Experimental study on H₂-CO combustion and recombination in severe accidents

Canadian Nuclear Laboratories (CNL, formerly Atomic Energy of Canada Limited - AECL) has established comprehensive R&D studies on hydrogen combustion and mitigation since 1980s. To prevent hydrogen explosion, various hydrogen mitigation measures have been implemented in the reactor containment buildings, including deliberate igniters and passive autocatalytic recombiners (PARs). Following the Fukushima accident, concerns on hydrogen safety for severe accidents (SAs) have been increasingly raised in the international community. In addition to hydrogen, a significant amount of carbon monoxide (CO) can be produced in SAs from ex-vessel molten core concrete interaction. CO is a combustible and toxic gas that creates safety issues. Assessment of scenarios involving ex-vessel interactions requires additional attention to the potential contribution of CO to combustion loads in containment. To reduce the risk of CO combustion, mitigation measures are desirable (for example, catalytic oxidation of CO to CO₂ using PARs or deliberate ignition). Experimental data on hydrogen combustion and recombination are abundant, but the data on H₂-CO-air mixtures are very limited. Collaborative research was initiated between the Canadian Nuclear Safety Commission (CNSC) and CNL to fill the knowledge gaps on H₂-CO combustion and recombination and to provide data for model validation. Two series of experiments were performed in CNL's large scale vented combustion test facility. The first series of tests examined the combustion behavior using H₂-CO-air mixtures with different H₂ vs. CO ratios under either initially quiescent or turbulent conditions. The second series of tests studied PAR performance in the presence of H₂-CO-air mixtures, including self-start behavior, recombination rate and efficiency, and ignition induced by hot catalyst plates. In this presentation, flammability measurements of H₂-CO-air mixtures with different H₂-CO ratios will be demonstrated. The combustion dynamics and overpressure of H₂-CO-air mixtures will be compared against the H₂-air mixtures under both quiescent and turbulent conditions. PAR characteristics (i.e., self-start, recombination efficiency and ignition) with H₂-CO-air mixtures will be compared against the H₂-air mixtures. The experimental data are important for severe accident management and useful for model validation and development. (author)