Multimodal x-ray microanalysis of a UFeO4 : evidence for the environmental stability of ternary U(v) oxides from depleted uranium munitions testing

An environmentally aged radioactive particle of UFeO4 recovered from soil contaminated with munitions depleted uranium (DU) was characterised by microbeam synchrotron X-ray analysis. Imaging of uranium speciation by spatially resolved X-ray diffraction (μ-XRD) and X-ray absorption spectroscopy (μ-XAS) was used to localise UFeO4 in the particle, which was coincident with a distribution of U(V). The U oxidation state was confirmed using X-ray Absorption Near Edge Structure (μ-XANES) spectroscopy as +4.9 ± 0.15. Le-Bail fitting of the particle powder XRD pattern confirmed the presence of UFeO4 and a minor alteration product identified as chernikovite (H3O)(UO2)(PO4)·3H2O. Refined unit cell parameters for UFeO4 were in good agreement with previously published values. Uranium–oxygen interatomic distances in the first co-ordination sphere were determined by fitting of Extended X-ray Absorption Fine Structure (μ-EXAFS) spectroscopy. The average first shell U–O distance was 2.148 ± 0.012 Å, corresponding to a U valence of +4.96 ± 0.13 using bond valence sum analysis. Using bond distances from the published structure of UFeO4, U and Fe bond valence sums were calculated as +5.00 and +2.83 respectively, supporting the spectroscopic analysis and confirming the presence of a U(V)/Fe(III) pair. Overall this investigation provides important evidence for the stability of U(V) ternary oxides, in oxic, variably moist surface environment conditions for at least 25 years

Neptunium and uranium interactions with environmentally and industrially relevant iron minerals

Neptunium (237Np) is an important radionuclide in the nuclear fuel cycle in areas such as effluent treatment and the geodisposal of radioactive waste. Due to neptunium’s redox sensitivity and its tendency to adsorb strongly to mineral phases, such as iron oxides/sulfides, the environmental mobility of Np can be altered significantly by a wide variety of chemical processes. Here, Np interactions with key iron minerals, ferrihydrite (Fe5O8H·4H2O), goethite (α-FeOOH), and mackinawite (FeS), are investigated using X-ray Absorption Spectroscopy (XAS) in order to explore the mobility of neptunyl(V) (Np(V)O2+) moiety in environmental (radioactive waste disposal) and industrial (effluent treatment plant) scenarios. Analysis of the Np LIII-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) showed that upon exposure to goethite and ferrihydrite, Np(V) adsorbed to the surface, likely as an inner-sphere complex. Interestingly, analysis showed that only the first two shells (Oax and Oeq) of the EXAFS could be modelled with a high degree of confidence, and there was no clear indication of Fe or carbonate in the fits. When Np(V)O2+ was added to a mackinawite-containing system, Np(V) was reduced to Np(IV) and formed a nanocrystalline Np(IV)O2 solid. An analogous experiment was also performed with U(VI)O22+, and a similar reduction was observed, with U(VI) being reduced to nanocrystalline uraninite (U(IV)O2). These results highlight that Np(V) may undergo a variety of speciation changes in environmental and engineered systems whilst also highlighting the need for multi-technique approaches to speciation determination for actinyl (for example, Np(V)O2+) species

Physico-chemical controls on artificial radionuclides in soil

Radionuclides in the environment come from a wide range of sources, some natural and some artificial. Their biogeochemical behaviour is influenced both by their own physico-chemical properties and by those of the soil with which they interact. The source of the radionuclides is important, as are any changes in physico-chemical characteristics that occur during transport and deposition. Once in the soil further reactions can occur as the radionuclides equilibrate. These include immobilization by ion exchange and precipitation and remobilization by complexing and dissolution. Land management and environmental changes that alter conditions in the soil result in changes in the interactions of radionuclides with the soil and thereby to changes in mobility in the soil

The influence of soil properties on the environmental mobility of caesium in Cumbria

The interaction of caesium isotopes with soil has been widely investigated and the influence of important soil properties studied. From the results of such work and a detailed knowledge of the physico-chemical properties of soils it is possible to classify Cumbrian soil according to its ability to immobilize caesium. The ‘immobilization capacity’ is a reflection principally of the clay mineral content and type, organic content, pH, ammonium content and potassium status. Although it is not quantifiable, the immobilization capacity permits ranking of the soils and indicates which areas may give rise to persistent caesium problems. Combination of the soil sensitivity classification with deposition data for Cumbria indicates that the mountainous region in the south-west of the county is the most vulnerable. This conclusion is supported by field evidence, since the area identified coincides closely with that where sheep movement and slaughter are restricted and where caesium remains persistently available to the plant-animal chain

Chemical associations of artificial radionuclides in Cumbrian soils

The distributions of radionuclides in the surface layer of four soils of contrasting types from west Cumbria, UK, have been studied in detail. All the soils contain enhanced levels of artificial radionuclides derived from low-level discharges both to sea and to atmosphere from the British Nuclear Fuels reprocessing plant at Sellafield. The bulk of the activity is held in the top 15 cm of soil profiles, suggesting that radionuclide mobility is limited. Sequential extraction experiments have identified major associations of plutonium with organic matter and of caesium with silicates but there is no preferential association of ruthenium with any single soil component

Particle size and radionuclide levels in some west Cumbrian soils

Four west Cumbrian soils of contrasting types, together with an estuarine silt sample, have been separated into different particle size fractions by a combination of sieving and settling techniques. The sub-samples thus isolated were analysed by quantitative gamma-ray spectrometry for several nuclides, principally 137Cs, 106Ru and 241Am, followed by chemical separation and alpha spectrometric determination of 238,239 & 240Pu. A simple empirical method of correction for differing sample sizes, and hence counting geometries, was developed for gamma spectrometry and was found to give good results. The correction technique is based on calibrating each detector over a range of sample depths in the container using a multi-nuclide standard, and hence calculating an efficiency correction factor. In all the samples, the radionuclides were concentrated into the finer size fractions, with the clay-sized (< 2 μm) particles showing enrichments of plutonium from 1.5 to 40 times that in the bulk samples, of americium from 2 to 40 times and of 137Cs from 3 to 35 times. The enhancement was greatest for all radionuclides in a sandy soil with a very low clay content (0.2% by weight) and it was found that, as the abundance of fine particles increased, so the concentration effect decreased. No evidence was found for a simple relationship between organic content and radionuclide activity, although the organic matter does have some effect

Sources of variation in environmental radiochemical analysis

Both collection and laboratory treatment of samples contribute to the uncertainty in the final analytical results. A detailed study of a pasture field showed that, for realistic sample numbers, field variation was in the range 20–40%. Routine analysis of a reference soil, coupled with the use of some simple criteria for the acceptance of analytical results, showed that laboratory variation could be maintained at well below 10%, generally much less than the field variation

Wet and dry deposition of 131I, 134Cs and 137Cs at an upland site in northern England.

Samples of cloud and rain water, soil and vegetation were collected from sites at up to 847 m altitude on Great Dun Fell in northern England during the period 2–6 May 1986. Wet deposition is the dominant depositional process for 103Ru and the caesium isotopes, whilst dry deposition is much more important for 131I. The observed deposition velocity for 131I is 3·4 mm s−1. The patchy distribution of activity is related to the convective nature of the rainfall, and there is evidence of direct deposition of contaminated orographic cloud water at the highest site