Advancements in PAR modelling: Major results of a national project performed at RWTH Aachen and JÜLICH

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1 Advancements in PAR modelling: Major results of a national project performed at RWTH Aachen and JÜLICH B. SIMON 1, E.-A. REINECKE 2,*, U. SCHWARZ 1, K. TROLLMANN 2, T. ZGAVC 2, H.-J. ALLELEIN 1,2 1 RWTH Aachen University, LRST, Aachen (GE) 2 Forschungszentrum Jülich, IEK-6, Jülich (GE) * corresponding author: e.reinecke@fz-juelich.de, phone ABSTRACT Passive auto-catalytic recombiners (PARs) play a key role in the hydrogen mitigation strategy of European LWRs, being designed for removing hydrogen released during a lossof-coolant accident in order to avoid possible threats related to hydrogen combustion. The goal of a national project performed in co-operation between RWTH Aachen and Forschungszentrum Jülich aimed to enhance numerical PAR modelling in order to enable better estimations regarding the operational behaviour of recombiners during postulated accident scenarios. A comprehensive database for model development and validation including a wide range of boundary conditions has been built from experiments under forced-flow conditions (REKO-3 facility) and under natural flow conditions in the new facility, which was established during the project. It was found that the results obtained from both facilities are consistent, thus confirming previous model assumptions with regard to the physical phenomena inside the PAR. The new database complements data from existing integral experiments (e.g. OECD-THAI). In the frame of the project, the REKO-DIREKT code, developed in Jülich and initially describing the phenomena inside the catalyst section of a recombiner, has been enhanced and optimised. The new code is now capable to calculate the PAR operational behaviour from the PAR inlet conditions and PAR geometry. It has been successfully tested against ThAI experiments with a full sized PAR. The present paper gives an overview of the working programme of the project and major results achieved. An outlook on the contents of the follow-up project is provided as well. 1 INTRODUCTION According to the recommendations of the German Reactor Safety Commission (RSK), passive auto-catalytic recombiners (PARs) are installed inside the containments of German nuclear power plants in order to remove hydrogen that may be released during a severe accident and to avoid possible threats related to hydrogen combustion. The reliability of numerical models describing the operational behaviour of PARs under relevant conditions is vital for the investigation of the efficiency of this countermeasure in different accident scenarios, as well as for the assessment of the optimum distribution of recombiners inside the containment. Several experimental programmes have been performed in the past in order to study the integral behaviour of PARs, like e.g. the French H2PAR (Ref. [1]) and KALI (Ref. [2]) experiments or the German BMC (Ref. [3]) and, more recently, ThAI (Ref. [4]) 1/12 pages

2 experiments. In addition, research activities at Jülich were devoted to understand the processes inside the PAR, e.g. (Ref. [5]), in order to provide a database for the validation of more detailed mechanistic PAR models (Figure 1). An overview of existing PAR models is given in (Ref. [6]). Integral experiments (e.g. BMC, THAI, KALI, H2PAR) Parameter correlations Integral experiments (e.g. BMC, THAI, KALI, H2PAR) Parameter correlations (e.g. Siemens-corr.) (e.g. Siemens-corr.) Mechanistic models (GRS-COCOSYS) Separateeffect tests (REKO) Mechanistic models (GRS-COCOSYS) REKO-DIREKT Figure 1: Integration of the project in the context of previous and on-going international research activities At the end of the year 29, the project Studies for the advancement of the methods for the safety-related evaluation of catalytic hydrogen recombination in containments of nuclear power plants during severe accidents was finalized. Funded by the German Federal Ministry of Economics and Technology (BMWi), the project was performed as a cooperation between RWTH Aachen and Forschungszentrum Jülich. The project aimed to deepen the understanding of the physical models and to validate them against experimental data, in order to achieve better estimations regarding the operational behaviour of recombiners during postulated accident scenarios. Experimental facilities in the hydrogen laboratory of the Institute of Energy and Climate Research Nuclear Waste Management and Reactor Safety (IEK-6) at Jülich and the existing REKO-3 database, which includes experimental data on hydrogen conversion and catalyst temperatures under different boundary conditions, have been used as a basis for the experimental part of the project (Ref. [7]). For the numerical modelling activities, the recombiner code REKO- DIREKT which describes the phenomena inside the catalyst section of a recombiner, was provided by Jülich (Ref. [8]). The working programme was complemented with generic investigations on issues related to e.g. alternative recombiner designs. However, these results are not part of this paper and will be published separately. 2 WORKING PROGRAMME The working programme included four work packages: WP1: Investigation of reaction kinetics (REKO-3 facility) WP2: Construction of a new facility ( facility) WP3: Investigation on catalyst-chimney-interaction ( facility) WP4: Model enhancement and validation (REKO-DIREKT code) The experimental and modelling activities were based on the basic understanding that the PAR can be principally divided into two sections: the catalyst section and the chimney section (Figure 2). Consequently, the model strategy includes a detailed forced-flow model of the processes inside the catalyst section and a chimney model which calculates the flow through the PAR box. For model validation, two experimental set-ups were taken into account: First, a section taken out of the recombiner catalyst section to be investigated 2/12 pages

3 under forced flow conditions (REKO-3 facility). Well defined conditions are needed to derive reaction kinetics and to be able to vary boundary conditions. Second, the catalyst section is combined with a chimney in order to study the interaction of both sections. For this purpose, a new facility had to be established ( facility). Chimney buoyancy flow interaction Catalyst section mass/heat transfer chemical (catalytical) reaction Figure 2: Fundamental principle of PAR modelling The interaction of the working packages is illustrated in Figure 3. The REKO-3 experiments serve as validation base for the catalyst section model while the interaction of the chimney and catalyst section is studied in. The full PAR model combines the implemented chimney model with the catalyst model. For the validation of the full PAR model, PAR operational data from large-scale integral experiments in the German ThAI facility (Becker Technologies) (Ref. [9]) were made available. WP2 Construction REKO-DIREKT Modelling WP4 WP3 Experiments WP1 REKO-3 Experiments Figure 3: Interaction of the work packages 3/12 pages

4 2.1 Reaction kinetics inside the catalyst section The REKO-3 facility represents a full scale section taken out of the recombiner catalyst section (Ref. [7]). This section consists of four catalyst sheets arranged inside a vertical flow channel (Figure 4). The gas mixture which is fed into the flow channel consists of different components and is well defined by means of mass flow controllers, pre-heater and steam generator. During the present project, the following conditions have been applied: hydrogen concentration: 1-7 vol.% oxygen concentration: 21-1 vol.% (oxygen starvation) flow velocity: m/s Basic measurements include gas temperatures, catalyst temperatures at 1 positions per sheet, hydrogen and oxygen concentration at 14 positions along the sheets. Together with the known inlet conditions, these data enable the validation of the reaction kinetics model describing the hydrogen conversion (Ref. [5]). Figure 4: REKO-3 experiments for reaction kinetics REKO-3 Typical measurement results of hydrogen concentration and catalyst temperature have been plotted against the catalyst sheet length in Figure 5. On the left side, the measured concentration curve shows the starting value of 4 vol.% dropping to the outlet value of approx. 1.2 vol.%. On the right side, the corresponding temperature profile is given. The maximum temperature at the leading edge amounts to about 58 C, while the temperature drops in direction of the upper edge to about 4 C. The run of the hydrogen concentration and catalyst temperature depend on the gas flow velocity. In the present case, a quite typical flow velocity through a PAR of 1. m/s was applied. 4/12 pages

5 Vol.-% O2 21 Vol.-% O Catalyst sheet length / mm Catalyst sheet length / mm vol.% H 2 1. m/s H 2 concentration / vol.% 2 4 vol.% H 2 1. m/s Catalyst temperature / C Figure 5: Typical REKO-3 data These detailed measurements provide valuable insights when studying the effect of oxygen starvation. Figure 6 shows the results for the same boundary conditions as in Figure 5 compared to an inlet gas mixture with only 3 vol.% oxygen (open symbols). It becomes obvious, that under these conditions the hydrogen conversion is less effective (outlet concentration about 2 vol.%) which affects especially the bottom part of the catalyst sheet, where the temperature is reduced to values slightly above 4 C Vol.-% O2 3 Vol.-% O Vol.-% O2 3 Vol.-% O2 Catalyst sheet length / mm Catalyst sheet length / mm vol.% H 2 1. m/s H 2 concentration / vol.% 2 4 vol.% H 2 1. m/s Catalyst temperature / C Figure 6: Oxygen starvation The findings achieved during the project confirmed and complemented former results, e.g. on oxygen starvation effect (Ref. [1]). Together with already existing data, there is now a comprehensive database for validation of the catalyst section that includes a wide range of boundary conditions. Table 1 gives an overview of the REKO-3 data base and the parameter field covered. 5/12 pages

6 Table I: REKO-3 database Parameters Flow velocity (m/s) Inlet gas temperature ( C) Hydrogen concentration (vol.%) Steam concentration (vol.%) Oxygen concentration (vol.%) Values,25,5,8 1, Interaction of catalyst section and chimney In order to study the interaction of the catalyst section with the chimney, the new facility has been built. consists of a 5.5 m³ vessel with a total of 35 flanges (Figure 7a). The facility may be operated at a pressure of up to 2.3 bar at 28 C. Two special types of measurements are applied here. The local hydrogen concentration is measured in-situ by means of mini-katharometers in order to avoid the removal of gas from the vessel. The flow velocity field of the PAR inflow is measured by means of Particle Image Velocimetry (PIV) in order to avoid disturbing the inflow by measurement installations. Inside the vessel, the PAR set-up is identical with the one used in the REKO-3 facility with an additional chimney on the top (Figure 7b). By this, a direct comparison of the measurement data from both facilities is possible. (a) (b) Figure 7: vessel (a), PAR set-up used in 6/12 pages

7 The course of a typical experiment is given in Figure 8. Hydrogen was injected over the time periods indicated by the yellow bars. The hydrogen concentration at the PAR inlet and outlet increases during the first injection phase until approx. 1,3 s when increasing catalyst temperatures indicate starting of PAR operation. Repeated injections enable quasi-steady-state conditions when PIV measurements are performed. This procedure provides a correlation of inlet hydrogen concentration with hydrogen conversion, catalyst temperatures, and flow velocities T_min sheet 1 Temperature ( C) H 2 concentration (vol.%) T_max sheet 1 T_min sheet 2 T_max sheet 2 T gas at PAR outlet T gas at PAR inlet H2 concentration at PAR outlet H2 concentration at PAR inlet 1 1 Experiment: R4-A Time (s) Figure 8: Data from experiment Catalyst Temperatur temperature ( C) ( C) vol.% 2. vol.% 5.4 vol.% 5.2 vol.% REKO-3 2. vol.% REKO-3 4. vol.% 3.8 vol.% 3.3 vol.% 3.2 vol.% REKO-3 3. vol.% Sheet Blechlänge length (mm) Figure 9: Comparison of REKO-3 and data A comparison of the measured catalyst temperatures from both experiments (REKO-3 and ) is given in Figure 9. The black closed symbols represent REKO-3 temperature data at 2 vol.%, 3 vol.%, and 4 vol.%, measured at a flow velocity of.25 m/s. For comparison, all measurement points of the experiment R4-A-9 are shown. The flow velocity measured 7/12 pages

8 during this test by PIV was between m/s. The good agreement of corresponding temperature profiles confirms the comparability between forced-flow (REKO-3) and natural convection () experiments. 2.3 REKO-DIREKT code enhancement and validation The pre-existing version of the REKO-DIREKT code modelled the catalyst section of a PAR and had been validated against the REKO-3 data base (Ref. [11]). The 2D-model in Cartesian coordinates considers the state equations in flow direction (x-direction) as well as between two adjacent catalyst sheets (y-direction) (Figure 1). x i Solid Solid (Catalyst) Gas Grid point Heat conduction Convection Heat radiation Enthalpy Heat source y n Figure 1: 2-dimensional numerical model of the catalyst section For the gaseous flow between the catalyst sheets, only one mesh in y-direction is applied in order to permit the use of established empirical heat and mass transfer laws in terms of λ q& α = Nu ΔT (1) d (heat conductivity λ, temperature difference ΔT, Nusselt number Nu) for the convective heat transfer and D d H 2 r& = Sh ΔCH 2 (2) (diffusion coefficient D H2, concentration difference ΔC H2, Sherwood number Sh) to describe the diffusion-controlled catalytic reaction rate (Ref. [12]). The characteristic dimensionless Nusselt and Sherwood numbers are calculated depending on the flow regime and according to classical semi-empiric correlations. The parameters of these correlations have been optimised according to the REKO-3 data base. Inside the flow channel the enthalpy flow, heat conduction inside the gas, heat transfer at the plate surface, and radiation between the walls are considered. Heat radiation is modelled as heat source/sink obtained from the net radiation exchange rate which enables considering the heat exchange with all surfaces including losses via the openings. Inside the catalyst sheets, heat is transported by heat conduction. The resulting steady-state equation system is transformed into a band matrix and then solved by means of a direct closed algorithm. As this approach doesn t require spatial iterations the numerical solutions are very stable and exact. In the frame of the present project, the code has been enhanced by a chimney model which is based on the circulation equation according to Unger (Ref. [13]). The calculation 8/12 pages

9 of the catalyst section is still based on a forced-convection approach, where the requested mass flow is provided by the new implemented chimney model. Consequently, the enhanced code is now entirely independent from scenario assumptions and only dependent on data which usually can be provided by the containment calculation, which are: inlet gas temperature and composition total pressure Figure 11 illustrates the basic input and output values of the code. Internally, the local distribution of the gas concentration and catalyst temperature along the catalyst sheets are also calculated. These particular data provide relevant information for the assessment of e.g. ignition at hot catalyst sheets. OUTPUT - Outlet gas temperature - Outlet gas composition - Mass flow through PAR OUTPUT y H2 / vol.% T / C INPUT - Inlet gas temperature - Inlet gas composition -Pressure Figure 11: REKO-DIREKT: Input and output data In order to check and demonstrate the capabilities of REKO-DIREKT, the code has been validated against several experiments performed in the ThAI facility operated by Becker Technologies in Eschborn, Germany (Ref. [9]). The ThAI vessel has a volume of approx. 6 m³ at a height of approx. 9 m and contains an additional vertical inner cylinder of 1.4 m diameter. A commercial PAR of Areva-Siemens design (type FR9-15) was mounted to the inner cylinder (Figure 12, left) in order to study the operational behaviour of the PAR under different operational conditions (hydrogen concentration, pressure, steam content). Five pre-tests (PAR-1 to PAR-5) were performed as reference cases for subsequent aerosol tests. At the PAR bottom inlet below the catalyst section, the inlet conditions flow velocity, hydrogen concentration and gas temperature were measured. Inside the PAR top outlet, the temperature and hydrogen concentration are measured. With these measurements, the relevant REKO-DIREKT input and output data are available, see Figure 11. Post-calculations of the PAR behaviour for all five tests have been performed. As an example, results of the post-calculation of the PAR-4 experiment will be given. A detailed description of the validation results against the remaining tests is given in (Ref. [14]). For the sake of simplicity, only the first phase of the experiment is described here. Figure 12 (right) shows the data progression which was used as REKO-DIREKT input: hydrogen concentration and gas temperature at the PAR inlet and the total pressure inside the vessel. During the first hydrogen injection phase, the hydrogen concentration reaches a 9/12 pages

10 maximum value of nearly 6 vol.% at the PAR inlet. Due to the PAR activity which reaches its state of full operation at approx. 1 s, the hydrogen concentration decreases continuously until approx. 1, s. Subsequently, for a time period of approx. 2 s, air is injected in order to raise the vessel pressure, and the second hydrogen injection phase begins. 5 5 Temperature ( C) Inlet hydrogen concentration Pressure Inlet gas temperature Hydrogen concentration (vol.%) -155 s 1 st hydrogen injection at ~1 s full PAR operation 1,-12, s air injection 12,-14,4 s 2 nd hydrogen injection Time (s) Figure 12: Input data for the post-calculation of the ThAI PAR-4 experiment 25 Symbols: Experiment Lines: Calculation 5 Temperature ( C) Outlet hydrogen concentration Inlet flow velocity Outlet gas temperature Hydrogen concentration (vol.%) Flow velocity (m/s) Time (s) Figure 13: Post-calculation of ThAI PAR-4 experiment The comparison of measured and calculated data for the hydrogen concentration at the PAR outlet, the gas temperature at the PAR outlet, and the flow velocity at the PAR inlet is given in Figure 13. The start-up of the PAR after hydrogen injection is represented well. For the whole time period, the inlet flow velocity is only slightly underpredicted. The agreement of the outlet hydrogen concentration is almost perfect besides a period between approx. 15 s and 4 s. At the same time period, the calculated outlet temperature differs significantly between calculation and experiment. Due to the long time span between experiment and calculation, it was not possible to determine whether these significant deviations could be attributed to the experimental conditions. Despite some deviations, the post-calculations of all five experiments yield very satisfying overall performance of the code in these first test calculations. The general agreement 1/12 pages

11 between calculation and experiment was found to be good, with some exceptions. Although the basic validation against REKO-3 tests have included data at ambient pressure only, the calculations have provided good results for higher pressures as well. 3 CONCLUSIONS AND OUTLOOK ON FOLLOW-UP PROJECT RWTH Aachen and Forschungszentrum Jülich have successfully concluded a project which aimed to enhance numerical PAR modelling in order to achieve better future estimations regarding the operational behaviour of PARs during postulated accident scenarios. Three main goals have been achieved: The REKO-3 database for validation of catalyst section models has been significantly enhanced. The database complements data from existing integral experiments and serves as validation of detailed models describing the processes inside PARs. A new facility () has been constructed in order to study the interaction of catalyst section and chimney. It was found that the results are in agreement with REKO-3 data under forced-flow conditions, thus confirming the fundamental assumptions of the physical PAR model. Based on the experimental results, the REKO-DIREKT code is capable to describe the PAR operational behaviour in a high level of detail. It has been successfully tested against ThAI experiments performed with a full sized PAR. In continuation of the presented successful project, a follow-up has started in October 21. The project Enhancement and validation of models describing the operational behaviour of passive autocatalytic recombiners in the containments of nuclear power plants is again funded by the BMWi. The working programme aims at further consolidation of the model concept, validation of numerical models against experimental data, and applicability check of the PAR model for the use in accident analyses by means of implementation in a containment code. The programme includes: Experimental studies and model development o Optimisation of the chimney model o Parallel recombination of hydrogen and carbon monoxide o PAR ignition REKO-DIREKT validation REKO-DIREKT application o interpretation of OECD-THAI experiments Implementation of REKO-DIREKT in COCOSYS The project is embedded in national and international activities through e.g. the link to the COCOSYS development by GRS, the co-operation with IRSN on PAR research, esp. ignition issues and CO recombination, and through the SARNET2 Containment activity PAR benchmark. ACKNOWLEDGEMENTS The presented projects were/are funded by the German Federal Ministry of Economics and Technology (BMWi, project nos , 15138, and ) on the basis of a decision by the German Bundestag. 11/12 pages

12 REFERENCES [1] P. Rongier, E. Studer, M. Petit: Studies of catalytic recombiner performances in H2PAR facility, Proc. Int. Cooperative Exchange Meeting on hydrogen in reactor safety, Toronto, Canada, June 18-2, 1997 [2] O. Braillard, S. Guieu, J. Hosler, G. Sliter, Tests of passive catalytic recombiners (PARs) for combustible gas control in nuclear power plants, Proc. ANS 2nd Int. Top. Meeting on Advanced Reactor Safety ARS 97, Orlando, Florida, Juni 1997 [3] T. Kanzleiter, Multiple hydrogen-recombiner experiments performed in the Battelle Model Containment: Multi-Reco Experiments Zx2 and Zx5, Final Report BF-R (1997) [4] G. Poss, T. Kanzleiter, S. Gupta, G. Langrock, Experimental investigation of passive autocatalytic recombiner (PAR) units under accidental scenarios, Proc. 2nd Int. Top. Mtg. Safety and Technology of Nuclear Hydrogen Production, Control and Management, June 13-19, 21, San Diego, CA, USA, PaperID 152 [5] E.-A. Reinecke, J. Boehm, P. Drinovac, S. Struth, I.M. Tragsdorf, Numerical and experimental investigations on catalytic recombiners, Proc. 13th Int. Conf. Nuclear Engineering ICONE-13, Beijing, China, May 16-2, 25, ICONE [6] E.-A. Reinecke, A. Bentaib, S. Kelm, W. Jahn, N. Meynet, C. Caroli, Open issues in the applicability of recombiner experiments and modeling to reactor simulations, Progress in Nuclear Energy 52 (21) [7] P. Drinovac, Experimental studies on catalytic recombiners for light water reactors, PhD Dissertation RWTH Aachen University (26) [8] J. Boehm, Modeling of the processes in catalytic recombiners, Report Forschungszentrum Jülich (Energy Technology), Volume 61 (26) [9] T. Kanzleiter et al., Experimental facility and program for the investigation of open questions on fission product behaviour in the containment - ThAI Phase II (ThAI = Thermal Hydraulics, Aerosols, Iodine), Final Report, BMWi , Eschborn, Germany (27). [1] E.-A. Reinecke, S. Kelm, S. Struth, U. Schwarz, I.M. Tragsdorf, Performance of catalytic recombiners for hydrogen mitigation in severe accidents under oxygen depletion conditions, Proc. 15th Int. Conf. Nuclear Engineering ICONE-15, Nagoya, Japan, April 22-26, 27, ICONE [11] E.-A. Reinecke, J. Boehm, P. Drinovac, S. Struth, I.M. Tragsdorf, First validation results of the recombiner code REKO-DIREKT, Proc. Annual Meeting on Nuclear Technology (KTG 26), Aachen, Germany, May 16-18, 26 [12] E.-A. Reinecke, J. Boehm, P. Drinovac, S. Struth, I.M. Tragsdorf, Modelling of catalytic recombiners: Comparison of REKO-DIREKT calculations with REKO-3 experiments, Proc. Int. Conf. Nuclear Energy for New Europe, Bled, Slowenia, September 5-8, 25 [13] J. Unger, Konvektionsstroemungen (Convective Flows), Teubner, Stuttgart (1988) [14] E.-A. Reinecke, B. Simon, H.-J. Allelein, Validation of the PAR code REKO-DIREKT: Post-calculation of integral PAR experiments in the ThAI facility, Proc. 2nd International Meeting on the Safety and Technology of Nuclear Hydrogen Production, Control, and Management, San Diego, CA, June 13-17, 21 12/12 pages