Investigation of Flammable Gas Releases from High Level Waste Tanks during Periodic Mixing

The Savannah River Site processes high-level radioactive waste through precipitation by the addition of sodium tetraphenylborate in a large (approximately 1.3 million gallon) High Level Waste Tank. Radiolysis of water produces a significant amount of hydrogen gas in this slurry. During quiescent periods the tetraphenylborate slurry retains large amounts of hydrogen as dissolved gas and small bubbles. When mixing pumps start, large amounts of hydrogen release due to agitation of the slurry. Flammability concerns necessitate an understanding of the hydrogen retention mechanism in the slurry and a model of how the hydrogen releases from the slurry during pump operation. Hydrogen concentration data collected from the slurry tank confirmed this behavior in the full-scale system. These measurements also provide mass transfer results for the hydrogen release during operation. The authors compared these data to an existing literature model for mass transfer in small, agitated reactors and developed factors to scale this existing model to the 1.3 million gallon tanks in use at the Savannah River Site. The information provides guidance for facility operations