa. The investigation of hydrogen behavior in the reactor containment atmosphere reveals that the simultaneous operation of the spray and the hydrogen recombiner systems is the most effective strategy for hydrogen mitigation. However, activating the spray system without the passive autocatalytic recombiners (PARs) function can lead to an increase in the hydrogen mole fraction in all containment compartments, thereby increasing the likelihood of hydrogen combustion.

Evaluation of hydrogen recombination characteristics of a PAR using SPARC PAR experimental results

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.

Tritium transport in the vacuum vessel pressure suppression system for helium cooled pebble bed

In the frame of the safety studies for the EU-DEMO reactor, attention is paid to the hydrogen concentration in the vacuum vessel and connected volumes since it would lead to a possible hazard of releasing tritium and activated dust. The risk of explosion cannot be excluded a priori if H2 stockpiles. For this reason, in both water (WCLL) and helium (HCPB) cooled breeding blanket concepts of EU-DEMO, the problem is under investigation with a cross-reference between the available technologies in fission (such as the Passive Autocatalytic Recombiners – PAR) and fusion application. In particular, the recent analyses pointed out the implementation of the PARs into the Vacuum Vessel Pressure Suppression System or linked systems.This paper evaluates the Hydrogen behavior (main mobilized tritium source term) for the Helium-Cooled Pebble Bed (HCPB) VVPSS concept. The analyses preliminary investigate the stratification of the hydrogen mass inventory inside the PSS. In particular, a MELCOR 1.8.6 model of the PSS, based on past activities aimed at dust transport and thermohydraulic analyses, is adopted. The paper also introduces the applicability of PAR technology in the operation range of fusion devices, analyzing the problem of the recombination rate due to the dilution of Hydrogen after a Helium blowdown.

An approach for an extension of the deflagration model in containment code system COCOSYS to separate burned and unburned atmosphere via junctions

Abstract In case of a postulated severe accident in a water-cooled nuclear power plant significant amounts of hydrogen (H2) and carbon monoxide (CO) can be generated and released into the containment or reactor building where it might form a combustible mixture with air assuming passive autocatalytic recombiners are not available. In case of ignition, pressure peaks might occur, that are relevant for the integrity of safety relevant equipment and the containment or reactor building. It is therefore important for safety analysis to be able to correctly predict combustion phenomena that might occur. The accident analysis code AC2 2021.0 which is developed by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) includes the Containment Code System (COCOSYS version 3.1) for the simulation of containment phenomena. COCOSYS contains the model FRONT for the simulation of premixed deflagration of H2 and CO. Recent code validation using H2 deflagration tests conducted in the multi-compartment THAI+ test facility shows that the flame propagation stops prematurely in simulations of some tests. This is partly attributed to the missing separation of burned and unburned atmosphere which leads to a reduction in fuel concentration in not yet burning zones connected to a burning zone. Model improvement potential was identified which is addressed in this paper. A model extension to separate burned and unburned atmosphere via a junction model is proposed and implemented into a development version of COCOSYS 3.1. First validation results using the THAI test HD-39 are discussed that show improved prediction capability by the extended model.

Development of Hydrogen Oxidation Reaction Catalysts to Overcome CO Poisoning and Elucidation of Reaction Mechanism

Passive autocatalytic recombiner (PAR) represents a potential technology for ensuring process safety in the hydrogen society. PARs function through the catalytic oxidation of generated or leaked hydrogen, facilitating its conversion into water and effectively mitigating the risk of hydrogen explosions. CO is recognized as a catalyst poison that hampers surface catalytic reactions. To investigate the negative effects of CO on the local structure of platinum metal nanoparticle catalysts during water formation, in situ and time-resolved X-ray absorption spectroscopy analyses were conducted. The results revealed that the Pt–Fe/CZY catalyst exhibited notable hydrogen oxidation activity even in the presence of CO. The enhanced performance can be attributed to the combined effects of Pt–Fe alloy composition and CZY support materials.

Development and validation of diffusion based CFD model for modelling of hydrogen and carbon monoxide recombination in passive autocatalytic recombiner

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

Thermal-hydraulic optimization of a proposed EU-DEMO hydrogen passive removal system

As the R&D of magnetic fusion power demonstrating plants are approaching important steps toward concept designs, analysts are working parallelly on the safety assessment of such concepts to identify any potential risk. One of the safety concerns involves the confinement of radioactive substances during normal operation and accidental conditions. Several accident sequences inside the tokamak vacuum vessel or pressure suppression systems are characterized by the risk of hydrogen buildup and subsequent ignition that could threaten the structural confinement integrity. In the Safety and Environment work package of the EUROfusion consortium, possible approaches to mitigate the hydrogen explosion risk are under investigation. One of the exploratory solutions is based on limiting the hydrogen concentration that could reach flammable gas mixture conditions and using Passive Autocatalytic Recombiners (PARs) installed into the atmosphere of the pressure suppression systems tanks to recombine hydrogen.This paper examines the theoretical effectiveness of the PARs intervention during an in-vessel loss of coolant accident without the intervention of the decay heat removal system for the Water-Cooled Lithium Lead (WCLL) concept of EU-DEMO, using an optimization methodology. The involved systems have been modelled in MELCOR to estimate the PARs recombination capability as a function of the thermal-hydraulic parameters of the suppression tanks. Furthermore, the optimizer entity of the RAVEN tool is applied to perform optimization studies on the hydrogen recombination system design parameters. The goal is to explore the geometrical and thermal-hydraulic parameters that maximize the capability of the hydrogen removal system for the WCLL concept.

Backfitting of Dukovany NPP for design extension conditions

The contribution deals with the backfitting of Dukovany NPP for design extension conditions with emphasis on severe accidents. This NPP, located in Czech Republic, is equipped with 4 units of VVER-440/V213 reactors. During the last decades significant attention was paid to backfitting of these units. New safety systems aimed at mitigating the consequences of severe accidents have been designed and have been (are being) implemented. At first, special features of VVER-440/V213 reactor and containment equipped with bubbler condenser pressure suppression system are briefly described. The systematic process of plant backfitting for design extension condition is summarized further. Attention is paid to description of crucial severe accident management strategies that have been adopted and design and implementation of new safety systems that are necessary to fulfil these strategies. Than the requirements of Czech regulatory body (SUJB) set for practical elimination of early or large radioactive releases are discussed. Finally, the performances of severe accident management strategies are illustrated using the specific case of blackout scenario. For this purpose, an analysis was performed using ASTEC code developed by the French IRSN. The results of analysis are briefly described and discussed further. From the obtained results it follows that the high-pressure melting scenario is reliably avoided, corium is localised and stabilised inside the reactor vessel and the hydrogen in containment atmosphere is safely controlled by passive autocatalytic recombiners. Thus, stable plant state with preserved containment integrity is reached. Source term that has been estimated under these conditions indicates that the predicted releases are well below the criteria set by SUJB for early or large radioactive releases, and therefore long-term contamination of the environment is avoided.

Experimental assessment of hydrogen removal rate for passive auto catalytic recombiners

Generation and accumulation of hydrogen in containment atmosphere could pose a potential threat to the integrity of containment as hydrogen can form flammable mixture with air in the containment during postulated severe accident scenario involving multiple failures. Studies on hydrogen generation and distribution for various accident scenarios in reactor containment of Indian pressurized heavy water reactors (PHWRs) indicate that during severe accident with core damage, the possibility of global deflagration in absence of mitigating features is high and may challenge the integrity of containment. Therefore, it is essential to incorporate mitigating provisions inside the containment to prevent the hydrogen reaching deflagration/ detonation limits during such a scenario.Out of several options available for hydrogen mitigation, Passive Auto-catalytic Recombiner (PAR) is one of the most effective and recommended (IAEA -TECDOC-1661(2011)) technology for the removal of hydrogen inside the containment. The testing and evaluation of the performance of PAR for different conditions envisaged in the containment (such as pressure, temperature, steam concentration and poison) is a vital requirement for the deployment of these devices in the nuclear power plants (NPPs). The behavior of catalytic recombiners in terms of hydrogen removal rate with varying hydrogen concentrations is required to be evaluated essentially to work out the numbers and location of recombiners inside the containment. Hence, for testing of catalytic recombiners, an instrumented test facility simulating different reactor containment environment under controlled conditions, has been commissioned. Recombiner Test Facility (RTF) is a test rig for testing of full scale prototypes of catalytic recombiners.Results of Hydrogen Removal Rates (HRR) for varying hydrogen concentration, method for the calculation of HRR and its validation, model equation developed for Corrected Hydrogen Removal Rates (CHRR) usable in numerical simulations are presented in this article. With the proposed method for CHRR, it was established that the Recombiner performance can be evaluated.

Passive Hydrogen Recombination during a Beyond Design Basis Accident in a Fusion DEMO Plant

One of the most important environmental and safety concerns in nuclear fusion plants is the confinement of radioactive substances into the reactor buildings during both normal operations and accidental conditions. For this reason, hydrogen build-up and subsequent ignition must be avoided, since the pressure and energy generated may threaten the integrity of the confinement structures, causing the dispersion of radioactive and toxic products toward the public environment. Potentially dangerous sources of hydrogen are related to the exothermal oxidation reactions between steam and plasma-facing components or hot dust, which could occur during accidents such as the in-vessel loss of coolant or a wet bypass. The research of technical solutions to avoid the risk of a hydrogen explosion in large fusion power plants is still in progress. In the safety and environment work package of the EUROfusion consortium, activities are ongoing to study solutions to mitigate the hydrogen explosion risk. The main objective is to preclude the occurrence of flammable gas mixtures. One identified solution could deal with the installation of passive autocatalytic recombiners into the atmosphere of the vacuum vessel pressure suppression system tanks. A model to control the PARs recombination capacity as a function of thermal-hydraulic parameters of suppression tanks has been modeled in MELCOR. This paper aims to test the theoretical effectiveness of the PAR intervention during an in-vessel loss of coolant accident without the intervention of the decay heat removal system for the Water-Cooled LithiumLead concept of EU-DEMO.