U-rich Phases Research Articles

Phase-Field Simulation of Spinodal Decomposition in U-50Zr Metallic Nuclear Fuel.

During the γ phase-δ phase transition, U-50Zr fuel experiences spinodal decomposition, which has a significant effect on fuel properties. However, little is known about the spinodal decomposition of U-50Zr. The spinodal decomposition behavior in U-50Zr is studied in this research using the phase-field approach. The mechanism of spinodal decomposition from a thermodynamic perspective is studied, and the effects of temperature and grain boundary (GB) on spinodal decomposition are analyzed. It is found that the concentration of U atoms in the U-rich phase formed during spinodal decomposition is as high as 90%. The U-rich phase first appears at the GB position, and precipitation phases appear inside the grain later. Ostwald ripening occurs when larger precipitation phases on the GB absorb isolated smaller precipitation phases inside the grain. The coarsening rate of precipitation phases and the time it takes for spinodal decomposition to achieve equilibrium are both influenced by temperature.

Engineered mineralogical interfaces as radionuclide repositories

Effective capture of fugitive actinides and daughter radionuclides constitutes a major remediation challenge at legacy or nuclear accident sites globally. The ability of double-layered, anionic clay minerals known as hydrotalcites (HTC) to contemporaneously sequester a range of contaminants from solution offers a unique remedy. However, HTC do not provide a robust repository for actinide isolation over the long term. In this study, we formed HTC by in-situ precipitation in a barren lixiviant from a uranium mine and thermally transformed the resulting radionuclide-laden, nanoscale HTC. Atomic-scale forensic examination of the amorphized/recrystallised product reveals segregation of U to nanometre-wide mineral interfaces and the local formation of interface-hosted mineral grains. This U-phase is enriched in rare earth elements, a geochemical analogue of actinides such as Np and Pu, and represents a previously unreported radionuclide interfacial segregation. U-rich phases associated with the mineral interfaces record a U concentration factor of ~ 50,000 relative to the original solute demonstrating high extraction and concentration efficiencies. In addition, the co-existing host mineral suite of periclase, spinel-, and olivine-group minerals that equate to a lower mantle, high P–T mineral assemblage have geochemical and geotechnical properties suitable for disposal in a nuclear waste repository. Our results record the efficient sequestering of radionuclides from contaminated water and this novel, broad-spectrum, nanoscale HTC capture and concentration process constitutes a rapid solute decontamination pathway and solids containment option in perpetuity.

Combined U-Pb isotopic signatures of U mill tailings from France and Gabon: A new potential tracer to assess their fingerprint on the environment

Uranium milling activities have produced high volumes of long-lived radioactive processed wastes stored worldwide in near surface environment. The aim of this study is to highlight relevant tracers that can be used for environmental impact assessment studies involving U mill tailings. A multi-tracer study involving elemental content, 238U decay products disequilibria and stable Pb isotopes was performed in different types of U mill tailings (alkaline, acid, neutralized acid) collected from five Tailings Management Facilities in France (Le Bosc, L’Ecarpière, Le Bernardan, and Bellezane) and Gabon (Mounana). Our results showed that U and Pb concentrations range between 30 and 594 ppm and 66–805 ppm, respectively. These tailings have a strong disequilibrium of (234U/238U) and (230Th/238U) activity ratios (1.27–1.87 and 6–65, respectively), as well as higher 206Pb/207Pb (1.86–7.15) and lower 208Pb/207Pb (0.22–2.39) compared to geochemical background ((234U/238U) and (230Th/238U) equal to unity; 206Pb/207Pb = 1.20; 208Pb/207Pb = 2.47). In situ analyzes (SEM, SIMS) showed that Pb-bearing phases with high 206Pb/207Pb are related to remaining U-rich phases, S-rich phases and potentially clay minerals or oxyhydroxides. We suggest that the combination of the 206Pb/207Pb with the (234U/238U) ratio is a relevant tool for the fingerprinting of the impact of U milling activities on the environment.

XRD and SEM/EDS characterization of two quaternary fuel alloys (U-2.5Mo-2.5Ti-5.0Zr and U-1.5Mo-1.5Ti-7.0Zr in wt. %) for fast reactors

The current study focuses on characterization of two potential fuel alloys for sodium-cooled fast reactors: U-2.5Mo-2.5Ti-5.0Zr (U-MT5Z) and U-1.5Mo-1.5Ti-7.0Zr (U-MT7Z) in wt. %. As-cast alloys and annealed (600 °C, 500 h) alloys are studied by X-ray powder diffraction (XRD) for their bulk structures, and scanning electron microscopy / energy dispersive X-Ray spectroscopy (SEM/EDS) for their microstructures and phase constituents. For as-cast alloys, the XRD results show that U-MT5Z mainly contains γ, while U-MT7Z alloy contains α. In both alloys, there are three phases indicated by EDS, a U-rich phase, secondary phase, and Zr-rich phase. The elemental compositions varied in different phases and in different alloys. After annealing, both alloys exhibit the structures of α-U and U2Ti according to XRD. The EDS results suggest there are five phases found in both alloys.

Visual and in situ Raman spectroscopic observations of the liquid–liquid immiscibility in aqueous uranyl sulfate solutions at temperatures up to 420 °C

The phase behaviors of aqueous UO2SO4 solutions were investigated in situ with a microscope and a Raman spectrometer at temperatures from 25 to 420°C. Results show that aqueous UO2SO4 solution separated into UO2SO4-rich (Urich) and UO2SO4-poor (Upoor) liquid phases coexisted with a vapor phase at ≥285.8±0.5°C. Both visual and Raman spectroscopic investigations suggest that a reversible strong UO22+–SO42− association was responsible for the liquid–liquid immiscibility in aqueous UO2SO4 solutions. Main evidences were summarized as: (1) the liquid–liquid phase separation temperature decreases with increasing UO2SO4 concentration up to 0.54mol/kg, and then increased at greater concentrations, characterizing a lower critical solution temperature (LCST) at 285.8°C±0.5°C. LCST is commonly accepted as a diagnostic feature of polymer solutions; (2) analyses of the shapes of the Raman spectra of v1(UO22+) and v1(SO42−) bands show that the UO22+–SO42− association becomes stronger at elevated temperatures, especially in the immiscible Urich phase; and (3) with increasing temperature, the Urich phase becomes more concentrated, whereas the Upoor phase becomes more dilute, indicating that the hydration of UO22+ and SO42− cannot be maintained in the Urich phase. Destruction of the hydration spheres of UO22+ and SO42− further favors the ion association in the Urich phase. These results are important for describing similar sulfate solutions at elevated temperatures, especially under supercritical conditions.

Transport of 234U in the Opalinus Clay on centimetre to decimetre scales

Abstract The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements. Samples were digested using aqua regia followed by total HF dissolution, yielding two fractions. In all studied samples U was found to be concentrated in the HF digestion fraction. It has a high U/Th ratio and a study by SEM-EDS points to sub-μm up to several μm in size zircon grains as the main U-rich phase. This fraction consistently has 234U/238U activity ratios below unity. The minute zircon grains constitute the major reservoir of U in the rock and act as constant rate suppliers of 234U into the rock matrix and the pore water. The aqua regia leach fraction was found to be enriched in Th, and complementary to the HF fraction, having 234U/238U activity ratios above unity. It is believed that these U activity ratios reflect the surplus of 234U delivered from the zircon grains. Some cm-spaced samples show bulk rock 234U/238U activity ratios that are markedly out of equilibrium. In most of them a striking negative correlation between the total U content and the bulk rock 234U/238U activity ratios is observed. This is interpreted to indicate net 234U transfer from regions of higher supply of 234U towards those of lower supply which is, in most cases, equivalent to transfer from clayey towards carbonate/sandy portions of the rock. In contrast, the 25 cm averaged samples all have uniform bulk rock 234U/238U activity ratios in equilibrium, indicating U immobility in the last 1–1.5 Ma on this spatial scale. It is concluded that the small-scale lithological variations which govern U spatial distribution in the Opalinus Clay are the major factor determining 234U in situ supply rates, regulating its diffusive fluxes and controlling the observed bulk rock 234U/238U activity ratios. A simple box-model is presented to simulate the measured bulk rock 234U/238U activity ratios and to give an additional insight into the studied system.

Uranium and other trace elements’ distribution in Korean granite: implications for the influence of iron oxides on uranium migration

To understand trace radionuclide (uranium) migration occurring in rocks, a granitic batholith located at the Korea Atomic Energy Research Institute (KAERI) site was selected and investigated. The rock samples obtained from this site were examined using mineralogical methods, including scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The changes in the distribution pattern of uranium (U) and small amounts of trace elements, and the mineralogical textures affected by weathering, were examined. Based on the element distribution analyses, it was found that Fe2+ released from fresh biotite is oxidized in short geological time, forming amorphous iron oxides, such as ferrihydrite, around silicate minerals. In that case, the amorphous ferrihydrite does not show distinct adsorption for U. However, as it gradually crystallizes to goethite or hematite, the most U-rich phases were found to be associated with the secondary iron oxides having granular forms. This evidence suggests that the geological subsurface environment is favorable for the crystallized iron oxides to keep their structures more stable for a long time as compared with the amorphous phases. There is a possibility that the long residence of U which is in contact with the stable crystalline phases of iron may finally lead to the partial sequestration of U in their structure. Consequently, it seems that Fe-oxide crystallization can be a dominating mechanism for U uptake and controls long-term U transport in granites with low U contents.

THE APPLICATION OF HRTEM TECHNIQUES AND NANOSIMS TO CHEMICALLY AND ISOTOPICALLY CHARACTERIZE GEOBACTER SULFURREDUCENS SURFACES

Bioprecipitated minerals are typically at the nanometer scale, hydrous, and beam-sensitive (i.e., can recrystallize during analysis), making them diffi cult to characterize using standard spectroscopic or electron-beam techniques. We have combined the ion-imaging capabilities of nanoscale secondary-ion mass spectrometry (NanoSIMS) and advanced high-resolution transmission electron microscopy (HRTEM) in order to characterize the surfaces of Geobacter sulfurreducens and the bioprecipitated uranium phases. Our results reveal the association between nutrient uptake and precipitation of uranium minerals. Biosequestration of uranium is enhanced by addition of nutrients such as acetic acid, and uranium is precipitated on the surface of the bacteria as nanocrystals of uraninite (UO2). The bioprecipitation of this anhydrous U-rich phase is signifi cant; although UO2 is thermodynamically stable over a range of pH values (2 to 12) and oxidizing conditions [Eh 0.2 to –0.2, or log f(O2) of approximately –50 to –125], thermodynamic models of inorganic systems suggest that U 6+ oxyhydroxide minerals should be stable. Our results suggest that the biofi lm shielded the UO2 from re-oxidation and that bacteria can immobilize uranium for extended periods, even under relatively oxidizing conditions in the subsurface.

The effects of secular disequilibrium on (U–Th)/He systematics and dating of Quaternary volcanic zircon and apatite

The (U–Th)/He dating method applied to U-rich phases such as zircon and apatite has sufficient sensitivity and precision to be of potential use for dating relatively recent geologic events such as volcanic eruptions. However, in phases with crystallization ages less than ∼1 Ma, chemical fractionation within the 238U decay series may modify the He ingrowth rate, causing He ages computed from the secular equilibrium age equation to be incorrect. The resulting systematic error depends on the [ 230Th/ 238U] activity ratio of the dated phase when it is erupted, and on the eruption age. Zircons, which exclude Th relative to U, will likely have secular equilibrium He ‘ages’ that underestimate the eruption age by up to a few tens of %, decreasing with increasing eruption age. Apatites tend to accommodate U and Th with little fractionation, so apatite secular equilibrium He ages will be nearly concordant with eruption age. If minerals are erupted immediately after crystallization, the disequilibrium effect can be reasonably accounted for based on Th/U systematics. However, crystals are likely to reside for unknown but potentially long periods in a magma chamber, such that the degree of secular disequilibrium will be reduced prior to the onset of He accumulation. (U–Th)/He analyses of co-genetic phases that fractionate the U/Th ratio differently, like apatite and zircon, can be used to better constrain eruption age, as well as to provide insights into magma chamber residence time. We illustrate this approach with (U–Th)/He analyses of zircons and apatites of the Pleistocene-age Rangitawa Tephra, New Zealand.

Durability of Pu-doped Titanate and Zirconate Ceramics Designed for Pu Immobilisation

ABSTRACTCeramics rich in pyrochlore-structured titanate and fluorite-structured zirconate phases designed for surplus Pu immobilisation, with and without process impurities, have been leach tested at 90°C in deionised water. The zirconates consist mainly of a defect-fluorite with secondary impurity-containing phase-powellite/scheelite (when sintered in Ar but not when sintered in air), a spinel or magnetoplumbite type phase, a glass forming silicate and a secondary U-rich phase (when sintered in air with added impurities). The pyrochlore-rich baseline titanate ceramic consists of pyrochlore, brannerite and Hf-rutile. When impurities are added zirconolite and a silicate glass are also present. The pyrochlore-rich titanate with 5 wt% of impurities sintered at 1300°C is highly durable. A well-densified zirconate version without impurities has comparable elemental releases to those of the titanate ceramic but a zirconate with 5 wt% of impurities sintered at 1400°C in air or Ar shows much higher U and Ca releases than the titanate ceramic. Sintering atmospheres, changing from Ar to air, can influence Pu and U release rates up to an order of magnitude. High Ga releases from zirconates with impurities show that the secondary phase containing Ga is not durable. The higher processing temperature and the apparent inability to incorporate many impurity elements suggest that zirconates are not as flexible as titanates in respect of processing conditions and aqueous durability.

Anomalous xenon in zone 13 Okelobondo

In situ laser extraction techniques were applied for the study of heavy noble gases in a polished section of Zone 13 from the natural nuclear reactor in Okelobondo. Three main mineral phases were identified in this polished section using SEM-EDX. The Xe and Kr isotopic structures were determined by multiple measurements in each of these phases. Twenty-four isotopic analyses of the gases extracted from two different U-rich phases revealed nearly normal fission spectra. All 9 analyses of a U-free phase, consisting mainly of alumophosphates, demonstrated an unusual isotopic composition ( 136Xe/ 134Xe/ 132Xe/ 131Xe/ 130Xe/ 129Xe/ 128Xe = 1/1.25/1.73/0.89/0.0045/0.274/0) with concentrations ranging up to 10 −2 cm 3 STP/g. This is the highest Xe concentration ever measured in a natural material. Kr was also anomalous, although to a lesser extent. These results confirm the presence of Chemical Fractionation of Fission Xe (CFF-Xe) in the Okelobondo alumophosphates. CFF-Xe is a decay product of intermediate fission fragments that have migrated out of the U-rich host phases into adjacent U-free minerals. The CFF-Xe spectra in the alumophosphates are also accompanied by 130Xe excesses, which are attributed to neutron capture on fissiogenic 129I that apparently migrated out of the nearby U-rich minerals. The 130Xe/ 129Xe ratio allows us to estimate the thermal equivalent neutron dose of 1.1 × 10 21 n/cm 2. The presence of an unknown fission component remarkably similar in composition to CFF-Xe can be inferred from the atmospheric and terrestrial data. This leads us to the hypothesis that the CFF process has operated on a global scale on the Earth.

Redistribution behavior of various constituents in U-Pu-Zr alloy and U-Pu-Zr alloy containing minor actinides and rare earths in a temperature gradient

U-Pu-Zr based metallic fuel is one of the likely possible candidate to transmute minor actinides, such as Np, Am and Cm (MA). MA can be recovered from the high-level waste by pyroprocess, where the presence of rare earths (RE) will not be avoided. One of the most important phenomena observed on U-Pu-Zr metallic fuel type is the redistribution of the elements in a temperature gradient. In order to evaluate the redistribution, especially of MA and RE, three samples of one U-Pu-Zr alloy and two U-Pu-Zr-MA-RE alloys were heated in a temperature gradient and were analyzed by metallography, α-autoradiography and electron microprobe. The results indicated that Zr migrated to the colder region in both kinds of specimens, and that in U-Pu-Zr-MA-RE, Am and RE did not significantly move at temperatures below 1020 K, whereas Pu and Np concentrations slightly increased in the region around 900 K. By contrast, a significant depletion of Am and RE was observed in the hotter region above 970 K, while they migrated to the colder region of the sample, when the highest temperature in U-Pu-Zr-MA-RE was at above 1270 K. The transformation of the γ phase into a Zr-rich and a U-rich phases was observed in the hotter region, probably corresponding to the γ 1-γ 2 miscibility gap region in the U-Zr system.

The chemical diffusion of Ta in γ-U in samples fabricated for positron studies

A tantalum-jacketed high-purity uranium sample has been thermally cycled into the γ phase. Porosity formation in the uranium and interdiffusion with the Ta jacket were observed. The interdiffusion coefficient in the U-rich phase was between 10 −14 and 10 −13 m 2 s −1 at 1037°C.

Uranium in the silicate inclusions of stony-iron and iron meteorites

The microdistribution of U has been studied, using fission track techniques, in eleven mesosiderites, seven pallasites and four iron meteorites with silicate inclusions. When concentrated, U is usually found in phosphates: merrillite and/or chlorapatite. As in stony meteorites, the U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also exhibit variations in the same meteorite. Uranium is found to be concentrated in merrillite (0.25 to 1.43 ppm) in all the mesosiderites except Bondoc where none was observed. No U-rich phase was identified in six of the seven pallasites. In the seventh, Marjalahti, there are merrillite grains with concentrations ranging from 0.06 to 0.14 ppm. Where observed, the phosphates from silicate inclusions in the irons appear to have U concentrations similar to the mesosiderites.

Single-grain [formula omitted] and U/Pb age determinations with a 10-μm spatial resolution using the ion microprobe mass analyzer (IMMA)

Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, 207 Pb 206 Pb and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb isotopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics. Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of 207 Pb 206 Pb ages is accomplished by simply measuring the intensities of these peaks and the background. An excellent correspondence of the ages determined using the IMMA with those from more conventional techniques is demonstrated for terrestrial, lunar and meteoritic phases. For example, the following 207 Pb 206 Pb ages were obtained from polished thin sections of crystalline lunar samples: 10047, 3.75 ± 0.05 (2 σ) Ga; 14310, 3.96 ± 0.03 Ga; and 15555, 3.36 ± 0.06 Ga. From small U-rich phases in CaAl-rich inclusions in the Allende carbonaceous chondrite, seven 207 Pb 206 Pb ages were obtained, averaging 4.60 ± 0.09 (2 σ) Ga. A method of correcting low-resolution Pb-isotope data for molecular ion interferences in zircon and baddeleyite is presented and shown to be practical through the analysis of terrestrial and lunar samples.

Initial Pb of the Amîtsoq gneiss, West Greenland, and implications for the age of the Earth

Pb isotopic abundances and U-Th-Pb concentrations are reported for feldspar megacrysts from the 3.59 AE old Amîtsoq gneisses, Godthaab District, West Greenland. The distinctive Pb in the feldspars is the most primitive terrestrial Pb so far observed. It is observed in feldspars which are from different geographic localities and which exhibit varying degrees of deformation and recrystallization. This appear to be either the initial Pb in the Amîtsoq gneiss or the initial Pb only slightly modified by subsequent metamorphism in a low 238U 204Pb environment. 238U 204Pb in the feldspars is low and the corrections for in situ produced Pb are only 0.4% for 207Pb 206Pb and 0.6% for 204Pb 206Pb . The mean corrected isotopic abundances are 204Pb 206Pb = 0.08720 , 207Pb 206Pb = 1.1513 , and 208Pb 206Pb = 2.7350 . The feldspars contain a very small amount of easily leachable radiogenic Pb which is correlated with U and which indicates the formation of U-rich phases at about 2.7 AE. The matrix surrounding the feldspar megacrysts contains Pb which is much evolved relative to the megacrysts and this matrix does not appear to have behaved as a simple closed system. Element redistribution and open system behavior at about 2.7 AE is also suggested by Pb in feldspar from a dike cutting across the gneiss. Assuming that the Amîtsoq gneiss feldspar Pb corrected for in situ U decay was the initial Pb in the gneiss at 3.59 AE ( Baadsgaard, 1973), a single-stage “age of the earth” is determined as 4.47 ± 0.05 AE and μ is 8.5. This is indistinguishable from the single-stage age for modern rocks and is distinctly younger than the 4.55 AE age of some meteorites. If the feldspar Pb was modified by metamorphism at 2.7 AE the model age of the earth is calculated as 4.53 AE which is similar to the 4.55 AE age of some meteorites. Two-stage models using the nominal 3.59 AE initial Pb indicate that if the earth is ~4.55 AE old then μ values were low in the early Earth's history and differentiation occurred within a few hundred million years after the planet formed.

Equilibrium nitrogen pressures and thermodynamic properties of uranium sesquinitride

Equilibrium nitrogen pressures over the two-phase region UN + U 2N 3 were measured between 750° and 1050 °C. Nitrogen pressures were also determined in the single-phase region of U 2N 3. The U-rich phase boundary was obtained as 1.542 of the N/U atom ratio from the tensiometric measurement. The decomposition pressure of U 2N 3 was given by log pN 2 (atm) = 7.201 −11736/ T. The standard heat of the decomposition, UN 1.54 = UN + 0.27 N 2, was calculated to be ΔH 0 298=16.74 kcal/mole.