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Comparison of ceramic waste forms produced by hot uniaxial pressing and by cold pressing and sintering

Description: Synroc C waste form specimens prepared using the Australian-developed technology are uniaxially pressed in stainless steel bellows at 1200{degrees}C and 20MPa. This produces a material with high chemical and physical durability and with the radioactivity enclosed inside both the waste form and the bellows. An alternative method of producing the ceramic product is to use cold pressing of pellets followed by reactive sintering to provide densification and mineralization. Depending on the scale of waste form preparation required and on the activity level and nature of the waste streams, the cold press and sinter method may have advantages. To evaluate the effects of production method on waste form characteristics, especially resistance to dissolution or leaching of waste elements, we have prepared two simulated waste samples for evaluation. Both samples were prepared from liquid precursor materials (alkoxides, nitrates, and colloidal silica) and then doped with waste elements. The precursor material in each case corresponded to a basic phase assemblage of 60% zirconolite, 15% nepheline, 10% spinel, 10% perovskite, and 5% rutile. One sample was doped with 25% by weight of U; the other with 10% by weight each of U and Gd. Each sample was calcined at 750{degrees}C for 1 hr. in a 3.5% H{sub 2} in N{sub 2} atmosphere. Then one portion of each sample was hot pressed at temperatures ranging from 1120 to 1250{degrees}C and 20MPa pressure in steel bellows. A separate portion of each sample was formed into pellets, cold pressed, and sintered in various atmospheres at 1200{degrees}C to produce final products about 2/3 cm in diameter. Samples were then examined to determine density of the product, grain sizes of the phases, phase assemblage, and the location of the U and Gd in the final phases. Density data indicate that sintering gives good results provided that the samples are ...
Date: September 1, 1994
Creator: Oversby, V.M. & Vance, E.R.
Partner: UNT Libraries Government Documents Department

Intergrowth Structure in U- and Hf-Bearing Pyrochlore and Zirconolite: TEM Investigation

Description: Transmission electron microscopy results from a sintered ceramics with stoichiometry of Ca(U{sub 0.5}Ce{sub 0.25}Hf{sub 0.25})Ti{sub 2}O{sub 7} show the material contains both pyrochlore and zirconolite phases and structural intergrowth of zirconolite lamellae within pyrochlore. (001) plane of zirconolite is parallel to (111) plane of pyrochlore because of their structural similarities. The pyrochlore is relatively rich in U, Ce, and Ca with respect to the coexisting zirconolite. Average compositions for the coexisting pyrochlore and zirconolite produced by sintering at 1350 C are (Ca{sub 1.01}Ce{sub 0.13}{sup 3+}Ce{sub 0.19}{sup 4+}U{sub 0.52}Hf{sub 0.18})(Ti{sub 1.95}Hf{sub 0.05})O{sub 7} (with U/(U+Hf) (in the AB sites) = 0.74) and (Ca{sub 0.91}Ce{sub 0.09})(Ce{sub 0.08}{sup 3+}U{sub 0.26}Hf{sub 0.66}Ti{sub 0.01})Ti{sub 2.00}O{sub 7} (with U/(U+Hf) = 0.28) respectively. A single pyrochlore ((Ca,U,Hf){sub 2}Ti{sub 2}O{sub 7}) phase may be synthesized at 1350 C if the ratio of U/(U+Hf) is greater than 0.72, and a single zirconolite (Ca(Hf,U)Ti{sub 2}O{sub 7}) phase may be synthesized at 1350 C if the ratio of U/(U+Hf) is less than 0.28. An amorphous leached layer that is rich in Ti and Hf forms on the surface after the ceramics has been leached in pH 4 buffered solution. The thickness of the layer ranges from 5 nm to 15 nm. It is suggested that under these conditions, the leached layer functions as a protective layer, and reduces the leaching rate over time.
Date: December 4, 2002
Creator: Xu, H; Wang, Y; Zhao, P; Bourcier, W L; Van Konynenburg, R & Shaw, H F
Partner: UNT Libraries Government Documents Department

Radiation Damage Effects in Candidate Ceramics for Plutonium Immobilization: Final Report

Description: In this document, we summarize our study of the effects of radiation induced damage to the titanate ceramics that were to be the immobilization form for surplus weapons-grade Pu. In this study, we made five ceramic materials: pure-phase pyrochlore, pure-phase zirconolite, pyrochlore-rich baseline, zirconolite-rich baseline, and impurity baseline. Two-hundred specimens were made of which 130 contained approximately 10 mass% 238Pu and 70 contained 10 mass% 239Pu. The specimens containing 239Pu served as materials against which the behavior of the 238Pu-bearing specimens could be compared. In our studies, we measured the true density (density exclusive of surface connected porosity), bulk density, crystalline-phase composition with X-ray diffraction (XRD), and dissolution rates as radiation induced damage accumulated in the 238Pu-bearing specimens. We routinely took photographs of the specimens during each characterization period. From our studies, we determined that these materials swell less than 10% and generally less than 5%. As the material swells, some open porosity can be converted to closed porosity, often causing the true density to decrease more rapidly than the bulk density. In general, 3?1018 a/g of damage accumulation were required for the materials to become amorphous as determined with the XRD method. The order in which the phases became amorphous was brannerite, pyrochlore, and zirconolite with brannerite being the most susceptible to radiation induced damage. However, we also show that Pu is not evenly distributed amongst the phases when multiple phases are present. We were unsuccessful in making a pure brannerite to study. Therefore, the brannerite was always present with other phases. For a material containing about 10 mass% 239Pu, 3?1018 a/g represent about 500 years in the geologic repository. At no time in our studies was there evidence for microcracking in these materials, even upon close examination in a scanning-electron microscope . Upon careful comparison between the dissolution ...
Date: February 1, 2004
Creator: Strachan, Denis M.; Scheele, Randall D.; Icenhower, Jonathan P.; Buck, Edgar C.; Kozelisky, Anne E.; Sell, Rachel L. et al.
Partner: UNT Libraries Government Documents Department

Development of a ceramic form for immobilization of excess plutonium

Description: Between 8 and 50 metric tonnes of excess plutonium are currently planned to be immobilized in a glass or ceramic waste form in the US. The immobilized Pu would then be encased in HLW glass (the can-in-canister alternative), which would provide a radiation barrier to enhance the proliferation resistance of the material. Associated with the plutonium are about 15 metric tonnes of uranium primarily {sup 238}U and a variety of other impurities (primarily Ga, Mo, Al, Mg, Si, and Cl) totaling about 1 metric tonne or less. Immobilization of this material is complicated by the fact that the uranium content in the various feed streams varies widely, from 0 to about 95%. The proposed ceramic form is composed of about 90% zirconolite (CaZrTi{sub 2}O{sub 7}) and/or pyrochlore (CaPuTi{sub 2}O{sub 7}) with about 10% other phases, typically hollandite (BaAl{sub 2}Ti{sub 6}O{sub 16}) and rutile (TiO{sub 2}). The form is a variation of Synroc-C, which contains nominally 30% zirconolite, 30% perovskite, 30% hollandite, and 10% rutile and noble metal alloys. Zirconolite and perovskite are the actinide host phases in Synroc-C with zirconolite being the more durable phase. The pyrochlore structure is closely related to zirconolite and forms at higher actinide loadings. Thus, this mineral is of interest for plutonium deposition in ceramic. Pyrochlore has the advantage that it is cubic rather the monoclinic like zirconolite. Cubic mineral swell isotropically when radiation damaged. As a result, differential strain in the microstructure will be minimal, leading to significantly less microcracking of the form after thousands of years in a repository. Zirconolites and pyrochlores containing uranium and.or thorium exist in nature and have demonstrated actinide immobilizations for periods exceeding 100 million years.
Date: April 22, 1997
Creator: Van Konynenburg, R.; Ebbinghaus, B.; Ryerson, F.; Shaw, H. & Curtis, P.
Partner: UNT Libraries Government Documents Department

Pu and Gd chemistry of zirconolite polytypes in a titanate ceramic

Description: Titanate-based ceramics are being developed as possible candidates for immobilizing excess plutonium from dismantled nuclear weapons. Evidence from testing of similar ceramics and natural analogues suggests that this material is very resistant to aqueous corrosion. The purpose of this work is to describe the phase(s) present in these ceramics. In particular the authors are interested in the disposition of important elements such as Pu and Gd (to be incorporated into the wasteform as a neutron absorber). In concert with data from corrosion tests, this characterization will allow one to describe the release behaviors of important elements from this type of ceramic. This is particularly difficult and important due to the heterogeneous nature of the material.
Date: October 1, 1997
Creator: Bakel, A.J.; Buck, E.C. & Ebbinghaus, B.
Partner: UNT Libraries Government Documents Department

EXAFS and XANES analysis of plutonium and cerium edges from titanate ceramics for fissile materials disposal.

Description: We report x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectra from the plutonium L{sub III} edge and XANES from the cerium L{sub II} edge in prototype titanate ceramic hosts. The titanate ceramics studied are based upon the hafnium-pyrochlore and zirconolite mineral structures and will serve as an immobilization host for surplus fissile materials, containing as much as 10 weight % fissile plutonium and 20 weight % (natural or depleted) uranium. Three ceramic formulations were studied: one employed cerium as a ''surrogate'' element, replacing both plutonium and uranium in the ceramic matrix, another formulation contained plutonium in a ''baseline'' ceramic formulation, and a third contained plutonium in a formulation representing a high-impurity plutonium stream. The cerium XANES from the surrogate ceramic clearly indicates a mixed III-IV oxidation state for the cerium. In contrast, XANES analysis of the two plutonium-bearing ceramics shows that the plutonium is present almost entirely as Pu(IV) and occupies the calcium site in the zirconolite and pyrochlore phases. The plutonium EXAFS real-space structure shows a strong second-shell peak, clearly distinct from that of PuO{sub 2}, with remarkably little difference in the plutonium crystal chemistry indicated between the baseline and high-impurity formulations.
Date: November 16, 1999
Creator: Fortner, J. A.; Kropf, A. J.; Bakel, A. J.; Hash, M. C.; Aase, S. B.; Buck, E. C. et al.
Partner: UNT Libraries Government Documents Department

Interim report on task 1.2: near equilibrium processing requirements part 1 of 2 to Lawrence Livermore National Laboratory for contract b345772

Description: The following ceramics were prepared for this study: the baseline ceramic, the baseline ceramic plus process impurities, a zirconolite-rich composition, a brannerite-rich composition, a composition designed to have {approx} 10% perovskite in addition to the normal baseline phases, and an {approx} 10% phosphate-doped batch. These samples were prepared by oxide (via wet or dry milling) and alkoxide-routes. The milling method has a direct effect on the samples. Incomplete milling leads to inhomogeneity in the microstructure. The main effect of incomplete milling is that unreacted actinide oxides remain in the microstructure, usually surrounded by brannerite. The phase composition also alters due to the actinide being tied up in the unreacted oxide, e.g., the pyrochlore has less actinide and additional phases, such as zirconolite may form. The approach to equilibrium is determined by a number of factors--the efficiency of the milling, the sintering time, the sintering temperature and the batch composition. The latter is very important, e.g., the zirconolite-rich batch has a slower approach to equilibrium than the baseline ceramic requiring higher sintering temperatures or longer sintering times. The addition of process impurities dramatically alters the approach to equilibrium. The additives produce a silicate liquid phase (at the sintering temperatures), which aids the dissolution and diffusion of the actinide oxide and densification such that even the relatively inhomogeneous dry milled oxide-route samples reach 90--95% of their equilibrium state on sintering at 1350 C for 4 hours. In terms of approach to equilibrium, the following summarizes the results. For 4 h at 1350 C samples: dry-milled oxide samples reach {approx} 50-80% of their equilibrium state, except for the baseline + additives batch which achieves {approx} 90-95% equilibrium. The wet-milled oxide route samples reach {approx} 90-100% equilibrium and alkoxide-route samples reach {approx} 95-100% equilibrium. The following refers to alkoxide or wet-milled oxide-routes. Sintering for ...
Date: April 5, 1999
Creator: Stewart, M W A; Vance, E R; Day, R A & Brownscombe, A
Partner: UNT Libraries Government Documents Department

Characterization of a Pu-bearing zirconolite-rich synroc

Description: A titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi{sub 2}O{sub 7}), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium- loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO{sub 2- x}. The Pu oxidation state in the zirconolite was determined to be mainly Pu{sup 4+}, although some Pu{sub 3+} was believed to be present.
Date: December 31, 1996
Creator: Buck, E.C.; Ebbinghaus, B.; Bakel, A.J. & Bates, J.K.
Partner: UNT Libraries Government Documents Department

Corrosion of a Pu-doped zirconolite-rich ceramic

Description: As part of a large Pu disposition program, a zirconolite-rich titanate ceramic is being developed at Lawrence Livermore National Laboratory (LLNL) as a possible immobilization material. This same material is being tested at Argonne National Laboratory (ANL). The goal of this study is to describe the corrosion behavior of this ceramic, particularly the release of Pu and Gd, using results from several static corrosion tests (MCC-1, PCT-A, and PCT-B). The release of relatively large amounts of Al, Ba, and Ca in short-term tests (3 day MCC-1 and 7 day PCT-A) indicates that these elements are released from grain boundaries or from highly soluble phases. Results from long-term (28, 98, and 182 day) PCT-B show that the releases of Al, Ba, and Ca decrease with time, the releases of U and Zr increase with time, and that the releases of Cs, Gd, Mo, and Pu remain fairly constant. Formation of alteration phases may lead to the decrease of Ba and Ca in leachate solutions. Due to the heterogeneous nature of the material, the formation of alteration phases, and the inherently low solubility of several elements, no element(s) could be recommended as good markers for the overall corrosion of this ceramic. Data show that, due to the complex nature of this material, the release of each element should be considered separately.
Date: June 1, 1997
Creator: Bakel, A.J.; Buck, E.C.; Wolf, S.F.; Chamberlain, D.B.; Bates, J.K. & Ebbinghaus, B.B.
Partner: UNT Libraries Government Documents Department

The relative radiation resistance of zirconolite, pyrochlore and perovskite to 1.5 MeV Kr{sup +} ions

Description: Zirconolite (CaZrTi2O7), pyrochlore (VIIIA2 VIB2 IV X6Y) and perovskite (CaTiO3) are candidate phases for the immobilisation of rare earth elements (REEs) and actinides (ACTs) in various high level radioactive waste (HLW) forms 1. The effect of radiation damage on the structure and consequently on the durability of these phases is important to predictive modelling of their behaviour in the repository environment and risk assessment.
Date: March 1, 1997
Creator: Smith, K.L.; Zaluzec, N.J. & Lumpkin, G.R.
Partner: UNT Libraries Government Documents Department

The impact of brannerite on the release of plutonium and gadolinium during the corrosion of zirconolite-rich titanate ceramics

Description: Titanate ceramics have been selected as the preferred waste form for the immobilization of excess plutonium. Corrosion tests are underway to try to understand the long-term behavior of this material. In this paper, results from PCT-B static dissolution tests are used to provide an explanation of the observed corrosion behavior of a zirconolite-based ceramic. Two important observations are made. First, Ca is released at a constant rate [7 x 10{sup {minus}5} g/(m{sup 2} day)] in PCT-B tests for up to two years. Second, the release rates for Pu and Gd increase with time (up to two years) in PCT-B tests. The first observation suggests that the ceramics continue to corrode at a low rate for at least two years in PCT-B tests. The second observation suggests that the release rates of Pu and Gd are controlled by some process or processes that do not affect the release rate of other elements. Evidence indicates that this is due to the preferential dissolution of brannerite from the ceramic.
Date: March 14, 2000
Creator: Chamberlain, D. B.; Hash, M. C.; Basco, J. K.; Bakel, A. J.; Metz, C. J.; Wolf, S. F. et al.
Partner: UNT Libraries Government Documents Department

HVEM-Tandem and EELS study of radiation damage in zirconolite

Description: Zirconolite (CaZrTi{sub 2}O{sub 7}) is the major host phase for actinides in Synroc, a promising waste form for the immobilization of high-level radioactive waste. The effect of radiation damage on the structure and durability of zirconolite are important to predictive modeling of zirconolite`s behavior in the repository environment and risk assessment. In this study, radiation damage effects in zirconolite were investigated by irradiating samples with 1.5 MeV Kr{sup +} ions using the HVEM-Tandem at Argonne National Laboratory (ANL) and energy loss electron spectroscopy (EELS). The HVEM-Tandem consists of a modified AEI high voltage transmission electron microscope interfaced to a 2 MV tandem ion accelerator. EELS spectra were collected using a Philips 420 TEM, operated at 120 kV, fitted with a Gatan Model 607 Serial EELS. EELS data were recorded at resolutions of {approximately} 1.0 eV and at a dispersion of about {approximately} 0.25 eV. Selected area diffraction patterns (SADs) of individual grains of various zirconolites were monitored as a function of dose to establish the critical dose for amorphization (D{sub c}). The authors found that (1) D{sub c}(zirconolite) is independent of the atomic weight of dopants in zirconolite and the mean atomic weight of the sample and that (2) the Bragg reflections in SAD patterns which persist to the highest doses are firstly those resulting from the fluorite sublattice and secondly the four (110)-type reflections which lie on the innermost of the two diffuse rings representative of amorphous zirconolite.
Date: March 1, 1997
Creator: Smith, K.L.; Lumpkin, G.R. & Zaluzec, N.J.
Partner: UNT Libraries Government Documents Department

Fundamental thermodynamics of actinide-bearing mineral waste forms.

Description: This report summarizes work after completion of a three-year project and our current ongoing efforts. Research efforts at UC Davis have focused on establishing the thermodynamic properties of zirconolite and pyrochlore, and the synthesis of other minerals relevant to storage of nuclear material. Heat capacity, entropy, enthalpy of formation, and fkee energy of formation data were established for zirconolite, CaZrTi{sub 2}O{sub 7}, in the range from 0 to 1500 K. The heat capacity, entropy, enthalpy of formation, and free energy of formation at 298 K for zirconolite are 211.9 J/K mol, 193.3 J/K mol, -3713.8 kJ/mol, and -35 14.6 kJ/mol, respectively. Solution calorimetry experiments with cerium pyrochlore, Ca{sub 0.8}Ce{sub 1.2}Ti{sub 2}O{sub 7}, are complete. Heat capacity data and confirmation of the pyrochlore composition are required for final data analysis. Synthesis and characterization of CaHfTi{sub 2}O{sub 7}, CaZr{sub 0.5}Hf{sub 0.5}Ti{sub 2}O{sub 7}, Gd{sub 2}Ti{sub 2}O{sub 7}, and CeTi{sub 2}O{sub 6} is complete. Research efforts at Los Alamos have focused on establishing synthesis techniques for actinide-bearing minerals and preparation of the calorimetry laboratory. The preparation of Pu-pyrochlore, nominally CaPuTi{sub 2}O{sub 7} has been achieved by Ebbinghaus at Lawrence Livermore National Laboratory. A sample of this material has been sent to Putnam at Los Alamos National Laboratory.
Date: January 1, 2002
Creator: Gallegos, U. F. (Ubaldo F.) & Putnam, R. L. (Robert L.)
Partner: UNT Libraries Government Documents Department

Ternary Phase Diagrams that Relate to the Plutonium Immobilization Ceramic

Description: The plutonium immobilization ceramic consists primarily of a pyrochlore titanate phase of the approximate composition Ca{sub 0.97}Hf{sub 0.17}Pu{sub 0.22}U{sub 0.39}Gd{sub 0.24} Ti{sub 2}O{sub 7}. In this study, a series of ternary phase diagrams was constructed to evaluate the relationship of various titanate phases (e.g., brannerite, zirconolite-2M, zirconolite-4M, and perovskite) to pyrochlore titanates, usually in the presence of excess TiO{sub 2} (rutile), and at temperatures in the vicinity of 1350 C. To facilitate the studies, U, Th, and Ce were used as surrogates for Pu in a number of the phase diagrams in addition to the use of Pu itself. The effects of impurity oxides, Al{sub 2}O{sub 3} and MgO, were also studied on pyrochlore (Gd{sub 2}Ti{sub 2}O{sub 7}) and zirconolite (CaHfTi{sub 2}O{sub 7}) mixtures. Either electron microprobe (at Lawrence Livermore National Laboratory) or quantitative SEM-EDS (at Australian Nuclear Science and Technology Organization) were used to evaluate the compositions of the phases.
Date: January 1, 2001
Creator: Ebbinghaus, B b; Krikorian, O H; Vance, E R & Stewart, M W
Partner: UNT Libraries Government Documents Department

The CaO-TiO{sub 2}-ZrO{sub 2} system at 1,200{degree}C and the solubilities of Hf and Gd in zirconolite

Description: In recent years, significant technological advancements have been made in the Synroc scheme for the immobilization high-level nuclear waste. However, many basic scientific issues related to Synroc fabrication have yet to be addressed. The CaO-TiO{sub 2}-ZrO{sub 2} system is an integral part of the Synroc formulation. Phase equilibria are established in the CaO-TiO{sub 2}-ZrO{sub 2} system at 1,200 C, using X-ray diffraction and electron probe microanalysis. The existence of two previously reported ternary phases, zirconolite (CaZrTi{sub 2}O{sub 7}) and calzirtite (Ca{sub 2}Zr{sub 5}Ti{sub 2}O{sub 16}), is confirmed. Each of these phases exhibits a significant range of homogeneity between TiO{sub 2} and ZrO{sub 2} while maintaining a nearly constant concentration of CaO. The ternary solubilities of the constituent binary phases are found to be negligible, with the exceptions of the perovskites, which display mutual solubility of at least 22 mol.% and may in fact form a series of continuous solid solutions. The solubilities of Hf and Gd in zirconolite are also investigated. While Hf-bearing samples did not reach thermodynamic equilibrium under the experimental conditions employed, the existence of a Hf analog to zirconolite, CaHfTi{sub 2}O{sub 7}, is conclusively demonstrated. The phase is stable at the stoichiometric composition, and its lattice parameters are very close to those reported in the literature for stoichiometric zirconolite. A Gd-bearing sample of the composition Ca{sub 0.88}Zr{sub 0.88}Gd{sub 9.24}Ti{sub 2}O{sub 7} is found to be essentially single phase zirconolite, in agreement with previous investigations at higher temperatures.
Date: December 1, 1995
Creator: Swenson, D.; Nieh, T.G. & Fournelle, J.H.
Partner: UNT Libraries Government Documents Department

Temperature dependence of ion irradiation induced amorphization of zirconolite

Description: Zirconolite is one of the major host phases for actinides in various wasteforms for immobilizing high level radioactive waste (HLW). Over time, zirconolite's crystalline matrix is damaged by {alpha}-particles and energetic recoil nuclei recoil resulting from {alpha}-decay events. The cumulative damage caused by these particles results in amorphization. Data from natural zirconolites suggest that radiation damage anneals over geologic time and is dependant on the thermal history of the material. Proposed HLW containment strategies rely on both a suitable wasteform and geologic isolation. Depending on the waste loading, depth of burial, and the repository-specific geothermal gradient, burial could result in a wasteform being exposed to temperatures of between 100--450 C. Consequently, it is important to assess the effect of temperature on radiation damage in synthetic zirconolite. Zirconolite containing wasteforms are likely to be hot pressed at or below 1,473 K (1,200 C) and/or sintered at or below 1,623 K (1,350 C). Zirconolite fabricated at temperatures below 1,523 K (1,250 C) contains many stacking faults. As there have been various attempts to link radiation resistance to structure, the authors decided it was also pertinent to assess the role of stacking faults in radiation resistance. In this study, they simulate {alpha}-decay damage in two zirconolite samples by irradiating them with 1.5 MeV Kr{sup +} ions using the High Voltage Electron Microscope-Tandem User Facility (HTUF) at Argonne National Laboratory (ANL) and measure the critical dose for amorphization (D{sub c}) at several temperatures between 20 and 773 K. One of the samples has a high degree of crystallographic perfection, the other contains many stacking faults on the unit cell scale. Previous authors proposed a model for estimating the activation energy of self annealing in zirconolite and for predicting the critical dose for amorphization at any temperature. The authors discuss their results and earlier published ...
Date: December 22, 1999
Creator: Smith, K. L.; Blackford, M. G.; Lumpkin, G. R. & Zaluzec, N. J.
Partner: UNT Libraries Government Documents Department

Corrosion behavior of pyroclore-rich titanate ceramics for plutonium disposition ; impurity effects.

Description: The baseline ceramic contains Ti, U, Ca, Hf, Gd, and Ce, and is made up of only four phases, pyrochlore, zirconolite, rutile, and brannerite. The impurities present in the three other ceramics represent impurities expected in the feed, and result in different phase distributions. The results from 3 day, 90 C MCC-1 tests with impurity ceramics were significantly different than the results from tests with the baseline ceramic. Overall, the addition of impurities to these titanate ceramics alters the phase distributions, which in turn, affects the corrosion behavior.
Date: January 13, 1999
Creator: Bakel, A. J.
Partner: UNT Libraries Government Documents Department

Intergrowth structures in synthetic pyrochlores : implications for radiation damage effects and waste form formulation.

Description: Titanate-based ceramic waste forms are currently under development for the immobilization of excess weapons plutonium. Both Hf and Gd are added to the ceramic formulation as neutron absorbers in order to satisfy a defense-in-depth concept for the waste form. The introduction of significant amounts of hafnium may be responsible for the presence of zirconolite-2M crystals in pyrochlore-based ceramics and the formation of zirconolite lamellae within pyrochlore. The zirconolite grows epitaxially on {l_brace}111{r_brace}planes of pyrochlore. Although the zirconolite lamellae within pyrochlore are non-cubic, any volume expansion due to radiation damage in the pyrochlore should still be isotropic; in addition, the presence of these intergrowths may allow some stress relief in the ceramic.
Date: March 30, 1999
Creator: Buck, E.
Partner: UNT Libraries Government Documents Department

Fundamental Thermodynamics of Actinide-Bearing Mineral Waste Forms

Description: The recent arms reduction treaties between the U.S. and Russia have resulted in inventories of plutonium in excess of current defense needs. Storage of this material poses significant, and unnecessary, risks of diversion, especially for Russia whose infrastructure for protecting these materials has been weakened since the collapse of the Soviet Union. Moreover, maintaining and protecting these materials in their current form is costly. The United States has about sixty metric tons of excess plutonium, half of which is high-purity weapon material. This high purity material will be converted into mixed oxide (MOX) fuel for use in nuclear reactors. The less pure excess plutonium does not meet the specifications for MOX fuel and will not be purified to meet the fuel specifications. Instead, it will be immobilized directly in a ceramic. The ceramic will be encased in a high level waste (HLW) glass monolith (i.e., the can-in-canister option) thus making a form that simulates the intrinsic security of spent nuclear fuel. The immobilized product will be placed in a HLW repository. To meet the repository requirements, the product must be shown to be durable for the intended storage time, the host matrix must be stable in the radiation environment, the solubility and leaching characteristics of the plutonium in the host material must be established, and optimum processing parameters must be determined for the entire compositional envelope of feed materials. In order to provide technically sound solutions to these issues, thermodynamic data are essential in developing an understanding of the chemistry and phase equilibria of the actinide-bearing mineral waste forms proposed as immobilization matrices. However, the relevant thermodynamic data (e.g., enthalpy, entropy, and heat capacity) for the ceramic forms are severely lacking and this information gap directly affects the Energy Department's ability to license the disposal matrices and methods. High-temperature solution ...
Date: June 1, 1999
Creator: Williamson, Mark A.; Ebbinghaus, Bartley B. & Navrotsky, Alexandria
Partner: UNT Libraries Government Documents Department

Technical Progress Report on Single Pass Flow Through Tests of Ceramic Waste Forms for Plutonium Immobilization

Description: This report updates work on measurements of the dissolution rates of single-phase and multi-phase ceramic waste forms in flow-through reactors at Lawrence Livermore National Laboratory. Previous results were reported in Bourcier (1999). Two types of tests are in progress: (1) tests of baseline pyrochlore-based multiphase ceramics; and (2) tests of single-phase pyrochlore, zirconolite, and brannerite (the three phases that will contain most of the actinides). Tests of the multi-phase material are all being run at 25 C. The single-phase tests are being run at 25, 50, and 75 C. All tests are being performed at ambient pressure. The as-made bulk compositions of the ceramics are given in Table 1. The single pass flow-through test procedure [Knauss, 1986 No.140] allows the powdered ceramic to react with pH buffer solutions traveling upward vertically through the powder. Gentle rocking during the course of the experiment keeps the powder suspended and avoids clumping, and allows the system to behave as a continuously stirred reactor. For each test, a cell is loaded with approximately one gram of the appropriate size fraction of powdered ceramic and reacted with a buffer solution of the desired pH. The buffer solution compositions are given in Table 2. All the ceramics tested were cold pressed and sintered at 1350 C in air, except brannerite, which was sintered at 1350 C in a CO/CO{sub 2} gas mixture. They were then crushed, sieved, rinsed repeatedly in alcohol and distilled water, and the desired particle size fraction collected for the single pass flow-through tests (SPFT). The surface area of the ceramics measured by BET ranged from 0.1-0.35 m{sup 2}/g. The measured surface area values, average particle size, and sample weights for each ceramic test are given in the Appendices.
Date: December 3, 2000
Creator: Zhao, P.; Roberts, S. & Bourcier, W.L.
Partner: UNT Libraries Government Documents Department

Characterization of phase assemblage and distribution in titanate ceramics with SEM/EDS and x-ray mapping.

Description: Titanate ceramics have been selected for the immobilization of excess plutonium. The baseline ceramic formulation leads to a multi-phase assemblage, which consists of a majority pyrochlore phase plus secondary phases. The phase distribution depends on processing conditions and impurity loading. In this paper, we report on the characterization of the phase assemblage and distribution in titanate ceramics using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and x-ray dot mapping. Two titanate ceramics were studied a baseline ceramic and a ceramic with impurities. In the baseline ceramic, the secondary phases that were observed include zirconolite, brannerite, and rutile. Additional phases, such as perovskite, an Al-Ti-Ca phase, and a silicate phase, formed in the impurity ceramic. The distribution of these phases was characterized with backscattered electron (BSE) imaging, except for zirconolite. While the zirconolite exhibited weak contrasts in BSE images and could not be easily distinguished from the pyrochlore matrix, its distribution was effectively characterized with x-ray mapping. Quantitative analyses of BSE images and x-ray maps reveal that the impurity ceramic contains less brannerite, rutile, and pores than the baseline ceramic.
Date: June 16, 1999
Creator: Luo, J. S.
Partner: UNT Libraries Government Documents Department

Improving iron-enriched basalt with additions of ZrO{sub 2} and TiO{sub 2}

Description: The iron-enriched basalt (IEB) waste form, developed at the Idaho National Engineering Laboratory a decade ago, was modified to IEB4 by adding sufficient ZrO{sub 2} and TiO{sub 2} to develop crystals of zirconolite upon cooling, in addition to the crystals that normally form in a cooling basalt. Zirconolite (CaZrTi{sub 2}O{sub 7}) is an extremely leach-resistant mineral with a strong affinity for actinides. Zirconolite crystals containing uranium and thorium have been found that have endured more than 2 billion years of natural processes. On this basis, zirconolite was considered to be an ideal host crystal for the actinides contained in transuranic (TRU)-contaminated wastes. Crystals of zirconolite were developed in laboratory melts of IEB4 that contained 5% each of ZrO{sub 2} and TiO{sub 2} and that were slow-cooled in the 1200--1000{degrees}C range. When actinide surrogates were added to IEB4, these oxides were incorporated into the crystals of zirconolite rather than precipitating in the residual glass phase. Zirconolite crystals developed in IEB4 should stabilize and immobilize the dilute TRUs in heterogeneous, buried low-level wastes as effectively as this same phase does in the various formulations of Synroc used for the more concentrated TRUs encountered in high-level wastes. Synroc requires hot-pressing equipment, while IEB4 precipitates zirconolite from a cooling basaltic melt.
Date: June 1, 1993
Creator: Reimann, G. A. & Kong, P. C.
Partner: UNT Libraries Government Documents Department

Viscosity-based high temperature waste form compositions

Description: High-temperature waste forms such as iron-enriched basalt are proposed to immobilize and stabilize a variety of low-level wastes stored at the Idaho National Engineering Laboratory. The combination of waste and soil anticipated for the waste form results in high SiO{sub 2} + Al{sub 2}O{sub 3} producing a viscous melt in an arc furnace. Adding a flux such as CaO to adjust the basicity ratio (the molar ratio of basic to acid oxides) enables tapping the furnace without resorting to extreme temperatures, but adds to the waste volume. Improved characterization of wastes will permit adjusting the basicity ratio to between 0.7 and 1.0 by blending of wastes and/or changing the waste-soil ratio. This minimizes waste form volume. Also, lower pouring temperatures will decrease electrode and refractory attrition, reduce vaporization from the melt, and, with suitable flux, facilitate crystallization. Results of laboratory tests were favorable and pilot-scale melts are planned; however, samples have not yet been subjected to leach testing.
Date: December 31, 1994
Creator: Reimann, G. A.
Partner: UNT Libraries Government Documents Department