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Measurement and Analysis of Gas Bubbles near a Reference Electrode in
Aqueous Solutions
Supathorn Phongikaroon,* Steven D. Herrmann, Shelly X. Li, and Michael F. Simpson
Pyroprocessing Technology Department, Idaho National Laboratory, Materials and Fuels Complex, MS 6180,
Idaho Falls, Idaho 83415
Bubble size distributions (BSDs) near a reference electrode (RE) in aqueous glycerol solutions of an electrolyte
NaCl have been investigated under various gas superficial velocities (U). BSD and voltage reading of the
solution were measured by using a high-speed digital camera and a pH/voltage meter, respectively. The results
show that bubble size (b) increases with liquid viscosity (u) and U,. Self-similarity is seen and can be described
by the log-normal form of the continuous number frequency distribution. The result shows that b controls the
voltage reading in each solution. As b increases, the voltage increases because of gas bubbles interrupting
their electrolyte paths in the solutions. An analysis of bubble rising velocity reveals that Stokes' Law should
be used cautiously to describe the system. The fundamental equation for bubble formation was developed via
Newton's second law of motion and shown to be the function of three dimensionless groups-Weber number,
Bond number, and Capillary number. After linking an electrochemical principle in the practical application,
the result indicates that the critical bubble size is 177 pm. Further analysis suggests that there may be 3 000
to 70 000 bubbles generated on the anode surface depending on the size of initial bubbles and provides the
potential cause of the efficiency drop observed in the practical application.1. Introduction
The electrometallurgical treatment of spent nuclear fuel in a
metal form has been demonstrated and is currently in operation
at the Idaho National Laboratory (INL). This treatment is based
on the anodic dissolution of an irradiated metal fuel in a molten
salt electrolyte and the simultaneous deposition and recovery
of uranium metal.' The fission products are separated from
the fuel in the process and are subsequently sequestered in
engineered waste forms.3 To extend the electrometallurgical
treatment technology to oxide-based fuels, a head-end operation
is required to first reduce an oxide fuel to metal. Such a head-
end process is referred to as oxide reduction. In the oxidc-
reduction process, uranium oxide is converted to uranium metal
(cathode) and to oxygen gas (anode) by electrolytic means
within a molten salt electrolyte (LiCI-Li2O) at 650 C.4 The
electrochemical reactions for this process in making U metal
are as follows:
Cathode: U02 + 4e - U + 202-
Anode: 202- 02 (g) + 4e
Net reaction: U02 - U + O (g)
Despite extensive research and development of the oxide-
reduction process, there is still concern regarding the generation
and adequate dispersion of oxygen bubbles from the anode,
which potentially lower the cell efficiency because the bubbles
will react to form Li20 if they are not removed from the system
efficiently. This issue has not been thoroughly investigated and,
therefore, provides the motivation to study the effect of physical
properties and device geometry on gas-liquid interaction in the
electrolytic reduction process. A fundamental mock-up study
for this process has been designed in this investigation by
generating bubbles through a glass frit in water and various
* Corresponding author. E-mail: supathorn.phongikaroon@inl.gov.aqueous glycerol solutions at different flow rates. In addition,
the reasons for choosing these liquid mediums relate to the
physical properties of actual LiCl used in oxide-reduction
operations.
The ultimate goal is to find a way of determining or predicting
bubbles size in molten salt solutions in order to help monitor
the process (events occurring in the oxide reduction). To advance
closely toward this objective, this paper covers the work on (1)
measurement and analysis of bubble size distribution (BSD),
(2) its effect on the reference electrode voltage reading, and
(3) the study of similitude to practical application.
First, a high-speed digital camera with image-analysis
software was used to measure the BSD in aqueous solutions.
Systematic experiments were conducted to determine the
dependency of bubble size on the relevant physicochemical
factors. Fundamental statistical analysis was applied. The bubble
mean diameter (blo) and the area-weighted mean diameter (b32)
were calculated to provide an interpretation of available data
and to facilitate the discussion. A test of similarity was analyzed
to examine the functional form of BSD based on these obtained
statistical parameters.
Second, the percent relative difference (PRD) based on the
baseline voltage reading from the reference electrode of the
solutions at different conditions was calculated and correlated
with mean bubble size. In this section, the idea is to find the
impact of the mean bubble sizes on the voltage reading of the
solutions obtained from the reference electrode. This information
would help in predicting the bubble size in the closed system
based on only the measured voltage reading values from the
reference electrode in the solution.
Last, the recorded images were further used to calculate
bubble rising velocity. Friction factor and flow regime were
determined from bubble size based on physical properties of
the system. In addition, these interpreted data were used to assist
in developing a model for bubble formation via dimensionless
groups and Newton's second law of motion incorporating
electrochemical generation of gas and related to a practical
application of the hot fuel dissolution apparatus (HFDA)
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Phongikaroon, Supathorn; Herrmann, Steve; Li, Shelly & Simpson, Michael. Measurement and Analysis of Gas Bubbles Near a Reference Electrode in Aqueous Solutions, article, October 1, 2005; [Idaho Falls, Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc888649/m1/2/: accessed March 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.