Durability of Zirconolite in Hydrothermal Fluids: Implications for Nuclear Waste Disposal

2000 ◽  
Vol 663 ◽  
Author(s):  
R. Gieré ◽  
J. Malmström ◽  
E. Reusser ◽  
G.R. Lumpkin ◽  
M. Düggelin ◽  
...  
Keyword(s):  
Rare Earth ◽  
Elevated Temperature ◽  
Nuclear Waste ◽  
Laboratory Tests ◽  
Hydrothermal Fluids ◽  
Fluid Composition ◽  
Secondary Phases ◽  
Natural Systems ◽  
Waste Forms ◽  
Temperature And Pressure

ABSTRACTSynthetic zirconolite (CaZrTi2O7) doped with rare earth elements (REEs) and Hf has been subjected to corrosion tests in a closed system at elevated temperature and pressure in fluids with various compositions. Together with previous studies, the results indicate only a very weak corrosion below 250°C at a pressure of 50 MPa. Above that temperature and up to 500°C zirconolite suffers from relatively rapid corrosion and, depending on the fluid composition, it may be covered by various secondary phases. Above 500°C in Na-rich fluids, zirconolite is replaced by perovskiteand calzirtite (nominally CaTiO3 and Ca2r5Ti2O16, respectively), but theREEs and Hf (acting as actinide analogues and/or neutron absorbers) are almost quantitatively incorporated into the secondary phases. The breakdown of zirconolite and its replacement by other phases in the laboratory tests are comparable to reactions observed in natural systems. Additional experiments with U-doped zirconolite revealed differences in the behavior of the used actinide analogues (Nd, Ce, Gd)and U during corrosion. The results of this study, together with observations on natural samples, strongly support the use of zirconolite-based ceramic waste forms for actinide-rich wastes.

scholarly journals Thermodynamic Constraints on REE Mineral Paragenesis in the Bayan Obo REE-Nb-Fe Deposit, China

Minerals ◽  
2020 ◽  
Vol 10 (6) ◽  
pp. 495
Author(s):  
Shang Liu ◽  
Lin Ding ◽  
Hong-Rui Fan
Keyword(s):  
Rare Earth ◽  
Aqueous System ◽  
Hydrothermal Fluids ◽  
Fluid Composition ◽  
Physico Chemical ◽  
Composition Variation ◽  
Bayan Obo ◽  
Chemical Conditions ◽  
The Cost ◽  
Hydrothermal Activities

Hydrothermal processes have played a significant role in rare earth element (REE) precipitation in the Bayan Obo REE-Nb-Fe deposit. The poor preservation of primary fluid inclusions and superposition or modification by multiphase hydrothermal activities have made identification of physico-chemical conditions of ore-forming fluids extremely difficult. Fortunately, with more and more reliable thermodynamic properties of aqueous REE species and REE minerals reported in recent years, a series of thermodynamic calculations are conducted in this study to provide constraints on REE precipitation in hydrothermal solutions, and provide an explanation of typical paragenesis of REE and gangue minerals at Bayan Obo. During the competition between fluocerite and monazite for LREE in the modelled solution (0.1 M HCl, 0.1 M HF and 0.1 M trichloride of light rare earth elements (LREE) from La to Sm), all LREE would eventually be hosted by monazite at a temperature over 300 °C, with continuous introduction of H3PO4. Additionally, monazite of heavier LREE would precipitate earlier, indicating that the Ce- and La-enriched monazite at Bayan Obo was crystallized from Ce and La pre-enriched hydrothermal fluids. The fractionation among LREE occurred before the ore-forming fluids infiltrating ore-hosting dolomite. When CO2 (aq) was introduced to the aqueous system (model 1), bastnaesite would eventually and completely replace monazite-(Ce). Cooling of hot hydrothermal fluids (>400 °C) would significantly promote this replacement, with only about one third the cost of CO2 for the entire replacement when temperature dropped from 430 °C to 400 °C. Sole dolomite addition (model 2) would make bastnaesite replace monazite and then be replaced by parisite. The monazite-(Ce) replaced by associated bastnaesite and apatite is an indicator of very hot hydrothermal fluids (>400 °C) and specific dolomite/fluid ratios (e.g., initial dolomite at 1 kbar: 0.049–0.068 M and 0.083–0.105 M at 400 °C and 430 °C). In hot solution (>430 °C) that continuously interacts with dolomite, apatite precipitates predating the bastnaesite, but it behaves oppositely at <400 °C. The former paragenesis is in accord with petrography observed in this study. Some mineral pairs, such as monazite-(Ce)-fluorite and monazite-(Ce)-parisite would never co-precipitate at any calculated temperature or pressure. Therefore, their association implies multiphase hydrothermal activities. Pressure variation would have rather limited influence on the paragenesis of REE minerals. However, temperature and fluid composition variation (e.g., CO2 (aq), dolomite, H3PO4) would cause significantly different associations between REE and gangue minerals.

Leaching of Actinides from Simulated Nuclear Waste Glass

1981 ◽  
Vol 11 ◽  
Author(s):  
S. Pickering ◽  
C.T. Walker ◽  
P. Offermann
Keyword(s):  
Elevated Temperature ◽  
Nuclear Waste ◽  
Salt Solution ◽  
Waste Glass ◽  
Nuclear Waste Glass ◽  
Temperature And Pressure

The aim of the work was to measure the resistance of simulated nuclear waste glasses to leaching by water and salt solution under the conditions of elevated temperature and pressure. Of particular interest was the behaviour of Am and Np.

scholarly journals The role of Th-U minerals in assessing the performance of nuclear waste forms

2014 ◽  
Vol 78 (5) ◽  
pp. 1071-1095 ◽  
Author(s):  
G. R. Lumpkin ◽  
Yan Gao ◽  
R. Gieré ◽  
C. T. Williams ◽  
A. N. Mariano ◽  
...  
Keyword(s):  
Radiation Damage ◽  
Nuclear Waste ◽  
Low Temperatures ◽  
Elevated Temperatures ◽  
Glass Ceramics ◽  
Hydrothermal Systems ◽  
Fluid Composition ◽  
Geological Time ◽  
Glass Waste ◽  
Waste Forms

AbstractMaterials designed for nuclear waste disposal include a range of ceramics, glass ceramics and glass waste forms. Those with crystalline phases have provided the momentum for studies of minerals as a means to understand aspects of waste-form crystal chemistry, behaviour in aqueous systems and radiation damage over geological periods of time. Although the utility of natural analogue studies varies, depending upon the degree of analogy to the proposed geological repository and other factors such as chemical composition, the available data suggest that Th-U host phases such as brannerite, monazite, pyrochlore, zircon and zirconolite are resistant generally to dissolution in aqueous fluids at low temperatures. Geochemical durability may or may not extend to hydrothermal systems depending on the specifics of fluid composition, temperature and pressure. At elevated temperatures, for example, davidite may break down to new phase assemblages including titanite, ilmenite and rutile. Perovskite is generally less resistant to dissolution at low temperatures and breaks down to TiO2, releasing A-site cations to the aqueous fluid. Studies of radiation damage indicate that the oxide and silicate phases become amorphous as a result of the gradual accumulation of alpha-recoil collision cascades. Monazite tends to remain crystalline on geological time scales, a very attractive property that potentially eliminates major changes in physical properties such as density and volume, thereby reducing the potential for cracking, which is a major concern for zircon. In spite of recent success in describing the behaviour of Th-U minerals in geological systems, considerable work remains in order to understand the P-T-X conditions during alteration and T-t history of the host rocks.

Interpreting Uranyl Mineral Diffraction Patterns

1998 ◽  
Vol 4 (S2) ◽  
pp. 560-561
Author(s):  
Edgar C. Buck
Keyword(s):  
Corrosion Behavior ◽  
Nuclear Waste ◽  
Equatorial Plane ◽  
Waste Form ◽  
Spent Fuel ◽  
Secondary Phases ◽  
Form Complex ◽  
Waste Forms ◽  
Diffraction Patterns ◽  
Nuclear Waste Forms

Secondary phases that form during the corrosion of nuclear waste forms may influence both the rate of waste form dissolution and the release of radionuclides [1]. The identification of these phases is critical in developing models for the corrosion behavior of nuclear waste forms. In particular, the secondary uranyl (VI) minerals that form during waste form alteration may control uranium solubility and release of radionuclides incorporated into these phases [2].The U6+ cation in uranyl minerals is almost always present as a linear (UO2)2+ ion [3]. This uranyl (Ur) ion is coordinated by four, five, or six anions (ϕ) in the equatorial plane resulting in the formation of square (Urϕ4), pentagonal (Urϕ5), and hexagonal (Urϕ6) bipyramids, respectively [3]. These bipyramid polyhedra may polymerize to form complex infinite sheet structures. The linking of Urϕ5 is observed in a number of uranyl minerals formed during waste glass and spent fuel corrosion [2,4], such as weeksite [Na,K(UO2)2(Si205)3*4H2O] and β-uranophane [Ca[(UO2)(SiO3OH)]2*5H2O].

scholarly journals Rare Earth Elements in Mineral Deposits: Speciation in Hydrothermal Fluids and Partitioning in Calcite

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-19 ◽  
Author(s):  
Emily P. Perry ◽  
Alexander P. Gysi
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Hydrothermal Fluids ◽  
Mineral Deposits ◽  
Chloride Complexes ◽  
Natural Systems ◽  
Low Salinity ◽  
Numerical Predictions ◽  
Hydrothermal Processes ◽  
Bayan Obo

Studying the speciation and mineral-fluid partitioning of the rare earth elements (REE) allows us to delineate the key processes responsible for the formation of economic REE mineral deposits in natural systems. Hydrothermal REE-bearing calcite is typically hosted in carbonatites and alkaline rocks, such as the giant Bayan Obo REE deposit in China and potential REE deposits such as Bear Lodge, WY. The compositions of these hydrothermal veins yield valuable information regarding pressure (P), temperature (T), salinity, and other physicochemical conditions under which the REE can be fractionated and concentrated in crustal fluids. This study presents numerical simulation results of the speciation of REE in aqueous NaCl-H2O-CO2-bearing hydrothermal fluids and a new partitioning model between calcite and fluids at different P-T-x conditions. Results show that, in a high CO2 and low salinity system, bicarbonate/carbonate are the main transporting ligands for the REE, but predominance shifts to chloride complexes in systems with high CO2 and high salinity. Hydroxyl REE complexes may be important for the solubility and transport of the REE in alkaline fluids. These numerical predictions allow us to make quantitative interpretations of hydrothermal processes in REE mineral deposits, particularly in carbonatites, and show where future experimental work will be essential in improving our modeling capabilities for these ore-forming processes.

scholarly journals Rare Earth Element Phases in Bauxite Residue

2018 ◽  
Author(s):  
Johannes Vind ◽  
Annelies Malfliet ◽  
Bart Blanpain ◽  
Petros E. Tsakiridis ◽  
Alan H. Tkaczyk ◽  
...  
Keyword(s):  
Solid Solution ◽  
Rare Earth ◽  
Rare Earth Element ◽  
Earth Element ◽  
Bauxite Residue ◽  
Bayer Process ◽  
Minor Amount ◽  
Secondary Phases ◽  
Wavelength Dispersive Spectroscopy ◽  
Natural Systems

The purpose of present work was to provide mineralogical insight in the rare earth element (REE) phases in bauxite residue to improve REE recovering technologies. Experimental work was performed by electron probe microanalysis with energy dispersive as well as wavelength dispersive spectroscopy and transmission electron microscopy. REEs are found as discrete mineral particles in bauxite residue. Their sizes range from &lt; 1 &mu;m to about 40 &mu;m. In bauxite residue, the most abundant REE bearing phases are light REE (LREE) ferrotitanates, that form a solid solution between the phases with major compositions (REE,Ca,Na)(Ti,Fe)O3 and (Ca,Na)(Ti,Fe)O3. These are secondary phases formed during the Bayer process by an in-situ transformation of the precursor bauxite LREE phases. Comparing to natural systems, the indicated solid solution resembles loparite-perovskite series. LREE particles often have a calcium ferrotitanate shell surrounding them, that probably hinders their solubility. Minor amount of LREE carbonate and phosphate minerals as well as manganese-associated LREE phases are also present in bauxite residue. Heavy REEs occur in the same form as in bauxites, namely as yttrium phosphates. These results show that Bayer process has an impact on the initial REE mineralogy contained in bauxite. Bauxite residue as well as selected bauxites are potentially good sources of REEs.

scholarly journals A large fixed bed reactor for MRI operando experiments at elevated temperature and pressure

2021 ◽  
Vol 92 (4) ◽  
pp. 043711
Author(s):  
Harm Ridder ◽  
Christoph Sinn ◽  
Georg R. Pesch ◽  
Jan Ilsemann ◽  
Wolfgang Dreher ◽  
...  
Keyword(s):  
Elevated Temperature ◽  
Fixed Bed ◽  
Fixed Bed Reactor ◽  
Temperature And Pressure
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