Ceramic Nuclear Waste Forms Research Articles

Enhanced crevice corrosion of stainless steel 316 by degradation of Cr-containing hollandite crevice former

This study explores the corrosion interactions between stainless steel 316 (SS316), a candidate nuclear waste canister material, and Cr-containing hollandite (Ba1.15Cr2.3Ti5.7O16), a model ceramic nuclear waste form. The two materials were corroded in close proximity in 0.6 M NaCl solution at 90 °C for 28 days. Severe crevice corrosion was observed on the stainless steel surface after the corrosion experiment, which is likely associated with the rapid accumulation of Cr3+ cations originating from both SS316 and the Cr-containing hollandite. The enhanced corrosion of SS316 by the corrosion products of hollandite is quantitatively explained using a classical crevice corrosion model.

Order–disorder structural tailoring and its effects on the chemical stability of (Gd, Nd)2(Zr, Ce)2O7 pyrochlore ceramic for nuclear waste forms

Series of unequal quantity Nd/Ce co-doped ceramic nuclear waste forms, (Gd, Nd)2(Zr, Ce)2O7, were prepared to tailor its ordered pyrochlore or disordered fluorite structure. The phase transition, microtopography, and elemental composition of the ceramic samples were systematically investigated, especially the effect of order-disorder structure on the chemical stability. It was confirmed that unequal quantity of Nd/Ce could synchronously replace the Gd/Zr-sites of Gd2Zr2O7. And the phase transition of order-disorder structure could be successfully tailored by regulating the average cationic radius ratio of (Gd, Nd)2(Zr, Ce)2O7 series. The elements of Gd, Nd, Zr, and Ce are uniformly distributed in the ordered or disordered structures. The MCC-1 leaching results showed that (Gd, Nd)2(Zr, Ce)2O7 pyrochlore ceramic nuclear waste forms had excellent chemical stability, whose elements' normalized leaching rates were as low as 10−4 -10−7 g‧m−2‧d−1 after 7 days. In particular, the chemical stability of disordered structure was superior to that of ordered structure. It was proposed that the force constant and the closest packing were changed with the structure transformation resulting the chemical stability difference.

Computational study of the substitution of early actinides and Ce into zirconolite

Substitutions of tetravalent early actinides (Th – Am) and Ce into the ceramic nuclear waste form zirconolite, CaZrTi2O7, have been analysed using hybrid Density Functional Theory in conjunction with the Periodic Electrostatic Embedded Cluster Method. Substitutions on to both Zr4+ and Ca2+ sites were modelled, with Fe3+ and Al3+ used for charge balancing. The mean displacements of a central region of ions in the cluster indicated redox behaviour which was then confirmed through analysis of spin density data. Strong correlations were found between the substitution energies of these ions and the ratio of the ionic radii of substituents and substituted ions. Similar redox behaviour and substitution energies were seen for substitutions of Ce4+ and Pu4+ into the lattice. Spin density data of molecular M(OH)n (M = Ce, Th - Am, n = 3–5) showed a preference for the +3 oxidation state for later actinides - as seen in the zirconolite clusters.

Partitioning of Ce, as a simulant for Pu, in a multiphase ceramic nuclear waste form

AbstractA pyrochlore‐rich multiphase ceramic waste form was prepared by the alkoxide‐route and consolidated by sintering in air. Cerium (Ce) was used as a surrogate for plutonium (Pu), which was found in both the major pyrochlore and minor perovskite phases, but not the minor rutile phase. A synchrotron‐based fluorescence X‐ray absorption near‐edge spectroscopy (XANES) imaging technique was employed to determine the Ce oxidation state in each of the two Ce‐containing phases. It was shown that the major pyrochlore phase contains only tetravalent Ce, whereas the minor perovskite phase incorporates exclusively trivalent Ce. The effect of sintering atmosphere was also investigated, which revealed a very different phase assemblage when sintered in argon (Ar). A major perovskite phase was produced under Ar with a minor zirconolite‐4M phase, both containing trivalent Ce. The composition of the minor rutile phase remained the same whether it was sintered in air or Ar. The results demonstrate the importance of controlling the oxidation state of multivalent waste ions in the design of ceramic waste forms.

CaTh(PO4)2 cheralite as a candidate ceramic nuclear waste form: Spark plasma sintering and physicochemical characterisation

In order to fulfil the requirements as an actinide waste form, cheralite (CaTh(PO4)2) needs to be stabilised into a dense material. Thermal stability studies indicated that heat treatment causes gradual decomposition of CaTh(PO4)2 into the corresponding oxides above 1550K. Thus, conventional sintering of this material cannot be applied. However, we demonstrate that rapid densification without decomposition can be achieved by using spark plasma sintering. High density pellets of pure cheralite were produced and the thermal diffusivity and mechanical properties (hardness and elastic modulus) were measured. Within this study, 31P MAS NMR was used to complete the structural characterisation of the synthetic cheralite.

Glass–ceramic nuclear waste forms obtained by crystallization of SiO 2–Al 2O 3–CaO–ZrO 2–TiO 2 glasses containing lanthanides (Ce, Nd, Eu, Gd, Yb) and actinides (Th): Study of the crystallization from the surface

Glass–ceramic materials containing zirconolite (nominally CaZrTi 2O 7) crystals in their bulk can be envisaged as potential waste forms for minor actinides (Np, Am, Cm) and Pu immobilization. In this study such matrices are synthesized by crystallization of SiO 2–Al 2O 3–CaO–ZrO 2–TiO 2 glasses containing lanthanides (Ce, Nd, Eu, Gd, Yb) and actinides (Th) as surrogates. A thin partially crystallized layer containing titanite and anorthite (nominally CaTiSiO 5 and CaAl 2Si 2O 8, respectively) growing from glass surface is also observed. The effect of the nature and concentration of surrogates on the structure, the microstructure and the composition of the crystals formed in the surface layer is presented in this paper. Titanite is the only crystalline phase able to significantly incorporate trivalent lanthanides whereas ThO 2 precipitates in the layer. The crystal growth thermal treatment duration (2–300 h) at high temperature (1050–1200 °C) is shown to strongly affect glass–ceramics microstructure. For the system studied in this paper, it appears that zirconolite is not thermodynamically stable in comparison with titanite growing form glass surface. Nevertheless, for kinetic reasons, such transformation (i.e. zirconolite disappearance to the benefit of titanite) is not expected to occur during interim storage and disposal of the glass–ceramic waste forms because their temperature will never exceed a few hundred degrees.

Molecular dynamics simulation of energetic uranium recoil damage in zircon

Defect production and amorphisation due to energetic uranium recoils in zircon (ZrSiO4), which is a promising ceramic nuclear waste form, is studied using molecular dynamics (MD) simulations with a partial charge model. An algorithm that distinguishes between undamaged crystal, crystalline defects and amorphous regions is used to develop a fundamental understanding of the primary damage state. The amorphous cascade core is separated from the surrounding crystal by a defect-rich region. Small, chemically inhomogeneous amorphous clusters are also produced around the core. The amorphous regions consist of under-coordinated Zr and polymerised Si leading to amorphisation and phase separation on a nanometer scale into Zr- and Si-rich regions. This separation could play an important role in the experimentally observed formation of nanoscale ZrO2 in ZrSiO4 irradiated at elevated temperatures.

Glass–ceramic nuclear waste forms obtained from SiO 2–Al 2O 3–CaO–ZrO 2–TiO 2 glasses containing lanthanides (Ce, Nd, Eu, Gd, Yb) and actinides (Th): study of internal crystallization

Glass–ceramic waste forms such as zirconolite (nominally CaZrTi 2O 7) based ones can be envisaged as good candidates for minor actinides or Pu immobilization. Such materials, in which the actinides (or lanthanides used as actinide surrogates) would be preferentially incorporated into zirconolite crystals homogeneously dispersed in a durable glassy matrix, can be prepared by controlled crystallization (nucleation + crystal growth) of parent glasses belonging to the SiO 2–Al 2O 3–CaO–ZrO 2–TiO 2 system. In this work we present the effects of the nature of the minor actinide surrogate (Ce, Nd, Eu, Gd, Yb, Th) on the structure, the microstructure and the composition of the zirconolite crystals formed in the bulk of the glass–ceramics. The amount of lanthanides and thorium incorporated into zirconolite crystals is discussed in relation with the capacity of the glass to accommodate these elements and of the crystals to incorporate them in the calcium and zirconium sites of their structure.

Geochemical behaviour of host phases for actinides and fission products in crystalline ceramic nuclear waste forms

Abstract A number of polyphase or single-phase ceramic waste forms have been considered as options for the disposal of nuclear waste in geological repositories. Of critical concern in the scientific evaluation of these materials is their performance in natural systems over long periods of time (e.g., 10 3 to 10 6 years). This paper gives an overview of the aqueous durability of the major titanate host phases for actinides (e.g., Th, U, Np, Pu, Cm) and important fission products (e.g., Sr and Cs) in alternative crystalline ceramic waste forms. These host phases are compared with reference to some basic acceptance criteria, including the long-term behaviour determined from studies of natural samples. The available data indicate that zirconolite and pyrochlore are excellent candidate host phases for actinides. These structures exhibit excellent aqueous durability, crystal chemical flexibility, high waste loadings, and well-known processing conditions. Although both pyrochlore and zirconolite become amorphous due to alpha-decay processes, the total volume swelling is only 5–6% and there is no significant effect of radiation damage on aqueous durability. Hollandite also appears to be an excellent candidate host phase for radioactive Cs isotopes. Brannerite and perovskite, on the other hand, are more prone to alteration in aqueous fluids and have a lower degree of chemical flexibility. With the exception of hollandite, many of the properties of these potential host phases have been confirmed through studies of natural samples.

Characterization of a Cerium-Rich Pyrochlore-Based Ceramic Nuclear Waste Form

ABSTRACTTitanate ceramics have been proposed as candidate materials for immobilizing excess weapons plutonium. This study focuses on the characterization of a titanate-based ceramic through X-ray diffraction (XRD), electron probe microanalysis, and electron energy-loss spectroscopy (EELS). Three distinct phases have been identified, and their volume fraction was determined from element distribution maps using Scionimage-NIH Analysis software. This analysis revealed that the pyrochlore-group phase betafite (A2Ti2O7) forms the matrix of the ceramic and occupies 90.4% of the volume. Uniformly distributed in this matrix are perovskite (A2Ti2O6) and Hf-enriched rutile (TiO2), which account for 6.4 vol% and 3.1 vol%, respectively. The studied ceramic exhibits a very low porosity (0.3 vol%), which is characterized by small (<6 μm), rounded and isolated voids. In the studied ceramic, A-site cations are represented by Ca, rare earth elements, and Hf. The powder XRD pattern of the ceramic allowed refining the unit cell parameters for the cubic betafite, which is characterized by a cell edge of 10.132±0.003Å. The EELS data indicate that Ce is present as both Ce3+ and Ce4+ in betafite, whereas in perovskite, all Ce is trivalent.

Element Partitioning in a Pyrochlore-Based Ceramic Nuclear Waste form

ABSTRACTPyrochlore-rich ceramics containing additional zirconolite, brannerite and rutile have been developed for immobilizing excess weapons plutonium. This study reports data for a ceramic containing small amounts of Al and Mo in addition to the major oxide components of Ti, U, Ca, Hf, Gd and Ce. Hafnium and Gd are added as neutron absorbers, Al and Mo represent impurities. Quantitative electron microprobe data demonstrate that UO2 is strongly partitioned into brannerite (45 wt%), which is present as euhedral crystals with inclusions of unreacted UO2. Pyrochlore forms the groundmass and has an average UO2 content of 28 wt%. Zirconolite contains only 15 wt% UO2, but is significantly more effective in accommodating Hf and Gd than both brannerite and pyrochlore. Zirconolite incorporates U together with Hf and Ce in one structural site, whereas brannerite and pyrochlore accommodate U with Gd in a site that is distinct from that occupied by Hf. Incorporation of Gd into zirconolite takes place via a coupled substitution involving Al, thus explaining the high Al2O3 contents (3 wt%). Molybdenum was not detected in the major oxides, and it might be present in the accessory rutile. Although the studied waste form was designed to incorporate Pu, the present dataset is also valuable because immobilization of highly fissile U (e.g., U-233) might need to be considered in the future.

First Rb Silicotitanate Phase and Its K-Structural Analogue: New Members of the SNL-A Family (Cc-A2TiSi6O15; A = K, Rb, Cs)

Alkali silicotitanate ternary phases have been studied for a wide variety of applications including catalysis, selective ion exchange, and ceramic nuclear waste form materials. We report here the synthesis and structural characterization of Rb2TiSi6O15 and K2TiSi6O15, structural analogues to SNL-A (Cc-Cs2TiSi6O15). The Rb analogue is the first reported ternary oxide, the rubidium silicotitanate phase. Both the Rb and the K analogues were synthesized nearly pure by hydrothermal treatment of oxide precursor mixtures seeded with SNL-A and could not be synthesized without the addition of the seed. Rietveld refinement gave good structural models in the Cc space group with unit cell parameters: Rb2TiSi6O15, V = 1357.9 Å3, a = 12.736(2) Å, b = 7.3392(3) Å, c = 15.061(3) Å, β = 105.29(2)° and K2TiSi6O15, V = 1318.5 Å3, a = 12.570(2) Å, b = 7.2534(3) Å, c = 15.082(3) Å, β = 106.49(3)°. The two isostructural phases were also characterized by thermogravimetry and 29Si NMR. Unit cell parameters, melting temperatures, and 29Si chemical shifts were compared to those of the Cs analogue and related to variation in alkali radius (Cs > Rb > K). The volume, the a-axis, and the b-axis increase linearly with alkali radius, which is directly related to a “relaxation” of the folds of the corrugated silicate layers that lie in the xy plane.

Cerium, uranium, and plutonium behavior in glass-bonded sodalite, a ceramic nuclear waste form

Glass-bonded sodalite is being developed as a ceramic waste form (CWF) to immobilize radioactive fission products, actinides, and salt residues from electrometallurgical treatment of spent nuclear reactor fuel. The CWF consists of about 75 mass% sodalite, 25 mass% glass, and small amounts of other phases. This paper presents some results and interpretation of physical measurements to characterize the CWF structure, and dissolution tests to measure the release of matrix components and radionuclides from the waste form. Tests have been carried out with specimens of the CWF that contain rare earths at concentrations similar to those expected in the waste form. Parallel tests have been carried out on specimens that have uranium or plutonium as well as the rare earths at concentrations similar to those expected in the waste forms; in these specimens UCl 3 forms UO 2 and PuCl 3 forms PuO 2. The normalized releases of rare earths in dissolution tests were found to be much lower than those of matrix elements (B, Si, Al, Na). When there is no uranium in the CWF, the release of cerium is two to ten times lower than the release of the other rare earths. The low release of cerium may be due to its tetravalent state in uranium-free CWF. However, when there is uranium in the CWF, the release of cerium is similar to that of the other rare earths. This trivalent behavior of cerium is attributed to charge transfer or covalent interactions among cerium, uranium, and oxygen in (U,Ce)O 2.

Geochemistry of Hydrothermal Veins Containing Zirconolite and Betafite at Adamello, Italy

ABSTRACTHydrothermal veins containing zirconolite and betafite occur in dolomite marbles at the contact with a tonalite intrusion of the Adamello batholith, Italy. The veins display four distinct mineral zones and are highly enriched in Th, U, Ti, Zr, Nb, and rare earth elements (REE) relative to their host rock. Wide ranges in concentration of these elements and distinct inter- element fractionation trends exist across the four vein zones. The behavior of Th closely reflects that of P, Ti, Nb, and heavy REE, but was distinct from that of U, Zr and light REE (La to Sm). The presence and composition of refractory minerals such as zirconolite, betafite, uraninite, thorianite, baddeleyite, rutile, allanite, and aeschynite provide evidence for the transport of Ti, Zr, Nb, REE, and actinides by a fluid, which was further characterized by a low pH and high contents of F, P, Cl and H2S. Thermodynamic analysis of these veins indicates that they were formed at 550-600°C, 200 MPa total pressure, and from a fluid derived from the tonalite. Mineralogical and textural observations suggest that crystallization of the refractory minerals was in part induced by precipitation of fluorapatite and sulfides. The geochemical data further indicate that precipitation of light REE was induced by the fluid/wall-rock interaction which led to a significant dilution of the fluid by CO2. The studied veins provide an example of high- temperature transport of actinides and REE in the Earth's crust and show that these elements can be precipitated from a fluid and subsequently immobilized by zirconolite and betafite, two important actinide hosts in ceramic nuclear waste forms.

Response of Zircon to Electron and Ne+ Irradiation

ABSTRACTZircon (ZrSiO4) is an actinide host phase in vitreous ceramic nuclear waste forms and a potential host phase for the disposition of excess weapons plutonium. In the present work, the effects of 800 and 900 keV electron, and 1 MeV Ne+ irradiations on the structure of single crystals of ZrSiO4 have been investigated. The microstructural evolution during the irradiations was studied in situ using a high-voltage electron microscope interfaced to an ion accelerator at Argonne National Laboratory. The results indicate that electron irradiation at 15 K cannot amorphize ZrSiO4 even at fluences an order of magnitude higher than that required for amorphization by 1.5 MeV Kr + ions. However, the material is readily amorphized by I MeV Ne+ irradiation at 15 K. The temperature dependence of this amorphization is discussed in light of previous studies of radiation Zdamage in ZrSiO4.

HRTEM observations on electron irradiation damage in F-apatite

Radiation damage in crystalline phases with the apatite structure is of interest in the biomedical, geoscience, and materials science communities. Phases close to hydroxyl apatite [Ca5(PO4)3OH] in composition are volumetrically the most significant in dental enamel and bone. Natural apatites with variable halogen compositions [Ca5(PO4)3(F,OH,Cl)] are used by geoscientists to place constraints on the thermochronologic conditions under which portions of the Earth evolved. Rare-earth-silicate isomorphs of these naturally occurring apatites are also of interest because they may be actinide hosts in ceramic nuclear waste forms.

High-level nuclear waste immobilization with ceramics

Although borosilicate glass remains the waste form of choice for many countries with nuclear waste, there has been a substantial effort to develop alternative ceramic nuclear waste forms. The alternative waste forms may be single or polyphase ceramics with several barriers against the release of radionuclides. Principal ceramic nuclear waste forms include: Synroc; tailored ceramics; TiO 2-matrix ceramics; glass ceramics; monazite; and FUETAP concrete. In addition, there are a number of ‘novel’ ceramic waste forms which have been developed to only the most preliminary stages, and there are several multi-barrier strategies which encapsulate one ceramic waste form in another. Finally, in recent years, spent fuel has become an important waste form. This paper will briefly describe the types of ceramic waste forms that have been developed and review the advantages and disadvantages of ceramic nuclear waste forms.

Metamict minerals: Natural analogues for radiation damage effects in ceramic nuclear waste forms

Metamict minerals: Natural analogues for radiation damage effects in ceramic nuclear waste forms

Alpha-Decay—Induced Fracturing in Zircon: The Transition from the Crystalline to the Metamict State

A natural single crystal of zircon, ZrSiO(4,) from Sri Lanka exhibited zonation due to alpha-decay damage. The zones vary in thickness on a scale from one to hundreds of micrometers. The uranium and thorium concentrations vary from zone to zone such that the alpha-decay dose is between 0.2 x 10(16) and 0.8 x 10(16) alpha-events per milligram (0.15 to 0.60 displacement per atom). The transition from the crystalline to the aperiodic metamict state occurs over this dose range. Differential expansion of individual layers due to variations in their alpha-decay dose caused a systematic pattern of fractures that do not propagate across aperiodic layers. High-resolution transmission electron microscopy revealed a systematic change in the microstructure from a periodic atomic array to an aperiodic array with increasing alpha-decay dose. At doses greater than 0.8 x 10(16) alpha-events per milligram there is no evidence for long-range order. This type of damage will accumulate in actinide-bearing, ceramic nuclear waste forms. The systematic pattern of fractures would occur in crystalline phases that are zoned with respect to actinide radionuclides.

Magnesium Titanate Spinel: A Ceramic Phase for Immobilization of Technetium‐99 from Radioactive Wastes

Technetium‐99 can be dissolved in Mg2TiO4 spinels to form solid solutions with low dissolution rates. These materials are possible host phases for 99Tc in ceramic nuclear waste forms.