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].