Stephan Kelm

SIMULATION OF MIXING INDUCED BY A HOT PAR EXHAUST PLUME

Passive Autocatalytic Recombiners (PARs) are installed in various reactor containment designs to mitigate the hydrogen risk. For the evaluation of the effectiveness of these devices, validated computational tools are needed. To build confidence in the codes, their capability must also be assessed against separate effect tests addressing specific phenomena. Within the OECD SETH 2 project three experiments have been performed in the large-scale PANDA facility, where the thermal effect of a PAR was simulated by means of a heater and the plume generated by the heat source interacted with an initially stratified ambient. In these tests, helium was used instead of hydrogen. The position of the heater and the presence of simultaneous injection of steam were varied in these tests. These experiments have been analyzed with the GOTHIC and the ANSYS CFX codes. This paper reports only the results obtained with the GOTHIC code. In general, the GOTHIC code in conjunction with a coarse mesh could predict the mixing process reasonably well. The only substantial discrepancy with the experiments was the overprediction of the velocity at the inlet of the heater case, but this had little effect on the simulation of the overall mixing.Copyright

Validation and Application of the REKO-DIREKT Code for the Simulation of Passive Autocatalytic Recombiner Operational Behavior

Abstract In order to reduce the accumulation of hydrogen and thus to mitigate the risk of combustion, many countries have installed passive autocatalytic recombiners (PARs) within light water reactor containments. The severe hydrogen combustion events of the recent Fukushima Daiichi accident are likely to incentivize an increased demand in upgrading nuclear power plants with PARs. Numerical simulation is an important tool for assessing PAR operation during a severe accident in terms of efficiency and proper installation. Advanced numerical PAR models are required for the challenging boundary conditions during a severe accident, for example, low oxygen amount, high steam amount, and presence of carbon monoxide. The REKO-DIREKT code has been developed in order to provide a PAR model capable of simulating complex PAR phenomena and at the same time being suitable for implementation in thermal-hydraulic codes. The development of REKO-DIREKT was supported by small-scale experiments performed at Forschungszentrum Juelich in the REKO facilities. These facilities allow the study of PAR-related single phenomena such as reaction kinetics under different conditions including variation of steam, oxygen, and carbon monoxide (REKO-3) and the chimney effect (REKO-4). Recently, the code has been validated against full-scale experiments performed in the Thermal-Hydraulics, Hydrogen, Aerosols, Iodine (THAI) facility at Eschborn, Germany, in the framework of the Organisation for Economic Co-operation and Development/Nuclear Energy Agency THAI project. By this, the code has proven its applicability for different PAR designs and for a broad range of boundary conditions (pressure of up to 3 bars, steam amount up to 60 vol %, low-oxygen conditions). REKO-DIREKT has been successfully implemented in the commercial computational fluid dynamics code ANSYS-CFX as well as in the LP code COCOSYS [Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Germany].

COMBINED ANALYTICAL AND EXPERIMENTAL INVESTIGATIONS FOR LWR CONTAINMENT PHENOMENA

Main focus of the combined nuclear research activities at Aachen University (RWTH) and the Research Center Julich (JULICH) is the experimental and analytical investigation of containment phenomena and processes. We are deeply convinced that reliable simulations for operation, design basis and beyond-design basis accidents of nuclear power plants need the application of so-called lumped-parameter (LP) based codes as well as computational fluid dynamics (CFD) codes in an indispensable manner. The LP code being used at our institutions is the GRS code COCOSYS and the CFD tool is ANSYS CFX mostly used in German nuclear research. Both codes are applied for safety analyses especially of beyond design accidents. Focal point of the work is containment thermal-hydraulics, but source term relevant investigations for aerosol and iodine behavior are performed as well. To increase the capability of COCOSYS and CFX detailed models for specific features, e.g. recombiner behavior including chimney effect, building condenser, and wall condensation are developed and validated against facilities at different scales. The close connection between analytical and experimental activities is notable and identifying feature of the RWTH/JULICH activities.

CFD Simulation of Passive Auto-Catalytic Recombiner Operational Behaviour

In order to mitigate the impact of a possible hydrogen combustion and to avoid containment failure, passive auto-catalytic recombiners (PAR) are used for hydrogen removal in an increasing number of European nuclear power plants. Hydrogen and oxygen react exothermally even below conventional flammability limits on the catalytic surfaces inside a PAR generating steam and heat. Modelling of the operational behaviour of PAR is one part of development issues in order to achieve reliable predictions of local atmosphere conditions. International research activities, e.g. in the European FP-6/FP-7 Network of Excellence SARNET (Severe Accident Research NETwork), investigate these aspects. At Forschungszentrum Julich, two strategies are pursued. First, the detailed evaluation of the reaction kinetics and heat and mass transport phenomena on a single catalyst element is performed by a direct implementation of the transport and kinetic approaches in ANSYS CFX-11. Second, in order to model the interaction of PAR with the containment, REKO-DIREKT, a detailed user model of the relevant processes inside a PAR based on Fortran 90 will be implemented in CFX. At present, validation against the available experimental database is performed. The validity of the numerical models strongly depends on the experimental data available. For this purpose, detailed experiments are performed at Julich. In the small-scale test facility REKO-3, representing a recombiner section, detailed investigations of the reaction kinetics under well-defined and steady-state conditions have been performed. In co-operation with RWTH Aachen University, a new test vessel REKO-4 is currently under preparation for testing PAR behaviour under natural convection conditions. It will provide various possibilities for instrumentation to measure temperatures and gas compositions and in particular it will be equipped with particle image velocimetry (PIV) for measuring the flow field around the PAR.© 2009 ASME

A Review of the CFD Modeling Progress Triggered by ISP-47 on Containment Thermal Hydraulics

Abstract The Organisation for Economic Co-operation and Development (OECD)/Nuclear Energy Agency International Standard Problem 47 (ISP-47) was aimed at assessing the predictive capabilities of computational fluid dynamics (CFD) and lumped-parameter codes regarding hydrogen mixing under representative thermal-hydraulic conditions of a loss-of-coolant-accident. The benchmark consisted of two systematic steps. The first step was a fundamental model assessment based on quasi-steady-state separate-effects tests in the French TOSQAN facility (7 m3, IRSN, Saclay) and MISTRA facility (100 m3, CEA, Saclay) regarding steam condensation, buoyant turbulent flows, and mixed atmospheric conditions. The second step was based on a more realistic experimental transient in the multicompartmented German Thermal-hydraulics, Hydrogen, Aerosols and Iodine (THAI) facility (60 m3, Becker Technologies, Eschborn). At that time, the blind and open analysis revealed that CFD codes needed further improvement regarding modeling of turbulence in buoyant flows, steam condensation, temperature and species concentration, and stratification buildup as well as their dissolution. This result triggered a comprehensive experimental and analytical effort, e.g., within the German national THAI, the OECD-THAI, and the OECD-SETH-1 and OECD-SETH-2 projects. Now, 10 years later, this paper aims to benchmark the state-of-the-art containment CFD model, developed at Forschungszentrum Juelich and RWTH Aachen University, and to highlight the progress made and the remaining open issues.

CFD Model Development and Validation for VHTR Air Ingress Scenarios

The Institute for Nuclear Waste Management and Reactor Safety (IEK-6) at the Forschungszentrum Juelich (JUELICH) investigates accident scenarios of gas-cooled High-Temperature Reactors (HTR), especially the air ingress scenario. Fluid mechanic codes and models are developed and validated using in-house experimental databases.This paper describes the development of a computational fluid dynamics (CFD) model with the porous media approach in order to investigate the air ingress scenario: “rupture of the pressure compensation line” at the HTR-Modul.Bases on a brief introduction on this chosen air ingress scenario, the used model equations are explained, as well as the necessary simplifications of the physical model. Afterwards, an overview of the used experiments for validation purposes is given. As an outlook, the assumptions and boundary conditions of the chosen scenario are shown and the setup for the air ingress calculation is presented.Copyright