American Nuclear Society 2014 Student Conference – Pennsylvania State University
State College, Pennsylvania, USA, April 3-5, 2013
Application of High Powered Ultrasonics for Tritium Capture and Removal in
Fluoride Salt Cooled High Temperature Reactors
Emory Brown, Floren Rubio, Danny Chaita, Seung Jun Kim, Edward Blandford
Department of Nuclear Engineering
University of New Mexico
1 University of New Mexico, Albuquerque NM
[email protected], [email protected], [email protected], [email protected]
1. INTRODUCTION
Liquid fluoride salts are a leading candidate
heat transport medium for high-temperature
nuclear application. The molten salt mixture
(LiF-BeF2), commonly referred to as FLiBe, is
one of a few fluoride salts under consideration
for use in the Fluoride salt High-temperature
Reactor (FHR) as well as Magnetic Fusion
Energy (MFE) reactor design [1,2]. Molten
salt as a primary coolant for high temperature
reactors has several engineering and safety
advantages (i.e., high thermal efficiency,
optically clear, well understood fluid flow
properties) [2]. However, FLiBe presents
tritium management challenges to high
temperature reactors. One of the criticisms of
the original ORNL MSR program from a
commercialization standpoint was the issue
surrounding tritium management [2]. Tritium
is produced in the FLiBe as lithium and
beryllium atoms absorb neutrons. The
techniques for reducing tritium inventory can
generally be thought of in four successive
stages: tritium production, tritium transport,
tritium barriers, and tritium recovery. Over
several decades, molten salt tritium recovery
systems have been investigated using various
separation techniques.
There are two distinct approaches to dealing
with tritium transportation. One is to prevent
the migration of tritium though the secondary
and tertiary loops. This can be done by special
oxide coatings on the walls of all components
in contact with the tritiated FLiBe [3]. If this
can be done, then the tritium can be dealt with
chemically while replenishing or purifying the
FLiBe coolant. Although this approach will
almost certainly makes its way into most
working designs as a redundant safety feature,
the possibility of degradation (mechanical and
chemical) in the coating makes it unfeasible by
itself. The other way to manage tritium
migration is with tritium ‘getters’. There are a
handful of different methods of ‘getting’ the
tritium from the salt, but all of them work by
actively removing the tritium from the salt.
Graphite is a known getter of tritium from the
MSRE project at ORNL. Approximately 15%
of the tritium inventory was found to be
absorbed into the graphite moderator [4].
Another getter design is a double walled heat
exchanger with yttrium as the intermediate
fluid between the walls [5]. Yttrium readily
forms with tritium to YT2 and can be pumped
to a chemical separation facility to release the
tritium and recycle the yttrium. A third option
for getting tritium in the FHR is to sparge inert
(helium) gas into the liquid phase of FLiBe to
strip the dissolved tritium by bubbly flow mass
transfer between bubble and liquid in
horizontal pipe using by-pass flow.
Gas-liquid contacting two-phase flow in
horizontal pipe is a widely studied field with
broad interest across the chemical and
separation process industries. A number of
gas-liquid contacting separation devices
operate under bubbly flow conditions in order
Emory Brown, et al.,
to attain large interfacial surface areas between
bubble and liquid for enhanced mass transfer.
Experimental observations are also difficult in
this case due to the upward migration of
dispersed bubbles towards the top of the pipe
due to buoyancy. This causes a highly nonsymmetric volume fraction distribution in the
pipe cross-section and makes visualization of
bubble dynamics challenging [6]. In this
paper, we investigate the use of ultrasound
technology for enhancing both mass transfer as
well as bubble extraction efficiency
2. Current Work
The objectives for this experiment are to better
understand
the
key
non-dimensional
parameters that govern mass transfer in a
bubbly two-phase flow and to apply this
understanding to improve the overall in-situ
tritium removal in a commercial FHR. This
understanding can be applied to various other
chemical and nuclear engineering applications
dealing with gas removal from an insoluble
fluid. To understand the governing processes
in two-phase bubbly flow, three physical
models have been reviewed.
2.1. Physical Models
After an extensive literature review, the
Master’s thesis of T.S Kress [7] revealed four
models that were key in understanding the
mass transfer in a two-phase bubbly flow. Of
the four, three were chosen as relevant to our
objective and are detailed below.
2.1.1. Surface Renewal Model
The surface renewal model can generally be
envisioned by imagining the interface as being
adjacent to a semi-infinite fluid through which
turbulent eddies having uniform concentration
characteristic of the continuous phase,
periodically penetrate to “renew” the surface.
The mass transfer them depends on the rate
and depth of eddy penetration and the eddy
residence time near the surface or the
distribution of eddy ages. For a given, the
original models are essentially solutions of the
diffusion equation [8,9]:
𝜕𝐶
𝜕!𝐶
=𝐷 !
𝜕𝑡
𝜕𝑦
Where, C is the concentration of dissolved gas
in liquid continuum, and D is the diffusivity of
dissolved gas. To establish an overall mass
transfer rate, it is necessary to assign a
frequency with which the surface are renewed
or the distribution of eddy ages. This model
does not give significant information as to the
effect of bubble size, conduit size, or Reynolds
number. Therefore the surface renewal model
was not the selected model for studying the
hydrodynamic effect on mass transfer.
2.1.2. Modeling of the Eddy Structure
If the fluid velocity field in the vicinity of the
interface could be completely described, then
the computation of transfer rates would be
straightforward. However, there are no
satisfactory descriptions of the details of a
turbulent velocity field and even if such were
available; the mathematical accounting of the
differential transfer processes might become
intractable. Consequently, there have been
idealizations for the eddy structure with
unrealistic fields and mass transfer behavior
has been computed based on these
idealizations of eddy structure [10].
Lamont calculated the mass transfer
coefficient for an individual eddy cell as a
function of the damping condition, fluid
properties, the wave properties, and the eddy
American Nuclear Society 2014 Student Conference – Pennsylvania State University
State College, Pennsylvania, USA, April 3-5, 2013
2/5
Short version of title as entered by author on web page
energy. His calculation for overall mass
transfer coefficient were
Sh~𝑆𝑐!/! 𝑅𝑒 !.!"
2.2. Enhancements
2.1.3. Turbulence Interactions
Some researchers have attempted to analyze
the forces and interactions between spheres
and fluid elements in a turbulent field to arrive
at equations for the fluctuating motion of the
spheres. These equations are solved to obtain a
“mean” relative velocity between the bubble
and the fluid, which is then substituted into a
steady-flow equation to establish the mass
transfer coefficients. The work of Levish is of
this nature and Peebles used same approach in
his dissertation [11,12]. For example, Peebles
used the result of Hinze for small gas bubbles.
𝑣!! ≅ 3 𝑣!!
Eq. 1
The relative velocity is then
𝑣! =
considered by adding ratio of mean bubble
diameter to conduit diameter.
𝑣!! − 𝑣!! = 2 𝑣!!
Peebles used the approximations:
𝑣!! ~𝑉 𝑓/2 𝑎𝑛𝑑 𝑓~𝑅𝑒 !!/!
Eq. 2
Where 𝑣! , 𝑣! , 𝑎𝑛𝑑 𝑣! are the velocity of gas,
liquid and relative bubble velocity.
Then substituted equations (1) to (2) into a
steady-state equation to obtain mass transfer
rate,
𝑑 !!/!
!.!"
!/!
Sh~𝑅𝑒 𝑆𝑐
𝐷
where, d is the mean bubble diameter, and D is
the diameter of pipe channel. In this model, the
hydrodynamic effect on the mass transfer is
Through study of the three different models, it
becomes apparent that three non-dimensional
numbers, Reynolds, Weber, and Schmidt, must
be matched to achieve a physically realizable
model. To manipulate the mass transfer (𝐾! )
in gas-liquid system we can vary the solution’s
velocity (𝑣), density (𝜌), viscosity (𝜇), the
average bubble diameter (d), the pipe channel
diameter (D), the solution’s diffusivity (𝐷! ),
and surface tension (σ) in the following
equations.
𝐾!
𝐷 !
~ 𝑅𝑒 !
𝑣
𝑑
𝐾! 𝑑
~ 𝑅𝑒 ! 𝑆𝑐 !
𝐷!
𝐾!
~ 𝑊𝑒 ! 𝑅𝑒 !
𝑣
2.2.1. In-line mixing vanes
After making preliminary runs on our simulant
fluid loop with a horizontal test section, it
became apparent that the buoyant forces
quickly overcame the turbulent forces, leading
to a stratified two-phase flow. An ideal
solution would be to decrease the bubble size
to the point that the buoyant forces aren’t as
large, while also increasing the surface area to
volume ratio. While this is a step we will take,
another solution was to install in-line mixing
vanes. Doing so will allow us to study the
effect of convective mass transfer.
2.2.2. High-powered ultrasonics
From the physical models previously
mentioned, the convective and diffusive
properties govern mass transfer in two-phase
Emory Brown, et al.,
bubbly flow. We hypothesize that the sonomechanical effects, when applied to a bubbly
flow mixture, will increase both the convective
and diffusive properties.
done using LiCl-KCl eutectic salt in a
transparent furnace with a ultrasonic
transducer coupled to the quartz crucible.
By introducing acoustic energy into the fluid,
we expect two phenomena to occur. The first
is increased turbulence in the mixture. This
can be readily seen in any submerged highpowered ultrasonic device as the energy
dissipates through the medium. This increases
the Reynolds number of the fluid and as the
Eddy structure and turbulence interaction
models predict, will increase the mass transfer.
The second phenomena we expect to see is the
oscillation of bubble size. Typically,
ultrasonics are used to induce cavitation in the
liquid. This would be detrimental to the mass
transfer as the gas is forced back into the
liquid, as well as increasing mechanical wear
as the shockwave from the bubble collapse
attacks the pipe wall. However, if you operate
just below the cavitation threshold we expect
the diffusive properties of the mixture will be
increased, as the surface renewal model
suggests.
To gather data on these phenomena, two
experiments will be conducted. The first will
be conducted on the current recirculating loop.
It will consist of a ring transducer test section
followed by a dissolved oxygen metering
section to observe the changes in the gas
concentration.
Figure 1: Ring transducer model
The other experiment that will be conducted
will use prototypical salts to better understand
the non-dimensional scaling of ultrasonic
energy in a two-phase system. This will be
Figure 2. Molten salt ultrasonic experiment
setup
Similar to the first experiment, an oxygen
meter will be used to observe the sonomechanical enhancements in mass transfer.
3. CONCLUSION
Tritium is formed in relatively large amounts in
MSRs and FHR’s. By understanding the current
bubble mass transfer models, the first steps in
tritium control and mitigation system designs can
be taken. In the context of the FHR project, this
work can be seen as the beginning of an optimized
tritium sparging and capture system. In the near
term, the next steps in this project will be to
thoroughly
characterize
the
effects
and
enhancements of inline swirling vanes and highpowered ultrasonics on the dissolved gas to bubble
mass transfer behavior.
ACKNOWLEDGMENTS
This template was adapted from the template
for PHYSOR 2002 posted on the Internet.
Acknowledge the help of colleagues, and
sources of funding, if you wish.
American Nuclear Society 2014 Student Conference – Pennsylvania State University
State College, Pennsylvania, USA, April 3-5, 2013
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Short version of title as entered by author on web page
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