Influence of Passive Autocatalytic Recombiners on Iodine Volatility: THAI Technical Scale Experiments

Passive Autocatalytic Recombiners (PARs) in PWR-Containments remove hydrogen released in case of a severe accident with core damage by catalytic oxidation with the oxygen of the containment atmosphere. The removal of hydrogen in autocatalytic recombiners (PARs) results in temperature levels at the catalytic surfaces up to 900 °C and leads to elevated temperatures up to several hundred degrees of the gas flowing over these surfaces. Under such operating conditions suspended CsI and other iodide particles transported with the convective gas flow through PARs can be converted into volatile iodine thus influencing the iodine source term. Even low conversion rates might lead to a significant influence on the concentration of gaseous iodine in the early phase of an accident where high CsI/I₂ ratios of 100:1 or even higher can be expected. In the frame of the German national THAI programme two technical-scale experiments, AER-2 and AER-5, have been performed to investigate the conversion rates CsI → I₂ occurring under realistic PAR operating conditions and beyond, and the influence of PARs on CsI aerosol parameters. An original SIEMENS (now AREVA NP) type PAR has been operated under realistic thermalhydraulic conditions. CsI aerosol has been generated by evaporating ultra pure CsI in an inductive furnace and monitored in the THAI vessel, following re-condensation, by low pressure impactors and filters. Experiments with hydrogen concentrations of up to 5 vol% H₂ under normal conditions and of up to 10.5 vol% H₂ in a steam-inerted atmosphere have been performed. An approved and qualified sampling and detection method has been applied to determine gas borne molecular iodine concentrations. CsI → I₂ conversion rates of 3% have been observed as an upper limit under realistic thermalhydraulic conditions. The results are compared to CsI → I₂ conversion rates achieved in the French RECI experiments. These experiments investigated the interaction of catalytic material and metal iodides (CsI, CdI) in a lab-scale setup showing conversion rates up to 60%. However, these experiments might deviate from representative thermochemical conditions. A comparison of THAI and RECI results — as far as appropriate concerning aerosol parameters and gas temperatures — shows conversion rates in comparable dimensions. In other cases they differ significantly. The potential contribution of an operating PAR (metal iodide interaction) to the iodine source term together with the restricted knowledge from both THAI and RECI tests, means that further investigations are required.