Article pubs.acs.org/est
Temperature Distribution within a Cold Cap during Nuclear Waste Vitrification Derek R. Dixon,*,† Michael J. Schweiger,† Brian J. Riley,† Richard Pokorny,‡ and Pavel Hrma† †
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States Department of Chemical Engineering, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic
‡
ABSTRACT: The kinetics of the feed-to-glass conversion affects the waste vitrification rate in an electric glass melter. The primary area of interest in this conversion process is the cold cap, a layer of reacting feed on top of the molten glass. The work presented here provides an experimental determination of the temperature distribution within the cold cap. Because direct measurement of the temperature field within the cold cap is impracticable, an indirect method was developed in which the textural features in a laboratory-made cold cap with a simulated high-level waste feed were mapped as a function of position using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The temperature distribution within the cold cap was established by correlating microstructures of cold-cap regions with heat-treated feed samples of nearly identical structures at known temperatures. This temperature profile was compared with a mathematically simulated profile generated by a cold-cap model that has been developed to assess the rate of glass production in a melter.
1. INTRODUCTION The current plan for immobilizing the 200 000 m3 of radioactive nuclear waste stored in tanks at the Hanford Site is through vitrification into glass.1−5 Vitrification will be performed in the Hanford Tank Waste Treatment and Immobilization Plant (WTP), which is currently being constructed by Bechtel National, Inc., for the U.S. Department of Energy.1 The WTP facility will receive the waste from the storage tanks, separate it into low-activity waste (∼90% of the waste mass) and high-level waste (HLW) fractions (∼95% of the waste radioactivity), add the desired glass-forming chemicals, and melt the waste into glass in joule-heated ceramic melters.2 As the waste slurry feed is charged into a melter, a layer of reacting feed floats on the molten glass forming a cold cap.6−11 Gases from the reacting feed and glass oxidation−reduction reactions accumulate around ∼700 °C within the newly connected, transient glass-forming melt to form a foam layer. This foam layer restricts the heat transfer to the reacting feed and ultimately limits the rate of glass production.9−11 Mathematical models of glass melting in an electric furnace tend to ignore the effect of the cold cap on melter performance.12−14 A few models have considered the interactions between the reacting feed and the melt surface,15,16 but until recently, the foam layer between the reacting feed layer and the melt has not been considered.17 Pokorny and Hrma9,10,18 developed a model that accounts for these factors and their effects on glass production. The model calculates the temperature distribution within the cold cap along with various © 2015 American Chemical Society
other parameters, such as the total cold-cap thickness and the melting rate (kg m−2 day−1).10 To accomplish this, the model splits the cold cap into four regions: (1) the reacting feed layer, (2) the primary foam layer, (3) cavities, and (4) the secondary foam layer (the latter three regions constitute the foam layer mentioned above).10 As initial input, the model requires values of the transition temperatures between (1) the primary foam and cavities and (2) the cavities and secondary foam.10 For model verification, values for the thickness of the coldcap regions are desired. The key to obtaining these values is to know the temperature distribution within the cold cap. The laboratory-scale melter (LSM), shown in Figure 1, was designed to simulate the vitrification process of nuclear waste glass in an electric melter.6,7,19,20 Because the LSM produces cold-cap samples suitable for sectioning and analysis, we attempted to determine the cold-cap temperature distribution based on the analysis of one of these samples. Previous work focused on the effect of the charging rate on the cold-cap structure6 and on comparing the cold-cap features with heattreated feed samples.7 This work takes it a step further by obtaining structural details as a function of position within the cold cap and then estimating the temperature distribution by comparing them with structural features of feed samples with well-defined temperature histories. The following sections Received: Revised: Accepted: Published: 8856
February 20, 2015 June 11, 2015 June 12, 2015 June 25, 2015 DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
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Environmental Science & Technology Table 1. A0 Feed Composition for 400 g of Glass compound
amount (g)
Al(OH)3 B(OH)3 Bi(OH)3 CaO Fe(H2PO2)3 Fe(OH)3 KNO3 Li2CO3 Mg(OH)2 Na2C2O4 Na2CrO4 Na2SO4 NaF NaNO2 NaOH NiCO3 Pb(NO3)2 SiO2 Zn(NO3)2·4H2O Zr(OH)4·0.65H2O total
146.99 107.93 5.12 24.31 4.97 29.53 1.22 35.32 0.68 0.50 4.46 1.42 5.91 1.35 39.77 2.54 2.43 122.02 1.06 2.19 539.74
LSM crucible (Figure 1). The crucible was placed into a Deltech furnace (Deltech, Inc., Denver, CO) preheated to 1200 °C through a 10.2 cm diameter hole in the top to rest on a platinum support. Two thermocouples were inserted through the side port of the crucible. One was positioned by the wall in the molten glass and the other was bent so the tip was located at the center of the crucible in the glass melt directly under the cold cap. After the initial glass in the crucible melted and reached a plateau temperature between 1100 and 1200 °C as recorded by the melt-edge thermocouple, the feed line was connected to an injection nozzle, and the feed pump was started at a charging rate of 7.5 mL min−1. The feed injection nozzle was cooled by the 10 °C chiller water through an external Chrom Tech Flash pump (Chrom Tech, Inc., Apple Valley, MN). When the first drop of feed slurry contacted the molten glass, the nozzle was fastened to the crucible, and the run time began. The feed slurry was continuously stirred and pumped using a Masterflex L/S Precision Pump (Cole-Parmer, Vernon Hills, IL) to fill the feed line. Water for the off-gas condensation column was cooled to 10 °C in a ThermoFlex 900 Chiller (Thermo Fisher Scientific, Inc., Waltham, MA). The thermocouples in the crucible were attached to a data acquisition system (HYDRA 2620, Fluke Corporation, Everett, WA) set to record the temperature in 20-s intervals. The angled off-gas port was attached to the off-gas condensation column with the exhaust port vented into a fume hood. Charging the feed into the crucible resulted in vigorous evaporation of the water and evolution of batch gases. Slow accumulation of a coating on the inside of the crucible with spattered feed made it increasingly difficult to view the cold cap and the melt as the run progressed beyond ∼30 min. The feed was charged for a 35 min run-time. At this point, the feed pump was turned off, the crucible off-gas port was disconnected from the condensation column, and finally the crucible was removed from the furnace and rapidly quenched on a large copper block. As the LSM crucible cooled, thermal expansion mismatches between the fused-silica and the glass and between the glass and
Figure 1. (color online) Schematic of the LSM. The pump on the left charges slurry feed into the crucible, and the pump on the right circulates cooling water from the chiller to the feed injection nozzle and back.
cover the LSM setup, the creation of a cold cap in the LSM, the techniques used to analyze the cold cap, the results of the analysis, and a discussion of the results including their impact on the environment.
2. EXPERIMENTAL SECTION 2.1. Feed Composition and Preparation. A simulated high-alumina HLW feed called A0 was used in these experiments (Table 1).6−8,10,18,21 For the LSM run, a 1-L batch of A0 slurry was prepared at 400 g L−1 of glass by mixing the components listed in Table 1 with ∼800 mL of deionized water. Iron hydroxide was added as 13-mass% slurry, and the silica source was crushed quartz of particle size ≤75 μm (SilCo-Sil-75; U.S. Silica, Frederick, MD). The feed slurry was transferred into a graduated cylinder and filled to 1000 mL with deionized water. 2.2. Cold Cap Formation in the Laboratory-Scale Melter. Using a fused-silica crucible, the LSM simulated the glass melt heating, which is provided by electric power dissipation in large-scale melters, by partly submerging the crucible into the hot zone of a laboratory furnace. The LSM crucible rested on a platinum support that allowed the melt to receive heat from the furnace, while the LSM head space remained surrounded by insulation protruding from the furnace. Because the LSM crucible did not have a discharge port for molten glass produced from feed charging, the furnace position was periodically adjusted to maintain a constant position of the cold cap. The cold cap was observed through the top of the transparent crucible during charging. In this work, two hundred grams of previously melted A0 “initial” glass were crushed into chunks and added into the 8857
DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
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Environmental Science & Technology the cold cap caused the cold cap to crack and split into several sections. Once the crucible cooled to room temperature, the A0 glass and cold cap were separated from the top portion of the crucible. 2.3. Slurry Heat Treatments. In a separate set of experiments, A0 slurry was subjected to isothermal heat treatments. Here, nine ∼1-g aliquots of the A0 slurry were transferred into separate ∼1 cm3 platinum crucibles. These slurry samples were placed in an oven at 105 °C for 24 h. The batch was stirred at 30 min intervals as it thickened and then left in the oven to dry. The mass of dried feed in each crucible weighed ∼0.5 g. Each crucible was covered with platinum foil and placed on a raised platinum base in a furnace. Starting from room temperature, the furnace was heated at 5 K min−1 up to 1200 °C. Beginning at 400 °C, samples were removed at 100 °C intervals without being held isothermally. Using this slurry drying method, capillary action allowed the water-soluble salts to migrate toward the top of the samples before the feed paste was completely dry (see section 3.5). However, this effect was only evident in the feed heat treated at 700 °C under these conditions (see section 3.6). 2.4. Analytical Techniques. A section from the LSM cold cap and sections from samples heat treated in platinum crucibles were mounted in epoxy, polished, and analyzed with scanning electron microscopy (SEM; JSM-7001F, JEOL USA, Inc. Peabody, MA) in backscattered electron imaging mode and energy dispersive spectroscopy (EDS; EDAX Apollo II 30 mm2 silicon drift detector, AMETEK, Berwyn, PA). Specimens were analyzed both uncoated and coated with gold/palladium or platinum. Optical microscopy was performed with a VHX2000E digital microscope (Keyence Corporation of America, Itasca, IL). X-ray diffraction (XRD) analysis was conducted on all of the heat-treated feed samples with a Bruker D8 Advance diffractometer (Bruker AXS Inc., Madison, WI) equipped with Cu Kα emission (λ = 1.5406 Å) at a power level of 40 kV and 40 mA. The instrument used a Lynx-Eye position-sensitive detector with a collection window of 3° 2θ. The scan parameters for the powdered feed were 10 to 75° 2θ with a step of 0.015° 2θ and a 1-s dwell at each step using a 12 mm variable divergent slit. Five weight percent of a CaF2 standard was added to each sample for quantitative analysis. The mass fractions of crystalline phases from the XRD patterns were identified with Bruker AXS DIFFRACplus EVA software and quantified with TOPAS software. Micro-XRD analysis was conducted on the polished cold-cap sample with a Rigaku D/Max Rapid II micro diffraction system (Rigaku Americas Corporation, The Woodlands, TX). X-rays were generated (MicroMax 007HF operated at 35 kV and 25 mA) from a rotating chromium target (λ = 2.2910 Å), focused through a 100-μm-diameter collimator, and the incident beam direction was fixed at 25° to the specimen surface. The scan parameters for each 100-μm step along the cold-cap sample were 25° to 160° 2θ with a step of 0.025° 2θ and a 0.2-s dwell at each step. To reach from the top of the cold-cap sample down into the glass region required three 6.1 mm scans.
Figure 2. (Color online) (a) Cross section of the cold cap from the LSM run at 7.5 mL min−1. The red outline shows the polished coldcap section shown in (b). (b) Polished cold-cap section as marked in (a) with the seven areas marked for comparison. Note that the features on the polished surface (b) do not exactly line up with those on the unpolished surface (a) because the top surface layer of (a) was removed (altered) during polishing.
from the top of the cold cap down to the glass melt that was clear of any obscuring bubbles. Along this continuous region, seven areas were chosen with identifiable microstructural features similar to those of feed samples heat treated to different temperatures. These areas are marked □1 to □7 in Figure 2b. Comparable microstructures were selected from the center of the heat-treated samples to avoid any incongruity resulting from the possible salt flux; the flux only affected the microstructures in the sample at 700 °C as discussed in section 2.3. 3.2. Feed EDS Mapping. The elemental maps from EDS analysis of areas □1 and □5 from Figure 2b are shown in Figure 3, parts a and b, respectively. The connected void space in Figure 3a (dark region in the backscatter electron [BSE] micrograph) is epoxy, indicating the absence of a connected glass-forming phase. In Figure 3b, the feed appears connected into a continuous melt with crystalline inclusions, mainly dissolving particles of quartz surrounded with silica-rich dissolution layers in which Fe/Ni-containing spinel and Zrcontaining particles are absent. In these dissolution layers, Na and Al are diffusing toward the silica grains from the bulk glass melt as noted in previous work.22 The Bi fraction, 5 to 25-μm agglomerates in Figure 3a, is incorporated into the melt in Figure 3b. The Al-containing particles were boehmite crystals8 at 400 °C, Figure 3a. They became partly amorphous23 and partly formed nepheline at 900 °C as seen in Figure 3b. 3.3. Sample Comparison. Figure 4 shows BSE micrographs of all seven areas marked on the polished cold cap in Figure 2b alongside a comparable BSE micrograph of the feed
3. RESULTS AND DISCUSSION 3.1. Cold Cap. A cold-cap fracture surface can be seen in Figure 2a. From this cross section, a piece of the cold cap, as marked by the outline in Figure 2a, was broken off by hand, mounted in epoxy, and polished (shown in Figure 2b). A narrow, vertical region was identified on the polished surface 8858
DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
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Environmental Science & Technology
Figure 3. (Color online) Elemental maps of (a) Area 1 and (b) Area 5 shown on the cold cap in Figure 2b. The scale-bar is valid for all micrographs.
heated to specified temperatures. As seen in Figure 4a (Area 1 at 400 °C), the particles remain loose. Large particles containing Si, Al, Fe, Bi, as well as sodium borate crystals are roughly the same size in both samples. The sodium borate crystals were found in the unreacted feed and appeared as large (∼50 μm) dark particles in the BSE micrographs (Figure 3a) as described in previously published work.7 Energy dispersive spectroscopy of Figure 4b (Area 2 at 500 °C) shows similarly sized Si- and Al-containing particles as in the 400 °C sample, but without the large Fe- and Bi-containing particles, and sodium borate crystals. At this temperature, the feed remained unconnected. Around the quartz particles in Figure 4c (Area 3 at 600 °C), other components, such as sodium and boron salts, as identified by EDS, began to connect and form small necks. Figure 5 shows close-up views comparing the quartz particles at 500 °C (Figure 5a) and 600 °C (Figure 5b). The boundaries of the quartz particles in the 500 °C-feed sample are sharp and smooth, while boundaries are less well-defined in the 600 °Cfeed sample as quartz particles are surrounded with traces of melt in which small particles of various components begin to dissolve. The fully connected glass-forming melt seen in Figure 4d (Area 4 at 800 °C) contains various solid inclusions and bubbles of different sizes (∼5 to 75 μm), some of which are well-rounded while others are irregularly shaped. At this temperature, dissolution layers around the quartz particles are not yet developed or are thin (85% of the total phase composition. 8860
DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
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Environmental Science & Technology The micro-XRD measured fractions of crystalline phases based on the total crystallinity, not as fractions of the sample. To convert these fractions to actual concentrations, quartz, spinel, and nepheline fractions were renormalized to match the maximum fraction in heat-treated samples. The result is shown in Figure 7c, together with averaged values of heat-treated feed samples. The crystalline-phase distribution in the cold-cap and heat-treated samples was in good agreement from the reacting feed layer, through the foam layer, and into the glass, thus confirming the initial placement of the temperature regions via SEM micrograph comparison. 3.5. Reacting Feed Layer. On the basis of thermal analyses of the feed,27 the reacting feed layer exists from the upper region of the cold cap to 600 °C (Area 3). The foam layer spans the region from 800 °C (Area 4) to 1100 °C (Area 7) as seen in Figure 2b. As discussed by Pokorny and Hrma,9 the thickness of the reacting feed layer is ∼1 to 2 cm, depending on the rate of melting, for a cold cap that is completely covered by slurry as established by the reaction kinetics and the heat transfer. Under boiling slurry typically containing 50 to 60% water, the reacting feed layer must be thin enough for heat conduction needed to evaporate the water. In the LSM, the extent of the free surface of molten glass radiates enough heat to keep most of the top surface of the cold cap dry. On the dry top surface of the cold cap, a reacting feed layer with a nearly constant temperature near that of the crucible atmosphere (∼400 °C) can accumulate, limited only by the charging rate of the feed into the melter. Thus, if the feed charging rate is increased, the reacting feed layer will increase proportionally while the foam layer remains relatively unchanged in size. This accounts for the relatively thick (∼10 mm) dry feed layer above ∼500 °C in Figure 2b. 3.6. Transition Temperature from Unconnected Feed to Connected Melt. The glass-forming melt becomes connected in the feed within a narrow temperature interval (∼20 to 50 °C) that, in the A0 feed, lies between 700 and 800 °C, depending on the heating rate.26 Within the cold cap, the interface between the connected and unconnected melt is not strictly horizontal. The forces of gravity and capillary action can drive salt through the cold cap both as a solution before the water fully evaporates and as a salt melt that is highly mobile. The alkali content in the salt is a powerful flux.27 Thus, local nonuniformities caused by salt migration affect the temperature at which the glass-forming melt is connected as shown in the 700 °C heat-treated sample (Figure 8). Because of the migration of salts, the upper section of the 700 °C sample (∼200 μm) contained ∼40% more sodium than the bottom section as quantified by EDS. 3.7. Comparison with Mathematical Model. The comparison of the cold cap observed in the LSM with the cold-cap characteristics calculated by the model developed by Pokorny and Hrma10 is not straightforward. This is because (1) their model considered that the heat flux from the melter head space is rather small, thus the heat necessary to evaporate the slurry needs to be transferred across the cold cap, and (2) the thermal shrinkage of the gas bubbles during sample quenching results in a thinner foam layer. If the melter head space heat flux (QU) is lower than the amount of heat necessary to evaporate the slurry (QUC), the top of the cold cap has a temperature, TT ≈ 100 °C, and the remaining heat for slurry evaporation is transferred through the cold cap, thus limiting its thickness. However, if QU > QUC, then all the water is evaporated on the top, which results in a dry
Figure 8. BSE micrograph of the A0 feed heat treated to 700 °C. The feed remains unconnected toward the bottom of the sample, similar to the lower temperature samples (400 °C600 °C), but connects near the top like the higher temperature samples (800 and 900 °C).
cold cap with a surface temperature TT > 100 °C. To address this situation, observed in both LSM and pilot plant melters,28 the cold-cap model was modified to account for the dry coldcap surface. Assuming TT = 400 °C, the updated model, which will be reported in a follow-up study,29 predicted the cold-cap thickness between temperatures 500 °C and TB = 1100 °C (the cold-cap bottom temperature) to be ∼1.8 cm. The predicted value (∼1.8 cm) is slightly greater than ∼1 cm observed in the LSM cold cap, which is in part due to TB ≈ 1200 °C in the LSM, as opposed to 1150 °C in the model. In addition, the gas bubbles undergo shrinkage during quenching. Considering the average temperature of gas bubbles to be 1000 °C during melting and 500 °C when the glass freezes, the bubble volume can decrease by a factor of ∼2 based on the ideal gas law. Thus, considering the average porosity (p, the gas-phase volume fraction) of the foamy layer to be p ≈ 0.5,11,26 the thickness of the foamy layer at melting conditions can be ∼1.5 times higher than after quenching. If some bubbles escaped from the cold-cap bottom just before or during quenching, then the foam layer thickness during melting would be even higher. Furthermore, additional melting reactions and feed compaction during quenching can result in a thinner measured cold cap. Thus, both the thick, open-porosity reacting feed layer and the thin, foamy layer of the quenched cold cap from the LSM 8861
DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
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Environmental Science & Technology
Kim and Jaehun Chun for their help in discussions about this research.
test, seen in Figure 2b, reasonably correspond to predictions made by the cold-cap model. The thickness of the bottom layer is determined by the heat transfer and conversion kinetics, while a pile of dry feed is accumulating above. The thickness of this pile can grow without limits in direct relationship to the charging rate. The temperature of this upper layer is practically constant and only defined by the amount of heat supplied from the melter head space.
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(1) Bechtel National Inc. About the Project; http://www. hanfordvitplant.com/page/the-project/. (2) Kruger, A. A. Advances in Glass Formulations for Hanford HighAluminum, High-Iron and Enhanced Sulphate Management in HLW Streams; WM2013 Conference, Paper 13000: Phoenix, AZ, 2013. (3) Certa, P. J.; Empey, P. A.; Wells, M. N. River Protection Project System Plan; ORP-11242 Revision 6; Office of River Protection, U.S. Department of Energy: Richland, WA, 2011. (4) Sylvester, P.; Rutherford, L. A.; Gonzalez-Martin, A.; Kim, J.; Rapko, B. M.; Lumetta, G. J. Ferrate treatment for removing chromium from high-level radioactive tank waste. Environ. Sci. Technol. 2001, 35 (1), 216−221, DOI: 10.1021/es001340n. (5) McCloy, J. S.; Riley, B. J.; Goel, A.; Liezers, M.; Schweiger, M. J.; Rodriguez, C. P.; Hrma, P.; Kim, D.-S.; Lukens, W. W.; Kruger, A. A. Rhenium solubility in borosilicate nuclear waste glass: Implications for the processing and immobilization of technetium-99. Environ. Sci. Technol. 2012, 46 (22), 12616−12622, DOI: 10.1021/es302734y. (6) Dixon, D. R.; Schweiger, M. J.; Hrma, P. Effect of Feeding Rate on the Cold Cap Configuration in a Laboratory-Scale Melter; WM2013 Conference, Paper 13362: Phoenix, AZ, 2013. (7) Dixon, D. R.; Schweiger, M. J.; Hrma, P. In Characterizing a High-Level Waste Cold Cap via Elemental and Structural Configuration, WM2014 Conference, Paper 14185: Phoenix, AZ, 2014. (8) Pierce, D. A.; Hrma, P.; Marcial, J.; Riley, B. J.; Schweiger, M. J. Effect of alumina source on the rate of melting demonstrated with nuclear waste glass batch. Int. J. Appl. Glass Sci. 2012, 3 (1), 59−68, DOI: 10.1111/j.2041-1294.2012.00079.x. (9) Pokorny, R.; Hrma, P. Mathematical modeling of cold cap. J. Nucl. Mater. 2012, 429 (1−3), 245−256, DOI: 10.1016/j.jnucmat.2012.06.013. (10) Pokorny, R.; Hrma, P. Model for the conversion of nuclear waste melter feed to glass. J. Nucl. Mater. 2014, 445 (1−3), 190−199, DOI: 10.1016/j.jnucmat.2013.11.009. (11) Marcial, J.; Chun, J.; Hrma, P.; Schweiger, M. Effect of bubbles and silica dissolution on melter feed rheology during conversion to glass. Environ. Sci. Technol. 2014, 48 (20), 12173−12180, DOI: 10.1021/es5018625. (12) Feng, Z.; Li, D.; Qin, G.; Liu, S. Study of the float glass melting process: Combining fluid dynamics simulation and glass homogeneity inspection. J. Am. Ceram. Soc. 2008, 91 (10), 3229−3234, DOI: 10.1111/j.1551-2916.2008.02606.x. (13) Yen, C.-C.; Hwang, W.-S. Numerical study of fluid flow behaviors in an alkali-free glass melting furnace. Mater. Trans. 2008, 49 (4), 766−773, DOI: 10.2320/matertrans.MRA2007259. (14) Abbassi, A.; Khoshmanesh, K. Numerical simulation and experimental analysis of an industrial glass melting furnace. Appl. Therm. Eng. 2008, 28 (5−6), 450−459, DOI: 10.1016/j.applthermaleng.2007.05.011. (15) Hrma, P. Thermodynamics of batch melting. Glastechn. Ber. 1982, 55 (7), 138−150. (16) Schill, P.; Chmelar, J. Use of computer flow dynamics in glass technology. J. Non-Cryst. Solids 2004, 345&346, 771−776, DOI: 10.1016/j.jnoncrysol.2004.08.199. (17) Agarwal, V.; Guillen, D. P. Incorporating Cold Cap Behavior in a Joule-Heated Waste Glass Melter Model; INL/EXT-13−29794; Idaho National Laboratory: Idaho Falls, ID, 2013. (18) Hrma, P.; Kruger, A. A.; Pokorny, R. Nuclear waste vitrification efficiency: Cold cap reactions. J. Non-Cryst. Solids 2012, 358 (24), 3559−3562, DOI: 10.1016/j.jnoncrysol.2012.01.051. (19) Kim, D.-S.; Schweiger, M. J.; Buchmiller, W. C.; Matyas, J. Laboratory-Scale Melter for Determination of Melting Rate of Waste Glass Feeds; PNNL-21005; Pacific Northwest National Laboratory: Richland, WA, 2012. (20) Riley, B. J.; Crum, J. V.; Buchmiller, W. C.; Rieck, B. T.; Schweiger, M. J.; Vienna, J. D. Initial Laboratory-Scale Melter Test
4. ENVIRONMENTAL IMPACT The temperature distribution within a simulated HLW cold cap was determined by comparing the microstructure of a cold cap produced in a LSM with microstructures of samples of identical feed heat treated to different temperatures and then confirmed with XRD results from the major crystalline phases across the same region. The heating conditions in the LSM allowed a ∼400 °C feed layer to accumulate in the cold cap. The cold-cap area spanning the temperature interval from ∼500 °C to ∼1150 °C corresponds reasonably with the temperature profile predicted with the mathematical model after the model was modified to allow a dry cold-cap top surface. The continued expansion of the mathematical model, which accounts for the effect of heat transfer limiting foam, is crucial to understanding the characteristics of the cold cap and their effect on the glass production rate in electric melters. The HLW electric melters designed for the Hanford WTP30 will process wastes containing volatile radionuclides like cesium-137, strontium-90, and technetium-99.5 The volatility of such components is inhibited by the presence of the cold cap, which would cover 90 to 100% of the glass melt surface.30 While the cold cap is necessary for restricting radionuclide volatility, it should not hinder the rate of melting. The mathematical model9,10 is capable of estimating cold-cap conditions and helping to optimize hundreds of different feeds anticipated for vitrification at the WTP.11 To achieve this, the model must be verified by both laboratory and simulated melter experiments at various scales before it can be broadly applied. The ultimate goal is to shorten the life cycle of waste cleanup at Hanford, thus minimizing the risk of additional environmental contamination.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (509) 375-2376; fax: (509) 372-5997; e-mail:derek.
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy’s (DOE) Waste Treatment and Immobilization Plant Federal Project Office under the direction of Dr. Albert A. Kruger. Pacific Northwest National Laboratory (PNNL) is operated for the DOE by Battelle Memorial Institute under contract DE AC05 76RL0 1830. A portion of the research was performed at the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Office of Biological and Environmental Research and located at PNNL. The authors would like to thank Shelley Carlson for mounting and polishing all specimens for SEM work, Mark Bowden for running micro-XRD on the cold-cap sample, and Dong-Sang 8862
DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
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DOI: 10.1021/acs.est.5b00931 Environ. Sci. Technol. 2015, 49, 8856−8863
