L. Pérez del Villar

The uranium ore from Mina Fe (Salamanca, Spain) as a natural analogue of processes in a spent fuel repository

Abstract In the frame of the ENRESA natural analogue programme, the uranium ore from the “Mina Fe” (Salamanca, Spain) has been studied as a natural analogue of radioactive spent fuel behaviour. This uranium mine is hosted in highly fractured schistose rocks, a geological setting that has not been envisaged in the Spanish options for radioactive waste burial. However, some analogies with the processes that might be involved in the evolution of these geological repositories suggested this investigation. The pitchblende–pyrite–carbonate paragenesis has been studied “in situ” as natural analogue of the nuclear spent fuel behaviour under extremely oxidative dissolution conditions. Similarly, secondary Fe oxyhydroxides and clay minerals have also been considered as relevant analogue materials for the retention of uranium and other analogous trace metals. A multidisciplinary characterisation of the site has been performed in order to study these processes. Though the intense mining activities in the site hindered precise determination of the original hydrogeological and hydrochemical features of the investigated zone (Boa fault zone), the mineralogy and geochemistry of fracture fillings, mineralisation and associated clayey materials have allowed the geochemical evolution of the system to be established. Three geochemical zones have been clearly differentiated: (i) the oxidised zone, from the surface to approximately 20 m depth, (ii) the redox transition zone, from 20 to 50 m depth, and (iii) the reduced zone, located below the transition zone. The oxidised zone is characterised by the presence of the typical mineral association resulting from the strong acid conditions caused by the total oxidation of pyrite and other sulphides. The total oxidation, dissolution and leaching of U(IV), as uranyl–sulphate aqueous complexes, prevailed in this oxidised zone. The redox transition zone is characterised by the coexistence of the primary uranium paragenesis, oxidised minerals, as well as numerous secondary solid phases as a result of the physico-chemical changes in the environment. The optimal physico-chemical conditions for the coffinitisation of pitchblende and the co-precipitation of Fe(III)–U(VI) took place in this zone. In the reduced zone, where the primary uranium paragenesis is present, we currently find the necessary physico-chemical conditions to stabilise pitchblende, pyrite and carbonates. The physico-chemical conditions of the oxidised zone are not relevant to disposal conditions. In the transition zone, two main geochemical processes take place: (i) the coffinitisation of pitchblende, which may be an important process for the stability of spent fuel in reducing conditions, and (ii) the co-precipitation of the Fe(III) and U(VI) as oxyhydroxides, another relevant mechanism for the retention of uranium. The physico-chemical conditions that prevail below 50 m depth should be sufficient to stabilise a spent nuclear fuel repository, in the same way as they have been able to preserve the 34-Ma-old uranium deposit of the Mina Fe.

Estimation of the concentrations of trace metals in natural systems: The application of codissolution and coprecipitation approaches to El Berrocal (Spain) and Poços de Caldas (Brazil)

Abstract Trace element concentrations in natural systems indicate that, in most cases, solubility controls are not exerted by pure solid phases. The cycling of trace metals in the environment is coupled to the chemistry of the major components. To model this behaviour we have developed a methodology based on codissolution and coprecipitation approaches. We have applied this methodology to the El Berrocal (Spain) and Pocos de Caldas (Brazil) sites. Trace metals under investigation were: Ba, Cu, Mn, Ni, Sr, and U. The models took into account: (1) the observed association of copper and uranium with iron oxyhydroxides (both in El Berrocal and in Pocos de Caldas), (2) the interaction of barium and manganese with calcite in El Berrocal, and (3) the association between strontium and fluorite in Pocos de Caldas. In most cases, the results were in good agreement with field data, showing that application of these approaches can reproduce more accurately the measured concentration of trace metals in groundwater than the assumption of a pure solid phase controlling the concentration of trace metals in solution. However, some problems still remain, such as the description of nickel concentrations in groundwater.

Carbonatation processes at the El Berrocal natural analogue / granitic system Spain : inferences from mineralogical and stable isotope studies

The El Berrocal granite/U-bearing quartz vein system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective is to understand the geochemical behaviour of natural radionuclides occurring under natural conditions. In this framework, the carbonatation processes have been studied from a mineralogical and isotopic ( and ) point of view, since carbonate anions are powerful complexing agents for U(VI) under both low-temperature hydrothermal and environmental conditions. The carbonatation processes in the system are identified by the presence of secondary ankerite, with minor calcite, scattered in the hydrothermally altered granite, and Mn calcite in fracture filling materials. The isotopic signatures of these carbonates lead us to conclude that ankerite and calcite from the former were formed at the end of the same hydrothermal process that altered the granite, at a temperature range of between 72° and 61°C for ankerite, and between 52° and 35°C for calcite. The effect of edaphic CO2 on both carbonates, greater on calcite than on ankerite, is demonstrated. Calcites from fracture fillings are, at least, binary mixtures, in different proportions, of hydrothermal calcite, formed between 25° and <100°C, and supergenic calcite, formed at ≤25°C. According to their signatures, the effect of edaphic CO2 in both calcites is also evident. It is assumed that: (i) hydrothermal calcite from fracture fillings and ankerite from the hydrothermally altered granite are the result of the same hydrothermal process, their chemical differences being due to the intensity of the water/rock interaction which was stronger in the altered granite than in the fractures; and (ii) all of these carbonatation processes are responsible for ancient and recent migration/retention of uranium observed in the hydrothermally altered granite and fracture fillings.

Mineralogical and geochemical evidence of the migration/retention processes of U and Th in fracture fillings from the El Berrocal granitic site (Spain)

Abstract The fracture fillings in the El Berrocal site are the result of several overlapping alteration processes of the granite since the formation of the fractures. They are essentially composed of quartz, feldspars, clay minerals, carbonates and minor pyrite and Fe oxyhydroxides. The U(IV) oxides occur only in some samples, whereas uranyl phosphates and silicates, sometimes with Th and REE, are present in almost all the samples. Adsorption and/or coprecipitation of U onto Fe oxyhydroxides were also observed. In relation to the granite, the fracture fillings are enriched in U by a factor ranging between 60 and 2, and in Th by a factor of 3. δ18O values in clay minerals, quartz and carbonates and δ13C in the latter indicate that quartz, illite and kaolinite were formed at a minimum temperature of 100 °C; the CO32−, of edaphic origin, was formed between 15 °C and at least 60 °C. Smectite and a second generation of kaolinite were formed at ambient temperature. Just under half of the bulk samples of the fracture fillings are in secular equilibrium with respect to U isotopes. The rest have at least one activity ratio less or larger than unity. The existence of uranyl minerals and the UTh enrichment factors observed indicate retention processes mainly by precipitation of both natural radionuclides. The different values of the 234 U 238 U activity ratio observed show that fixation or leaching of U may be currently in progress at various locations in the fracture system. However, some samples in equilibrium for the 234 U 238 U pair are taking up 228Ra (228Th) from the waters, whereas samples with 234 U 238 U ≠ 1 generally function as a close system for the 228 Ra ( 228 Th ) 232 Th pair. This is consistent with the extremely open character of the site studied.

Uranium isotopic distribution in the mineral phases of granitic fracture fillings by a sequential extraction procedure

In order to study the recent rock-water interaction processes in the E1 Berrocal site, a sequential leaching method has been applied to granitic fracture fillings to obtain the U isotopic distribution in the mineral phases of these samples. Based on the mineralogical composition of these materials, six dissolution steps have been chosen to extract U as exchangeable cation, from carbonates, amorphous Fe-oxyhydroxides, labile resistates and highly insoluble resistates. In this way, the processes involved in the rock-water interaction phenomena, mainly dissolution, precipitation, coprecipitation and adsorption can be distinguished and even approximately dated.

Argillization processes at the El Berrocal analogue granitic system (Spain): mineralogy, isotopic study and implications for the performance assessment of radwaste geological disposal

Abstract The El Berrocal granite/U-bearing quartz vein (UQV) system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective was to understand the geochemical behaviour of natural nuclides under different physicochemical conditions. Within this framework, the argillization processes related to fracturing and formation of the uranium–quartz vein were studied from a mineralogical and isotopic standpoint in order to establish their temperatures of formation and thus complete the geothermal history of the system. For this purpose, δ 18 O values were determined for pure mineral from the unaltered granite and quartz from the uranium–quartz vein, as well as for mixture samples from the hydrothermally altered granite (sericitised granite) and clayey samples from fracture fillings, including the clayey walls of the uranium–quartz vein. The isotopic signature of quartz from the uranium–quartz vein and the monophasic nature of its fluid inclusions led us to conclude that the isotopic signature of water in equilibrium with quartz was approximately in the range from −8.3‰ to −5.7‰ V-SMOV, its temperature of formation being around 85–120 °C. The δ 18 O values of pure sericite from the hydrothermally altered granite, calculated by means of the oxygen fraction molar method, indicate that its temperature of formation, in equilibrium with the aforementioned waters, is also in the range from 70 °C to approximately 120 °C. Clays from fracture fillings and clayey walls of the uranium–quartz vein are usually mixtures, in different proportions, of illite, approximately formed between 70 and 125 °C; two generations of kaolinite formed at approximately 90–130 °C and at around 25 °C, respectively; smectite, formed at ≤25 °C; and occasionally palygorskite, formed either between 30 and 45 °C or 19 and 32 °C, depending on the fractionation equation used. These data suggest that sericite from the hydrothermally altered granite, quartz from the uranium–quartz vein, illite and the first generation of kaolinite from the fracture fillings resulted from the same hydrothermal process affecting the El Berrocal granite in relation to fracturing. Under certain physicochemical conditions ( T ≈100 °C, pH≈8 and log [H 4 SiO 4 ] between −4 and −3), illite and kaolinite can be paragenetic. As a result of weathering processes, smectite was formed from hydrothermal illite and inherited albite under alkaline weathering, while the second generation of kaolinite was formed from smectite, under acid conditions and close to the sulphide-rich uranium–quartz vein. Palygorskite is an occasional mineral formed probably either during the thermal tail of the above-described hydrothermal process or during weathering processes. In both cases, palygorskite must have formed from alkaline Si–Mg-rich solutions. Finally, these data and processes are discussed in terms of natural analogue processes, drawing some implications for the performance assessment of a deep geological radwaste repository (DGRR).

U-series in Fe-U-rich fracture fillings from the oxidised cap of the “Mina Fe” uranium deposit (Spain): implications for processes in a radwaste repository

Abstract Within the framework of the ENRESA (Spain) natural analogue programme, the U-ore deposit of “Mina Fe” is being studied as a natural analogue of radioactive spent fuel behaviour. In this context, the knowledge of the role played by fracture minerals as scavengers of certain analogue elements, mainly U, and the establishment of the time scale of the rock-water interaction processes controlling the uptakes or losses of U in the system are two relevant objectives. Fracture-infill materials from the site have first been mineralogically characterised, then the upper part of the U-series determined in both bulk samples and U-rich leachates obtained by sequential leaching. Uranium-series of the bulk samples indicate that most of the fractures remained as closed systems in the last 1.6 Ma, while in other fractures water/rock interaction processes affecting the upper part of the U-series have been identified. These processes indicate recent or rapid U accumulation or losses ( 102 ka) or 234U+230Th recoil gain. The apparently random distribution in depth of fractures where these processes occurred corroborates the different hydraulic behaviour of fractures, as a result of their varying degree of sealing. Uranium concentrations and 234U/238U ARs of the leachates obtained with Morgans solution and 6N HCl indicate that minerals dissolved with these reagents (U(IV/VI) oxyhydroxides and goethite+clays, respectively) are responsible for the retention of almost all of the U in the bulk samples. Furthermore, the 234U lost by the U minerals dissolved with Morgans solution seems to be fixed onto goethite–clay mixtures, the intersticial water being the vehicle for the isotopic transfer, which in turn is a recent or recent-past process.

U and Th series disequilibrium in unaltered and hydrothermally-altered granites from the El Berrocal site (Spain): Weathering effects

Abstract Within the framework of analysis of the safety assessment of high level radioactive waste repositories, the migration of natural radionuclides in a fissured granitic environment, the El Berrocal site, is being studied in Spain. In order to determine the weathering effects on the natural radionuclides of the site, the top parts of the 238 U and 232 Th decay series have been measured in bulk samples of unaltered and differently altered granites by α-particle spectrometry. The results have established the recent geochemical processed involving 238 U, 234 U, 230 Th, 232 Th and 228 Th such as: 234 U recoil loss, bulk U assimilation or 234 U recoil gain, equilibrium states, bulk U leaching without fractionation, bulk uranium assimilation with activity ratio near unity, and loss or gain of 228 Ra ( 228 Th) as a function of the hydrochemical characteristics of the percolating water. These results have been interpreted in terms of the different degrees of weathering according to literature data.

Preparation and characterisation of a (226)Ra spiked slag as reference material for radioactive control of steelworks.

One of the issues of the European Research Project MetroMetal is to develop reference materials in order to provide SI-traceable radioactivity monitoring in foundries. For this purpose, a protocol for preparing a set of identical standard slag samples, containing known activity concentrations of (226)Ra, has been developed. This paper describes the preparation of the raw material, the characterisation in terms of its mineralogical, chemical and radiological features, the spiking procedure and the homogeneity testing of the spiked samples.

Distribution of natural radioactivity within an estuary affected by releases from the phosphate industry

Abstract The distribution and behaviour of the radionuclides 210 Pb, 210 Po and 226 Ra in the Odiel and Tinto river estuaries, southwest Spain, have been studied. This system receives large quantities of solid and liquid effluents in the form of phosphogypsum waste from the phosphate industry containing natural radionuclides. Ranges of activities from 20 to 5000 Bq kg −1 (d.w.) in riverine sediments were observed. The ratios of radionuclides were also determined (uncertainties quoted to 2 σ), showing clear disequilibria in the effluents from the factories ( 210 Pb/ 210 Po = 2.7±0.6, 210 Pb/ 226 Ra = 0.6±0.1, 210 Po/ 226 Ra = 0.24±.03) and from the phosphogypsum piles ( 210 Pb/ 210 Po = 2.1±0.6, 210 Pb/ 226 Ra = 1.5±0.6, 210 Po/ 226 Ra = 0.7±0.2). However, the sampling sites on the rivers affected directly by the discharges exhibit activity ratios other than those found in the effluents, manifesting their different behaviour in the estuarine environment. The estimated inventory in sediments for these radionuclides was 0.92±0.20 TBq (1993) which can be compared with the annual release of 0.52±.04 TBq (1993) from the phosphogypsum piles and factories. Preferential deposition of 210 Po and 210 Pb rather than 226 Ra onto the river bed was observed. The proportion of total sediment 210 Pb activity associated with particles less than 2 μm in diameter, which is one of the most resuspendable and transportable sizes, is 5%-15%.