(284f) Hydrodynamic Correlations with Experimental Results from Cold Mockup Spouted Beds for Advanced Fuel Particle Coating

The U. S. Department of Energy (DOE) has a large initiative to develop options for the next generation of nuclear reactors. By 2010, one goal is to certify processes for making several types of nuclear reactor fuels that are inherently safe with no melt down issues, have a planned life cycle for all of the initial fuel and fission products by utilizing several additional technologies, and are hardened against proliferation. The AGR (Advanced Gas Reactor) and its fuel production process is the nearest term option. The fuel for AGRs will be a small (~350-micron) uranium fuel particles that are coated by 4 layers: an amorphous (soft) carbon layer, then a pyrolytic (hard) carbon layer, then Silicon Carbide (SiC) layer, and finally another pyrolytic carbon layer. This results in particles with diameters from 500 to 700-micron that are called TRISO particles. The SiC layer provides a containment vessel for the radioactive fission products including gas. The coating process uses a spouted bed for CVD (Chemical Vapor Deposition) at temperatures typically from 650 to 1800 degree C [1]. For the particles to meet quality specifications less than 1 in 10,000 particles can be defective. In order to produce large quantities of fuel to support advanced gas reactors, the spouted bed coaters will have to be scaled-up from the present size of 5-cm diameter. There is no universal way to scale-up fluidized beds even for hydrodynamics [2]. The state-of-the-art in fluidized bed design and scale-up is to establish the hydrodynamics from cold bed studies without reaction [3]. Many researchers are utilizing numerical simulations to address the scale-up problem [4]. As shown by the papers in this and related sessions at this meeting, a combination of detailed experimental measurements and numerical research shows promise in solving parts of the complex scale-up issues. Recent research has identified dimensionless numbers that must be maintained to scale certain characteristics of fluidized beds [5,6]. Our future work includes research to address scaling in spouted fluidized beds. In the high temperature nuclear particle coaters, it is nearly impossible to optically observe and monitor the coater due to the opaque carbon wall of the bed and dense carbon soot inside the bed. The high temperature also limits the use of most sensors. In general, it is expensive to set up and run a high-temperature surrogate coater. Given these constraints, we correlate results obtained from ambient room-temperature coaters using dimensionless groups. In the future, we will test the correlations with selected measurements from a high-temperature coater. Experimental work is underway at the University of Tennessee in Knoxville in several size diameter cold spouted beds with conical bases for six particles sizes, several gas flow rates, temperatures up to 250 degree C, four cone angles, different throat diameters, different particle inventories (weight of particles in bed), and designed distributors (more sophisticated than a conical bottom). The cylindrical column is made out of quartz tubes to have optical access to capture the fountain shape and size with simultaneous 60 fps video images recorded from top and side views or side views 90 degrees apart by two image acquisition cards. Both Zirconium Oxide (ZrO2) and Hafnium Oxide (HfO2) particles are used as surrogates for the uranium particles. The work reported in this paper includes results for a 5-cm spouted bed with different conical bases having a 0.4-cm throat diameter. The experimental matrix is reduced to three cone angles, four particle radii, several gas (humidified air) flow rates, and four particle inventories. Since the fluctuation of the total pressure drop across the spouted bed is the most easily obtained information characterizing the ongoing hot coating process, pressure drop across the cold mockup coater is studied systematically. The total fluctuating pressure drop across the spouted bed over the experimental matrix is measured, analog filtered and archived. The data acquisition system and operator interface are based on National Instrument's PXI 1101 computer, PXI IO cards and LabVIEW. The air velocity to run the bed spans the full spouted condition to minimum spouted condition. The pressure information is analyzed not only with regular statistical methods but also in frequency domain. As expected the total pressure drop decreases as air velocity decreases. Our Umf measurements follow the Wen and Yu correlation closely. Due to the high density of our particles a new Ums correlation is needed. Our dimensionless Ums correlation for ZrO2 particles is presented. A new quantitative method for Ums evaluation is presented. A dimensionless correlation for average pressure drop across the spouted bed covering the experimental matrix is given in terms of U/Ums. The first four moments and the power spectral density of the pressure time series are calculated. The main frequency peak of the pressure drop shifts from high to low as air velocity drops from fully spouted velocity to near to minimum spouting velocity. Meanwhile, the absolute value of the skewness of the pressure increases quickly from 0.1 to 0.4 or even higher than 0.9 depending on particle size. For 53.9 grams of 500 micron particles in 60 degree spouted bed, the main frequency peak is 17.4 ~ 21.4 Hz and the skewness is around 0.1 when the air velocity is between 1.3 and 2.3 Ums. As air velocity is lowered to 1.2 Ums, the main frequency peak increases to 25.5 Hz and the skewness is around 0.49. The frequency and skewness of the total pressure drop could be used to monitor and control the real coating process.

References

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[2] T. M. Knowlton, S. B. R. Karri, A. Issangya, Scale-up of fluidized-bed hydrodynamics, Powder Technology, 150(2005), 72-77.

[3] T. M. Knowlton, S. B. R. Karri, A. Issangya, Scale-up of fluidized-bed hydrodynamics, Powder Technology, 150, 72-77, 2005.

[4] S. Pannala, C. S. Daw, C. E. A. Finney, and D. Boyalakuntla, ?ORNL FY05 Process Modeling Summary Report for the Advanced Gas Reactor Fuel Development and Qualification Program,? Oak Ridge National Laboratory Technical Report, September 2005.

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[6] Glicksman, L. R., Hyre, M. and Woloshun, K., ?Simplified scaling relationships for fluidized beds,? Powder Technology, 77, 177-199, 1993.