Janusz Janeczek

Uraninite and UO2 in spent nuclear fuel: a comparison

Abstract Similarities and differences between uraninite and uranium dioxide in spent nuclear fuel are discussed in detail. Uraninite and its morphological variety, pitchblende, display structural and chemical properties and features that can be considered analogous to UO 2 in spent nuclear fuel, despite the different thermal histories and irradiation conditions. The similarities include topologically identical structures, high resistance to radiation damage, solid solutions with ThO 2 , CaO, and RE 2 O 3 , similar behavior during oxidation, and presence of fission products in uraninite from natural fission reactors in Gabon. The behavior of fission product residues during alteration of uraninite to coffinite in natural fission reactors may aid in estimation of the long-term behavior of fission products from spent fuels under repository disposal conditions.

Mechanisms of lead release from uraninite in the natural fission reactors in Gabon

Twenty-four samples of uranium ore from the natural fission reactors in Gabon were studied by detailed electron microprobe analysis and backscattered electron imaging in order to determine the behavior of radiogenic Pb and fissiongenic nuclides. Lead content in uraninite varies from 19 wt% PbO in relicts of pristine uraninite, which were found only in reactor zone 10, to less than 5 wt% in altered uraninites. Different mechanisms of Pb loss from uraninite prevailed in different reactor zones and included leaching, grain boundary diffusion, exsolution via continuous precipitation, and volume diffusion. As a result of these processes, Pb content in uraninites from all the reactor zones, except for reactor zone 10, are similar and vary around a mean value of 5.2 wt% PbO. All of these processes were thermally activated and episodic. The predominance of any single mechanism in a particular reactor zone was controlled by the accessibility of solutions to the uranium ore. The thermal event which caused Pb mobilization in the deposits resulted from regional igneous activity in the Franceville Basin more than 1100 Ma after the reactors sustained spontaneous fission reactions. Reducing conditions prevented the long distance migration of Pb, as well as of fissiongenic Mo and Ru.

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

Abstract 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 δ18O values that range from −30.1 ‰ to −15.2‰; whereas, stage 3 uraninite has δ18O values ranging from −10.0‰ to −3.4‰. Uranyl oxide hydrate minerals have δ18O values that range from −11.3‰ to −8.2‰; whereas, uranyl minerals have much higher δ18O values. Based on U Pb chemical ages,δ18O 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.

Vorlanite (CaU6+O4) - A new mineral from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Abstract The new mineral vorlanite, (CaU6+)O4, Dcalc = 7.29 g/cm3, H = 4-5, VHN10 = 360 kg/mm2, was found near the top of Mt. Vorlan in a calcareous skarn xenolith in ignimbrite of the Upper Chegem caldera in the Northern Caucasus, Kabardino-Balkaria, Russia. Vorlanite occurs as aggregates of black platy crystals up to 0.3 mm long with external symmetry 3̄m. The strongest powder diffraction lines are [d(Å)/(hkl)]: 3.107/(111), 2.691/(200), 1.903/(220), 1.623/(311), 1.235/(331), 1.203/(420), 1.098/(422), 0.910/(531). Single-crystal X-ray study gives isometric symmetry, space group Fm3̄m, a = 5.3813(2) Å, V = 155.834(10) Å3, and Z = 2. X-ray photoelectron spectroscopy indicate that all U in vorlanite is hexavalent. The mineral is isostructural with fluorite and uraninite (U4+O2). In contrast to synthetic rhombohedral CaUO4, and most U6+ minerals, the U6+ cations in vorlanite are present as disordered uranyl ions. [8]Ca2+ and [8]U6+ are disordered over a single site with average M-O = 2.33 Å. Vorlanite is believed to be a pseudomorphic replacement of originally rhombohedral CaUO4. We assume that this rhombohedral phase transformed by radiation damage to cubic CaUO4 (vorlanite). The new mineral is associated with larnite, chegemite, reinhardbraunsite, lakargiite, rondorfite, and wadalite, which are indicative of high-temperature formation (>800 °C) at shallow depth.

PHOSPHATIAN COFFINITE WITH RARE EARTH ELEMENTS AND CE-RICH FRANCOISITE-(ND) FROM SANDSTONE BENEATH A NATURAL FISSION REACTOR AT BANGOMBE, GABON

being obtained, indicate the precipitation of vaterite and amorphous calcium carbonate at the same time. The latter has the main absorption (v3) at 1492 cm l , in good agreement with Andersen and Brecevic (1991). The spectra, however, show other absorptions in the 1410-1470 cm -1 interval, where intensity and frequencies are strictly related to the synthesis followed (Fig. 2). Further studies on this topic, and particularly, about the conditions of crystallization of vaterite in hydraulic mortars are in progress.

Florencite-(La) with fissiogenic REEs from a natural fission reactor at Bangombe, Gabon

Abstract Florencite-(La) (La/Ce = 1.09) with fissiogenic REEs and florencite-(Ce) (La/Ce = 0.62) have been identified in illite from the clay mantle surrounding a natural, 2 Ga fission reactor at Bangombe and in sandstone beneath the reactor zone, respectively. Florencite- (Ce) is apparently unrelated to nuclear processes and occurs with monazite-(Ce), apatite, Ti02 (probably anatase), zircon, and illite. Grains of florencite-(Ce) contain inclusions of thorite, chalcopyrite, and galena. Florencite-(La) was found 5 cm from the “core” of the reactor and contains inclusions of galena and U-Ti-bearing phases. Secondary uraninite and coffinite have precipitated on some of the florencite grains. The chemical composition of florencite-(La) as determined by electron microprobe analysis is (La0.38Ce0.35Nd0.06Sm0.01Ca0.03Sr0.17)(Al2,98Fe3+0.02)(PO4)[PO3.80(OH)0.20](OH)6. Secondary ion mass spectrometry revealed that between 27 and 30% of Nd and 67 and 71% of Sm in florencite-(La) is fissiogenic. The presence of fissiogenic REEs in “florencite” from the reactor zone in Bangombe and their preferential concentration in florencite relative to the bulk sample of clay demonstrate that aluminous phosphates may have played a more significant role in the fixation of fissiogenic REEs released from uraninite after the sustained fission reactions than sorption onto clays.

An analytical electron microscope study of airborne industrial particles in Sosnowiec, Poland

Abstract The types and the relative amounts of airborne particles in the city of Sosnowiec (Poland) during 21–22 June, 1994 were identified by analytical electron microscope analyses. They are mostly aspherical angular Al-bearing silica particles (0.1–5.15 μm) and clusters thereof. Carbonaceous particles form sheets of soluble volatile-rich materials (0.3–33.9 μm) and rare soot. Numerous nanometer-sized Al-bearing silica grains and salt minerals are associated with the larger particles. They resulted from inefficient combustion of low-grade coals by the local industries whereby the silica particles are coal impurities that survived combustion. The total particle emission was constant during a 24 h period but silica shards dominated the nighttime emission while carbonaceous particles abounded during the daytime. This study showed that tropospheric particles in regions dominated by inefficient coal combustion are fundamentally different from typical coal fly ash spheres.

Oxidation of uraninite: Does tetragonal U3O7 occur in nature?

Abstract Samples of uraninite and pitchblende annealed at 1200°C in H 2 , and untreated pitchblende were sequentially oxidized in air at 180–190°C, 230°C, and 300°C. Uraninite and untreated pitchblende oxidized to the U 4 O 9 -type oxide, and their X-ray symmetry remained isometric up to 300°C. Reduced pitchblende after oxidation to UO 2 + x and U 4 O 9 -type oxides transformed into α-U 3 O 8 at 300°C. Two major mechanisms control uraninite and untreated pitchblende stability during oxidation: (1) Th and/or REE maintain charge balance and block oxygen interstitials near impurity cations; (2) the uraninite structure saturates with respect to excess oxygen and radiation-induced oxygen interstitials. Untreated pitchblende during oxidation behaved similarly to irradiated UO 2 in spent nuclear fuel; whereas, reduced pitchblende resembled nonirradiated UO 2 . An analysis of the data in the literature, as well as our own efforts to identify U 3 O 7 in samples from Cigar Lake, Canada, failed to provide conclusive evidence of the natural occurrence of tetragonal α-U 3 O 7 . Most probably, reported occurrences of U 3 O 7 are mixtures of isometric uraninites of slightly different compositions.

Crandallites and Coffinite: Retardation of Nuclear Reaction Products at the Bangombé Natural Fission Reactor

Various REE-Sr-(Pb)-crandallites, uraninite, and coffinite in the near-field of the 2 Ga old super-gene-altered Bangombe U-deposit and its natural fission reactor (RZB) have been examined. The crandallite minerals may have formed during syncriticality host-rock alteration, continous alteration of phosphates, episodic Pb-loss and/or supergene weathering. Coffinitization with P 2 O 5 and SO 4 -substitution has occurred immediately below RZB and resulted in extensive loss of U (≤ 46%) and enrichment of Ce (≤ 190%) and Nd (≤ 780%). Additional loss of U during coffinitization also may have occurred due to dissolution. Current alteration under oxidizing conditions has resulted in partial dissolution of uraninite and coffinite and the formation of uranyl phases. Despite supergene alteration, the hydrogeochemistry (3.09 ppt U [ 235 U/ 238 U = 0.7012 to 0.7019%], 4.96 ppt Ce, and 1.92 ppt Nd) suggests a remarkable retardation of lanthanides and depleted uranium by REE-Sr-(Pb)-crandallites, uraninite, coffinite, and uranyl phases at RZB.

Seasonal changes in the mineral compositions of tropospheric dust in the industrial region of Upper Silesia, Poland

Abstract The tropospheric dust loading in Upper Silesia (Poland) shows a steady, annually averaged supply of minerals from natural and anthropogenic sources, industrial dust emitters, domestic heating, transportation, but with superimposed seasonal changes for some dust types. Samples of airborne and deposited particles were collected at monthly intervals between 1996 and 2001 in several cities of Upper Silesia. Dust samples were examined by X-ray powder diffraction, analytical transmission electron microscopy, analytical scanning electron microscopy, and electron microprobe microanalysis. The most common dust included quartz, gypsum, coke, soot, hematite, magnetite, wüstite, bassanite, graphite and various K-, Fe- and Mg-bearing aluminosilicates, in order of decreasing abundance. Minor phases included α-iron, sulphur, sphalerite, halite, sylvite, hercynite, franklinite, baryte, dolomite, ankerite, apatite, olivine and feldspars. Quartz, and specific industrial minerals, e.g. spinels, sphalerite, olivine and iron, occurred throughout the year in almost constant abundances. The amounts of all other dust components show seasonal variations; gypsum, baryte and other sulphates are particularly abundant in winter. In general, minerals related to low-emission sources are abundant in the winter time, while both natural dusts and dust from high-emission sources are predominant during the summer.