Passive Catalytic Hydrogen Mitigation

Abstract

Hydrogen accumulating in reactor containment during postulated accidents has the potential, if ignited, to produce temperatures and overpressures that could damage structures or equipment in the vicinity. Currently, the management of hydrogen produced in the aftermath of a loss-of-coolant accident depends on rapid dilution of the hydrogen with containment air and active, deliberate ignition of local flammable volumes at safe concentrations. A means of removing the hydrogen before flammable concentrations are reached would be preferred. Catalytic oxidation is a common way of removing hydrogen from gas streams, but conventional catalyst formulations are deactivated by water at ambient temperatures. However AECL has been working to develop wetproofed catalysts that retain their activity in the presence of water and water vapour, with the results that passive catalytic removal of hydrogen is now possible. These catalysts operate with a low pressure drop, and new catalyst formulations enable high-temperature operation without damaging the catalyst. This paper examines the feasibility of systems for passive hydrogen mitigation below flammable concentrations. Two concepts are considered: installation of recombiner catalyst beds in sections of forced circulation ducts, and natural convection-driven recombiner modules in containment subvolumes where mixing may be impaired. The performance of a prototype natural convection recombiner in a humid atmosphere was demonstrated in tests conducted in the Containment Test Facility at AECL's Whiteshell Laboratories. A 5-L cylindrical catalyst module was used, and continuous hydrogen consumption rates of 36 std L/h were observed in a 5-m3 volume in a 1.3% hydrogen air steam atmosphere. The catalyst temperature remained safely below 125 degrees C. The catalyst is used more efficiently with forced convection. Experiments and modelling using forced convection show the catalyst is capable of maintaining its high conversion efficiency with gas residence times lower than 0.1 s.