Gas Trapping Associated with the Tri-Lab Process to Reduce Emissions

Abstract

To reduce radioactive-gas emissions associated with processing thorium (Th) targets generated through the Tri-Lab process, the gas-trapping system was evaluated, modified, and reassessed to validate improved gas-trapping efficiency. Historical gamma-spectroscopy data from the Oak Ridge National Laboratory (ORNL) Building 4501 stack indicated the release of multiple antimony (Sb) and iodine (I) isotopes, with the two major isotopes being Sb-126 and I-131. During the gas-trapping assessment, ruthenium (Ru) was included with Sb and I in the gas assessment, as it is a product of the Tri-Lab process and is analogous to Osmium (Os), which was inadvertently released from another radiological process in July 2020. The gas-trapping system was altered to pull a slight vacuum on the flask through trapping media to prevent the escape of radioactive gases into the processing hood. In addition, a metered reagent introduction capability was incorporated using a peristaltic-pump line through a port on the dissolution flask to eliminate the requirement for open-flask conditions to the ambient air during Th-target dissolution. A one-liter bubbler trap with sparging frit to reduce bubble size replaced the traditional Erlenmeyer flask bubbler that contained the caustic-trap solution. This modification increased the surface area of the gaseous bubbles in the trap solution and ensured sufficient time for phase transfer of the specified gases from air to liquid. The apparatus was first tested using compounds to generate volatile forms of Sb, I2, and Ru as non-radioactive “cold” gases, separately followed by a combination of the three gases under normal target-dissolution conditions. Spectroscopic methods were utilized to obtain a quick estimate of the amount of gases trapped by the apparatus. A more sensitive ICP-MS method was employed for final quantification and mass balance. Once the “cold” gases were evaluated, radioactive forms of the gases were tested to verify improved gas-trapping efficiency of the specific isotopes. The amount of cold carrier material and gas flow rate was varied to determine the limits for effective gas trapping. The system trapped all radioactive gases at a mass 10-times higher than the amount of gases expected to be produced by the largest Th targets. Although a high flow rate caused problems with the dissolution itself, gas trapping was effective at a gas flow rate up to 5 LPM. The gas trapping system was first implemented in the April 2021 Tri-Lab processing campaign and reduced radioactive emissions by a factor of 10. This system will be used in all further Tri-Lab processing campaigns.