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) . 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 . 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 . 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 . Another getter design is a double walled heat exchanger with yttrium as the intermediate fluid between the walls . 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 . 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  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]: ! = ! 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 . 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~!/! !.!" 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. !! ≅ 3 !! Eq. 1 The relative velocity is then ! = considered by adding ratio of mean bubble diameter to conduit diameter. !! − !! = 2 !! Peebles used the approximations: !! ~ /2 ~ !!/! Eq. 2 Where ! , ! , ! 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, !!/! !.!" !/! Sh~ 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 (! ) in gas-liquid system we can vary the solution’s velocity (), density (), viscosity (), the average bubble diameter (d), the pipe channel diameter (D), the solution’s diffusivity (! ), and surface tension (σ) in the following equations. ! ! ~ ! ! ~ ! ! ! ! ~ ! ! 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. 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