High Uranium Content Research Articles

Uranium at the Cretaceous‐Tertiary boundary in Denmark

ABSTRACTUranium is significantly enriched (up to two orders of magnitude) in the Danish Cretaceous‐Tertiary boundary beds relative to the underand overlaying sediments. Both the predominant oxic and some anoxic deposits have a high uranium content. To investigate the geochemical behaviour of the element, a series of sections was analysed for uranium by means of the delayed‐neutron counting technique.Uranium contents in the carbonate rocks from different parts of the Danish Sub‐basin are generally low but show slight regional trends. Experiments that involve cold acid extractions suggest that uranium is associated with the non‐carbonate residues.The uranium distribution with depth in the offshore drill core from the Central Graben area (North Sea) suggests that the element is associated with clay in the Danian part.Cretaceous‐Tertiary boundary beds from Stevns Klint and other localities in the Danish sub‐basin have anomalously high uranium contents compared with the values for chalk. Uranium is not correlated with iron, non‐carbonate carbon or aluminium. The highest uranium values within boundary sections are not found in the lower part of the sections as is the case for iridium.The only boundary beds that show typical anoxic depositional affinity have a total uranium accumulation that is one order of magnitude lower than that found in the oxic sections.High amounts of uranium within Cretaceous‐Tertiary boundary beds are suggested to be related to diagenetic processes such as compaction and dehydration.Uranium is thought not to cause the extinctions at the Cretaceous‐Tertiary boundary because of the relatively low contents found at the actual extinction level.

Effects of Chlorite Alteration on Uranium Redistribution in Koongarra, Australia

ABSTRACTThe relevance of alteration of chlorite, one of the major constituent minerals of the host rock to uranium ore at Koongarra, to the redistribution of uranium in the vicinity of the ore deposit has been examined. The chlorite alteration is produced by weathering; chlorite is transformed to vermiculite, through regularly interstratified chlorite/vermiculite. The Fe released from chlorite reprecipitates to form iron minerals, possibly ferrihydrite. Vermiculite is then replaced by kaolinite (and possibly smectite), further releasing Fe. On the millimeter scale, an alpha autoradiography study has shown that uranium concentrations are qualitatively proportional to the extent of the alteration; altered chlorite grains having higher uranium concentration. On the meter scale, the X-ray diffraction study has revealed that the abundances of chlorite, vermiculite, and kaolinite correspond well to the low, intermediate, and high uranium concentration zones, respectively. These suggest that the interaction of the uraniferous solution with chlorite causes the alteration of chlorite and the precipitation and sorption of uranium in the alteration products from the solution, and thus, the uranium migration is retarded at Koongarra.

The Solubility of Unirradiated UO2In Both Perchlorate And Chloride Test Solutions. Influence of the Ionic Medium

ABSTRACTThe solubility of a crystalline unirradiated UO2(s) has been studied under reducing conditions at 25°C in three different ionic media: 0.008 mol dm–3NaCIO4, 1 and 5 mol dm-3NaCl+. The species responsible for the solubility in perchlorate medium are U(OH)3and U(OH)4, with the stability constants log β13= -0.4(±0.2) and log β14= -5.7(±0.1), respectively. The solubility in both1! and 5 M chloride media is explained at pH values higher than 6 by the species U(OH)4with log β14= -5.5(±0.2) while at lower pH values an abnormal behaviour is observed, with higher uranium concentrations in solution and a proton independent solubility for pH values lower than 4. The XPS observations of the reacted solids do not show the presence of solid surface phases other than uranium dioxide, with an upper oxidation state of UO2.1.

Natural fission reactors of Oklo

Fourteen natural fission reactors are known to date in three different ore deposits located in the Franceville basin. The most famous and the first discovered are those in the Oklo uranium ore deposit. Petrographic, geochemical, and oxygen isotope studies have been performed in an attempt to answer these questions: (1) what are the main characteristics of the uranium ores in which fission reactions took place? (2) what are the processes that created such exceptionally high-grade ores? and (3) what are the effects of such fission reactions on the host rocks?The ore which hosts the fission reactions at Oklo is characterized by a gangue of clay minerals, whereas the normal ore of Oklo consists of sandstones. These clay minerals are the result of hydrothermal alteration of sandstone; hydrothermal circulation was induced by the heat produced by fission reactions. The reactor ores are further characterized by very high uranium contents (20-60%) and high uranium accumulation. By means of silica depletion and by uranium enrichment of the normal ore (without uranium depletion), hydrothermal convective circulation, induced by the fission reactions, created very high grade ore next to the reactors. Hydrothermal processes are thus critical to the propagation of the fission reactions.

The continuous recovery of uranium from biologically leached solutions using immobilized biomass

The potential of uranium recovery from the dilute uranium ore bioleach solutions of the Elliot Lake district of Canada was examined using immobilized microbial biomass. Batch and continuous laboratory scale pilot plant experiments were carried out. The results have shown that the immobilized microbial biomass can successfully recover all of the uranium from dilute (less than 300 mg U/L) solutions. The uranium can subsequently be eluted producing a high uranium concentration eluate perhaps exceeding 5000 mg U/L. The biomass maintained its biosorption capacity of about 50 mg U/g over 12 examined successive adsorption-elution cycles with no apparent indication of failure.

Irradiation of organic matter by uranium decay in the Alum Shale, Sweden

The Alum Shale of Sweden contains black shales with anomalously high uranium concentrations in excess of 100 ppm. Syngenetic or early diagenetic origin of this uranium indicates that organic matter within these shales has been irradiated by decaying uranium for approximately 500 Ma. Radiation-induced polymerization of alkanes through a free-radical cross-linking mechanism appears to be responsible for major alterations within the irradiated organic matter. Specific radiation-induced alterations include generation of condensate-like oils at reduced yields from hydrous pyrolysis experiments, decrease in atomic H/C ratios of kerogens, decrease in bitumen/organic-carbon ratios, and a relative increase in low-molecular weight triaromatic steroid hydrocarbons. Conversely, stable carbon isotopes of kerogens, reflectance of vitrinite-like macerais, oil-generation kinetics, and isomerization of 20R to 20S αα C 29-steranes were not affected by radiation. The radiation dosage needed to cause the alterations observed in the Alum Shale has been estimated to be in excess of 10 5 Mrads with respect to organic carbon. This value is used to estimate the potential for radiation damage to thermally immature organic matter in black shales through the geological rock record. High potential for radiation damage is not likely in Cenozoic and Mesozoic black shales but becomes more likely in lower Paleozoic and Precambrian black shales.

Chronologie Th/U des concrétions calcaires des varves du lac glaciaire de Deschaillons (Wisconsinien inférieur)

Late Pleistocene Lake Deschaillons varves from the central St. Lawrence Lowlands represent a lithostratigraphic unit whose upper limit is defined by the overlying Gentilly Till, which was laid down during the last Laurentide Ice advance over the area. Calcareous concretions are imbedded in the varves. These concretions reflect two distinct phases of CaCO3 precipitation. The first phase was during the early diagenesis of the interstitial organic matter. This phase is characterized by high uranium contents (~ 8 μg∙g−1), low carbon- and oxygen-isotope composition (−22 > δ13C > −24‰, −16 > δ18O > −9‰, vs. PDB), and 14C activities lower than 3% (vs. 13.56 dpm∙g−1). The second phase of CaCO3 precipitation occurred during postglacial times, as shown by the higher 14C activity (~30%) and stable-isotope composition (δ13C > −16‰; δ18O > −9‰). No significant U uptake was observed in the corresponding concretions. Th and U activity ratios were determined. The results allow evaluation of the analytical artefacts resulting from partial leaching of the detrital particles incorporated in the concretions by the weak acid treatment used to solubilize calcite. Since the early diagenetic concretions contain a significant but variable detrital fraction, 230Th/232Th and 234U/232Th isochrons from activity ratios measured in several samples are not representative of the age of the carbonate fraction. Isochrons from activity ratios of leachate and residue from individual samples yield, however, a reproducible slope (mean value: 0.52 ± 0.01) representative of the 230Th/234U activity ratio of the pure carbonate fraction. An age of 79 800 ± 1400 years is calculated for the relevant phase of CaCO3 precipitation. It is considered, within a few hundred years, the age of the Lake Deschaillons episode, which would therefore correspond to the first Wisconsinian ice advance in southern Quebec, at the transition between isotopic stages 5a and 4 of the oceanic 18O stratigraphy.

Chemical study on the separation and purification of promethium-147

A chemical process for the separation of147Nd/147Pm from fission products of synthetic radioactive waste solution has been developed. The process includes: (1) denitration, (2) removal of high concentration of uranium by 30% TBP/kerosene extraction, (3) removal of95Nb,103Ru,137Cs and part of90Sr by 50% TBP/dodecane extraction, (4) separation of147Nd/147Pm from part of90Sr and95Zr by oxalic acid precipitation, and (5) removal of144Ce by mixture of 0.4M D2EHPA and 0.2M TBP extraction. Experimental results indicate that the recovery of147Nd/147Pm in the final separated solution is about 90%. The purification of147Nd and147Pm from some other rare earth elements, viz.153Sm,154Eu and144Ce was further investigated by using a Dowex 50W×8 ion-exchanger. Parameters of flow rate, eluent concentration and pH were examined. The results show that the recovery and radionuclide purity of147Nd plus147Pm under the present separation conditions are 77.8% and 98.6% for diethylenetriaminepentaacetic acid (DTPA) and 87.3% and 99.5% for nitrilotriacetic acid (NTA), respectively.

Direct acid elution of anionic exchange resins for uranium recovery

An effective elution technique has been developed for recovering uranium from ion exchange resins loaded in the carbonate leaching process. The procedure employs direct elution with acid. In comparison with conventional base elution, the procedure yields eluate of higher uranium concentration, results in a more complete elution, keeps the performance of the ion exchange resin at the fresh stage, reduces acid consumption, eliminates carbonate consumption, and reduces waste for disposal. The technique has been successfully demonstrated in the field and patented. It lends itself to commercialization.

Aqueous geochemistry of ground water with high concentrations of Arsenic and Uranium, Carson River Basin, Nevada

Aqueous geochemistry of ground water with high concentrations of Arsenic and Uranium, Carson River Basin, Nevada

Radon and Geology in the United States

Abstract Regional geology and indoor radon concentrations appear to be related in the US. Using data from probability based samples, indoor radon frequency distributions have been developed in areas with similar regional geology. These frequency distributions can be used to define relative radon potential in the US. The geologic factors which determine regional radon potential are: (1) Lithologic: Rock types with known or potentially high uranium concentrations have a high probability of generating elevated indoor radon levels. Lithologies with US DOE National Uranium Resource Evaluation (NURE) equivalent uranium concentrations greater than 3 ppm may constitute high radon potential areas. (2) Soils: Soil permeability greatly influences radon potential. For Example, evidence suggests that high permeabilities (>0.01 cm.s-1) can cause elevated indoor radon levels even if radium concentrations are low.

The influence of high uranium, plutonium and gadolinium concentrations, respectively, on emission spectroscopy assays for selected elements

The influences of high uranium, plutonium, and gadolinium concentrations, respectively, on emission spectroscopy assays for traces mainly stem from the abundance of lines found in these spectra. In the UV range, especially a rise in background is observed, while line interferences occur preferably in the wavelength range > 300 nm. The performance capabilities of two types of spectrometers of different resolutions are demonstrated by the example of a boron assay.

Effects of irradiation from uranium decay on extractable organic matter in the Alum Shales of Sweden

Extractable organic matter in the Alum Shales of Sweden is believed to have been altered by irradiation from the natural decay of uranium. The amount of extractable bitumen is inversely proportional to uranium concentration suggesting conversion of extractable bitumen by irradiation. The ratio of aromatic to saturated hy hydrocarbons increased with increasing uranium concentration, presumably because aromatic hydrocarbons are more stable in the presence of radiationthan saturated hydrocarbons. Bitumen from samples with high uranium concentrations tends to be relatively enriched in aromatic hydrocarbons and contains a larger percentage of polars, asphaltenes and resins. Acyclic isoprenoid hydrocarbons are absent in extracts and kerogen pyrolysates of samples with high uranium concentrations. They therefore appear more susceptible to irradiation induced alteration than normal alkanes, which are present in almost all extracts and kerogen pyrolysates. Parent/alkylated polyaomatic hydrocarbon (PAH) ratios are highest in shallow samples with high uranium concentrations. Extracts from samples low in uranium concentration contain C 20, C 21, C 26, C 27 and C 28 triaromatic steroids, whereas extracts from samples of high uranium concentration contain only C 20 and C 21. Alkyl side chain cleavage by irradiation is the mechanism presumed responsible. Ratios of pyrene/phenanthrene and pyrene/chrysene are highest whereas ratios of 1+9/2+3 methylphenanthrenes are lowest in shallow samples with high uranium concentrations.

Error in neutron activation analysis from recoil-implanted fission products from uranium in aluminum foil.

Commercial aluminum foil may contain a high concentration of uranium, up to 1.4 μg/g. Samples or sample tubes in contact with aluminum foil during neutron irradiation can be contaminated with recoil-implanted fission products of 235U from uranium in the foil. This can lead to errors in the determination of Zr, Mo, La, Ce, Nd in neutron activation analysis.

An investigation of radon exhalation from fly ash produced in the combustion of coal

The exhalation of 222 Rn from fly ash produced by coal-burning power stations has been measured and compared with that from different kinds of soil and from coal itself. It is found that the radon exhalation rate from fly ash is less than that from soil and coal, despite the fact that fly ash contains a higher concentration of uranium and its decay products (especially 226 Ra) than does typical soil, as measured spectrometrically.

The Speciation of Uranium and Thorium at the Broubster Natural Analogue Site, Caithness, Scotland

ABSTRACTAnomalously high concentrations of uranium, thorium and the rare earth elements (REE) are associated with peat-rich soils near Broubster, northern Scotland. The source of the anomaly has been traced to a hydrocarbon-containing mineralised zone within a uraniferous, calcareous Devonian sedimentary sequence. From there, the elements have been transported approximately 100 metres downslope and fixed within waterlogged peat deposits. The degree of concentration and fractionation accompanying actinide transport is extreme. This paper describes the results of a theoretical modelling exercise and complementary experimental study carried out to determine the chemical speciation of actinides at the Broubster site. Efforts made to identify mobilisation and retention mechanisms for uranium and thorium have led to the development of a simple model for actinide behaviour in the peat soils. The work, which forms part of an on-going programme of research into natural radionuclide migration, is primarily aimed at improving confidence in predictive models used for radiological assessment.

The application of photon activation analysis for the characterization of uranium minerals

Instrumental photon activation analysis can be used advantageously for the homogeneity control and the determination of uranium concentration in minerals and ores to be certified as uranium reference materials. The homogeneity control of high contents of uranium can also be performed by passive gamma-ray spectrometry with high energy resolution. Interferences of237U characteristic lines with gamma-rays from other radionuclides produced in the 44 MeV bremsstrahlung are evaluated.

Ultrastructural localization of uranium biosorption in Penicillium digitatum by stem X-ray microanalysis

When Penicillium digitatum Saccardo cultures are exposed to aqueous solutions containing soluble uranium salts, considerable amounts of this element are accumulated in the fungal mycelium. The accumulated uranium is retained after thorough rinsing with distilled water but is removed by alkali carbonate solutions. Analysis of thick sections (0·5 μm) of the fungal hyphae with TEM, after incubation in UO 2Cl 2 solutions of varying concentrations under both light and dark conditions, revealed conspicuous crystal-like deposits in UO 2Cl 2-exposed hyphae, but none in the control hyphae. Thick sections were necessary for crystal visualization. Using energy-dispersive X-ray analysis, uranium was detected as the only heavy element in these crystals. Uranium crystal biosorption was localized on the outside surface of the hyphal cell wall (following short exposures to relatively low uranium concentrations) or inside the cell wall (following long exposure to relatively high uranium concentrations). In some cases, crystal-like deposits of uranium salts were located on the outside surface as well as inside the cell.

Radon and its decay products in the indoor environment: radiation exposure and risk estimation.

Radon, a member of the uranium decay chains, enters the indoor environment from the building subsoil, walls and the drinking water. In areas with high radium content of rocks and soil, considerable indoor levels of radon are measured. A sample of 32 conventional homes in Southeastern Switzerland, an area with generally high uranium concentrations in the subsoil, showed an arithmetic mean radon concentration in living quarters and cellars of 307 Bq/m3 (8.3 pCi/l) and 1410 Bq/m3 (38.1 pCi/l), respectively. Using UNSCEAR82 conversion factors, the annual effective dose equivalent in this area amounts to 9 mSv (900 mrem). The corresponding dose to the stem cells of the tracheobronchial tissue is about 100 mSv/a (10 rem/a). Life time exposure in such dwellings is in the range of occupational doses collected in modern uranium mines and leading to a significant increase in the lung cancer risk.

The behaviour of uranium, thorium and tin during leaching from a coarse-grained porphyritic granite in an arid environment

During the routine analysis of chip samples from water boreholes it was noted that the DeBakken granite south of Kenhardt has high uranium and tin contents. Rock samples collected on surface, however, revealed low concentrations of both these elements indicating that they have been leached from the surface samples. The distribution of thorium in the borehole profiles showed that while both tin and uranium have been removed above the water table, other “immobile” elements and thorium remained unaffected. The geological data showed that the leaching took place since the start of the Tertiary. Initially, the movement of uranium was vertical and accumulated in pedogenic calcrete above the granite. This vertical leaching was controlled by fluctuations in the water table and took place from the Pliocene to Pleistocene. Later, in the Holocene, the pedogenic calcrete was removed and deposited as non-pedogenic calcrete along the rivers. These nodular calcrete deposits, which are abundant in Namaqualand, are mineralized only along those rivers which drain the DeBakken granite, indicating that this granite was the source of the uranium. When the water table reached depths of ten metres or more the vertical migration of uranium ceased and horizontal leaching caused by the movement of ground water became active. Where the water table cuts the surface, such as in the pans, uranium deposition due to evaporation of groundwater is still active. Leaching of uranium from this granite, together with the formation of the secondary deposits is mineralogically controlled, and is ascribed to the fact that uranium is not hosted in zircon. Granites such as the DeBakken granite, in which the uranium is hosted in biotite, monazite and apatite, cannot be recognized by surface sampling and hence a lithogeochemical approach, using sub-surface samples has to be adopted. Calculations showed that approximately 1400 tons of uranium metal has been leached from this granite since the start of the Tertiary and that the maximum reserve of the province as a whole is relatively small. The distribution of tin is erratic, and although it has been leached from the surface samples, it has not been transported for any significant distance. Thus the tin distribution in the profiles does not show the same degree of leaching as does uranium. This is ascribed to the fact that samples from a percussion drill would include both the rock fragments from which tin has been leached and the clay-rich alteration products in which it has been trapped. Tin is generally immobile when present as cassiterite, but when enclosed in biotite, and when the groundwater is enriched in chlorine and fluorine, it leaches readily from the biotite.