Modeling the start-up behavior of passive auto-catalytic recombiners in REKO-DIREKT

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1 Modeling the start-up behavior of passive auto-catalytic recombiners in REKO-DIREKT E.-A. Reinecke a,*, St. Kelm a, D. Heidelberg b, M. Klauck b, P.-M. Steffen a, H.-J. Allelein a,b a Forschungszentrum Jülich GmbH, IEK-6, Jülich, Germany b RWTH Aachen University, LRST, Kackertstr. 9, Aachen, Germany Abstract Passive auto-catalytic recombiners (PARs) are installed inside the containments of LWRs worldwide as key element of the hydrogen mitigation strategy. For many years, research efforts have been focused on understanding the operational behavior of PARs under relevant boundary conditions in order to derive numerical models for the simulation of PAR operation beyond initial simplified parameter correlations. The REKO-DIREKT code developed at Forschungszentrum Jülich in cooperation with RWTH Aachen University represents one of these advanced modeling approaches. Developed with the support of small-scale experiments in the REKO facilities, the code is coupled with the thermal hydraulics codes CFX (ANSYS) and COCOSYS (GRS) to simulate accident scenarios including PAR operation. Both stand alone and coupled versions have been validated against full-scale experiments. While quasi-steady-state PAR operation and the processes involved are well understood and sufficiently modeled, the transient start-up behavior still lacks a proper treatment in numerical PAR models. However, the start-up phase is critical for the overall PAR performance due to comparatively low catalyst temperatures, which make the PAR considerably susceptible for the impact of possible adverse processes, e.g. catalyst poisoning or fouling, causing performance reduction such as start-up delay or deactivation. In the framework of the German national project H2REKO ( Database for modeling the start-up and ignition behavior of passive auto-catalytic recombiners inside the containment of nuclear power plants ) funded by the Federal Ministry for Economic Affairs and Energy (BMWi), an experimental program is performed to study the start-up behavior under challenging boundary conditions, in particular in the presence of humidity, in the presence of carbon monoxide, and under the influence of counter flow. The experimental data are used to further develop the REKO-DIREKT code to describe the processes relevant for PAR start-up. The enhanced model is used to simulate experiments performed in the THAI facility in order to demonstrate the capability and applicability of the modeling approach. The present paper gives an overview of the experimental program, the results obtained, and describes the modeling approaches and validation results. Keywords: Passive auto-catalytic recombiner, PAR, Hydrogen, Simulation 1. Introduction Hydrogen mitigation remains one of the highly safety relevant topics in severe accident research for light water reactors. With research starting with the TMI accident in 1979, the Fukushima Daiichi accident has in 2011 again highlighted the importance of efficient hydrogen mitigation measures. An increasing number of research activities worldwide demonstrates the growing interest in suitable safety devices. Passive auto-catalytic recombiners (PARs) have been established as key element of the hydrogen mitigation strategy and are installed inside the containments of LWRs worldwide [1]. For many years, research efforts have been focused on understanding the operational behavior of PARs under relevant boundary conditions, including international reactor safety projects, e.g. OECD/NEA-THAI/THAI2, ERCOSAM and SARNET/SARNET2 (both EU-FP7). Numerical PAR models are required for both CFD and LP codes in order to allow for the assessment of a large variety of scenarios (LP) as well as to investigate specific scenarios in more detail (CFD). For this purpose, PAR codes have been developed with model approaches at different levels, from simple parameter models, socalled manufacturer s correlations, to detailed chemistry and thermal hydraulic modeling [2]. Amongst the most detailed approaches are the codes REKO-DIREKT (JÜLICH) [3], SPARK (IRSN) [4], and the model proposed by Rozen [5]. * Corresponding author address: e.reinecke@fz-juelich.de (E.-A. Reinecke)

2 While quasi-steady-state PAR operation and the processes involved are well understood and sufficiently modeled [6], the transient start-up behavior still lacks a proper treatment in numerical PAR models, although the start-up phase is critical for the overall PAR performance due to comparatively low catalyst temperatures, which make the PAR considerably susceptible for the impact of possible adverse processes, e.g. catalyst poisoning or fouling, causing performance reduction such as start-up delay or deactivation. Furthermore, there is a lack of experimental data, as most of the relevant qualification tests were not published in sufficient detail. In the framework of the German national project H2REKO ( Database for modeling the start-up and ignition behavior of passive auto-catalytic recombiners inside the containment of nuclear power plants ) funded by the Federal Ministry for Economic Affairs and Energy (BMWi), an experimental program has been performed to study the start-up behavior under challenging boundary conditions, in particular in the presence of humidity, in the presence of carbon monoxide, and under the influence of counter flow. The experimental database has been used to further develop the REKO-DIREKT code to describe the processes relevant for PAR start-up. 2. Basic modelling approach The REKO-DIREKT code developed at JÜLICH in cooperation with RWTH Aachen University represents one of the advanced PAR modeling approaches. Developed with the support of small-scale experiments in the REKO facilities, the code has been coupled with the thermal hydraulics codes CFX (ANSYS) and COCOSYS (GRS) to simulate accident scenarios including PAR operation. Both stand alone and coupled versions have been validated against full-scale PAR experiments [7]. The PAR start-up phase includes the time interval required to heat up the catalyst sheets from ambient temperature (approx C) to the operational temperature of several 100 C due to the exothermal reaction. Adverse effects potentially reducing the catalyst activity and causing a delay of the start-up time are effective especially during this time period. The second relevant process during PAR start-up is the establishment of the chimney flow through the PAR box Experimental experience Insights into PAR start-up can be gained from several well-instrumented PAR experiments. In the framework of the OECD/NEA-THAI project [8], more than 30 tests of PAR performance with different commercial devices have been performed. In these tests, hydrogen concentration and temperature at the PAR inlet and outlet, catalyst temperatures, and inlet flow velocity have been measured. Furthermore, experiments in the REKO-4 facility with a down-scaled PAR prototype [9] provide even more detailed data with additional PIV measurement of the flow velocity field at the PAR inlet and measurement of the catalyst temperature profiles. Figure 1 shows typical PAR start-up data obtained from THAI experiments (no axis values given due to contractual provisions). The start-up process can be described in 2 phases. After injection of hydrogen starts, the hydrogen concentration increases simultaneously at both the PAR inlet and outlet. Almost immediately, reaction start on the catalyst surface is observed as the catalyst temperature slowly increases (Phase I). Nevertheless, no decrease of the hydrogen concentration at the PAR outlet is observed and the chimney gas temperature remains at the initial level. The beginning of the Phase II is characterized by establishment of the chimney flow indicated by the velocity measurement signal and a steep increase of the chimney gas temperature. At the same time, the catalyst temperature is strongly rising followed by a significant decrease of the outlet hydrogen concentration. This highly transient process is then developing towards quasi-steady-state operation, depending on the environmental boundary conditions.

3 Figure 1 Phases of PAR start-up (OECD/NEA-THAI data) Similar phenomena can be observed in the REKO-4 experiments. Figure 2 shows the relevant data for the start-up process of a typical experiment. We can observe that the catalytic reaction is initially more pronounced at the upper edge of the catalyst sheets where the increase of the catalyst temperature starts earlier than at the lower edge. However, at approx. 450 s the catalyst temperature at the lower edge is significantly increasing, followed by strong fluctuations of the hydrogen concentration signal at the PAR outlet. During this time, the gas temperature inside the PAR chimney remains almost constant. The start of the chimney flow can be observed around 660 s with a steep increase of both catalyst and gas temperature as well as a drop of the hydrogen concentration at the PAR outlet. Figure 2 Phases of PAR start-up (REKO-4 data)

4 The transient catalyst temperature distribution during REKO-4 experiments shows more details of the process (Fig. 3). Initially, the highest catalyst temperature is observed at the upper edge (300 s). Within the next 300 s, the catalyst temperature at the lower edge significantly increases. With the start of the chimney flow (between 600 and 700 s), the catalyst profile in flow direction is established reaching almost steady-state temperature values after 1200 s. Figure 3 Catalyst temperature profiles at selected times (REKO-4 data) The most relevant finding for Phase I of PAR start-up is that the chimney atmosphere remains at initial conditions despite increasing catalyst temperatures. Phase II is characterized by enhanced transfer of depleted gas mixture from the catalyst section to the chimney leading to the required buoyancy effect starting the chimney flow. Once established, the chimney flow drives PAR operation at elevated catalyst temperatures due to the enhanced transport of hydrogen/air mixture into the catalyst section Modelling approach The REKO-DIREKT code consists of two parts, as catalyst section and chimney are treated in separate modules (Figure 4). Inside the catalyst section, heat and mass transfer processes are modeled in detail, in order to determine the catalyst temperature distribution and local hydrogen and carbon monoxide reaction rates [3]. The chimney model consists of a 1D approach determining the integral mass flow through the PAR chimney [9]. Figure 4 Basic approach of REKO-DIREKT PAR modelling

5 The development of the REKO-DIREKT code has pursued the objective to provide a detailed model at limited numerical cost. For this purpose, the model of the catalyst section is based on heat and mass transfer phenomena, omitting the complex surface or gas-phase chemistry. However, start-up processes as well as relevant processes causing start-up delay (e.g. impact of humidity, cable und oil fire products, and carbon monoxide) are driven by surface reactions. Consequently, corresponding models have to describe the reaction delay with physical variables or based on empirical correlations. Furthermore, the existing version of REKO- DIREKT requires the definition of an initial mass flow. Consequently, the code provides relevant results only after chimney flow development taking into account forced flow heat and mass transfer [3]. In order to model PAR start-up (Phase I), the following model strategy has been adopted: as long as the gas density inside the catalyst section is higher than in the chimney, the catalyst section exchanges gas only with the surrounding atmosphere. During this time, heat transfer from the catalyst surface to the gas phase occurs by natural convection inside the catalyst section. During this first phase, the surface reaction rate is of Arrhenius type, however probably limited by diffusive mass transfer. When the density inside the catalyst section decreases due to intensified heat transfer from the catalyst sheets, gas exchange between catalyst section and chimney leads to temperature increase inside the chimney section and the establishment of the chimney flow. From this moment, forced flow heat and mass transfer is driving the processes. The definition of suitable transition criteria is one crucial aspect in the model development. Figure 5 shows calculation results with this new approach. However, the basic approach still needs significant fine tuning. For this reason, no detailed parameter values are given. Calculated catalyst temperatures at the lower edge ( R-D_Tcat_bottom ) and at the upper edge ( R-D_Tcat_top ) are compared with measured values ( TR_4_55_RK and TR_4_54_RK ) from REKO-4. Although there are still qualitative and quantitative discrepancies between the experimental and numerical data, the modification allows the new program structure to simulate start-up in quiescent atmosphere. Figure 5 Comparison of numerical and experimental catalyst temperature transients during PAR start-up (REKO-4 data) 3. Specific start-up phenomena Several possible influences on the PAR start-up during the first phase have been investigated experimentally in the framework of the H2REKO project. The considered impact on the cold catalyst is scenario dependent. Most likely, water condensate formation on the cold catalyst surface is blocking active sites and increasing the catalyst heat capacity. Aerosols from early fire events, such as e.g. cable fires, may deposit on the catalyst surface while carbon monoxide may poison the catalyst. Counter flow is possible due to global and local containment thermal hydraulics and may interfere with the chimney flow formation (currently also investigated in the framework of the OECD/NEA-THAI3 project). Models for these phenomena have been developed and implemented as described in the next sub-sections. The corresponding experimental procedures have been presented and published in [10].

6 3.1. Start-up delay due to water condensate To study the effect of water condensate on the catalyst surface, experiments under humid and wet boundary conditions have been performed in the GRART facility with alternating phases of application of humid or wet atmosphere to catalyst samples and injection of hydrogen/air mixtures [11]. The measurement results reveal the dependence of the amount of water deposited on the catalyst surface from relative humidity and the sample surface characteristics. The experimental data were successfully correlated with isotherms of type 2 according to IUPAC (International Union of Pure and Applied Chemistry) [12]. Figure 6 shows exemplarily the correlation between relative humidity and water mass for 3 x 3 cm² reference samples with a BET value of m²/gwashcoat. Figure 6 Adsorption isotherm for the reference catalyst (GRART data [11]) Figure 7 shows the start-up time for both reference samples as a function of the condensate mass on the catalyst surface. The start-up delay caused by water adsorption was found to increase linearly with the condensate mass. Figure 7 Start-up delay as a function of the water mass adsorbed on the catalyst surface (GRART data [11])

7 Based on the experimental results, an empirical model describing the start-up delay as a function of humidity has been developed taking into account the correlations given in Figs. 6 and 7 to determine the amount of water adsorbed on the catalyst surface and the corresponding start-up delay time. For a mechanistic model, further modelling of surface chemistry processes such as adsorption/desorption is required. However, as REKO- DIREKT is tailored for fast numerical performance especially for the use in accident simulations at containment scale such a numerically demanding model enhancement appears to be unrewarding Catalyst poisoning by carbon monoxide A test series on the start-up behavior in the presence of carbon monoxide (CO) has been performed in the REKO-3 facility. The REKO-3 facility has been used in the past to study the parallel conversion of CO and hydrogen [13]. If both CO and hydrogen are injected simultaneously, the cold catalyst is initially poisoned. However, at more elevated temperatures the adverse effect is removed. To study the effect of different gas compositions on the start-up temperature, gaseous mixtures including hydrogen (1-6 vol.%), carbon monoxide ( ppm) and air were injected into the reaction channel and preheated until reaction start-up was observed [10]. The tests data available allow the derivation of an empirical correlation to predict the poisoning effect. Figure 8 shows the correlation of start-up temperature and an expression involving hydrogen and CO concentration. The correlation has been experimentally confirmed for CO concentrations between 700 ppm und 2000 ppm and hydrogen concentrations between 1 vol.% and 6 vol.%. Figure 8 Required start-up temperature as a function of hydrogen and carbon monoxide concentration (REKO-3 data, symbols: measurement points; line: linear fit function) The empirical correlation given in Fig. 8 has been implemented in REKO-DIREKT. Calculated ( R- D_Tcat_bottom and R-D_Tcat_top ) and measured ( TR 3.01 and TR 3.06 catalyst temperatures of a typical REKO-3 experiment are shown in Fig. 9. In this test, the catalyst sheets are exposed to a gas mixture of 2 vol.% hydrogen and 1500 ppm carbon monoxide in air. During gas pre-heating in the CO poisoning phase (starting at 500 s), the catalyst temperature is steadily increasing. At approx s, the required temperature threshold is surpassed and the catalyst starts operation.

8 Figure 9 REKO-DIREKT calculation of PAR start-up in the presence of carbon monoxide 3.3. Start-up under counter flow As a follow-up to first orienting investigations performed in the REKO-4 facility [14], counter flow experiments have been realized by a radiator fan unit consisting of a blower with flow conditioner und guide tube. The test matrix includes a total number of 32 experiments with different flow velocities up to 0.9 m/s with three different counter flow configurations under different angles of incidence [10]. Figure 10 shows exemplarily the course of a test with a horizontal counter flow of 0.9 m/s. After initiating the hydrogen injection into the vessel (1100 s), the catalytic reaction starts. Due to the superposed counter flow, the hydrogen concentration at the recombiner inlet decreases. As the flow passes the recombiner in downwards direction, the catalyst temperature at the upper edge is higher than the temperature at the leading edge. After approx s, the chimney flow dominates the counter flow, and the concentration as well as the temperature measurements turn around according to the now upwards directed flow. Figure 10 General scheme of counter flow start-up (REKO-4 data [10]) A comparison of the conversion efficiency observed under different flow angles demonstrates that a PAR of the present design (with top hood) is not negatively affected by the counter flow as soon as the chimney flow has developed. PIV measurements reveal that the counter flow is deflected by the PAR outlet flow. Figure 11 exemplarily shows hydrogen depletion curves for three different flow angles (vertical, horizontal and 30 ) with

9 the highest fan power (corresponding to a flow velocity of 0.9 m/s) compared with reference data. In the presented diagram, the time axes have been synchronized to illustrate the common depletion gradient. Figure 11 Hydrogen concentration history measured inside the REKO-4 vessel at maximum counter flow (0.9 m/s) from different flow directions Based on the available data, empirical criteria defining the point of flow reversal between downward directed forced flow and upwards directed chimney flow have been derived. The primary criterion for the flow reversal is based on the Richardson number (Ri = Gr / Re²) which characterizes the ratio of buoyancy-induced and forced flow. As secondary criterion, the inlet hydrogen concentration must exceed 2.5 vol.%. A critical number of Ri = 13.5 gives a good representation of the point of flow reversal observed in the experiments. For post-calculation of the experiments with REKO-DIREKT, the criteria defining the point of flow reversal have been implemented. First calculations are in good agreement with the experimental data. Figure 12 shows measured ( TR_4_55_RK and TR_4_54_RK ) and calculated ( R-D_Tcat_bottom and R-D_Tcat_top ) catalyst temperatures at the lower and upper edges for a typical REKO-4 experiment. In the phase of downward flow (Ri < 13.5), catalyst temperatures are higher at the upper edge. The model then predicts well the moment of flow reversal and the following normal PAR operation (Ri > 13.5). Figure 12 Simulation result of start-up under counter flow conditions

10 4. Summary and conclusions The start-up phase is critical for the overall PAR performance due to low catalyst temperatures, which make the PAR considerably susceptible for the impact of possible adverse processes, e.g. catalyst poisoning or fouling, causing performance reduction such as start-up delay or deactivation. In the framework of the German national project H2REKO, the start-up behavior under challenging boundary conditions has been investigated within a comprehensive experimental program. The experimental database has been used to further develop the REKO- DIREKT code to describe the processes relevant for PAR start-up. In the presence of humidity, start-up delay was found to be proportional to the water mass adsorbed on the catalyst surface. The delay time can be modeled empirically by taking into account correlations for the adsorption isotherms and the mentioned delay time derived from the experimental data for different surface characteristics. In the presence of carbon monoxide, the catalyst is initially poisoned and requires elevated temperature to start hydrogen conversion. The corresponding threshold temperature as a function of different gas compositions has been derived from experimental data and implemented as empirical model. The correlation has been experimentally confirmed for CO concentrations between 700 ppm und 2000 ppm and hydrogen concentrations between 1 vol.% and 6 vol.%. Based on a series of counter flow experiments, empirical criteria defining the point of flow reversal between downward directed forced flow and upwards directed chimney flow have been derived based on the Richardson number which characterizes the ratio of buoyancy-induced and forced flow. For post-calculation of the experiments with REKO-DIREKT, a critical number of Ri = 13.5 has been implemented. The model predicts well both flow phases and the moment of flow reversal. Generally, the empirical models presented allow a good representation of the phenomena within the tested boundary conditions. However, more mechanistic modeling approaches are required to enhance the range of validity. The limitation of the REKO-DIREKT code on heat and mass transfer phenomena reveals modeling drawbacks when surface processes become more relevant, e.g. during PAR start-up. Consequently, the experimental matrix will be continuously enhanced in the future to enhance the validation range of the models and correlations. Acknowledgements The presented research project is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi, project no ) on the basis of a decision by the German Bundestag. References [1] Z. Liang, M. Sonnenkalb, A. Bentaïb, and M. Sangiorgi, Status report on hydrogen management and related computer codes, OECD/NEA Nuclear Safety Report, NEA/CSNI/R (2014) 8. [2] E.-A. Reinecke, A. Bentaïb, S. Kelm, W. Jahn, N. Meynet, and C. Caroli, Open issues in the applicability of recombiner experiments and modeling to reactor simulations, Progress in Nuclear Energy 52 (2010) [3] J. Boehm, Modelling of processes in catalytic recombiners, Forschungszentrum Jülich, Energy Technologies 61 (2007). [4] N. Meynet, A. Bentaïb Numerical study of hydrogen ignition by passive autocatalytic recombiners Nuclear Technology, 178 (2012) [5] Antoni Rożeń, A mechanistic model of a passive autocatalytic hydrogen recombiner, Chemical and Process Engineering 2015, 36 (1), [6] B. Simon, E.-A. Reinecke, U. Schwarz, K. Trollmann, T. Zgavc, H.-J. Allelein, Advancements in PAR modelling: Major results of a national project performed at RWTH Aachen and JÜLICH, Proc. European Review Meeting on Severe Accident Research (ERMSAR-2012), Cologne, Germany, March 21-23, 2012.

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