(585a) Prediction of Radiolytic Benzene Generation in a Waste Tank at the Savannah River Site | AIChE

(585a) Prediction of Radiolytic Benzene Generation in a Waste Tank at the Savannah River Site


Hang, T. - Presenter, Savannah River National Laboratory
Peters, T. B. - Presenter, Savannah River National Laboratory

Back in the 1990s, an in-tank precipitation process at the Savannah River Site (SRS) was designed to remove radioactive cesium and strontium from aqueous high-activity waste.  The process begins when saltcake stored in the SRS Tank Farms is dissolved in water and transferred to the processing tank.  Dilution water is added to adjust the salt concentration.  Sodium tetraphenylborate and sodium titanate slurry are added to the salt solution.  Cs-137 is removed by precipitation with sodium tetraphenylborate, and Sr-90 is removed by adsorption on sodium titanate particles.  The tank dilute slurry is concentrated by filtration.  The resulting decontaminated filtrate salt solution is processed as a low-activity waste.  The concentrated precipitate remaining in Tank 48 is ultimately processed as high-activity waste in the Defense Waste Processing Facility (DWPF) vitrification plant.

The radiolytic generation of benzene in soluble and solid tetraphenylborate (TPB) compounds raised safety concerns.  Benzene, when released from solution, may exceed the lower flammable limit if adequate purge air flow through the tank gas space is not maintained.  Benzene reaches the vapor space by the following mechanisms: (1) formation of dissolved (“free”) benzene by radiolysis of slurries containing TPB compounds; (2) formation of “trapped” benzene by radiolysis of solid TPB compounds; (3) release of “trapped” benzene during dissolution of the solids; (4) adsorption of “free” benzene on the surface of the solids; and (5) evaporation of benzene into the gas space.  To predict benzene emissions, a FORTRAN dynamic model was first developed, and later converted into Aspen Custom Modeler (ACM).  Due to safety issues, the in-tank precipitation process was replaced by other processes (i.e., Actinide Removal Process (ARP) to remove strontium, Modular Caustic Side Cesium Extraction Unit (MCU) to remove cesium).  The processing tank has remained idle since 2000.

This paper discusses the model development and presents the computational results for the designed in-tank precipitation process and for the current processing tank waste content.