Dr Victoria Frankland

Laser-based spectroscopic techniques offer potential for characterising the alteration products of spent nuclear fuel in settings where the use of more traditional analytical methods is impracticable. Among these alteration products, uranyl phosphate phases have long attracted interest owing to their potential to form passivating surfaces on primary uranium phases inhibiting further uranium dissolution. Two strontium-rich meta-autunite ((Ca,Sr)(UO2)2(PO4)2·2–8(H2O)) samples from the Mount Spokane uranium deposit, Washington, USA were characterised by multiple laser wavelength Raman and time-resolved laser fluorescence spectroscopy. Well-defined Raman features were obtained, particularly at a laser wavelength of 785 nm, but partially hydrated meta-autunite phases could not be differentiated by Raman alone. However, subtle differences in three key modes were observed between meta-autunite and published data for fully hydrated autunite specimens enabling these minerals to be distinguished. Seven luminescence emission and several excitation features were resolved for the two samples, with the latter being the first reported excitation data for meta-autunite. The luminescence decay lifetime was found to be significantly longer than previously reported and sensitive to the meta-autunite dehydration phase. [Display omitted] •Alteration of spent nuclear fuel in the presence of phosphate may result in meta-autunite formation.•Type mineral specimens were characterised by SEM-EDXA, XRD, Raman and TRLFS.•Fluorescence excitation spectrum for meta-autunite is reported for the first time.•Fluorescence decay may be sensitive to meta-autunite dehydration.•Raman features are sufficient to discriminate the phase from fully hydrated autunite.

Studtite is known to exist at the back-end of the nuclear fuel cycle as an intermediate phase formed in the reprocessing of spent nuclear fuel. In the thermal decomposition of studtite, an amorphous phase is obtained at calcination temperatures between 200 and 500 degrees C. This amorphous compound, referred to elsewhere in the literature as U2O7, has been characterised by analytical spectroscopic methods. The local structure of the amorphous compound has been found to contain uranyl bonding by X-ray absorption near edge (XANES), Fourier transform infrared and Raman spectroscopy. Changes in bond distances in the uranyl group are discussed with respect to studtite calcination temperature. The reaction of the amorphous compound with water to form metaschoepite is also discussed and compared with the structure of schoepite and metaschoepite by X-ray diffraction. A novel schematic reaction mechanism for the thermal decomposition of studtite is proposed.

A sample of meta-autunite (Ca(UO2)2(PO4)2·6-8(H2O)) from a national reference collection was characterised by Raman spectroscopy as a representation of a potential spent nuclear fuel corrosion product. Raman spectra were collected at 457, 532, 633 and 785 nm; all exhibited some fluorescence effects, though to a lesser extent at 785 nm. The phosphate (v2(PO4)3−, v3(PO4)3−, v4(PO4)3−) and uranyl (v1(UO2)2+ and v2(UO2)2+) features could be unambiguously assigned in the resolved 785 nm spectrum. The position of the v3(UO2)2+ mode was predicted but not observed. The uranyl bond lengths and force constants were determined from the v1(UO2)2+ dominant and shoulder peak, as 1.78±0.01 and 1.79±0.01 Å and 5.69±0.08 and 5.29±0.08 millidynes Å−1, respectively.

Uranyl sulphate minerals are common alteration phases in uranium mines and uraniferous waste deposits where they occur in conjunction with other products of acidic drainage such as jarosite. Although not persistent in nature due to their high solubility, they may play an important role in governing uranium mobility during the operational and immediate post-closure environment of an engineered radioactive waste repository where oxidising conditions prevail. One such mineral, johannite (Cu(UO2)(2)(SO4)(2)(OH)(2)center dot 8H(2)O), is of particular interest given the stated intention of several countries to use copper canisters in the disposal of spent nuclear fuel. A museum reference sample of johannite has been characterised by luminescence and multiple-laser Raman spectroscopy, resulting in the first reported luminescence excitation and emission spectra for this mineral. Well-defined Raman features were observed using 785, 633, and 532 nm lasers with the resolved peaks corresponding well to the published spectra. The Raman spectrum measured with the 457 nm laser was mostly masked by a series of repeating doublets attributed to the luminescence emission features, from which band spacing values of 831 and 823 cm(-1) were extracted; the former corresponded to both the resolved 785 nm nu(1)(UO2)(2+) peak position and the band spacing value obtained from the first reported luminescence emission spectrum for johannite. Four emission and nine excitation peaks were resolved from the luminescence spectra. The findings indicate that a suite of complementary laser-based techniques offer the potential for real-time characterisation of johannite formed in environments where intrusive sampling, transportation, and 'off-site' laboratory analysis are not feasible.

Non-invasive techniques capable of distinguishing the alteration products of spent nuclear fuel during prolonged storage would be of great benefit when assessing the potential mobility of uranium from the fuel matrix. Two potential alteration products, becquerelite (Ca(UO2)6O4(OH)6·8(H2O)) and vandendriesscheite (Pb1.5(UO2)10O6(OH)11·11(H2O)), were characterised by multiple laser-wavelength Raman and time-resolved laser fluorescence spectroscopy (TRLFS). Chemical composition was confirmed by scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction. Well-defined Raman features were obtained, particularly with a 785 nm laser, allowing subtle differences between the two minerals to be observed. Differences were also found in the Raman spectra using lasers at 457, 532 and 633 nm, with the degree of luminescence depending on the metal cation present. Chemical quenching of luminescence in Raman data correlated with luminescence excitation and emission spectra results. Five and seven luminescence emission peaks were resolved for becquerelite and vandendriesscheite, respectively. The first reported luminescence excitation spectra were collected for each mineral but not all of the spectral ranges could be resolved into individual peaks. A luminescence decay lifetime of 5.5 ± 0.9 μs was obtained for vandendriesscheite. This study demonstrates that Raman spectroscopy and TRLFS are potentially valuable techniques for characterising uranyl oxy-hydroxides and could be used in conjunction with other methods, such as laser induced breakdown spectroscopy (LIBS), as part of a remote analysis package in nuclear waste management. [Display omitted]

Among the wide range of spent nuclear fuel alteration products, uranophane-type uranyl silicates have attracted particular attention due to their ability to incorporate additional ions into their crystal structures, including other radionuclides. Two such mineral phases, uranophane (s.s.) (Ca(UO2)2(SiO3OH)2.5(H2O)) and boltwoodite ((K, Na)(UO2)(SiO3OH).1.5H(2)O), have been characterised by Raman, luminescence and laser-induced breakdown spectroscopy. Well-defined Raman features were observed in each case with the two minerals being differentiated by detection of the nu(3)(SiO4)(2-)and delta(SiOH) modes for uranophane but not boltwoodite. Some distinction between uranophane-alpha and -beta was observed by Raman, but the phase sensitivity was more apparent in the luminescence emission peak positions. The luminescence signal emitted for boltwoodite was much weaker than with uranophane, suggesting that the associated K+ and Na+ cations in the former are more efficient at chemical quenching than Ca2+. This study also reports the first luminescence excitation data for uranophane and boltwoodite.

Among the wide range of natural uranium minerals, uranyl vanadates have attracted particular attention due to the economic viability of co-producing uranium and vanadium for industrial applications The typically remote locations of the ore deposits favour in situ analytical techniques offering rapid, minimally destructive charac-terisation wherever possible. This study reports on the use of luminescence, Raman and laser-induced breakdown spectroscopy to characterise the two most commonly found uranyl vanadates in nature, carnotite (K2(UO2)2V2O8.3H2O) and tyuyamunite (Ca(UO2)2V2O8.5-8H2O); the first attempt to use all three laser-based techniques in tandem for these phases. Significant differences in the luminescence emission signal intensity along with a noticeable shift in the resolved emission peak positions enable carnotite and tyuyamunite to be readily distinguished. Extraction of the band spacing from luminescence emission spectra provides confirmation of the position of the equivalent Raman uranyl symmetric stretch, v1(UO2)2+, vibration for each mineral. Raman itself, was unable to differentiate carnotite and tyuyamunite owing to the weak signals obtained; however, the degree of splitting in the vanadate symmetric stretch, v1(VO3)3-, feature and, to a lesser extent, the position of the v1(UO2)2+ peak might be used to distinguish between tyuyamunite and metatyuyamunite, although further studies are required for confirmation. Compositional LIBS analysis was successful in identifying minor quartz, gypsum, Mg-and Fe-bearing inclusions, subsequently confirmed by optical and scanning electron microscopy. The findings indicate that multiple laser-based techniques offer the potential for real-time characterisation of uranyl vanadate phases where intrusive sampling, transportation and 'off-site' laboratory analysis is impracticable.

Andersonite (Na2Ca[UO2(CO3)(3)].(5+x)(H2O) where x

The use of copper canisters in the Swedish KBS-3 concept for spent nuclear fuel disposal could result in the formation of copper-bearing uranyl phases should a canister suffer from defects or if the containment were to fail before reducing conditions are established in the repository. Most uranyl species would be expected to display higher solubility than the original uranium(IV) dioxide fuel, leading to enhanced release, though this would depend on the phase and prevailing groundwater conditions. Secondary alteration products may also be poorly crystalline or even amorphous, making characterisation difficult during the pre-closure period owing to the high radiation field close to the canister. Vandenbrandeite, (CuUO2(OH)4), is a rare mineral in nature but known to form by alteration of primary uraninite through interaction with oxidising groundwater containing dissolved copper Consequently, an attempt has been made to characterise two vandenbrandeite specimens of varying crystallinity by luminescence and multiple-laser Raman spectroscopy; techniques amenable to remote, robotic deployment and which have proved useful in discriminating other uranyl oxy-hydroxides, silicates and phosphates. The first reported luminescence emission and excitation spectra for vandenbrandeite revealed near-negligible luminescence, with a slightly enhanced signal for the specimen displaying poorer crystallinity. This observation agrees well with density functional theory calculations. The simulated projected density of state and band structure show an unlikely transition from the U f-orbitals to Cu d-orbitals, or O states, would be required for luminescence to be detectable; this probably improves for poorly crystalline specimens as the spatial overlap between the orbitals increases. Further, negligible differences in the number of peaks and peak positions were detected in the laser wavelength-dependent Raman spectra although again, variation in background noise and peak shape was observed based on the degree of crystallinity. Good agreement was obtained between experimental and simulated Raman spectra, particularly with the environmentally sensitive axial uranyl stretching modes, validating the crystal system derived in this study. The findings of this study suggest luminescence spectroscopy, when combined with Raman spectroscopy, may be able to both identify vandenbrandeite and distinguish between crystalline and amorphous forms based on their relative luminescence intensity.

Polar mesospheric clouds form in the summer high latitude mesopause region and are primarily comprised of H2O ice, forming at temperatures below 150 K. Average summertime temperatures in the polar mesosphere (78°N) are approximately 125 K and can be driven lower than 100 K by gravity waves. Under these extreme temperature conditions and given the relative mesospheric concentrations of CO2 and H2O (~360 ppmv and ~10 ppmv, respectively) it has been hypothesised that CO2 molecules could become trapped within amorphous mesospheric ice particles, possibly making a significant contribution to the total condensed volume. Studies of CO2 trapping in co-deposited gas mixtures of increasing CO2:H2O ratio (deposited at 98 K) were analysed via temperature programmed desorption. CO2 trapping was found to be negligible when the H2O flux to the surface was reduced to 4.8×1013 molecules cm−2 s−1. This corresponds to an average of 0.4 H2O molecules depositing on an adsorbed CO2 molecule and thereby trapping it in amorphous ice. Extrapolating the experimental data to mesospheric conditions shows that a mesospheric temperature of 100 K would be required (at a maximum mesospheric H2O concentration of 10 ppmv) in order to trap CO2 in the ice particles. Given the rarity of this temperature being reached in the mesosphere, this process would be an unlikely occurrence.

Icy particles containing a variety of Fe compounds are present in the upper atmospheres of planets such as the Earth and Saturn. In order to explore the role of ice sublimation and energetic ion bombardment in releasing Fe species into the gas phase, Fe-dosed ice films were prepared under UHV conditions in the laboratory. Temperature-programmed desorption studies of Fe/H2O films revealed that no Fe atoms or Fe-containing species co-desorbed along with the H2O molecules. This implies that when noctilucent ice cloud particles sublimate in the terrestrial mesosphere, the metallic species embedded in them will coalesce to form residual particles. Sputtering of the Fe-ice films by energetic Ar+ ions was shown to be an efficient mechanism for releasing Fe into the gas phase, with a yield of 0.08 (Ar+ energy=600 eV). Extrapolating with a semi-empirical sputtering model to the conditions of a proton aurora indicates that sputtering by energetic protons (>100 keV) should also be efficient. However, the proton flux in even an intense aurora will be too low for the resulting injection of Fe species into the gas phase to compete with that from meteoric ablation. In contrast, sputtering of the icy particles in the main rings of Saturn by energetic O+ ions may be the source of recently observed Fe+ in the Saturnian magnetosphere. Electron sputtering (9.5 keV) produced no detectable Fe atoms or Fe-containing species. Finally, it was observed that Fe(OH)2 was produced when Fe was dosed onto an ice film at 140 K (but not at 95 K). Electronic structure theory shows that the reaction which forms this hydroxide from adsorbed Fe has a large barrier of about 0.7 eV, from which we conclude that the reaction requires both translationally hot Fe atoms and mobile H2O molecules on the ice surface.

The uptake of HNO3, H2O, NO2 and NO was studied on meteoric smoke particle analogues using a low-pressure Knudsen cell operating at 295 K. The analogues used were olivine (MgFeSiO4) and a haematite/goethite (Fe2O3/FeO(OH)) mixture synthesised by the sol–gel process. For uptake on MgFeSiO4, the following uptake coefficients were obtained: γ(HNO3)=(1.8±0.3)×10−3, γ(H2O)=(4.0±1.3)×10−3, γ(NO2)=(5.7±0.2)×10−4 and γ(NO)

Analogues have been developed and characterised for both interplanetary dust and meteoric smoke particles. These include amorphous materials with elemental compositions similar to the olivine mineral solid solution series, a variety of iron oxides, undifferentiated meteorites (chondrites) and minerals which can be considered good terrestrial proxies to some phases present in meteorites. The products have been subjected to a suite of analytical techniques to demonstrate their suitability as analogues for the target materials.

The uptake and potential reactivity of metal atoms on water ice can be an important process in planetary atmospheres and on icy bodies in the interplanetary and interstellar medium. For instance, metal atom uptake affects the gas-phase chemistry of the Earth's mesosphere, and has been proposed to influence the agglomeration of matter into planets in protoplanetary disks. In this study the fate of Mg and K atoms incorporated into water-ice films, prepared under ultra-high vacuum conditions at temperatures of 110–140 K, was investigated. Temperature-programmed desorption experiments reveal that Mg- and K-containing species do not co-desorb when the ice sublimates, demonstrating that uptake on ice particles causes irreversible removal of the metals from the gas phase. This implies that uptake on ice particles in terrestrial polar mesospheric clouds accelerates the formation of large meteoric smoke particles (≥1 nm radius above 80 km) following sublimation of the ice. Energetic sputtering of metal-dosed ice layers by 500 eV Ar+ and Kr+ ions shows that whereas K reacts on (or within) the ice surface to form KOH, adsorbed Mg atoms are chemically inert. These experimental results are consistent with electronic structure calculations of the metals bound to an ice surface, where theoretical adsorption energies on ice are calculated to be −68 kJ mol−1 for K, −91 kJ mol−1 for Mg, and −306 kJ mol−1 for Fe. K can also insert into a surface H2O to produce KOH and a dangling H atom, in a reaction that is slightly exothermic.

The heterogeneous oxidation of CO by O2 on olivine, Fe sulfate and Fe oxide particles was studied using a flow tube apparatus between 300 and 680 K. These particles were chosen as possible analogues of unablated cosmic dust and meteoric smoke in Venus’ atmosphere. On olivine and Fe oxides, the rate of CO oxidation to CO2 only becomes significant above 450 K. For iron sulfates, CO2 production was not observed until these dust analogues had decomposed into iron oxides at ∼ 540 K. The CO oxidation rate increases significantly with a higher Fe content in the dust, implying that oxidation occurs through Fe active sites (no reaction was observed on Mg2SiO4). The oxidation kinetics can be explained by CO reacting with chemi-sorbed O2 through an Eley–Rideal mechanism, which is supported by electronic structure calculations. Uptake coefficients were measured from 450 to 680 K, yielding: log10(γ (CO on MgFeSiO4)) = (2.9 ± 0.1) × 10-3 T(K) – (8.2 ± 0.1); log10(γ (CO on Fe2SiO4)) = (2.3 ± 0.3) × 10-3 T(K) – (7.7 ± 0.2); log10(γ (CO on FeOOH/Fe2O3)) = (5.6 ± 0.8) × 10-3 T(K) – (9.3 ± 0.4). A 1-D atmospheric model of Venus was then constructed to explore the role of heterogeneous oxidation. The cosmic dust input to Venus, mostly originating from Jupiter Family Comets, is around 32 tonnes per Earth day. A chemical ablation model was used to show that ∼34% of this incoming mass ablates, forming meteoric smoke particles which, together with unablated dust particles, provide a significant surface for the heterogeneous oxidation of CO to CO2 in Venus’ troposphere. This process should cause almost complete removal of O2 below 40 km, but have a relatively small impact on the CO mixing ratio (since CO is in large excess over O2). Theoretical quantum calculations indicate that the gas-phase oxidation of CO by SO2 in the lower troposphere is not competitive with the heterogeneous oxidation of CO. Finally, the substantial number density of meteoric smoke particles predicted to occur above the cloud tops may facilitate the low temperature heterogeneous chemistry of other species.

The kinetics of heterogeneous HO2 uptake onto meteoric smoke particles (MSPs) has been studied in the laboratory using analogues of MSP aerosol entrained into a flow tube. The uptake coefficient, γ, was determined on synthetic amorphous olivine (MgFeSiO4) to be (6.9 ± 1.2) × 10−2 at a relative humidity (RH) of 10%. On forsterite (Mg2SiO4), γ = (4.3 ± 0.4) × 10−3 at RH = 11.6% and (7.3 ± 0.4) × 10−2 at RH = 9.9% on fayalite (Fe2SiO4). These results indicate that Fe plays a more important mechanistic role than Mg in the removal of HO2 from the gas phase. Electronic structure calculations show that Fe atoms exposed at the particle surface provide a catalytic site where HO2 is converted to H2O2 via an Eley-Rideal mechanism, but this does not occur on exposed surface Mg atoms. The impact of this heterogeneous process in the middle atmosphere was then investigated using a whole atmosphere chemistry-climate model which incorporates a microphysical treatment of MSPs. Using a global MSP production rate from meteoric ablation of 44 t/day, heterogeneous uptake (with γ = 0.2) on MSPs significantly alters the HOx budget in the nighttime polar vortex. This impact is highly latitude dependent and thus could not be confirmed using currently available satellite measurements of HO2, which are largely unavailable at latitudes greater than 70°.

A low-temperature flow tube and ultra-high vacuum apparatus were used to explore the uptake and heterogeneous chemistry of acetylene (C2H2) on cosmic dust analogues over the temperature range encountered in Titan's atmosphere below 600 km. The uptake coefficient, γ, was measured at 181 K to be (1.6 ± 0.4) × 10-4, (1.9 ± 0.4) × 10−4 and (1.5 ± 0.4) × 10−4 for the uptake of C2H2 on Mg2SiO4, MgFeSiO4 and Fe2SiO4, respectively, indicating that γ is independent of Mg or Fe active sites. The uptake of C2H2 was also measured on SiO2 and SiC as analogues for meteoric smoke particles in Titan's atmosphere, but was found to be below the detection limit (γ < 6 × 10−8 and < 4 × 10-7, respectively). The rate of cyclo-trimerization of C2H2 to C6H6 was found to be 2.6 × 10-5 exp(-741/T) s−1, with an uncertainty ranging from ± 27 % at 115 K to ± 49 % at 181 K. A chemical ablation model was used to show that the bulk of cosmic dust particles (radius 0.02–10 µm) entering Titan's atmosphere do not ablate (< 1% mass loss through sputtering), thereby providing a significant surface for heterogeneous chemistry. A 1D model of dust sedimentation shows that the production of C6H6 via uptake of C2H2 on cosmic dust, followed by cyclo-trimerization and desorption, is probably competitive with gas-phase production of C6H6 between 80 and 120 km.

Although several research groups have studied the formation of H2 on interstellar dust grains using surface science techniques, few have explored the formation of more complex molecules. A small number of these reactions produce molecules that remain on the surface of interstellar dust grains and, over time, lead to the formation of icy mantles. The most abundant of these species within the ice is H2O and is of particular interest as the observed molecular abundance cannot be accounted for using gas-phase chemistry alone. This article provides a brief introduction to the astronomical implications and motivations behind this research and the requirement for a new dual atomic beam ultrahigh vacuum (UHV) system. Further details of the apparatus design, characterisation, and calibration of the system are provided along with preliminary data from atomic O and O2 beam dosing on bare silica substrate and subsequent temperature programmed desorption measurements. The results obtained in this ongoing research may enable more chemically accurate surface formation mechanisms to be deduced for this and other species before simulating the kinetic data under interstellar conditions.

Temperature-programmed desorption and reflection-absorption infrared spectroscopy have been used to explore the interaction of oxygen (O2), nitrogen (N2), carbon monoxide (CO) and water (H2O) with an amorphous silica film as a demonstration of the detailed characterization of the silicate surfaces that might be present in the interstellar medium. The simple diatomic adsorbates are found to wet the silica surface and exhibit first-order desorption kinetics in the regime up to monolayer coverage. Beyond that, they exhibit zero-order kinetics as might be expected for sublimation of bulk solids. Water, in contrast, does not wet the silica surface and exhibits zero-order desorption kinetics at all coverages consistent with the formation of an islanded structure. Kinetic parameters for use in astrophysical modelling were obtained by inversion of the experimental data at sub-monolayer coverages and by comparison with models in the multilayer regime. Spectroscopic studies in the sub-monolayer regime show that the C–O stretching mode is at around 2137 cm−1 (5.43 μm), a position consistent with a linear surface–CO interaction, and is inhomogenously broadened as resulting from the heterogeneity of the surface. These studies also reveal, for the first time, direct evidence for the thermal activation of diffusion, and hence de-wetting, of H2O on the silica surface. Astrophysical implications of these findings could account for a part of the missing oxygen budget in dense interstellar clouds, and suggest that studies of the sub-monolayer adsorption of these simple molecules might be a useful probe of surface chemistry on more complex silicate materials.

Experimental measurements on the thermal and nonthermal behavior of water and other simple molecules, including organic compounds such as methanol and benzene, on model interstellar dust grain and solid water ice surfaces using science techniques and methodologies are reviewed. A simple qualitative model of the early stages of mantle growth arising from a synthesis of the results of such investigations from our own laboratory and others is presented.