Cigar Lake Research Articles

The long-term effect of hydrogen on the UO2 spent fuel stability under anoxic conditions: Findings from the Cigar Lake Natural Analogue study

The present paradigm on UO2 spent fuel stability under anoxic conditions assumes that the potential oxidative alteration of the matrix is suppressed in the presence of the hydrogen generated by the anoxic corrosion of iron by water. The observations from the Cigar Lake Natural Analogue project indicated the long-term stability of the uraninite ore under anoxic conditions and with substantial hydrogen generation. The radiolytic models developed in the analogue project have been used to test some of the hypothesis concerning the activation of hydrogen on the uranium(IV) oxide surface. Suggestions to pathways of radiolytic oxidant consumption by other processes than uranium dioxide or sulphide oxidation are presented. The stability of the ore body for billions of year indicates the presence of processes which neutralise radiolytic oxidants and one major factor may be the presence of dissolved hydrogen in the groundwaters contacting the ore body. The results from this test would indicate that hydrogen is activated on the surface of the Cigar Lake uraninites by alpha radiation consuming the generated radiolytic oxidants.

Geochemistry and electron spin resonance of hydrothermal dickite (Nowa Ruda, Lower Silesia, Poland): vanadium and chromium

Geochemistry and electron spin resonance of hydrothermal dickite (Nowa Ruda, Lower Silesia, Poland): vanadium and chromium Geochemical analyses for trace V and Cr have been done on a representative sample of a typical hydrothermal dickite/kaolinite filling vein at Nowa Ruda. The mineralogy of the sample is comparatively simple, dickite being the principal component (ca. 91 % of the total sample). Geochemical fractionation and inductively coupled plasma-optical emission spectrometry (ICP-OES) indicate that most (> 90 % of total metal) of the V and Cr reside in the dickite. Electron Spin Resonance (ESR) shows that most (> 70 %) of the V in the dickite structure is in the form of vanadyl (VO2+) ions. A high concentration of Cr3+ is also detected in this structure by ESR. The combination of geochemical and spectroscopic tools applied to VO2+ and Cr3+ allow one to specify the Eh (> 0.4 V, highly oxidizing) and pH (≤4.0, highly acidic) of the solution during the formation of dickite from the Nowa Ruda Basin. Substantial proportions of the V and Cr (as well as VO2+ and Cr3+) in the dickite structure were probably contained in an original hydrothermal acid water. We suggest that hot hydrothermal waters leached the surrounding varieties of gabbroids enriched in V and Cr for the dickite-forming solution. The results of this work have shown V and Cr are potentially reliable indicators for geochemical characterization of the physicochemical conditions of their formation. The bulk-rock V/Cr ratio in hydrothermal dickites and kaolinites from Nowa Ruda, Sonoma (California, USA), Cigar Lake (Saskatchewan, Canada) and Teslić (Bosnia and Hercegovina) is also briefly explored here as a potential tracer of redox state during their formation.

Low grade uranium deposits of India – a bane or boon

Abstract Uranium resources of the world is estimated to be 5.5 million tonnes and the proven resources in India forms 3% of the world resources. The biggest uranium deposit is the Olympic dam deposit in Australia, which contains nearly 2.45 million tonnes of 0.023% U3O8, while the highest average grade of nearly 21% is established in the McArthur river deposit, Canada. Another very high grade deposit, the Cigar lake deposit, is also in Canada with an average grade of nearly 18%. Most of the uranium deposits established in India so far falls under the category of low grade. These low grade uranium deposits are distributed mainly in Singhbhum Shear Zone, Jharkhand; in parts of Chhattisgarh; Southern parts of Meghalaya; Cuddapah Basin, Andhra Pradesh; in parts of Karnataka and Aravalli- and Delhi Supergroups, Rajasthan and Haryana. These deposits are mainly hydrothermal vein type, stratabound type and unconformity related. The Singhbhum Shear Zone, Jharkhand hosts seventeen low grade vein type uranium deposits, aggregating about 30% of Indian uranium resources. The stratabound uranium mineralisation hosted by Vempalle dolostone extends over 160 km belt along southwestern margin of Cuddapah Basin in Andhra Pradesh and accounts 23% of the Indian resources. Though the dolostone hosted Tummalapalle uranium deposit was established in the early nineties, because of techno -economic constraints, the deposit remained dormant. As a consequence of the development of an innovative pre ssure alkali leaching process, the deposit became economically viable and a mine and mill are being constructed here. Recent exploration inputs are leading more such strata-bound low grade uranium deposits in the extension areas of Tummalapalle. Nearly 10 blocks have been identified within a 30 km belt which are being actively explored and a large uranium deposit has already been proved in this province. The deposit at Tummalapalle and adjoining areas is likely to emerge as one of the larger deposits in the world. The northern parts of Cuddapah basin in Andhra Pradesh is the host for unconformity related low grade uranium deposits. Four low grade low to medium tonnage deposits viz. Lambapur, Peddagattu, Chitrial and Koppunuru have been established in this province along the contact between basement granite and Proterozoic sedimentary rocks. This province also holds promise for large tonnage-high grade mineralization in the deeper unexplored parts of the basin. The North Delhi Fold Belt in the states of Rajas than and Haryana is the host for albitite/vein type uranium mineralization. A 320 km long NE-SW trending zone of deep seated fractures is the locus of mineralization and the mineralization is caused by soda metasomatism. A low grade uranium deposit is already established at Rohil associated with albitites and vast areas in this zone is open for exploration. The Cigar lake deposit, one of the two very high grade deposits in the world, though discovered in the year 1991 remains unexploited because of it high grade and requirement of remote handling and high end mining technology. The Mc Arthur deposit is being mined by raisebore mining method and the entire operation is carried out by 750 employees. Because of the high grade nature of the ore, it is diluted with barren rocks to bring the grade into manageable level before the beneficiation. The Rossing uranium deposit in Namibia, the biggest opencast mine for uranium, is of low grade and produces nearly 8% of the global output. As a result of the massive minin g and beneficiation operation, the employment of manpower is also very high compared to the high grade deposits. India, being a developing country with a high rate of unemployment, should treat the low grade deposits as an opportunity for employment. The operation of Rossing mine in Namibia is a shining example of as to how a low grade deposit could be economically exploited in the national interest and still be among the top five countries in terms of production. Inspite of mining the low grade resources, uranium mining industry in India provides direct and indirect employment to thousands of people and still operates in profit mode. Environmental concerns and radiation exposure to mining and milling personnel is negligible in the case of low grade deposits. The new mines, which are going to be opened in Tummalapalle, Andhra Pradesh; KPM (Domiasiat), Meghalaya; Gogi, Karnataka; Rohil, Rajasthan etc. will create substantial direct and indirect employment opportunities and transform the economy of these regions.

36Cl production and mobility in the Cigar Lake uranium deposit

Abstract Can accelerator mass spectrometry (AMS) studies validate risk assessments of the long-term behaviour of contaminants such as radionuclides? AMS measurements on samples from the 1.3 billion-year-old Cigar Lake uranium ore deposit provide one approach to address this question. In Cigar Lake, elevated concentrations of uranium enhance the in situ neutron flux that produces 36 Cl and other radionuclides. We calculated the production of 36 Cl using a Monte Carlo neutron transport code. We then tested for the loss of 36 Cl from ore samples collected from an 8 m stratigraphic section through the deposit by comparing the predicted values (assuming equilibrium between production and decay) with the concentrations measured by AMS. The 36 Cl:Cl atom ratios within the ore were more than two orders of magnitude higher than in the surrounding host rock and ranged from 4 to 64 × 10 −12 . The 36 Cl concentrations in the ore, rock, clay and fracture infilling minerals all agree with the values predicted by the Monte Carlo simulations. We conclude that 36 Cl has very limited mobility. Even in matrix adjacent to more permeable fractures, there is no evidence that the measured isotopic ratios deviate significantly from the predicted values.

Special cases of natural analogues: The Gabon and Cigar Lake U ore deposits

Abstract The Gabon and Cigar Lake uranium deposits may be used as natural analogues for nuclear waste because they both provide information on actinide immobilization in a geological system over a very long period of time. These deposits contain high-grade uranium ores (40–60% UO 2 ) embedded in a clay matrix. Futhermore, the Oklo and Bangombé uranium deposits in Gabon were natural nuclear fission reactors that operated 2000 Ma ago. This offers the unique opportunity to study the behaviour of fission products and actinides in a geological environment. The geological stability of the sites is one of the most important parameters that can explain the preservation of the natural reactors of Gabon and of the Cigar Lake deposit. The mineralogy of the hosted rocks and the chemical composition of the fluids are likewise important parameters. It can be shown that clays, Fe-sulphides and organic matters provide effective redox buffering and consequently play an important role in the long-term preservation of the ores. Adsorption of actinides and fission products on clays and Ti-oxides is also an important process for retention of these elements. This process is even more efficient when it is followed by the formation of coffinite, USiO 4 · n H 2 O. Coffinitization is common at the Cigar Lake deposit and in the natural reactors of Gabon. Coffinitization may be related to hydrothermal alteration (Cigar Lake) and to weathering (Bangombé).

The applicability and limitations of thermodynamic geochemical models to simulate trace element behaviour in natural waters. Lessons learned from natural analogue studies

Natural analogue investigations aim to understand key phenomena and processes in natural systems related to those expected to occur in radioactive waste repositories. One of the key applications of natural analogue studies has been the possibility to test the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. To this end, several geochemical modelling testing exercises (commonly denoted as blind predictive modelling, BPM) have formed an integral part of these studies over the last decade. We have reviewed, discussed and compared the results obtained from geochemical modelling BPM exercises carried out within six natural analogue studies: Poços de Caldas, Cigar Lake, Maqarin, El Berrocal, Oklo and Palmottu. To make this comparison meaningful, we present the main geochemical characteristics of each site in order to highlight the most relevant mineralogical and hydrochemical differences. The elements selected for discussion are: Sr, Ba, Sn, Pb, Se, Ni, Zn, REEs, Th and U. We have based our discussion on the results obtained from the calculated aqueous speciation as well as by comparing solubility calculations with the actually observed concentrations. Results can be differentiated into two categories of elemental behaviour: 1. those elements like Th and U under reducing conditions that can be fairly well described by assuming solubility control exerted by pure solid phases as their oxyhydroxides; 2. elements such as Sr, Zn, REEs and U under oxidising conditions for which the association to major geochemical components of the system must be considered in order to explain their concentrations in groundwaters. Additionally, we discuss the main improvements made to the thermodynamic databases and the geochemical calculation methodologies due to the BPM exercises. Furthermore, the most important characterisation geochemical data needed to complete predictive solubility and speciation calculations are identified.

O and Pb isotopic analyses of uranium minerals by ion microprobe and U–Pb ages from the Cigar Lake deposit

We apply a rapid and accurate in situ technique to make U–Pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of uraninite from the unconformity-type Cigar Lake uranium deposit. Secondary uranium minerals intergrown with uraninite, such as coffinite, USiO 4· nH 2O and calciouranoite, CaU 2O 7·5H 2O, were identified by high-resolution transmission electron microscopy (HRTEM). In situ U–Pb results from three stages of uraninite and coffinite define well-correlated arrays on concordia with upper intercepts of 1461±47, 1176±9, and 876±14 Ma (±1 σ). These ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration (∼1477 Ma) associated with these unconformity-type uranium deposits, the ages of magnetization events at 1600–1450 and 900 Ma from the Athabasca Basin, and the Grenvillian Orogeny at ∼1100 Ma. In situ U–Pb isotopic analyses of uraninite and coffinite can document the Pb*/U heterogeneities that occur on the scale of 15–30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits. The high spatial resolution and precision of the ion microprobe allow us to measure δ 18O values of 20–100 μm unaltered portions of uraninites from Cigar Lake. The range of δ 18O values (−33.9 to −20.4‰) are among the lowest reported for unconformity-type deposits, confirming that conventional fluorination analyses of material sampled at the mm-scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite.

A Rapid In Situ Method for Determining the Ages of Uranium Oxide Minerals: Evolution of the Cigar Lake Deposit, Athabasca Basin

We present a rapid and accurate technique for making in situ U-Pb isotopic measurements of uranium oxide minerals that utilizes both electron and ion microprobes. U and Pb concentrations are determined using an electron microprobe, whereas the isotopic composition of Pb for the same area is measured using a high-resolution ion microprobe. The advantages of this approach are: mineral separation and chemical digestion are unnecessary; homogenous uranium oxide standards, which are difficult to obtain, are not required; and precise and accurate U-Pb ages on ∼10 μm spots can be obtained in a matter of hours. We have applied our method to study the distribution of U-Pb ages in complexly intergrown uranium oxides from the unconformity-type Cigar Lake uranium deposit, Saskatchewan, Canada. In situ U-Pb results from early formed uraninite define a well-correlated array on concordia with upper and lower intercepts of 1467 ± 63 Ma and 443 ± 96 Ma (±lσ), respectively. The 1467 Ma age is interpreted as the minimum age of mineralization and is consistent with the age of clay-mineral alteration (∼1477 Ma) and magnetization of diagenetic hematite (1650 to 1450 Ma) that is associated with these unconformity-type uranium deposits and early diagenesis of the Athabasca Basin sediments. In situ U-Pb isotopic analyses of uraninite and coffinite can document the Pb∗/U heterogeneities that can occur on a scale of 15 to 30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits.

Predicting arsenic concentrations in the porewaters of buried uranium mill tailings

The proposed JEB Tailings Management Facility (TMF) to be emplaced below the groundwater table in northern Saskatchewan, Canada, will contain uranium mill tailings from McClean Lake, Midwest and Cigar Lake ore bodies, which are high in arsenic (up to 10%) and nickel (up to 5%). A serious concern is the possibility that high arsenic and nickel concentrations may be released from the buried tailings, contaminating adjacent groundwaters and a nearby lake. Laboratory tests and geochemical modeling were performed to examine ways to reduce the arsenic and nickel concentrations in TMF porewaters so as to minimize such contamination from tailings buried for 50 years and longer. The tests were designed to mimic conditions in the mill neutralization circuit (3 hr tests at 25°C), and in the TMF after burial (5–49 day aging tests). The aging tests were run at, 50, 25 and 4°C (the temperature in the TMF). In order to optimize the removal of arsenic by adsorption and precipitation, ferric sulfate was added to tailings raffinates1 having Fe/As ratios of less that 3–5. The acid raffinates were then neutralized by addition of slaked lime to nominal pH values of 7, 8, or 9.Analysis and modeling of the test results showed that with slaked lime addition to acid tailings raffinates, relatively amorphous scorodite (ferric arsenate) precipitates near pH 1, and is the dominant form of arsenate in slake limed tailings solids except those high in Ni and As and low in Fe, in which cabrerite-annabergite (Ni, Mg, Fe(II) arsenate) may also precipitate near pH 5–6. In addition to the arsenate precipitates, smaller amounts of arsenate are also adsorbed onto tailings solids.The aging tests showed that after burial of the tailings, arsenic concentrations may increase with time from the breakdown of the arsenate phases (chiefly scorodite). However, the tests indicate that the rate of change decreases and approaches zero after 72 hrs at 25°C, and may equal zero at all times in the TMF at 4°C. Consistent with a kinetic model that describes the rate of breakdown of scorodite to form hydrous ferric oxide, the rate of release of dissolved arsenate to tailings porewaters from slake limed tailings: (1) is proportional to pH above pH 6–7; (2) decreases exponentially as the total molar Fe/As ratio of tailings raffinates is increased from 1/1 to greater than 5/1; and (3) is proportional to temperature with an average Arrhenius activation energy of 13.4 ± 4.2 kcal/mol.Study results suggest that if ferric sulfate and slaked lime are added in the tailings neutralization circuit to give a raffinate Fe/As molar ratio of at least 3–5 and a nominal (initial) pH of 8 (final pH of 7–8), arsenic and nickel concentrations of 2 mg/L or less, are probable in porewaters of individual tailings in the TMF for 50 to 10,000 yrs after tailings disposal. However, the tailings will be mixed in the TMF, which will contain about 35% tailings with Fe/As = 3.0, and 65% tailings with Fe/As = 5.0–7.7. Thus, it seems likely that average arsenic pore water concentrations in the TMF may not exceed 1 mg/L.

Radionuclides in the lichen-caribou-human food chain near uranium mining operations in northern Saskatchewan, Canada.

The richest uranium ore bodies ever discovered (Cigar Lake and McArthur River) are presently under development in northeastern Saskatchewan. This subarctic region is also home to several operating uranium mines and aboriginal communities, partly dependent upon caribou for subsistence. Because of concerns over mining impacts and the efficient transfer of airborne radionuclides through the lichen-caribou-human food chain, radionuclides were analyzed in tissues from 18 barren-ground caribou (Rangifer tarandus groenlandicus). Radionuclides included uranium (U), radium (226Ra), lead (210Pb), and polonium (210Po) from the uranium decay series; the fission product (137Cs) from fallout; and naturally occurring potassium (40K). Natural background radiation doses average 2-4 mSv/year from cosmic rays, external gamma rays, radon inhalation, and ingestion of food items. The ingestion of 210Po and 137Cs when caribou are consumed adds to these background doses. The dose increment was 0.85 mSv/year for adults who consumed 100 g of caribou meat per day and up to 1.7 mSv/year if one liver and 10 kidneys per year were also consumed. We discuss the cancer risk from these doses. Concentration ratios (CRs), relating caribou tissues to lichens or rumen (stomach) contents, were calculated to estimate food chain transfer. The CRs for caribou muscle ranged from 1 to 16% for U, 6 to 25% for 226Ra, 1 to 2% for 210Pb, 6 to 26% for 210Po, 260 to 370% for 137Cs, and 76 to 130% for 40K, with 137Cs biomagnifying by a factor of 3-4. These CRs are useful in predicting caribou meat concentrations from the lichens, measured in monitoring programs, for the future evaluation of uranium mining impacts on this critical food chain.

Radionuclides in the Lichen-Caribou-Human Food Chain near Uranium Mining Operations in Northern Saskatchewan, Canada

The richest uranium ore bodies ever discovered (Cigar Lake and McArthur River) are presently under development in northeastern Saskatchewan. This subarctic region is also home to several operating uranium mines and aboriginal communities, partly dependent upon caribou for subsistence. Because of concerns over mining impacts and the efficient transfer of airborne radionuclides through the lichen-caribou-human food chain, radionuclides were analyzed in tissues from 18 barren-ground caribou (Rangifer tarandus groenlandicus). Radionuclides included uranium (U), radium (226Ra), lead (210Pb), and polonium (210Po) from the uranium decay series; the fission product (137Cs) from fallout; and naturally occurring potassium (40K). Natural background radiation doses average 2-4 mSv/year from cosmic rays, external gamma rays, radon inhalation, and ingestion of food items. The ingestion of 210Po and 137Cs when caribou are consumed adds to these background doses. The dose increment was 0.85 mSv/year for adults who consumed 100 g of caribou meat per day and up to 1.7 mSv/year if one liver and 10 kidneys per year were also consumed. We discuss the cancer risk from these doses. Concentration ratios (CRs), relating caribou tissues to lichens or rumen (stomach) contents, were calculated to estimate food chain transfer. The CRs for caribou muscle ranged from 1 to 16% for U, 6 to 25% for 226Ra, 1 to 2% for 210Pb, 6 to 26% for 210Po, 260 to 370% for 137Cs, and 76 to 130% for 40K, with 137Cs biomagnifying by a factor of 3-4. These CRs are useful in predicting caribou meat concentrations from the lichens, measured in monitoring programs, for the future evaluation of uranium mining impacts on this critical food chain.

Evaluation of the Effect of Microbial Subsurface Ecosystems on Spent Nuclear Fuel Repositories

Suitable subsurface repositories for spent nuclear fuel are presently under investigation at several national underground research laboratories that have been excavated for this purpose. However, since spent fuel storage canisters can be expected to break down after a period of time, engineered barriers have been proposed to prevent radionuclides, leached from the canisters by groundwater, from escaping into the environment. Serious investigation of the effect of microbial activity on these barriers and the surrounding geosphere has only been undertaken fairly recently. This research has now provided evidence that most subsurface groundwater environments are reducing, due to the presence of indigenous microbial ecosystems. If such a reducing environment can be maintained during excavation, emplacement and storage of the waste canisters it could ensure that most of the radionuclides are in the reduced state, in which case, as the majority are only slightly soluble, they are unlikely to be very mobile in the groundwater system. Geological analogues at Cigar Lake, Canada, and Oklo, Gabon demonstrate that uranium ore and its fission products can be retained in situ for more than a billion years. Furthermore excavation of the laboratory at Äspö in Sweden has shown that, although the subsurface environment was disturbed during excavation, the indigenous microbial community was able to maintain the environment in a reduced state. Thus it appears to be important to characterize the natural ecosystem as well as the hydrological regime present in the subsurface sites that are being considered for storage. Natural rock barriers, which contain the indigenous groundwater and microorganisms, may provide a more resilient protection to the movement of radionuclides than that of the proposed engineered barriers.

Nature’s uncommon elements: plutonium and technetium

Natural 99Tc and 239Pu were measured in ores from the Cigar Lake uranium deposit, in which U concentrations ranged from 0.3 to 55 wt%. Atomic ratios ranged from 1.4 × 10 −12 to 51 × 10 −12 for 99Tc/U and 2.4 × 10 −12 to 44 × 10 −12 for 239Pu/U. Measured concentrations are compared to those expected if the ores had behaved as closed systems with respect to U and its products. Under conditions of secular equilibrium in closed systems, 99Tc and 239Pu concentrations are solely a function of the neutron flux, U content, and decay rates. The neutron production rate and physical and chemical parameters that control the in situ neutron flux were measured in several samples. Neutron transport modeling of the sample environs using the MCNP code indicated that about half of the samples showed apparent excesses of 99Tc and 239Pu beyond the amounts predicted for secular equilibrium. Although production by neutron-capture on 98Mo complicates the accuracy of the 99Tc predictions, the excess quantities nonetheless strongly suggest the redistribution of these elements within the deposit. The failure to observe complementary deficiencies within the deposit suggests that the redistribution processes enriched the elements, removing small proportions of the elements from large uraniferous masses of rock, and concentrating them in smaller, less uraniferous, masses. The consistency of 239Pu/U and 99Tc/U ratios in bulk rock suggests that the redistribution processes observed at Cigar Lake are highly localized and in no case result in large-scale losses or gains of these nuclear products from the deposit as a whole.

The role of pe, pH, and carbonate on the solubility of UO 2 and uraninite under nominally reducing conditions

Experimental data obtained from uranium dioxide solubility studies as a function of pH and under nominally reducing conditions in a 0.008 mol dm −3 perchlorate medium and in a 1 mol dm −3 chloride solution are presented. The solubility of extensively characterized uraninite samples from Cigar Lake (Canada), Jachymov (Czech Republic), and Oklo (Gabon) was determined in a solution matching the composition of a groundwater associated with granitic terrain. The redox potential of the test solution was monitored throughout the experimental period. The results obtained were modeled using aqueous formation constants compiled by the NEA, using stability constants corrected to appropriate ionic strengths. The solubility curves have been adjusted by calculating the value of K s4 (UO 2(s) + 2H 2O ⇔ U(OH) 4(aq)) that gave the best fit with the experimental data. For a low temperature synthetic UO 2, a value of logK s4 of −7.3 was determined, while for uraninites the best fit was obtained with a value of logK s4 of −8.5. A wide range of published UO 2 solubilities can be reproduced by the available database, where experimental conditions were adequately defined in the original experiments. A lower value of the solubility product of the uranium dioxide phase defined as fuel in the SKB uranium database provides reasonable solubilities for a wide span of experimental results at near to neutral pH. Based on the modeling and using the β 1,4 for the U(IV)-OH complexation given by Grenthe et al. (1992a), a logK s0 (UO 2(s) + 4H + ⇔ U 4+ + 2H 2O) value of −2.3 ± 0.2 is proposed. Differences in solubility between natural and synthetic samples are attributed to the presence of carbonate in the experiments performed with uraninites, while differences in solubility observed among the natural samples can be correlated to radiation effects at atomic scale.

Mineral chemistry and oxygen isotopic analyses of uraninite, pitchblende and uranium alteration minerals from the Cigar Lake deposit, Saskatchewan, Canada

The Cigar Lake unconformity-type U deposit is one of the largest and highest grade U deposits in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada. Cigar Lake has recently been the focus of an international, 3-a, collaborative program in which this U deposit was studied as a natural analogue for a spent nuclear fuel repository. The deposit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults and the unconformity between Athabasca group sandstones and Aphebian metasediments. Three stages of U mineralization have been identified based on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably altered to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coffinite. U Pb chemical ages of the 3 stages of U mineralization from Cigar Lake coincide with the 3 major fluid events that precipitated clay and silicate minerals at 1500 Ma, 950 Ma, and 300 Ma, throughout the entire Athabasca Basin. Stage 1 and 2 uraninite and pitchblende have the lowest δ 18O values that range from −30.1 ‰ to −15.2‰; whereas, stage 3 uraninite has δ 18O values ranging from −10.0‰ to −3.4‰. Uranyl oxide hydrate minerals have δ 18O values that range from −11.3‰ to −8.2‰; whereas, uranyl minerals have much higher δ 18O values. Based on U Pb chemical ages, δ 18O values, and petrographic relationships of U alteration minerals associated with primary U mineralization, the Cigar Lake U ore is similar to U ore from other unconformity-type U deposits in the Athabasca Basin. Therefore, the Cigar Lake ore deposit, although surrounded by clay and sandstone barriers, has been effected by the same fluid events that have altered other unconformity-type U deposits in the Athabasca Basin. The 3 stages of ore formation and associated alteration minerals permit the detailed study of fluids responsible for U deposition and alteration. This information provides the necessary context for the evaluation of the Cigar Lake deposit as a “natural analogue” for the disposal of spent nuclear fuel in underground vaults in rocks of the Canadian Shield.

Development and application of a model for the long-term alteration of UO 2 spent nuclear fuel Test of equilibrium and kinetic mass transfer models in the Cigar Lake ore deposit

In recent years, we have been building up the thermodynamic and kinetic basis necessary to quantitatively determine the pathway for the long-term oxidative alteration of the UO 2 spent fuel matrix. There is a need to complement our thermodynamic description of these processes with kinetic mass transfer models. The development of these kinetic mass transfer models to the long-term oxidative alteration of UO 2 has been recently illustrated. The Cigar Lake ore deposit constitutes a unique opportunity to test the validity of these alteration models, since they provide the appropriate time and space scales. The purpose of this paper will be to show the successful testing of equilibrium and kinetic mass transfer models to describe the long-time-low temperature evolution of the uraninite deposit and the contacting groundwaters. In this paper, we will discuss the basic kinetics and thermodynamics of the model as well as the encouraging results so far obtained. The kinetic model has been applied to the aluminosilicates system. The results show that the interactions with clay minerals control the bulk composition of groundwater. The kinetic approach is able to describe the evolution of the groundwater composition along the hydrogeological pathways studied. The uranium system has been modelled by simulating the dissolution of uraninite along the flowpaths investigated. This modelling is based on equilibrium assumptions. The evolution of the redox state of groundwaters along different flowpaths is well reproduced.

Ferriferous and vanadiferous kaolinites from the hydrothermal alteration halo of the Cigar Lake uranium deposit (Canada)

AbstractThe uranium deposit (1350 Ma) of Cigar Lake (Canada) is surrounded by a late hydrothermal alteration halo (330 Ma) containing Fe-illites and kaolinites. Crystallochemical characterization of the kaolinites has been carried on the microscale using XRD, electron microscopy (SEM and TEM) coupled with EDX spectrometry and EPR. The large, well-crystallized particles show large amounts of Fe (0.9–1.8%) and V (0.3–0.5%). According to EPR measurements performed on both random powders and oriented samples, V occurs as the vanadyl ion VO2+, in substitution within the octahedral sheet of the kaolinite structure in the same way as Fe3+. Kaolinite growth proceeded through the hydrothermal alteration of anterior phyllosilicates devoid of V, induced by fluids which leached V-rich titano-magnetites in the surrounding sandstones.

A model for radiation energy deposition in natural uranium-bearing systems and its consequences to water radiolysis

Water radiolysis is of great concern in the concept of final disposal of nuclear waste in geological formations. In this paper, a model is developed for radiation energy deposition and its consequences to water radiolysis. The model concepts are based on radiation energy deposition in the porewater and other constituents of a uranium ore (the Cigar Lake uranium deposit). Radiation is randomly generated within grains of uranium minerals in the ore, by the Monte Carlo method. The radiation energy is then allowed to deposit into the various constituents of the ore. The fraction of the total radiation energy absorbed by water is obtained, and the oxidant production rate is calculated with G-values of water. When back-reactions of the oxidants and reductants projected by other researchers in similar systems are accounted for, the calculated oxidant production rate is in agreement with that predicted by mass transport models.

In situ production of 36Cl in uranium ore: A hydrogeological assessment tool

In situ neutron activation of 35Cl within the rock and groundwater of geologic deposits that have elevated concentrations of uranium provides a hydrogeological tracer. We determine the production rate and mobility of 36Cl in the 1.3‐billion‐year‐old Cigar Lake uranium ore deposit. Accelerator mass spectrometry was used to map the concentrations of 36Cl in the ore and in the groundwater that were up to 100 times greater than those encountered in unmineralized portions of the host sandstone aquifer. The residence time of this mobile anion in groundwater within the mineralized zone ranged from 14 to 280 kyr. These residence times are consistent with the hydraulic and geochemical data, suggesting significant control of Cl− and groundwater movement by the clay‐rich matrix of the mineralized zone.

Transport modelling in the natural analogue study of the Cigar Lake uranium deposit (Saskatchewan, Canada)

A near-field release model is developed both conceptually and mathematically. The model is tested against known helium release from the Cigar Lake uranium ore deposit. The release and transport of various aqueous species (including some radionuclides) in the deposit are studied by using this model. The uranium release rate predicted by the model is extremely low, which is consistent with field observations. In modelling the release of three other radionuclides ( 3H, 14C and 36Cl), the in situ generation and decay are taken into account. The measured concentration gradients of hydrogen was used to estimate the net rate of radiolysis. The simultaneously formed oxidising species are found in sulphates formed by oxidation of sulphides. There is a good agreement of the estimated rate of formation of the reducing component hydrogen and the oxidising component as found in the sulphate.