Effect of Oxygen Bubble Distributions On Rare Earth Precipitates In Molten Salts

Effect of Oxygen Bubble Distributions on Rare Earth Precipitates in Molten Salts

Ryan W. Bezzant and Supathorn Phongikaroon

University of Idaho-Idaho Falls

Center for Advanced Energy Studies

1776 Science Center Dr. Idaho Falls, ID 83402   Michael F. Simpson Pyroprocessing Technology Department Idaho National Laboratory Idaho Falls, ID 83402

Abstract:

Pyroprocessing technology has been developed at the Idaho National Laboratory for treating spent nuclear fuel from Experimental Breeder Reactor-II.  An electrorefiner (ER) is the key unit operation of this process, recovering uranium metal via anodic dissolution and electrodeposition at a cathode.  During this process, minor actinides, fission products and rare earths (RE) form chlorides and accumulate in the LiCl-KCl eutectic salt.  This results in excess heat generation and other effects which lower the system efficiency.  To maintain ER performance, it is necessary to remove contaminated salt and refill with fresh salt.  But this approach is costly and generates a high waste volume.

Researchers at the Korea Atomic Energy Research Institute have demonstrated oxygen sparging as an effective way of separating rare earth chlorides from molten salt to partially prepare it for reuse.  Rare earth chlorides react with oxygen at high temperatures.  Rare earth oxides and oxychlorides form and precipitate to the bottom of the salt container. After the salt cools, the salt can be mechanically separated from the rare earth fission products.  Aside from a demonstration of the process, no fundamental aspects of this system are well understood. 

Thus, to gain insight into physiochemical parameters for systematic scale-up, such as mass transfer rate and gas holdup fraction, a transparent furnace with an oxygen sparging system was constructed with the capability of operating under an argon environment at 500 °C.  LiCl-KCl-RECl₃ ternary salt was loaded in the quartz crucible to study the effect of oxygen bubble sizes on rare earth precipitate in the system. A simple two blade, downward pitch turbine was used for mixing the bubbles in the medium with a rotational speed varying between 100 and 250 RPM. The oxygen flowrate was varied between 0.05 L/min and 0.25 L/min.  Oxygen concentration was detected by an oxygen sensor.  Bubble sizes were captured by using a high speed digital camera.  Detailed results will be presented and discussed.