Karl T. Chuang

Development of a wetproofed catalyst recombiner for removal ofairborne tritium

Abstract For cleanup of airborne tritium at tritium handling facilities, it is generally agreed that the most reliable method is to convert the tritium in a recombiner into water vapor followed by adsorption of the vapor in a molecular sieve drier. Decontamination factors of 103 to 106 have been reported. Wetproofed catalysts developed at Chalk River Nuclear Laboratories have been shown to maintain their activities when exposed to liquid water or air at 100% relative humidity. When a wetproofed catalyst recombiner is used, operation can be carried out at room temperatures thus greatly simplifying the system. Two catalysts, Pt/carbon and Pt/silica, were prepared for this study. The activity of Pt/carbon was measured with hydrogen and found to be comparable to the published results for conventional Pt/alumina catalysts at similar conditions. The results were best represented by a rate equation, rate = 0.045 (hydrogen concentration) 1.57. The activity of the Pt/silica catalyst was similar to the Pt/carbon catalyst; however, it could be increased by a factor of five by exposure to air at 150°C containing 50–200 ppm hydrogen. Experiments were carried out for the following range of operating conditions: flows from 0.3 to 3.0 m s−1, pressure from 100 to 500 kPa. Tritium was added to the air stream at 1–5 MBq m−3. No significant isotope and/or pressure effects were observed. To date lifetime data of greater than four months have been obtained.

SAFE RECOMBINATION OF HYDROGEN AND OXYGEN WITH WETPROOFED CATALYSTS

This paper describes a new and versatile recombiner technology based on the use of wetproofed catalysts developed at the Chalk River Nuclear Laboratories (CRNL) of Atomic Energy of Canada Limited (AECL). The catalysts consist of finely dispersed noble metals deposited on noncombustible hydrophobic substrates and coated with wetproofing agents such as silicones or polytetrafluoroethylene (PTFE). Depending on the hydraulic and efficiency specifications, the catalyst may be in the form of spheres, saddles, rings or ordered screens. n nWhen the hydrogen concentration in the feed stream exceeds 2%, the recombination is carried out in a trickle bed reactor where liquid water is used as a coolant to remove the heat of reaction. At concentrations below 2% the recombiner operates without supplementary cooling. n nThe process has been demonstrated both in the laboratory and pilot plants with hydrogen flow rates up to 19 L/s. Current applications range from off-gas treatment systems for nuclear fuel reprocessing plants and nuclear reactors to purification of electrolytic hydrogen or oxygen and removal of airborne tritium. Future uses such as passive safety devices for use in maintenance free storage batteries or accidental hydrogen production are also under investigation.