This study assesses the recombination rate of a passive autocatalytic recombiner (PAR) with lattice-shaped catalysts under severe accident conditions. In order to determine the recombination rate from the difference in the hydrogen mass flow rate at the inlet and outlet of the PAR, an equation was developed using experimentally measurable data on hydrogen concentration, flow rate, pressure, and temperature.

The PAR was installed within an 80-m3 test vessel capable of replicating the atmospheric conditions of a severe accident involving the rapid generation of hydrogen due to cladding oxidation following a loss of coolant in the OPR1000. The gas mixture was drawn through 1/4-in. tubing at the PAR inlet and outlet to measure the concentration of dry hydrogen, excluding steam, using a thermal conductivity sensor positioned outside the vessel. The hydrogen concentration was adjusted to account for factors such as time delays, thermohydraulic changes, and steam levels within the test vessel during the sampling process.

The average recombination rate was lower when the hydrogen concentration around the PAR was increasing than when it was decreasing. It was discovered that the calculation of the hydrogen recombination rate was largely influenced by the disparity in the dry hydrogen concentration between the PAR inlet and outlet. The continuous sampling of gas from the PAR outlet can lead to fluctuations in the calculation of the recombination rate due to condensation of high-temperature steam produced by the exothermic reaction.