Radionuclide concentrations in raw and purified phosphoric acids from Brazil and their processing wastes: implications for radiation exposures

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Potentialities and practical limitations of absolute neutron dosimetry using thin films of uranium and thorium applied to the fission track dating

Neutron dosimetry using natural uranium and thorium thin films makes possible that mineral dating by the fission-track method can be accomplished, even when poor thermalized neutron facilities are employed. In this case, the contributions of the fissions of (235)U, (238)U and (232)Th induced by thermal, epithermal and fast neutrons to the population of tracks produced during irradiation are quantified through the combined use of natural uranium and thorium films.If the Th/U ratio of the sample is known, only one irradiation (where the sample and the films of uranium and thorium are present) is necessary to perform the dating. However, if that ratio is unknown, it can be determined through another irradiation where the mineral to be dated and both films are placed inside a cadmium box.Problems related with film manufacturing and calibration are discussed. Special attention is given to the utilization of thin films having very low uranium content. The problems faced suggest that it may be better to substitute these films by uranium doped standard glasses calibrated with thicker uranium films (thickness greater than 1.5 x 10(13) mu m).

Radionuclides, heavy metals and fluorine incidence at Tapira phosphate rocks, Brazil, and their industrial (by) products

Inorganic phosphate fertilizers may contain radionuclides, heavy metals and fluorine. This paper presents the possible environmental hazards from Tapira phosphate rocks and their (by) products (Brazil) utilized as phosphate fertilizers. The activity concentration of U-238, U-234, Ra-226 and K-40 in Tapira phosphate rocks is within the world range for these rock types. The Th-232 activity concentration is higher than the mean reported in phosphate rocks. A value of 2184 nGy h(-1) was obtained for the exposure dose rate in Tapira phosphate deposit area, which is indicative of a high background radiation area. The flotation-separation process causes the incorporation of no more than 9%, 11 % and 24% of radionuclides, heavy metals and fluorine, respectively, into the phosphate concentrate. The radionuclides and heavy metals existing in phosphate fertilizers applied in Brazilian crops according to the recommended rates, do not raise their concentration in soils to harmful levels. (C) 2005 Elsevier Ltd. All rights reserved.

Seleção de linhagens de basidiomicetos resistentes aos herbicidas atrazina e diurom -produção de enzimas ligninolíticas e degradação dos compostos

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Diffusion-driven transport in clayrock formations

Clay mineral-rich sedimentary formations are currently under investigation to evaluate their potential use as host formations for installation of deep underground disposal facilities for radioactive waste (e.g. Boom Clay (BE), Opalinus Clay (CH), Callovo-Oxfordian argillite (FR)). The ultimate safety of the corresponding repository concepts depends largely on the capacity of the host formation to limit the flux towards the biosphere of radionuclides (RN) contained in the waste to acceptably low levels. Data for diffusion-driven transfer in these formations shows extreme differences in the measured or modelled behaviour for various radionuclides, e. g. between halogen RN (Cl-36, I-129) and actinides (U-238,U-235, Np-237, Th-232, etc.), which result from major differences between RN of the effects on transport of two phenomena: diffusion and sorption. This paper describes recent research aimed at improving understanding of these two phenomena, focusing on the results of studies carried out during the EC Funmig IP on clayrocks from the above three formations and from the Boda formation (HU). Project results regarding phenomena governing water, cation and anion distribution and mobility in the pore volumes influenced by the negatively-charged surfaces of clay minerals show a convergence of the modelling results for behaviour at the molecular scale and descriptions based on electrical double layer models. Transport models exist which couple ion distribution relative to the clay-solution interface and differentiated diffusive characteristics. These codes are able to reproduce the main trends in behaviour observed experimentally, e.g. D-e(anion) < D-e(HTO) < D-e(cation) and D-e(anion) variations as a function of ionic strength and material density. These trends are also well-explained by models of transport through ideal porous matrices made up of a charged surface material. Experimental validation of these models is good as regards monovalent alkaline cations, in progress for divalent electrostatically-interacting cations (e.g. Sr2+) and still relatively poor for 'strongly sorbing', high K-d cations. Funmig results have clarified understanding of how clayrock mineral composition, and the corresponding organisation of mineral grain assemblages and their associated porosity, can affect mobile solute (anions, HTO) diffusion at different scales (mm to geological formation). In particular, advances made in the capacity to map clayrock mineral grain-porosity organisation at high resolution provide additional elements for understanding diffusion anisotropy and for relating diffusion characteristics measured at different scales. On the other hand, the results of studies focusing on evaluating the potential effects of heterogeneity on mobile species diffusion at the formation scale tend to show that there is a minimal effect when compared to a homogeneous property model. Finally, the results of a natural tracer-based study carried out on the Opalinus Clay formation increase confidence in the use of diffusion parameters measured on laboratory scale samples for predicting diffusion over geological time-space scales. Much effort was placed on improving understanding of coupled sorption-diffusion phenomena for sorbing cations in clayrocks. Results regarding sorption equilibrium in dispersed and compacted materials for weakly to moderately sorbing cations (Sr2+, Cs+, Co2+) tend to show that the same sorption model probably holds in both systems. It was not possible to demonstrate this for highly sorbing elements such as Eu(III) because of the extremely long times needed to reach equilibrium conditions, but there does not seem to be any clear reason why such elements should not have similar behaviour. Diffusion experiments carried out with Sr2+, Cs+ and Eu(III) on all of the clayrocks gave mixed results and tend to show that coupled diffusion-sorption migration is much more complex than expected, leading generally to greater mobility than that predicted by coupling a batch-determined K-d and Ficks law based on the diffusion behaviour of HTO. If the K-d measured on equivalent dispersed systems holds as was shown to be the case for Sr, Cs (and probably Co) for Opalinus Clay, these results indicate that these cations have a D-e value higher than HTO (up to a factor of 10 for Cs+). Results are as yet very limited for very moderate to strongly sorbing species (e.g. Co(II), Eu(III), Cu(II)) because of their very slow transfer characteristics. (C) 2011 Elsevier Ltd. All rights reserved.

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

The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of U-238, U-234 and Th-230 was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim of U-234 was to assess the in situ, long-term migration behaviour 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 U-238 and Th-230 immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of U-234 from the rock matrix to the pore water by the decay of U-238 will be established. Diffusive redistribution separates U-234 from its immobile parent U-238 resulting in bulk rock U-234/U-238 activity disequilibria. These may provide a means of estimating the mobility of U-234 in the rock if the diffusion rate of U-234 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 beta-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-mu m up to several mu m in size zircon grains as the main U-rich phase. This fraction consistently has U-234/U-238 activity ratios below unity. The minute zircon grains constitute the major reservoir of U in the rock and act as constant rate suppliers of U-234 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 U-234/U-238 activity ratios above unity. It is believed that these U activity ratios reflect the surplus of having U-234 delivered from the zircon grains. Some cm-spaced samples show bulk rock U-234/U-238 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 U-234/U-238 activity ratios is observed. This is interpreted to indicate net U-234 transfer from regions of higher supply of U-234 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 U-234/U-238 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 U-234 in situ supply rates, regulating its diffusive fluxes and controlling the observed bulk rock U-234/U-238 activity ratios. A simple box-model is presented to simulate the measured bulk rock U-234/U-238 activity ratios and to give an additional insight into the studied system. (C) 2008 Elsevier Ltd. All rights reserved.

Measurement of isotopic uranium in water for compliance monitoring by liquid scintillation counting with alpha/beta discrimination

A simple and inexpensive method is described for analysis of uranium (U) activity and mass in water by liquid scintillation counting using $\alpha$/$\beta$ discrimination. This method appears to offer a solution to the need for an inexpensive protocol for monitoring U activity and mass simultaneously and an alternative to the potential inaccuracy involved when depending on the mass-to-activity conversion factor or activity screen.^ U is extracted virtually quantitatively into 20 ml extractive scintillator from a 1-$\ell$ aliquot of water acidified to less than pH 2. After phase separation, the sample is counted for a 20-minute screening count with a minimum detection level of 0.27 pCi $\ell\sp{-1}$. $\alpha$-particle emissions from the extracted U are counted with close to 100% efficiency with a Beckman LS6000 LL liquid scintillation counter equipped with pulse-shape discrimination electronics. Samples with activities higher than 10 pCi $\ell\sp-1$ are recounted for 500-1000 minutes for isotopic analysis. Isotopic analysis uses events that are automatically stored in spectral files and transferred to a computer during assay. The data can be transferred to a commercially available spreadsheet and retrieved for examination or data manipulation. Values for three readily observable spectral features can be rapidly identified by data examination and substituted into a simple formula to obtain $\sp{234}$U/$\sp{238}$U ratio for most samples. U mass is calculated by substituting the isotopic ratio value into a simple equation.^ The utility of this method for the proposed compliance monitoring of U in public drinking water supplies was field tested with a survey of drinking water from Texas supplies that had previously been known to contain elevated levels of gross $\alpha$ activity. U concentrations in 32 samples from 27 drinking water supplies ranged from 0.26 to 65.5 pCi $\ell\sp{-1}$, with seven samples exceeding the proposed Maximum Contaminant Level of 20 $\mu$g $\ell\sp{-1}$. Four exceeded the proposed activity screening level of 30 pCi $\ell\sp{-1}$. Isotopic ratios ranged from 0.87 to 41.8, while one sample contained $\sp{234}$U activity of 34.6 pCi $\ell\sp{-1}$ in the complete absence of its parent, $\sp{238}$U. U mass in the samples with elevated activity ranged from 0.0 to 103 $\mu$g $\ell\sp{-1}$. A limited test of screening surface and groundwaters for contamination by U from waste sites and natural processes was also successful. ^

Uranium chemistry of oceanic basalts

Measurements of uranium concentration and the 234U/238 U activity ratio in oceanic basalts which have undergone low-temperature seafloor alteration indicate that uranium uptake is a pervasive occurrence but that the various phases involved behave differently with respect to this process. Palagonite exhibits uranium contents 8-20 times higher than unaltered glass coupled with low 234U/238U, suggesting ongoing preferential leaching of 234U. Altered crystalline interiors of several old basalts have 234U/238U > 1, indicative of recent uranium exchange with seawater. The data also provide evidence for uranium sources with 234U/238U higher than the seawater value of 1.14. Manganese crusts on basalts of a variety of ages have isotopic ratios indicating that they either are recent deposits or also have experienced continuing uranium exchange with seawater.

Propagation of nuclear data uncertainties for PWR core analysis

An uncertainty propagation methodology based on the Monte Carlo method is applied to PWR nuclear design analysis to assess the impact of nuclear data uncertainties. The importance of the nuclear data uncertainties for 235,238 U, 239 Pu, and the thermal scattering library for hydrogen in water is analyzed. This uncertainty analysis is compared with the design and acceptance criteria to assure the adequacy of bounding estimates in safety margins.

Evaluation of the multi-element capabilities of collision/reaction cell inductively coupled plasma - mass spectrometry in wine analysis

This work explores the multi-element capabilities of inductively coupled plasma - mass spectrometry with collision/reaction cell technology (CCT-ICP-MS) for the simultaneous determination of both spectrally interfered and non-interfered nuclides in wine samples using a single set of experimental conditions. The influence of the cell gas type (i.e. He, He+H2 and He+NH3), cell gas flow rate and sample pre-treatment (i.e. water dilution or acid digestion) on the background-equivalent concentration (BEC) of several nuclides covering the mass range from 7 to 238 u has been studied. Results obtained in this work show that, operating the collision/reaction cell with a compromise cell gas flow rate (i.e. 4 mL min−1) improves BEC values for interfered nuclides without a significant effect on the BECs for non-interfered nuclides, with the exception of the light elements Li and Be. Among the different cell gas mixtures tested, the use of He or He+H2 is preferred over He+NH3 because NH3 generates new spectral interferences. No significant influence of the sample pre-treatment methodology (i.e. dilution or digestion) on the multi-element capabilities of CCT-ICP-MS in the context of simultaneous analysis of interfered and non-interfered nuclides was observed. Nonetheless, sample dilution should be kept at minimum to ensure that light nuclides (e.g. Li and Be) could be quantified in wine. Finally, a direct 5-fold aqueous dilution is recommended for the simultaneous trace and ultra-trace determination of spectrally interfered and non-interfered elements in wine by means of CCT-ICP-MS. The use of the CCT is mandatory for interference-free ultra-trace determination of Ti and Cr. Only Be could not be determined when using the CCT due to a deteriorated limit of detection when compared to conventional ICP-MS.

Nitrification processes in groundwaters evidenced by modification on the U contents and U-234/U-238 activity ratios

Groundwaters from the Botucatu-Piramboia aquifer system located at the Parana sedimentary basin in Brazil and sampled at Ribeirao Preto city, São Paulo State, were chemically and isotopically analysed, with the aim of evaluating how uranium isotopes U-234 and U-238 can be utilized to monitor the progress of nitrate, the main indicator of agrichemical pollution into an aquifer. Based on the isotope dilution analysis, the U-234/U-238 activity ratio and U content data in the groundwaters were utilized to deduce their proportions in a mixture, suggesting that the water not affected significantly by nitrification still dominates the exploitation system involving the studied wells.

A laboratory study of the transfer of U-234 and U-238 during water-rock interactions in the Carnmenellis granite (Cornwall, England) and implications for the interpretation of field data

Laboratory time-scale experiments were conducted on gravels from the Carnmenellis granite, Cornwall, England, with the purpose of evaluating the release of natural uranium isotopes to the water phase. The implications of these results for the production of enhanced U-234/U-238 activity ratios in Cornish groundwaters are discussed. It is suggested that the U-234/U-238 lab data can be used to interpret activity ratios from Cornwall, even when the observed inverse relationship between dissolved U and U-234/U-238 in leachates/etchates is taken into account. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Some constraints on the modeling using the uranium isotopes U-238 and U-234 for dating groundwaters from Guarany aquifer, South America

Groundwaters from the Guarany aquifer located at the South American continent and sampled at four wells with described geological sections in São Paulo State, Brazil, were chemically and isotopically analysed with two aims: to evaluate the quality of this important hydrological resource and to investigate the possibility of using the natural uranium isotopes U-234 and U-238 as a chronological tool, since the U-234/U-238 activity ratio and dissolved U content data in groundwater systems have generated models for dating purposes.

Transfer of radon and parent nuclides U-238 and U-234 from soils of the Mendip Hills area, England, to the water phase

Laboratory time scale experiments were conducted on soils from the Mendip Hills area, England, with the purpose of evaluating the release of Rn-222 and their parent nuclides U-238 and U-234 to the water phase and to determine the influence of parameters that can affect the geochemical behaviour of these nuclides in natural systems. The specific surface area of the samples ranged from 43.8 to 52.5 cm(2) g(-1), where the particle size for all soil horizons is lognormally distributed, with modal values of the particle radius undersize ranging from 107 up to 203 mu m. The values for the released radon were between 26 and 194 pCi, which allowed to estimate emanation coefficients for these materials between 0.1 and 0.2, within the context of other values reported elsewhere. Soils derived from Carboniferous limestone and characterized by higher pH, exchangeable calcium, and the presence of U, but with a lower U-231/U-238 activity ratio, yielded the highest values for released Rn; however, this trend was not observed for dissolved U and its respective U-234/U-238 activity ratio, when considering the less aggressive etchant. Uranium is mobilized from rock matrix to A and B horizons in the analysed soil profiles, where its enrichment is about 10 times higher in soils derived from Carboniferous limestone. These data also permitted an evaluation of a theoretical model for the generation of Rn in soils and its transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the Mendip Hills district, England. (C) 1999 Elsevier B.V. B.V. All lights reserved.