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INL/CON-13-28244PREPRINT

The Production of Advanced Glass Ceramic HLW Forms Using Cold Crucible Induction Melter

GLOBAL 2013

Veronica J. Rutledge Vince Maio

October 2013

THE PRODUCTION OF ADVANCED GLASS CERAMIC HLW FORMS USING COLD CRUCIBLE INDUCTION MELTER

Veronica J. Rutledge, Vince Maio

Idaho National Laboratory: P.O. Box 1625, Idaho Falls, ID, 83415-2110, veronica.rutledge@inl.gov

Cold Crucible Induction Melters (CCIMs) will favorably change how High-Level radioactive Waste (from nuclear fuel recovery) is treated in the 21st century. Unlike the existing Joule-Heated Melters (JHMs) currently in operation for the glass-based immobilization of High-Level Waste (HLW), CCIMs offer unique material features that will increase melt temperatures, increase throughput, increase mixing, increase loading in the waste form, lower melter foot prints, eliminate melter corrosion and lower costs. These features not only enhance the technology for producing HLW forms, but also provide advantageous attributes to the waste form by allowing more durable alternatives to glass. This paper discusses advantageous features of the CCIM, with emphasis on features that overcome the historical issues with the JHMs presently utilized, as well as the benefits of glass ceramic waste forms over borosilicate glass waste forms. These advantages are then validated based on recent INL testing to demonstrate a first-of-a-kind formulation of a non-radioactive ceramic-based waste form utilizing a CCIM.

I. INTRODUCTION The United States Department of Energy Nuclear

Energy (DOE-NE) Fuel Cycle Research and Development (FCRD) program is focusing efforts on evaluating and demonstrating an advanced nuclear fuel cycle (NFC) and are taking the modified open approach. A modified open nuclear fuel cycle recycles fissionable material from the used fuel, as opposed to the U.S.A. used once through fuel cycle in which the fuel is used once and then disposed of as high level waste (HLW) in a repository. The recycle approach would increase nuclear power production efficiency and reduce HLW generated. The fuel reprocessing technology used in the advanced NFC generates secondary waste streams partitioned into groups of UNF non-fissionable material with common chemistry.

The waste streams must be treated to immobilize the non-fissionable materials in a stable waste form. They can be treated either separately or combined when it would be advantageous to do so. According to a 2008 study, it was concluded that from a cost perspective, it

would be beneficial to combine waste streams and treat them using existing waste form technologies.1 Vitrification is the selected method for immobilizing the combined alkali/alkaline earths (CS), lanthanides (LN), and transition metals (TM) waste streams. Treating all three into a single waste form would be the simplest and most cost effective immobilization option from a reprocessing operations standpoint.

Borosilicate Glass (BSG) was initially selected as the preferred waste form for the CS/LN/TM combined waste streams generated by the uranium extraction (UREX+) separations process. However BSG waste forms had a low waste loading (~18 mass %) due to limited molybdenum and noble metal solubility and low thermal stability (glass transition temperature (Tg) = 500°C).1,2 Therefore, even though the use of BSG may simplify the processing, it will defeat the objective of minimizing HLW form volumes.

Studies on a BSG waste form for CS/LN combined waste streams, with high waste loadings, resulted in large amounts of crystallization upon slow cooling. When a Product Consistency test (PCT) was performed on the crystallized waste form, there was no noticeable impact on it when compared to the non-crystalline BSG waste form.3 These results led to increased interest in developing a glass ceramic waste form which had previously been avoided due to the possibility that the crystalline phase would form a non-durable glass phase by depleting the remaining glass in glass formers. The PCT tests showed the possibility of taking advantage of the crystallinity to form durable waste forms with high waste loadings.

Ceramic waste forms are tailored to create crystalline phases that favor the binding of radionuclides within their structures. There are many naturally occurring minerals, which are crystalline structures, containing radioactive and non-radioactive species similar to the radionuclides of interest in reprocessing waste streams. The synthetic ceramic waste forms are made to reflect these minerals. A multi-phase glass ceramic waste form can be tailored to accommodate the major waste components in the CS/LN/TM combined waste streams.

The amount of alkali, alkaline earths, lanthanides, and molybdenum in the waste streams exceed their respective solubility limits of a BSG waste form. However, at high temperatures they all dissolve and form a molten glass that is able to be processed via melter technology. The concentrations of the glass formers and the waste loading can be modified so that upon pouring the molten glass into controlled cooling canisters, it will transform into a glass ceramic encapsulating the major waste stream components.

The main benefits of a glass ceramic waste form over a glass waste form are:

• increased waste loadings dues to increased solubility limits in crystalline phases,

• increased heat tolerance due to eliminated constraints of Tg and elevated processing temperatures to the lowest crystalline phase melting temperature,

• increased durability due to reduced surface area caused by reduced cracking during waste form synthesis, and

• the ability to use baseline melter technology to process them.

Results from studies performed on glass ceramic waste forms for fission products in the CS/LN/TM combined waste streams showed that durable glass ceramic waste forms are achievable.2 They have a large increase in waste loadings and therefore, greatly reduce waste form volume. These glass ceramics can be processed utilizing baseline melter technology.

Over time, many different electrically-heated melters have been developed and used in multiple industries, including the treatment of radioactive wastes. Different designs utilize different ways to heat and contain the molten material. Joule heated melters (JHMs) are currently used in the U.S.A. for vitrifying liquid HLW.

The JHM operates by passing current between water- or air-cooled electrodes submerged in the molten glass in a refractory lined chamber. It uses resistance heating. The electrodes are typically made of metals that can withstand temperatures up to about 1100°C. The JHM has a limited operating life due to degradation, corrosion and melting of the electrodes and refractory caused by constant exposure to the glass melt. In addition, the glass chemistry needs to be carefully controlled to avoid increasing material problems and the potential of electrical short-circuiting and glass leaks.4

Cold crucible induction melters (CCIMs) have been used outside of the U.S.A. for years. In contrast to the JHM, the CCIM uses induction heating by coupling a water-cooled high frequency electrical coil with the glass melt, causing eddy currents that heat and mix the melt. In addition, the water-cooled crucible forms a shell of frozen glass, which contains the melt and protects the metal from degradation caused by contact with the molten glass. Not having in-melt electrodes allows higher operating

temperatures of up to around 2000°C. The CCIM is smaller, less expensive and generates less waste for ultimate disposal.4 It will also allow a more flexible glass chemistry than the JHM, including better accommodating the formation of glass ceramic waste forms, due to higher temperatures and increased tolerance to crystal formation. The higher temperatures will also allow for higher waste loadings, higher throughput capacities, and higher tolerances for noble metals and molybdenum.

In order to test the processability of CS/LN/TM combined waste streams in the CCIM to form a glass ceramic waste form, the Idaho National Laboratory’s (INL’s) pilot scale CCIM was utilized. The HLW simulant was made to represent the combination of the TRUEX raffinate and the TALSPEAK strip product streams from a proposed future aqueous-based used nuclear fuel recycle method, the UREX+ processes. It is based on a matrix developed at Pacific Northwest National Laboratory (PNNL)3, referred to as CS/LN/TM-Mo-6.25. The main objectives of this test were to demonstrate the ability to process HLW using CCIM technology and form a glass ceramic waste form immobilizing the fission products of the HLW utilizing post-pouring controlled cooling. II. EXPERIMENTAL

II.A. HLW Simulant for CCIM Feed

The waste simulant was based on a feed matrix

previously used at PNNL (CS/LN/TM-Mo-6.25)3. As stated, this matrix was based on a combination of the TRUEX raffinate and the TALSPEAK strip product streams generated by a representative uranium extraction group of aqueous extraction processes presently being developed by the Advanced Fuel Cycle Initiative (AFCI) within the DOE-NE. It combines the alkali and alkaline earth fission product (CS), the lanthanide fission product (LN), and the transition metal fission product (TM) waste streams of UREX+, along with any trace actinides not removed by the UREX+ process. Table I compares the compositions of the combined TRUEX raffinate and TALSPEAK strip product5 (the combined UREX+ waste streams) with the waste simulant used as the feed for the CCIM tests performed at the INL.

TABLE I. Comparison of UREX+ waste and INL simulant compositions

UREX+ Waste Streams

INL Waste Simulant

g/L wt% g/L wt% Actinides 8.79E-04 0.20 0.14 0.20 Lanthanides 2.16E-01 48.68 32.68 47.44 Alkali 4.93E-02 11.12 9.30 13.50 Alkaline Earths 6.60E-02 14.89 8.41 12.21 TMs 1.07E-01 24.12 16.48 23.93 Noble Metals 4.37E-03 0.99 0.14 0.21 Others 1.74 2.52

For the CCIM surrogate feed, Neodymium (Nd) was

substituted for the trace actinides on a mole basis. Therefore, Nd was representative of the actinides (i.e. U, Pu, Np, Am, and Cm) still remaining after solvent extraction recovery in the UREX + method, as well as the contribution of Nd to the fission product lanthanides. The discrepancy in the wt% of the noble metals is justified and explained later.

As shown by the contrasting g/L quantities in Table I, the solids in the CCIM surrogate feed were assumed to have been concentrated (de-watered and denitrated) for more efficient melting. This higher concentration removed some, but not all, of the heat and volume burden placed on the pilot plant CCIM’s melter and off-gas systems with respect to the large quantities of nitric acid and water existing in the simulated UREX+ waste stream feed.5

In the interest of reducing costs, modifications were made to the CS/LN/TM-Mo-6.25 original targeted glass ceramics waste form composition. The changes reduced costs and impacted neither the properties of the glass ceramics produced nor the impact the UREX+ waste stream had on the resultant glass ceramic immobilized waste form. Therefore, the waste simulant was still representative of the combined TRUEX raffinate and TALSPEAK strip product streams from UREX+ process, as shown in table I.

Due to the fact that the cadmium, antimony, and selenium (Cd, Sb, and Se) were present at very low concentrations, they were removed from the simulant. These three elements would have dissolved into the glass matrix and therefore, their removal did not impact the properties of the glass ceramic waste form. Noble metals are required in the feed because they are insoluble in glass, even at low concentrations, and become nucleation sites for crystallization, as required for ceramic mineral formation. However, the amount needed in the glass ceramics matrix was reduced to 0.1 wt% since that small of an amount was all that was needed to see the effect of the undissolved noble metals in the glass ceramic waste forms. The decrease in the amount of noble metals caused the INL waste simulant to have a noble metal wt%

less than that in the UREX+ waste stream it simulated (shown in table I). Even though the waste simulant had la lower noble metal concentration than the stream it was representing, there were still enough noble metals to see the representative effect of them on the properties of the glass ceramic waste form. Ruthenium (Ru) was chosen as the element to represent the noble metals in the waste simulant, while removing palladium and rhenium (Pd and Rh), since it was least expensive. After the modifications were made to the PNNL targeted glass ceramic composition, it was renormalized to obtain the INL targeted glass ceramic composition as shown in table II.

TABLE II. Compositions of the targeted glass ceramic

waste form produced by INL’s CCIM and the waste simulant fed to the CCIM.

Oxide in waste

form

INL targeted waste form composition

wt%

INL simulant additive

Simulant composition

wt %

Al2O3 5.04 Al2O3 3.13 B2O3 9.46 B2O3 10.43 CaO 4.29 CaCO3 4.75 Na2O 3.57 Na2CO3 3.79 SiO2 33.06 SiO2 20.52 MoO3 6.29 MoO3 3.91 SrO 1.58 Sr(NO3)2 2.00 BaO 3.55 Ba(NO3)2 3.76 Rb2O 0.68 RbNO3 0.67 Cs2O 4.63 CsNO3 3.98 Y2O3 1.01 Y(NO3)3*6H2O 2.13 Ce2O3 4.99 Ce(NO3)3*6H2O 8.20 Eu2O3 0.28 Eu(NO3)3*6H2O 0.43 Gd2O3 0.26 Gd(NO3)3*6H2O 0.39 La2O3 2.55 La(NO3)3*6H2O 4.20 Nd2O3 8.44 Nd(NO3)3*6H2O 13.65 Pr2O3 2.33 Pr(NO3)3*6H2O 3.82 Sm2O3 1.73 Sm(NO3)3*6H2O 2.74 ZrO2 4.81 ZrO(NO3)2*2H2O 6.47 RuO2 0.10 Ru(NO)(NO3)3 0.15 Ag2O 0.18 AgNO3 0.16 SnO2 0.11 SnO2 0.07 TeO2 1.06 TeO2 0.66 Sum 100.00 100.00 Waste loading 44.59%

In addition to the INL CCIM targeted glass ceramic

waste form composition, Table II shows the composition of the surrogate melter waste stream fed to the pilot-scale CCIM. The first five chemicals listed in the table were the glass ceramic additives and are therefore not part of the original surrogate HLW inventory. NOAH

Technologies of San Antonio, TX prepared the INL waste simulant.

The INL targeted glass ceramic composition had a waste loading of approximately 45 wt%. The oxide composition was used to calculate the amounts of the additives needed for the melter feed. The melter feed contained ~ 300 g of solids/L melter feed. Approximately 62 wt% of the solids in the feed formed the glass ceramics, resulting in ~186 g glass/L melter feed.

Melter starter glass was prepared and provided for the CCIM test by MO-SCI Corporation of Rolla, MO. The composition for the starter glass is that of the targeted waste form composition shown in table II.

II.B. Melter Operation

To begin the melter run, the starter glass was added to the crucible and was used for coupling the coils with the melt. When the entire starting bed was molten at the desired temperature (1300-1350°C), the feeding of the slurry feed (HLW simulant) started at rates from ½ to 1 kg/hr. The feed was well mixed in order to ensure the feeding of homogeneous slurry. The melter continued to operate at 1300-1350°C and consistently melt the waste simulant.

When stable CCIM operations were achieved, the system operated in a planned manner at a set radio frequency to induce and couple to the melt, which had a conductivity that was lower than normal and was therefore, tougher to couple to. Tapping of the drain and pouring of the melt was then undertaken. The drain was heated to 1200-1275°C to begin the pouring. To stop the pour a vacuum was pulled on the crucible and if that was not successful, a cooled rod was used to freeze the melt in the drain. Three taps and pours were intermittently and successfully performed.

Even though the waste simulant was assumed to be de-watered and de-nitrated, the feed was not enough of aslurry because it took a long time to boil out all of the excess liquid in the feed. II.C. Controlled Cooling for Crystal Growth

Each drain was poured into one of three specially designed graphite canisters, shown if Fig. 1, for controlled cooling of the melt to ensure formation of the desired crystalline phases.

Fig. 1. This is one of the 3 specially designed graphite canisters used for controlled cooling of the pour. The canisters were four inches in diameter and 10 inches tall. Each of the canisters was equipped with a thermocouple tree and was located in a heating mantle, seen in Fig. 2, to control the cooling profile.

Fig. 2. The bank of specially designed canisters in their heating mantles for controlled cooling of the pour.

The four-inch diameter canister in conjunction with the heating mantle is used to simulate the natural cooling, with no added heat, of a two-foot diameter canister. The significance of the two-foot diameter canister is that Savannah River Site Defense Waste Processing (SRS-DWP) and West Valley use this canister size to pour the vitrified waste, melted utilizing JHMs, into for cooling and disposal. SRS-DWP developed a worst-case scenario to create a centerline cooling profile that provided the slowest rate at which the two-foot canisters could possibly cool, called 1X, without the use of heaters. It is preferred to perform the cooling process without heaters because this is an in-cell process. Since SRS-DWP was forming BSG waste forms, crystal formation was an undesirable occurance. If the worst-case cooling scenario, 1X, which is the slowest, formed no crystals, then any other cooling

scenario would work as well because faster cooling results in fewer crystals forming.

Even though the CCIM, in this case, is being used for the formation of crystals, the 1X cooling rate is being used as a reference case because it is a standard that is already developed and being used in waste vitrification. If the CCIM pours cool at this rate, or more quickly, in the simulated two-foot canister, and forms the desired crystalline phases with all of the simulant components going into the desired phases, then it is shown that a two-foot canister can be used in-cell without the aid of heaters for the formation of crystalline HLW waste forms.

Each of the three pours were cooled under separate slow cooling schedules to form a four inch diameter glass ceramic log varying from six to eight inches in height. The first cooling scenario is the 1X cooling curve described above. The second cooling schedule, referred to as 2X was controlled to twice as fast as 1X. The third canister was allowed to cool naturally, referred to as NC, which is faster than the controlled cools because no heat is applied to cool it according to a specific cooling profile. The different cooling scenarios were chosen to try to bound the cooling time required to form the desired crystalline phases and have all of the components in the HLW simulant go into the correct phase. The end results will be a range of cooling rates at which the CCIM melt can cool to achieve the targeted results in a two-foot diameter canister without having to use heating mantles.

To accommodate the waste components that are above solubility limits of a single phase BSG waste form, the phases targeted by the slow cooling are powellite (XMoO4) where X can be Ca, Sr, Ba, and/or LN and oxyapatite (Yx,Z(10-x)Si6O26) where Y is alkaline earth and Z is LN. Bulk phase separation, achieved at high temperature, allows for the occurrence of nucleation of oxyapatite crystals. Controlled cooling slows the cooling process and allows enough time to crystallize the targeted phases. II.D. Sampling and Analysis

After the pours were cooled to room temperature and cured, they were sent to PNNL for sampling and analysis. Nine samples were cut from controlled cools 1X and NC. There was an error in the controlled cooling of 2X that caused it to become pure crystalline and was too hard to cut. Therefore, it was neither sampled nor analyzed. To obtain the nine samples, the cylindrical log was cored horizontally 3 times (top, middle, and bottom). Three cross-sections from each of the three cores were then made (left, middle, and right).

Analyses of the canisters were performed to determine how well the glass ceramic filled the canister, examine the distribution of phases in the canister, identify the crystal phases present, and determine the chemistry. The samples were analyzed by X-ray powder diffraction

(XRD), scanning electron microscopy (SEM), and SEM coupled with energy dispersive spectrometry (EDS).

XRD was used to identify the crystalline phases and determine crystal concentrations. The samples were ground into a fine powder and doped with a known concentration of rutile (SRM-674b 2007). The XRD patterns were collected on a Bruker D8 Advance system equipped with a Cu-target over a scan range of 5–75° 2θ using a step size of 0.015° 2θ at a hold time of 4 seconds per step.

The scans were analyzed with TOPAS version 4.2 (Bruker AXS) whole pattern fitting software according to the fundamental parameters approach. Structure patterns were selected from the Inorganic Crystal Structure Database with unit cell dimensions refined in the fitting process of each pattern. The amorphous content of each sample, the remainder after the crystalline phases were quantified, was calculated by the software based on concentration of the known internal standard.

SEM was performed to examine morphology and homogeneity of the samples with a JEOL JSM-7001F microscope. The SEM was also coupled with an Apollo XL Si drift detector (EDAX, USA) energy dispersive spectrometer (EDS) to perform elemental analysis. The elemental dot maps from SEM-EDS show component distribution in the various phases. SEM image analysis was performed to determine the volume percent of each phase with Scandium software. Phases were selected by grey scale thresholding and the software calculated the area fraction of each phase, which were then extrapolated into volume fractions. III. RESULTS AND DISCUSSION

The samples were labeled as follows: 1, 2, 3 for top,

middle, bottom (respectively) horizontal cores and A, M, B for left, middle, right (respectively) cross-sections of the cores. For example, 2B would be the sample for the middle core and right cross-section.

X-ray tomography was attempted to 3-dimensionally examine the interior of the glass ceramic specimens. However, the sample chemistry was too dense to penetrate with the x-ray source. Alternatively, the 1X controlled cooled and NC cooled waste form logs were cut vertically to expose the glass ceramic inside. The pieces were examined for any voids that might be created by rapid solidification of the melt due to crystallization upon cooling and no significant voids were found.

The XRD scans of all 9 samples of the 1X canister were very similar. Three crystalline phases were observed by XRD, 2 of which were powellite and the other was oxyapatite. Upon analyzing the scans with TOPAS, the quantity of each phase and total crystallinity were determined. The results are shown in Fig. 3.

Fig. 3. The amounts of each crystalline phase in each sample from 1X observed by XRD.

Fig. 3 shows the amounts of each crystalline phase and crystal distribution to be uniform from top to bottom and radially.

The SEM micrographs showed the crystalline phases formed in the glass ceramic waste form throughout the 1X canister to be homogenous in terms of the types and the observed distribution, seen in Fig. 4.

Fig. 4. SEM images taken at magnification of 300x of the 1X canister samples showing observed crystals.

SEM-EDS results of the 1X canister also showed three phases, two distinct powellite and an oxyapatite. Fig. 5 shows the elemental dot map of the SEM-EDS analysis.

Fig. 5. Elemental dot map of SEM-EDS results for 1X canister.

Fig. 5 shows the distribution of the elements within the glass ceramic waste form and the crystalline phases formed. It is seen that the Nd went into the oxyapatite as targeted and the Mo went into the targeted phase of powellite. Also, the elemental dot map distinctly shows the two powellite phases, one being Ba, Sr-rich and the other being Ca, Sr-rich. The Ba and Ca are seen in different areas of the powellite crystals, denoting the two powellite phases. The noble metal oxide (RuO2) is also observed in the phase image of Fig. 5.

Analysis of the NC canister showed, by XRD, that the concentration and distribution of the crystals were uniform from top to bottom and radially. XRD results also showed that there were three phases, however, they were all powellite phases. No oxyapatite was able to be identified via XRD in the NC glass ceramic waste form.

SEM images, seen in Fig. 6, did show small amounts of oxyapatite, however, the morphology of the NC canister is vastly different than that of the 1X canister as shown in Fig. 4.

Fig. 6. Backscattered electron image of NC canister at magnifications of 250x.

The long crystals in the left of Fig. 6 are oxyapatite, the small white droplets to the right of the oxyapatite are powellite crystals. The SEM-EDS analysis of the NC canister showed a few isolated pockets of oxyapatite and

μm-sized droplets and nm-sized droplets throughout. The droplets were too small to analyze be SEM-EDS; therefore, they were analyzed by scanning transmission electron microscopy (STEM). Fig. 7 shows the elemental dot map made from the STEM analysis results.

Fig. 7. Elemental dot map of STEM results for NC canister.

From Fig. 7 it can be seen that the Ba, Sr-rich and Ca,

Sr-rich powellite phases are still present. The droplets are identified as having a Cs-Al-silicate matrix and the phase around the droplets has a lanthanide-alkaline earth-Na-silicate matrix. The NC glass ceramic did not form the oxyapatite with the Nd as was targeted. IV. CONCLUSIONS AND FUTURE WORK

From the work reported within this report, it was

demonstrated that HLW can be processed using CCIM technology. It is concluded that glass ceramic waste forms that are tailored to immobilize fission products of HLW can be can be made from the HLW processed with the CCIM. The advantageous higher temperatures reached with the CCIM and unachievable with the JHM causes the lanthanides, alkali, alkaline earths, and molybdenum to dissolve into a molten glass. Upon controlled cooling they go into targeted crystalline phases to form a glass ceramic waste form with higher waste loadings than achievable with BSG waste forms. Natural cooling proves to be too fast for the formation of all targeted crystalline phases.

Future plans for glass ceramic waste form production utilizing the CCIM are to process a much more concentrated HLW simulant and to perform more controlled cools on the pours. These controlled cools will be quicker than the 1X but not as slow as the natural cooled from this test.

ACKNOWLEDGMENTS

The authors wish to thank Battelle Energy Alliance

personnel Bradley Benefiel, Clark Scott, Kip Archibald, Mike Ancho and Sabrina Morgan-all with INL. We also

would like to thank Dr. John Richardson (Vista Engineering) in regard to RF generator operation, theory, and mentoring and Jarrod Crum from PNNL for sample analysis and guidance on the feed.

REFERENCES

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2. J. V. CRUM, et.al. “Multi-phase glass ceramics as a waste form for combined fission products: alkalis, alkaline earths, lanthanides, and transition metals,” Journal of the American Ceramic Society, 95, 4 (2012).

3. J. V. CRUM, et.al. “Glass ceramic waste forms for combined CS+LN+TM fission products waste streams,” FCRD-WAST-2010-000181, PNNL-2010, Pacific Northwest National Laboratory (2010).

4. DR. D. GOMBERT, et.al “Cold-Crucible design parameters for next generation HLW melters,” Waste Management ’02 Conference, Tuscon, AZ, Feb. 24-28, (2002).

5. B. J. RILEY, et.al. “Initial laboratory-scale melter test results for combined FP waste,” AFCI-WAST-PMO-MI-DV-2009-000184, PNNL-2009, Pacific Northwest National Laboratory (2009).