Effective modeling of hydrogen mixing and catalytic recombination in containment atmosphere with an Eulerian containment code

Large amounts of hydrogen can be generated in the containment of a nuclear power plant following a postulated accident with significant fuel damage. Different strategies have been proposed and implemented to prevent violent hydrogen combustion. An attractive one aims to eliminate hydrogen without burning processes; it is based on the use of catalytic hydrogen recombiners. This paper describes a simulation methodology which is being developed by Ansaldo, to support the application of the above strategy, in the frame of two projects sponsored by the Commission of the European Communities within the IV Framework Program on Reactor Safety. Involved organizations also include the DCMN of Pisa University (Italy), Battelle Institute and GRS (Germany), Politechnical University of Madrid (Spain). The aims to make available a simulation approach, suitable for use for containment design at industrial level (i.e. with reasonable computer running time) and capable to correctly capture the relevant phenomenologies (e.g. multiflow convective flow patterns, hydrogen, air and steam distribution in the containment atmosphere as determined by containment structures and geometries as well as by heat and mass sources and sinks). Eulerian algorithms provide the capability of three dimensional modelling with a fairly accurate prediction, however lower than CFD codes with a full Navier Stokes formulation. Open linking of an Eulerian code as GOTHIC to a full Navier Stokes CFD code as CFX 4.1 allows to dinamically tune the solving strategies of the Eulerian code itself. The effort in progress is an application of this innovative methodology to detailed hydrogen recombination simulation and a validation of the approach itself by reproducing experimental data. The GOTHIC predicted wall surfaces temperatures are assigned to each patch of the CFX model. The GOTHIC gas mixture pressure, volume average temperature and gas composition is assigned as a uniform field in all the CFX domain (initial guess solution). Since the goal is to calculate a quasi-steady state condition in CFX a set of mathematical steady-state boundary conditions has to be defined for this calculation This is achieved by assuming that, at the selected time, the hydrogen injection rate and the steam injection rate are equal to hydrogen recombination and steam condensation rates calculated by GOTHIC, even if there is a mismatch between the quantities in the long term. The CFX simulation will consider a fully-compressible, multicomponent now (air, steam, hydrogen). The recombiner model will be included in the CFX model. The recombiner itself is modelled as a solid region and its physical inlet and outlet are the outlet and the inlet of the CFX computational domain respectively. The gas mixture conditions at the recombiners boundaries (temperature, composition and velocity) are calculated via the CFX FORTRAN-interface which represents the CFX recombiner model. The activity to tune this linking procedure by benchmarking against Zx- experiments is in progress.