Catalytic combustion of hydrogen as a countermeasure to the risk of deflagradation in a nuclear reactor containment in a severe accident situation: study of the poisoning of catalysts in a representative atmosphere
Franck Morfin has defended his thesis on the 30th June 2000 in the University Claude Bernard Lyon I.
In case of a severe nuclear reactor accident (beyond design basis accident), with extensive fuel rod damage, a significant quantity of hydrogen would be produced in the reactor primary circuit. Build-up of this combustible gas in the air-filled reactor building could eventually lead to an explosion and, in extreme conditions, to the failure of the containment. In situ catalytic combustion of this hydrogen is one of the mitigation techniques proposed to reduce the so-called hydrogen risk in post-accident PWR containments. In a nuclear safety perspective, we have studied the performances of Passive Autocatalytic (hydrogen) Recombiners (or PARs) in a representative atmosphere including, beside air, steam and hydrogen, a complex mixture of aerosol particles and vapours incorporating chemical elements (e.g., I or Te) known to be potential poisons of the catalytic materials making-up the recombiners. This study is based on 1) phenomenological (or "global") experiments, in a containment mock-up (H2-PAR) at the 1/22nd scale, making it possible to investigate into the efficiency of recombiners immersed in the representative atmosphere, and on 2) analytical experiments (catalyst testing) in a small laboratory reactor exposing catalysts to iodine and iodine compounds. The analytical experiments shed light on properties of the catalysts that could not be investigated in the containment mock-up, such as the relative catalytic activity of Pt, Pd and a Pt-Pd alloy, and their resistance to poisoning by I2 and by AgI, CsI, InI or CdI2. The remarkable resistance to poisoning of the industrial catalysts under scrutiny is interpreted as a consequence of the low poison to catalyst ratio and of the diffusion regime of recombiner operation. At last, experiments with iodides showed that a possible modification of the post-accident containment atmosphere induced by the catalysts is worth examining, since it could alter the iodine source-term in a containment fitted out with recombiners.