Scaled Testing of Hydrogen Gas Getters for Transuranic Waste

Alpha radiolysis of hydrogenous waste and packaging materials generates hydrogen gas in radioactive storage and shipment containers. Hydrogen forms a flammable mixture with air over a wide range of concentrations (5% to 75%), and very low energy is needed to ignite hydrogen-air mixtures. For these reasons, the concentration of hydrogen in waste shipment containers (Transuranic Package Transporter-II or TRUPACT-II containers) needs to remain below the lower explosion limit of hydrogen in air (5 vol%). Accident scenarios and the resulting safety analysis require that this limit not be exceeded. The use of 'hydrogen getters' is being investigated as a way to prevent the build up of hydrogen in TRUPACT-II containers. Preferred getters are solid materials that scavenge hydrogen from the gas phase and chemically and irreversibly bind it into the solid state. In this study, two getter systems are evaluated: a) 1,4-bis (phenylethynyl)benzene or DEB, characterized by the presence of carbon-carbon triple bonds; and b) a proprietary polymer hydrogen getter, VEI or TruGetter, characterized by carbon-carbon double bonds. Carbon in both getter types may, in the presence of suitable precious metal catalysts such as palladium, irreversibly react with and bind hydrogen. With oxygen present, the precious metal may also eliminate hydrogen by catalyzing the formation of water. This reaction is called catalytic recombination. DEB and VEI performed satisfactorily in lab scale tests using small test volumes (ml-scale), high hydrogen generation rates, and short time spans of hours to days. The purpose of this study is to evaluate whether DEB and VEI perform satisfactorily in actual drum-scale tests with realistic hydrogen generation rates and time frames. The two getter systems were evaluated in test vessels comprised of a Gas Generation Test Program-style bell-jar and a drum equipped with a composite drum filter. The vessels were scaled to replicate the ratio between void space in the inner containment vessel of a TRUPACT-II container and volume of a payload of seven 55-gallon drums. The tests were conducted in an atmosphere of air for 60 days at ambient temperature (15 to 27 deg. C) and a scaled hydrogen generation rate of 2.60 E-07 moles hydrogen per second (0.35 cc/min). Hydrogen was successfully 'gettered' by both systems. Hydrogen concentrations remained below 5 vol% (in air) for the duration of the tests. However, catalytic reaction of hydrogen with carbon triple or double bonds in the getter materials did not take place. Instead, catalytic recombination was the predominant mechanism in both getters as evidenced by 1) consumption of oxygen in the bell-jars; 2) production of free water in the bell-jars; and 3) absence of chemical changes in both getters as shown by NMR spectra. (authors)