Hydrogen recombiners are passive autocatalytic devices intended to mitigate the risk of a hydrogen explosion in the reactor building of a nuclear power plant under the hypothetical conditions of a severe (beyond design basis) accident. They are made up of catalytic materials (e.g., platinum and palladium on porous alumina) encased in a metallic "chimney" designed to boost the convective flow powered by the heat (242 kJ/mol) generated through the so-called recombination reaction H2 + ½ O2 → H2O on the catalyst. In the industrial plate-type recombiner (Heck and Hill, 1992), the gas circulates between catalyst-coated vertical sheets arranged in parallel, that can be heated up to 900°C or even more, depending on the hydrogen concentration. In case of a core-melt accident, the reactor containment would be filled with a mixture of hydrogen and steam in air laden with aerosol particles carrying the largest fraction of fission products. Performing as a very efficient chemical reactor, the recombiner in operation could likely tamper with aerosol chemistry in the containment atmosphere. For example, metal iodide particles (mainly caesium iodide, but also silver, indium and cadmium iodides), representing by far the largest radioactive iodine inventory in this atmosphere, could be partially converted into gaseous iodine upon crossing the hot recombiner. Since radioiodine (isotopes 129 to 135), and specially its gaseous forms dominate the radiological consequences of all core meltdown scenarios (Hosemann and Hassman, 1986), it is legitimate to investigate into the dissociation of metal iodides within the physical and chemical environment that characterises a recombiner in operation.
This work has been done with "Hemisphères", France.