• Energy Research

Abstract In case of a core melt-down accident in a light water nuclear reactor, hydrogen is produced during reactor core degradation and released into the reactor building. This subsequently creates a combustion hazard. A local ignition of the combustible mixture may generate standing flames or initially slow propagating flames. Depending on geometry, mixture composition and turbulence level, the flame can accelerate or be quenched after a certain distance. The loads generated by the combustion process (increase of the containment atmosphere pressure and temperature) may threaten the integrity of the containment building and of internal walls and equipment. Turbulent deflagration flames may generate high pressure pulses, temperature peaks, shock waves and large pressure gradients which could severely damage specific containment components, internal walls and/or safety equipment. The evaluation of such loads requires validated codes which can be used with a high level of confidence. Currently, turbulence and steam effect on flame acceleration, flame deceleration and flame quenching mechanisms are not well reproduced by combustion models usually implemented in safety tools and further model enhancement and validation are still needed. For this purpose, two hydrogen deflagration benchmark exercises have been organised in the framework of the SARNET network. The first benchmark was focused on turbulence effect on flame propagation. For this purpose, three tests performed in the ENACCEF facility were considered. They concern vertical flame propagation in an initially homogenous mixture with 13 vol.% hydrogen content and different geometrical configurations. Three blockage ratios of 0, 0.33 and 0.6 were considered to generate different levels of turbulence. The second benchmark objective was the investigation of the diluting effect on flame propagation. Thus, three tests performed in the ENACCEF facility using the same blockage ratio of 0.63 and three different initial gas compositions (with 10, 20 and 30 vol.% diluents) have been considered. Since ENACCEF runs at ambient temperature, a surrogate to steam was used consisting of a mixture of 0.6He + 0.4CO 2 on molar basis. This paper aims to present the benchmarks conclusions regarding the ability of LP and CFD combustion models to predict the effect of turbulence and diluent on flame propagation.

Abstract In this paper we present our investigations on biomass used as renewable energy. The numerical study of biomass gasification and methanation in fluidized bed was carried out in the framework of the GAYA project supported by ADEME (French Agency For Environmental Protection and Energy). We have developed a sharp expertise in the modeling of the hydrodynamic of single or multiple phase fluidized beds. Indeed, specific sub-models have been developed to represent particles/particles and fluid/particles interactions. Our tools are now able to help improving an industrial scale gasifier and methanation reactor unit design by defining optimal operating conditions and instrumentations location.

Abstract In the case of a severe accident in light-water reactors, a large amount of hydrogen could be generated from the reaction between steam and zirconium at high fuel clad temperature and also from reactions of molten core debris with concrete. The hydrogen generated will be released into the containment atmosphere, and mixed with air and steam possibly creating local flammable conditions. In order to prevent loads resulting from a possible hydrogen combustion, French and German reactor containments are equipped with passive autocatalytic recombiners (PARs), which recombine hydrogen with oxygen even at concentrations below the lower flammability limit. In common PAR designs, catalytic materials (platinum and palladium on ceramic washcoat) are housed in a metallic structure whose purpose is to optimise the circulation of gases in contact with the catalyst. Numerous tests have been conducted in the past to investigate PAR behaviour in situations representative of severe accidents (Battelle Model Containment in Germany, H2PAR and KALI-H2 in France, AECL Whiteshell Laboratories in Canada, etc.). Furthermore, these tests demonstrated that, provided special care is paid to the design and construction of the catalysts, catalyst poisoning by materials such as carbon monoxide, iodine and aerosols present in the containment atmosphere will not fundamentally reduce the effectiveness of the PARs. Some of the above-mentioned tests also show that PARs could ignite the flammable gas mixture at elevated hydrogen concentrations. These experimental results need however to be corroborated by more detailed experiments and by refined modelling of phenomena occurring in PARs. In order to better characterise the PAR-induced ignition risk, a series of dedicated experiments has started at the REKO-3 facility located in Forschungszentrum Julich. In parallel, a refined modelling of the recombiners has been developed by IRSN and will be used to gain insights into the phenomena occurring at the PAR catalyst plates. Furthermore, previous tests indicated that the position of the recombiners could have an impact on their overall efficiency. The installation of PARs in the reactor building is influenced by geometric and operational constraints. To this end, numerical models were developed from the experimental data for codes like COCOSYS or ASTEC in order to optimise the PAR location and to assess the efficiency of PAR implementation in different scenarios. However, these models are usually simple (black-box type) and based on the manufacturer's correlation to calculate the hydrogen depletion rate. Recently, enhanced CFD models have been developed at IRSN and Julich in order to take into account phenomena such as the PAR location effect, gas mixture ignition induced by PARs, and the oxygen starvation effect. A new specifically instrumented facility is also under construction at Julich to investigate these phenomena in more detail.

Abstract In a LWR severe accident, carbon monoxide (CO) may be generated inside the containment due to molten corium concrete interaction (MCCI). As a component of the accident atmosphere, CO will interact with passive auto-catalytic recombiners (PARs) which are installed inside LWR containments for hydrogen (H 2 ) removal. Depending on the boundary conditions, CO may either react with oxygen to carbon dioxide (CO 2 ) or act as catalyst poison, reducing the catalyst activity and hence the hydrogen conversion efficiency. A new experimental test programme performed in co-operation between JULICH and RWTH investigates these aspects aiming at providing data for model development for advanced severe accident analyses. In the first test series presented here, the parallel catalytic reaction of H 2 and CO on the catalyst surface has been studied, i.e. the hydrogen recombination reaction was started before CO was injected. In total, 33 steady state measurements have been performed. The test series was jointly evaluated by JULICH, RWTH and IRSN. The test results show that under the given conditions the conversion of CO into CO 2 has no negative impact on the parallel hydrogen conversion. The efficiency of the CO recombination in terms of molar rates is significantly smaller (by a factor of ∼2) than the corresponding H 2 conversion efficiency. Due to the exothermal reaction, the parallel CO conversion may also have an impact on the possible ignition of the flammable gases at hot PAR surfaces. The authors have used three different numerical codes for the simulation of the parallel CO/H 2 recombination. The codes REKO-DIREKT (JULICH/RWTH), SPARK (IRSN), and CFX (ANSYS) were able to capture the effects observed in the experiments, providing a versatile basis for further investigations in this important safety issue. The different model approaches and additional enhancements in order to simulate the CO test series are described in the paper.

Abstract A large amount of hydrogen can be released into the containment of light water reactors during a severe accident. Passive Auto-catalytic Recombiners (PARs) aim to avoid flame acceleration and excessive pressure loads on the containment in case of hydrogen combustion. Their operation is based on the catalytic recombination of hydrogen into steam in the presence of oxygen. Thus, the recombiners reduce the hydrogen but also the oxygen content in the containment atmosphere. As a consequence, the oxygen/nitrogen ratio diverts more and more from the standard 21 vol.% in air. This decreasing ratio may impact on the PAR efficiency. Additionally, the exothermic surface chemical mechanism leads to the overheating of the catalytic plates and activates the natural convection inside the recombiners. This heat source can also create local conditions for hydrogen combustion in the gas phase, as igniters do. Hence, the oxygen/nitrogen ratio may also determine the conditions for the gas-phase ignition inside PARs. This study deals with the numerical simulation of the impact of the oxygen starvation (i.e. low oxygen/nitrogen ratio) on the PAR efficiency and on the PAR gas-phase ignition limit. Calculations are performed with a dedicated CFD code named SPARK. We focus on the interaction of recombiners with any H2/O2/N2/H2O mixtures and thus establish a quite complete understanding of PAR operation. Calculations confirm the experimental oxygen surplus (i.e. twice more oxygen than stoichiometry) necessary to ensure an optimal PAR efficiency ( X O 2 ≈ X H 2 ) independently of the steam content. The PAR gas-phase ignition limit is then determined numerically in the classical H2/Air/H2O ternary diagram with a very good agreement with the available experimental database. It points out the importance of catalyst heat radiation, and more secondarily of species thermal diffusion (i.e. Soret effect). Finally, the PAR gas-phase ignition limit is determined for all oxygen/nitrogen ratios. The ignition domain appears to strongly contract when the oxygen content decreases, so that the steam threshold for inertization of the containment with respect to the recombiners ignition becomes very low.

AbstractDuring the course of a severe accident in a light water nuclear reactor, large amounts of hydrogen can be generated and released into the containment during reactor core degradation. Additional burnable gases [hydrogen (H2) and carbon monoxide (CO)] may be released into the containment in the corium/concrete interaction. This could subsequently raise a combustion hazard. As the Fukushima accidents revealed, hydrogen combustion can cause high pressure spikes that could challenge the reactor buildings and lead to failure of the surrounding buildings. To prevent the gas explosion hazard, most mitigation strategies adopted by European countries are based on the implementation of passive autocatalytic recombiners (PARs). Studies of representative accident sequences indicate that, despite the installation of PARs, it is difficult to prevent at all times and locations, the formation of a combustible mixture that potentially leads to local flame acceleration. Complementary research and development (R&D) projects were recently launched to understand better the phenomena associated with the combustion hazard and to address the issues highlighted after the Fukushima Daiichi events such as explosion hazard in the venting system and the potential flammable mixture migration into spaces beyond the primary containment. The expected results will be used to improve the modeling tools and methodology for hydrogen risk assessment and severe accident management guidelines. The present paper aims to present the methodology adopted by Institut de Radioprotection et de Sûreté Nucléaire to assess hydrogen risk in nuclear power plants, in particular French nuclear power plants, the open issues, and the ongoing R&D programs related to hydrogen distribution, mitigation, and combustion.