Production of a High-Level Waste Glass from Hanford Waste Samples

Transcription

1 S 1) WSRC-MS I Production of a High-Level Waste Glass from Hanford Waste Samples by C. L. Crawford Westinghouse Savannah River Company Savannah River Site Aiken, South Carolina D. M. Farrara B. C. Ha N. E. Bibler A document prepared for SPECTRUM '98 at Denver, CO, USA from 9/13/98-9/18/98. DOE Contract No. DE-AC09-96SR18500 This paper was prepared in connection with work done under the above contract number with the U. S. Department of Energy. By acceptance of this paper, the publisher and/or recipient acknowledges the U. S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper, along with the right to reproduce and to authorize others to reproduce all or part of the copyrighted paper.

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4 PRODUCTION OF A HIGH-LEVEL WASTE GLASS FROM HANFORD WASTE SAMPLES WSRC-MS Page 1 of 8 Charles L. Crawford Daro M. Ferrara Bao C. Ha Ned E. Bibler Westinghouse Savannah Westinghouse Savannah Westinghouse Savannah Westinghouse Savannah kver Co. River Co. River Co. River Co. Bldg. 773-A Bldg. 772-F Bldg. 773-A Bldg. 773-A Aiken, SC Aiken, SC Aiken, SC Aiken, SC (803) (803) (803) (803) ABSTRACT The HLW glass was produced from a HLW sludge slurry (Envelope D Waste), eluate waste streams containing high levels of Cs-137 and Tc-99, solids containing both Sr-90 and transuranics (TRU), and glass-forming chemicals. The eluates and Sr-9O/TRU solids were obtained from ion-exchange and precipitation pretreatments, respectively, of other Hanford supernate samples (Envelopes A, B and C Waste). The glass was vitrified by mixing the different waste streams with glass-forming chemicals in platinum/gold crucibles and heating the mixture to 1150 "C. Resulting glass analyses indicated that the HLW glass waste form composition was close to the target composition. The targeted waste loading of Envelope D sludge solids in the HLW glass was 30.7 wt%, exclusive of Na and Si oxides. Condensate samples from the off-gas condenser and off-gas dry-ice trap indicated that very little of the radionuclides were volatilized during vitrification. Microstructure analysis of the HLW glass using Scanning Electron Microscopy (SEM) and Energy Dispersive X- Ray Analysis (EDAX) showed what appeared to be iron spinel in the HLW glass. Further X-Ray Diffraction (XRD) analysis confirmed the presence of nickel spinel trevorite (NiFe,O,). These crystals did not degrade the leaching characteristics of the glass. The HLW glass waste form passed leach tests that included a standard 90 "C Product Consistency Test (PCT) and a modified version of the United States Environmental Protection Agency Toxicity Characteristic Leaching Procedure (TCLP). I. INTRODUCTION As part of the Tank Waste Remediation System (TWRS), the United States Department of Energy (DOE) issued a Request for Proposal (RFP)' to vendors interested in building and operating a privatized facility for immobilizing radioactive waste currently being stored in tanks at the DOE site in Hanford, Washington. British Nuclear Fuels Limited, Incorporated (BNFL, Inc.), responded to the RFP and was awarded a contract to perform the initial phase of this work., BNFL has teamed with SAIC, Bechtel National Inc., BNFL Engineering Ltd. (BEL), GTS Duratek and the Vitreous State Laboratory (VSL) at Catholic University to develop and demonstrate technology needed to meet the terms outlined in the TWRS RFP. In collaboration with the BNFL team, the Savannah River Technology Center (SRTC) was asked to perform radioactive demonstrations with four samples of the actual radioactive waste from Hanford. SRTC has produced and characterized three Low-Activity (LAW) glasses and a single High-Level Waste (HLW) glass from actual radioactive Hanford waste samples. The three waste samples used for the LAW glasses are referred to as Envelopes A, B and C. The HLW glass was produced from a mixture of the following waste streams: (1) HLW sludge slurry referred to as Envelope D from Hanford Tank 24 1-C- 106,' (2) eluate waste streams containing high levels of Cs- 137 and Tc-99 derived from ion exchange treatment of Envelopes A,B and C, (3) solids containing both Sr-90 and transuranics (TRU) derived from precipitation treatment of Envelope C, and (4) glass-forming chemicals.

5 W SRC-MS Page 2 of 8 This paper describes production and characterization of the HLW glass. Production and characterization of the Hanford wastecontaining LAW glasses are presented in a separate related report from this conference. 11. WORK DESCRIPTION Vitrification and leach testing for this demonstration was performed remotely in the SRTC Shielded Cells Facility (SCO). The demonstration was performed in four phases consisting of feed stream preparation, vitrification, glass analysis and leach testing. A. Feed Stream Preparation The Feed Stream Preparation phase involved mixing the waste forms (HLW filtered Envelope D sludge, (3-137 and Tc-99 eluate, and Sr-9O/TRU solids) and glass-forming chemicals in a 100-mL platinudgold crucible. Liquid eluate waste streams produced from ion exchange treatments of Envelopes A, B and C were combined and concentrated by evaporation by a factor of - 10X before they were mixed with the chemicals. Feed stream samples were analyzed using ICP-ES, ICP-MS, Atomic Absorption Spectroscopy, Gamma Spectroscopy and Scintillation Counting prior to batching. Results from the analyses were transmitted to VSL at Catholic University, where developmental work had been performed with waste sirnulant~.~~~ Results from this development work were the basis for glass formulations used at SRTC. Glass-forming chemical batches were dissolved by both NqO, and CsOH fusion, acidified and analyzed by ICP-ES prior to being mixed with the waste streams. B. Vitrification Vitrification of the mixture was performed in three steps inside a remotely operated electrically heated BarnsteadThermolyne Model furnace. Water was initially evaporated from the mixture and collected in one of the off-gas system traps. The off-gas system included a condenser, a dryice trap, charcoal filters, and a vacuum pump. The off-gas system was developed to keep hazardous species from the shielded cells environment. The evaporation stage involved heating the mixture up from 50 C to 200 C at a rate of 10 C per 30 minutes. The mixture was then calcined by heating to 900 C at a rate of about 100 C per hour. After the temperature had been held at 900 C for 30 minutes, the off-gas system was disconnected. Finally in the vitrification stage the material was brought to the melt temperature of between 1120 C and 1150 C and held there for typically 2 hours before the resulting glass waste form was taken from the furnace, quenched and sampled. After cooling of the vitrification system, the condensate trap and the cold trap were sampled and the mass of the charcoal filters was measured to determine adsorbtion of any volatile components on the filters. C. Glass Analysis The HLW product glass was dissolved by NqO, and CsOH fusion, acidified and analyzed using ICP-ES and ICP-MS for elemental and individual radionuclide isotope (La, Nd, Pr and Sm- 15 1) concentrations. A standard Uranium glass (Corning Glass Works, Coming, NY) was also dissolved and analyzed for quality assurance purposes. Liquid scintillation counting and gamma spectroscopy were also performed to obtain concentrations of radionuclides including Cs- 137Ba- 137m and Sr-90/Y-90. Microstructure analysis of the HLW glass used Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDAX) and X-Ray Diffiaction (XRD) Analysis. D. Leach Tests Two leach tests were performed remotely in the SCO on the HLW glasses. The durability was measured using the ASTM C-1285 standard nuclear waste glass durability test commonly referred to as the Product Consistency Test (PCT).6 This is a crushed glass leach test at 90 "C for 7 days using deionized water as leachate. Triplicate tests were performed in sealed stainless steel vessels. Final leachate ph's were measured and final elemental concentrations of the filtered, acidified leachates were measured by ICP-ES, ICP-MS, AA and Gamma Spectroscopy for total elemental and individual radionuclide isotope concentrations. Release rate measurements for toxic substances were determined using the United

6 WSRC-MS Page 3 of 8 States Environmental Protection Agency Toxicity Characteristic Leaching Procedure (TCLP).7 The TCLP is a standard room temperature crushed glass static leach procedure for 18 hours using acetic acid extraction fluid as leachate. Duplicate tests were performed in sealed polyethylene bottles that were constantly rotated at 30 rpm. A blank and a standard (TCLP Metals in Soil, Environmental Resource Associates, Arvada, CO) were also included with the duplicate glass TCLP tests. Final leachate ph's were measured and final elemental concentrations of the acid-digested leachates were measured by ICP-ES and AA for total elemental concentration RESULTS AND DISCUSION A. Feed Analysis Characterization of the feed streams was performed to determine the concentration of species important to the vitrification process. The major characteristics of the various radioactive feeds used in the vitrification of the HLW glass are described below. Table I shows characterization of the filtered HLW solids. These HLW solids contained total elemental compositions of primarily Fe, Na and Al, with minor components of Ag, Ca, K, Mn, Ni, P, Pb and Ti also present. Drying of the filtered HLW solids to no further mass change indicated these filtered solids contained 30 wt% water. The solids also contained about 700 pci/g of and 1,100 pci/g of Sr-90. Table I1 shows the calculated amounts of major species present in the concentrated eluate waste stream. These results were obtained from detailed characterization of the various Cs and Tc eluate streams that were combined and concentrated by 1OX to the final eluate concentrate. This eluate concentrate solution contained 2.3M Na, 1.4 MNO,-, 0.4M NO;, 0.2M of both A1 and K and 5.5M Total Organic Carbon (TOC). The eluate also contained about 2,200 pci/ml of Cs- 137 and 1.O pci/ml of Tc-99. Table I1 also shows the amounts of strontium and iron containing solids used in the HLW glass formulation. The Sr/Fe-solids contained total activity of about 17,000 pci of Sr-90 and 110 pci of TRU isotopes. The waste streams detailed in Tables I and I1 were combined with a non-radioactive glass former batch consisting of four solid reagent grade chemical compounds. Solid glass formers used to produce the HLW glass contained only B, CayLi and Si. The ratio of chemical compounds used in preparing the final 60 grams of HLW glass was determined from analyses of the HLW streams shown in Tables I and 11, and consideration of the corresponding target glass composition. Table I11 lists the amounts of each stream used in the HLW glass formulation. Target loadings in the HLW glass for (3-137, Tc-99 and Sr-90 derived from pretreatment of Envelopes A, B and C were based on calculations using data from Reference # 8. These target loadings, expressed as curies per gram of Envelope D waste oxides exclusive of Na and Si, were Cs-137 = 1. 3 ~ 1 0CUg, -~. ~ and Sr-90 = 7. 4 ~ 1 0Ci/g. -~ Tc-99 = 3. 6 ~ 1 0Ci/g Table I. Elemental and Radionuclide Analyses of Insoluble Solids in Composite Envelope D HLW Sludge Slurry Ba K 8.1 X1o4 7.2 xloz 3.9 X1o3 9.8 xloz 2.0 X i o x X1o3 7.2 X1o4 3.2 x x x x102 - <2 X1o3 6.9 x xi03

7 WSRC-MS Page 4 of 8 Table 11. Eluate Concentrate Composition and Sr/Fe/TRU Solids Composition Table 111. High Level Waste GIass Feed Streams I 1 1 Amount Mass of Weight Added 0xi;e % 58.5 g mL Formate 0.09 Oxalate c F Sulfate Phosphate CS-137 Tc-99 Total Activity Sr (pci/ml) 2,200 1.o I10 pci 17,000 pci Table I11 data indicates that the total Envelope D mass basis waste loading was 54.4 wt%. Target waste loading specifications do not credit any Na or Si from the Envelope D sludge solids.' The calculated Envelope D waste loading without consideration of Na and Si is lower (30.7%) than the overall Envelope D mass basis loading of 54.4 wt??. The product specification waste loading for the immobilized HLW borosilicate waste form is a minimum of 25 wt% on an oxide basis without Na and Si.' B. Glass Analysis A total of 60 grams of HLW glass was produced from the radioactive waste streams and batchforming boron, calcium, lithium and silicon- LiOH E, 3.0 s i o, I t 9.0g I 9.0 I I- Total I 60 I 100 containing compounds. The HL W glass elemental analysis is shown in Table IV. The glass was analyzed to contain on an oxide basis primarily Si, Fe, Na and A1.-Other significant elements present in the product glass were B, Ca, Li and Sr. Results shown in Table IV are averages of two separate glass samples from the -200 mesh fraction not used in the PCT durability test. Duplicate results agreed to 10% or better. The sodium value was not measurable using sodium peroxide hsion glass dissolution. Thus the sodium value shown in parenthesis for sodium peroxide fusion in Table IV was determined from analyses of the feed streams and should be considered to be a maximum. The results for Na and Si were low. Sodium analysis was also low in the standard glass whereas Si results for the standard glass were within 4%. Thus there could be considerable error in determining Na and the reason for the - 17% low result for Si in the HLW glass is not apparent. However, if the results for Na and Si are increased to their amounts in the target glass, the measured oxide sum increases to - 100%. The concentrations of elements La, Nd, Pr, Th and U were measured by ICP-MS. La, Nd and Pr are in the waste primarily as fission products from U-235. These elements were not

8 WSRC-MS Page 5 of 8 included in the glass formulation calculations. Thus there is no target composition listed in Table IV. Measured results for Cr and Zn were found to be - 1OX higher than target, and measured Ni was - 2.5X higher than target. These elements appear in the glass at higher than expected levels as result of slight contamination from both the stainless steel grinder (stainless steel nominally 18-20% Cr and 8-10% Ni) and the brass (CulZn alloy) sieves used in preparation of the ground glass used for both glass dissolution analysis and glass leaching tests. Indeed these contaminate particles could clearly be seen in the 1,000-3,OOOX magnification SEM photographs and the EDAX spectrums of these particles showed distinct spectrums for either Fe/Cr/Ni from the steel, or CdZn from the brass. Table V presents the HLW radionuclide results for the glass. Total alpha and beta were measured by liquid scintillation counting, Cs-137 by gamma spectroscopy. N. D. indicates that the specific radionuclide was Not Detected. The actinides, Sm-151 and Tc-99 were detected by ICP-MS. The total activity for Sr-90 and its daughter Y-90 were calculated by subtracting the Cs- 137 activity from the total beta. Sr-90 and Y-90 are in secular equilibrium and their activities are equal. Ba-137m is in secular equilibrium with Cs-137 and its activity is equal to 95% of that for Cs-137 (5% of the Cs-137 decays directly to the stable Ba-137). Table IV. Envelope D HLW Glass Composition (Weight Percent)

9 WSRC-MS Page 6 of 8 Table V. Radionuclide Composition of HLW Glass ~ U X I XIO x xi04 Pu-239 Pu-240 Am-241 Pu-242 Am-243 Cm x x1o x x104 N.D. 20 I 7.8 xlo-s 8.8 xlo N. D. 4.9 xlo-s 5.9 Xio- 1.0 x10-~ 3.O 8.7 xlo- 5.7 N. D. 2.2 x10-l 65 C. Microstructure Analysis Scanning electron microscopy (SEM) analysis was performed on several pieces of the mesh glass granules prepared for the PCT. Two phases were identified in the HLW glass - one that appeared featureless and apparently amorphous and one that was clearly a crystalline phase indicated by triangular shapes. These phases were examined by energy dispersive X-ray analysis (EDAX). The amorphous phase was the glass itself and the EDAX showed peaks resulting from the major glass components such as Si, Fe, Al, Na and Ca. The EDAX spectrum of the crystalline phase indicated that it was primarily Fe with minor peaks attributed to Ag, Mn and Ni. This EDAX method does not detect oxygen. Thus, this phase could be an oxide of Fe. Based on the triangular shape of the crystals, they were postulated to be i iron spinels of possibly hematite (Fe,O,), magnetite (FeFe,O, ) or trevorite (NiFe,O,) that crystallized from the glass while it was molten or as it cooled. No attempt was made to quantify the extent of crystallization; however, based on the PCT results it did not significantly affect the glass s durability compared to the EA glass. Further examination of the glass using X-Ray Diffraction (XRD) analysis has confirmed the presence of Fe primarily as NiFe,O,. Observation of crystalline phases has also been reported in simulant testing at VSLCatholic University involving continuously-fed melter runs using waste simulants of the TWRS Envelope D VSL researchers report that glasses drained from the melter contained trace amounts of Fe-Cr-Ni spinels, along with Ag-containing nodules and various noble metalscontaining spinels. The crystals were observed in only a few samples and the extent of combined crystals comprised typically less than 1 ~01%.They also report that observed crystals did not appear to degrade leaching characteristics of the glass as determined by either PCT or TCLP tests. D. Comparison of Cs/Sr Radionuclides in Waste Streams to Cs/Sr Radionuclides Measured in HLW Glass A comparison can be made of the major (3-137 and Sr-90 radionuclides detected in the glass to the amounts of these species added in the HLW glass formulation. Cs-137 was added predominately from the eluate concentrate stream and the HLW filtered sludge solids (See Tables I & 11). A total of approximately 1.4 xlos pci of Cs-137 was present in the feed streams used to produce the 60 grams of HLW glass. The concentration of Cs-137 in the HLW glass was in the range of (1.9 to 2.1) x103pci/g of glass (See Table V). The total 60 grams of glass contained (1.1 to 1.3) x105 pci of Relatively low levels of Cs-137 were detected in the HLW glass off-gas condensate and cold-trap samples. A total of only about 300 pci of Cs137 is estimated from analyses of the off-gas samples. Thus, the calculated amounts of Cs-137 in the feed streams is in very good agreement with the Cs-137 amounts found in the HLW glass. Sr-90 was added predominately from the TRU/Sr/Fe solids product from Envelope C ~

10 WSRC-MS Page 7 of 8 pretreatment and the HLW filtered sludge solids. A total of approximately 5.6 x104yci of Sr-90 was present in the HLW feed streams (See Tables I & 11). Of this Sr-90 source, about 1.4 xl O4 pci was in the TRU/Sr/Fe solids and about 4.2 x l O4 pci was in the HLW filtered sludge solids. The concentration of Sr-90 in the HLW glass was (1.1 to 1.2) x103pci/g of glass (See Table V). The total 60 grams of glass contained (6.6 to 7.2) x104 pci of Sr-90. Sr-90 was not detected in the HLW glass off-gas condensate or cold-trap samples. Therefore, the amount of Sr-90 measured in the feed streams was within 19% of the Sr-90 detected in the product HLW glass. E. Leach Tests Durability of the HLW glass was measured by the Product Consistency Test, or PCT (ASTM Test C ).6 Triplicate samples of the ground glass were subjected to the 90 "C PCT along with the appropriates blanks and standard Environmental Assessment (EA) glass. Normalized releases were calculated based on the average measured composition fi-om glass dissolution and analysis given previously in Table IV. The average normalized releases for B, Li, and Na, and the standard deviations from triplicate tests are reported in Table VI in units of grams glass leached per liter of leachate. These values can be converted to kg glass leached per square meter of glass surface area per second (kg/m2s),the corrosion rate units suggested in Reference #1, by multiplying them by the constant 8.3 x 10". Average measured values for the EA glass tested concurrently with the HLW glass are also presented. Published values are presented for comparison. Clearly, the HLW glass is more durable than the EA glass. Table VI. Normalized PCT Releases (grams glass/l) Measured for the HLW Glass, and Measured and Published Results for the EA Glass The toxic leach characteristics of the HLW glass waste form were measured by the TCLP7on duplicate HLW glass samples along with a blank and a standard. This test was performed remotely in the SRTC shielded cells. The tests were performed at ambient temperature of between 27 and 30 "C since temperature inside of the shielded cells environment could not be controlled to 23 f 2 "C. Because mobility increases with temperature, this was a more rigorous test than the standard TCLP. The average concentrations in leachates resulting from tests of the HLW glass are reported in Table VII. HLW glass was shown to pass the TCLP for the EPA limits on characteristic hazardous RCRA metals (Ag, As, Ba, Cd, Cr, Hg, Pb and Se). Although it is not required by the EPA procedure, a 'TCLP Metals in Soil' standard was tested in parallel to the glass waste forms. For each waste form the results from the standard analyses are presented along with the average concentrations in the leachates resulting from the TCLP in Table VII. Acceptable ranges (as defined by the standard vendor, Environmental Resource Associates, Arvada, CO) for the standard are also presented for comparison even though these ranges were developed for tests performed at 23 k 2 "C. Some of the metals were not present in the glass waste forms in concentrations that would have failed TCLP even if 100 % were to leach into the extraction fluid. For these elements, the maximum possible concentration (assuming 100 YOextraction) has been given. These included As, Hg, and Se. These three elements were measured in the glass dissolution analyses to be at or below the detection limits of about 2-5 x l o 4wt% in the HLW glass.

11 WSRC-MS Page 8 of 8 Table VII. Concentrations (ppm) of Characteristic Hazardous RCRA Metals Measured in the Envelope D HLW Glass TCLP Leachates IV. CONCLUSIONS Laboratory scale tests carried out at SRTC have successfully demonstrated the production of an immobilized high-level waste borosilicate glass product incorporating actual Envelope D HLW sludge from Hanford Tank 106-C and other Cs/Tc/Sr high-level waste components separated from other actual Hanford wastes streams. HLW glass analyses indicated that the HLW glass waste form composition was close to target composition. A waste loading of 30.7 wt% Envelope D solids, exclusive of Na and Si, was demonstrated. Microstructure analysis of the HLW glass showed the presence of nickel spinel trevorite (NiFe,O,) crystal formation. These crystals did not degrade the leaching characteristics of the glass since the glass waste form passed standard durability PCT leach tests and TCLP leach tests. REFERENCES 1. United States Department of Energy, TWRS Privatization Request for Proposals, DE-RP06-96RL13308, February British Nuclear Fuels Limited, Incorporated, Tank Waste Remediation System Privatization Technology Development and Demonstration Program, August M. W. Urie, Tank Waste Remediation System (TWRS) Privatization Contractor Samples - Waste Envelope D Material 241- C-106, Final Analytical Report, April 1, S. S. Fu, K. S. Matlack and I. L. Pegg, Results of Melter Tests Using TWRS HLW Envelope D Simulants, Vitreous State Laboratory, The Catholic University of America, Final Report, Rev. 0, January 9, S. S. Fu and I. L. Pegg, Glass Formulation and Testing with TWRS HLW, Vitreous State Laboratory, The Catholic University of America, Final Report, Rev. 0, January 18, Standard Test Methods for Determining Chemical Durability of Nuclear Waste Glasses: The Product Consistency Test (PCT), 1995 Annual Book of ASTM Standards, Vol , Philadelphia, Pa, 1995, pp United States Environmental Protection Agency, Test Methods for Evaluating Solid Waste, SW-846 Method 1311, July (a) M. E. Johnson, TWRS Privatization Facility Preliminary Basis of Design, Rev. 0, August 1996; (b) M. Page, M. Johnson, D. Hughes and I<. C. Colebrook, TWRS Hanford Basis of Design (LAW and HLW), BEL Report KO1 04_REP-O02_PRC, November C. M. Jantzen, N. E. Bibler, D. C. Beam, C. L. Crawford, and M. A. Pickett, Characterization of the Defense Waste Processing Facility (DWPF) Environmental Assessment (EA) Glass Standard Reference Material (U), WSRC-TR , Rev. 1, 1993.