Alpha-decay Dose Research Articles

In situ heating high-resolution TEM observation of structural recovery in metamict titanite

Besides radiation resistance, thermal recovery of radiation damage provides another critical criterion to evaluate the host phase for immobilizing high-level waste (HLW) produced by nuclear plants. Despite the increasing investigations, a comprehensive atomic-scale understanding remains elusive on thermal annealing of radiation damage in ceramics. Here, using in situ heating high-resolution transmission electron microscopy, we investigate the thermal annealing of radiation damage in natural metamict titanite (CaTiSiO5) caused by a high alpha decay dose from the incorporated U and Th during the geologic time. The recovery of the fully amorphous titanite precursor starts at 456 ℃ from the appearances of monoclinic nanoparticles 2–5 nanometers wide, and more nanoparticles form as the temperature gradually increases. At 1000 ℃, the amorphous phase recovers through epitaxial growth with titanite nanoparticles as templates. This study provides information for a unit-cell-scale understanding of the thermal stability of radiation damage in titanite-based ceramics, having significance for immobilizing HLW.

Zircon-to-reidite phase transition enhanced by minor radiation damage: Implications for hypervelocity impacts

Reidite, a high-pressure phase of zircon, is increasingly identified at terrestrial impact sites. Despite its growing recognition, the potential applications for estimating minimum impact pressure face impediments due to existing discrepancies in the condition of zircon-reidite transformation, controversial models governing the transformation mechanism, and unclear effects of pre-existing radiation damage on reidite formation. Here, we show enhanced reidite formation by synchrotron X-ray diffraction, Raman spectroscopy, and transmission electron microscopy analyses of zircon grains that have experienced different alpha-decay doses from U and Th impurities and subsequent pressurization in diamond anvil cells. Below ∼1 × 1018 α-decay events/g, the α-decay-induced isolated point defects in the still crystalline zircon facilitate the minor atomic readjustments required for reidite formation. However, above this dose, the loss of long-range periodicity in severely damaged or even metamict zircon inhibits the transformation. The enhanced reidite formation by minor radiation damage coincides with the more common occurrence of reidite at impact sites for which the precursor zircon has a relatively lower alpha-decay-event dose before the impact event. In addition, the detailed atomic-scale structures of twinned reidite provide unambiguous evidence for a characteristic internal stress-induced martensitic transition. These findings have important implications for interpreting the formation conditions of natural reidite due to the convergence of pressure from the static, shockwave, and natural reidite samples.

Monitoring of alpha-decay radiation damage in a 241Am-doped glass-ceramic material

The evolution under alpha-decay radiation of a 241Am doped aluminoborosilicate glass-ceramic was investigated in hot cells DHA-ATALANTE facility in CEA. The cumulative La2O3 - Am2O3 solubility limit was voluntarily exceeded, leading to the formation of apatite-like silicate crystals. The crystals, with a hexagonal-shaped morphology, that is characteristic of apatite crystals, have a composition and a cell parameter close to those expected for an apatite phase of stoichiometric composition Ca2(La,Am)8(SiO4)6O2. Structural and microstructural evolutions under alpha self-irradiation were followed for 8 years by regularly analyzing the crystals and the residual glassy matrix. The evolution of the X-Ray Diffraction (XRD) patterns is the result of a progressive radiation-induced amorphization in apatite crystals. The fully amorphous state is reached at an alpha-decay dose of around 3 × 1018 α/g. Raman analyses suggest a modification of the connectivity of the SiO4 tetrahedra of apatite crystals, with a transition from isolated SiO4 units in the crystalline state to connected SiO4 units in the metamict state. The crystalline-to-amorphous transformation is accompanied by an increase in macroscopic volume (swelling), highlighted by RAMAN imaging. This macroscopic dimensional change is also associated to a decohesion of the crystals from the glassy matrix, observed by Scanning Electron Microscopy (SEM). However, optical and SEM images of the glass-ceramic surface do not reveal any significant cracks in the residual glass under alpha self-irradiation, thus showing a quite good stability of this glass-ceramic material.

Quantitative evaluation of metamictisation of columbite-(Mn) from rare-element pegmatites using Raman spectroscopy

AbstractRaman spectroscopic analysis was performed on columbite-(Mn) samples from a variety of previously studied rare-element pegmatites in Xinjiang, China, including the Jing'erquan No. 1 spodumene-subtype, Dakalasu No. 1 beryl–columbite-subtype and Kalu'an spodumene-subtype pegmatites, to quantify the relationship between the degree of metamictisation of columbite and Raman spectra. For all of the analysed columbites-(Mn), the position (p) and the full width at half maximum (FWHM) of the strongest band, A1g vibration mode related to the Nb/Ta–O bond, in the Raman spectra have a negative correlation. Combined with previously determined U–Pb isotopic data and major–minor-element data for the columbites-(Mn), the degree of metamictisation was quantified using the alpha-decay dose (D) and displacement per atom (dpa), both of which were corrected for effects caused by annealing. The results demonstrate that the columbite-(Mn) from Jing'erquan and Kalu'an are very crystalline, whereas those from Dakalasu are transitional between crystalline and amorphous stages. The main factor influencing the key parameters, i.e. band position and FWHM, of the strongest Raman band of columbite-(Mn) is metamictisation caused by radiation damage, whereas composition and crystal orientation have limited influence. A set of equations are established to quantify the degree of metamictisation of columbite using the band position and the full width at half maximum: FWHM = 8.309 × ln(aD) + 30.11 (R2 = 0.9861); p = –5.187 × ln(aD) + 867.09 (R2 = 0.966); FWHM = 8.1453 × ln(adpa) + 48.425 (R2 = 0.9822); and p = –5.078 × ln(adpa) + 855.67 (R2 = 0.9594).

Combining detrital zircon shape and U–Pb–Hf isotope analyses for provenance studies – An example from the Aquiri region, Amazon Craton, Brazil

Provenance studies are commonly based on in situ U–Pb dating and Hf isotope analyses of detrital zircon grains, while leaving shape parameters unconsidered, even though these can provide valuable information about the hinterland and detritus transport. This is demonstrated herein by presenting a comprehensive data set derived from approximately 950 detrital zircon grains from three quartzite samples of the Cuxiú Formation of the Aquiri region, which is part of the eastern realm of the Amazon Craton. The data set combines results of zircon U–Th–Pb–Hf isotope analyses with zircon shape, and some physical parameters, including length, width, aspect ratio, roundness, typology, alpha-decay dose and density. Zircon U–Pb ages and field relationships indicate that pebbly sandstones of the Cuxiú Formation were deposited during the Neoarchaean between 2705 and 2665 Ma, likely in a fluvial or near-shoreline environment of a rift basin, with zircon grains supplied from igneous rocks of the surrounding Amazon Craton. The detrital zircon grains record crystallisation ages of 3100–2800 Ma (74%), and subordinately of 2800–2700 Ma (13%) and 3600–3100 Ma (3%). Hafnium isotope analyses further suggest crust formation in the hinterland at 4.01–3.65 Ga (14%), 3.60–3.10 Ga (85%) and 2.95 Ga (1%). Well preserved euhedral shapes of more than 50% of the detrital zircon grains, independent of U–Pb ages, suggest fast, direct supply from heterogeneous Archaean basement sources to the sedimentary basin, with a short-distance transport, plausibly in a fluvial setting. Correlation of zircon roundness with width in the size range from 50 to 150 µm indicates more intense abrasion of larger zircon grains compared to smaller ones, reflecting difference in transport mechanisms – i.e., abrasive-rolling transport vs. non-abrasive suspension transport. Similar average values for length (~120 µm), width (~66 µm), aspect ratio (1.9) and roundness (~2.5), obtained from zircon grains of very different ages (3600–2700 Ma), as well as log-normal distribution of the width parameter, suggest thorough hydrodynamic sorting during transport. Zircon typology analysis reveals a wide range of combinations of {100}:{110} prism faces with{101}:{301}:{211} pyramids, pointing to zircon crystallisation from 900 to 600 °C (average 755 °C). Uranium–Th contents indicate low α-decay doses of ≪1 × 10−15 decay events/mg for all zircon grains by the time of deposition, providing an additional explanation for the excellent zircon preservation.

Alpha dose rate and decay dose impacts on the long-term alteration of HLW nuclear glasses

In the prospect of deep geological disposal, the long-term behavior of high-level nuclear glasses has to be investigated regarding alpha radiation induced by long-life minor actinides. The present study focuses on the effects of alpha radiation on the long-term chemical reactivity of R7T7-type glasses, by separately considering the alpha dose rate and the alpha decay dose. Old SON68 glasses doped with 238/239PuO2 or 244CmO2 were studied to simulate high alpha dose rates corresponding to an early water ingress and a high level of alpha decay doses corresponding to long-term disposal conditions. A part of the 238/239Pu-doped glass block was annealed to fully recover the irradiation-induced damage accumulated since the glass was fabricated and to dissociate the effect of the alpha dose rate from that of the alpha decay dose. The glasses were then leached under static conditions at 90 °C for several years. The results showed that the residual alteration rate is not affected by the alpha dose rate over a wide range of dose rate values expected under disposal conditions: this glass remained relatively insensitive to the alpha radiolysis phenomena at the glass–water interface. However, over the duration of the experiments, the residual alteration rate of the damaged 238/239Pu-doped glass was enhanced compared to that of the annealed glass. This result is in agreement with those obtained on the 244Cm-doped glass and with reported values in the literature on simplified externally irradiated glasses, indicating that the ballistic effects of the recoil nuclei are responsible for this increase in the residual alteration rate.

LA-ICP-MS dating of high‑uranium columbite from no. 1 pegmatite at Dakalasu, the Chinese Altay orogen: Assessing effect of metamictization on age concordance

High‑uranium (~4200 to 8500 ppm U) columbite-group minerals (CGMs) from No. 1 pegmatite at Dakalasu, the Chinese Altay orogen were used to assess the effect of metamictization on the UPb isotopic systematics. Backscattered electron (BSE) imaging shows that the high U concentrations are due to the U in the crystal structure itself. In the rims, the part of CGM next to cracks or pores filled by titan-uranoan fluorcalciomicrolite is altered. The microlite was likely formed via a dissolution-reprecipitation process that resulted in fractionation between U and Pb and potential discordance of the UPb isotopic systematics in the altered CGM.Raman spectroscopy indicates that the columbite samples are metamictized to various extents. However, the CGM samples dated by laser ablation – inductively coupled plasma – mass spectrometer (LA-ICP-MS) show excellent UPb age concordance, with thirty eight out of forty analyses displaying concordant UPb ages (229.0 ± 1.0 Ma to 228.1 ± 0.6 Ma) overlapping within analytical uncertainty (2σ). The alpha-decay doses are estimated to be 1.53 to 2.31 × 1015 α-decay events/mg, indicating that the CGMs are at a transition stage from being crystalline to completely metamict states. Although spatial separation between U and radiogenic Pb isotopes caused by α-recoil is inevitable, the displacement is on sub-micron scale, much smaller than the scale of laser ablation pits (43 μm) for LA-ICP-MS dating. Therefore, the UPb isotopic systematics of partially metamict, U-rich CGM may still remain concordant for CGMs that have not suffered hydrothermal alteration or metasomatism and do not contain U- or Pb-rich inclusions within the laser ablation pits. Additionally, the chemical compositions of the CGM have no control over the UPb isotopic systematics.

Improved accuracy of LA-ICP-MS U-Pb ages of Cenozoic zircons by alpha dose correction

While Laser Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS) is the method of choice for U-Pb dating of zircons in provenance analysis, its application to young (<100Ma) zircons is hindered by systematic analytical bias. In magmatic petrology, where the majority of studied units are young, this often places zircon dates at odds with established 40Ar/39Ar eruption ages or high-precision ID-TIMS crystallization ages. Zircon lattice properties, particularly the degree of lattice damage caused by the radioactive decay of U and Th, impart analytical bias by causing differential ablation rates and therefore differential fractionation of U and Pb throughout each analysis. Although it is possible to normalize the zircon lattice strengths to calibration reference zircons by thermal annealing to some extent, this may not entirely alleviate the problem. In this study, the effects of alpha decay dose (i.e., degree of radiation damage) on analytical biases in age determination are examined by analyzing a number of zircon reference materials under well-constrained analytical parameters. A regression-based, multi-standard correction method is demonstrated, which improves the accuracy of age data, particularly in young (Cenozoic) zircons. A novel data reduction scheme (Dose_Corrector.ipf) is introduced, which runs in conjunction with the widely-used Igor Pro/Iolite platform and performs a correction for alpha dose and Th disequilibrium. This scheme improves the accuracy of age data for unannealed zircons, and its utility is demonstrated by applying it to zircons from several well-studied units.

Petrography and chemistry of tungsten-rich oxycalciobetafite in hydrothermal veins of the Adamello contact aureole, northern Italy

Tungsten-rich oxycalciobetafite occurs in complex Ti-rich hydrothermal veins emplaced within dolomite marble in the contact aureole of the Adamello batholith, northern Italy, where it occurs as overgrowths on zirconolite. The betafite is weakly zoned and contains 29–34 wt% UO2. In terms of end-members, the betafite contains approximately 50 mol% CaUTi2O7 and is one of the closest known natural compositions to the pyrochlore phase proposed for use in titanate nuclear waste forms. Amorphization and volume expansion of the betafite caused cracks to form in the enclosing silicate mineral grains. Backscattered electron images reveal that betafite was subsequently altered along crystal rims, particularly near the cracks. Electron probe microanalyses reveal little difference in composition between altered and unaltered areas, except for lower totals, suggesting that alteration is primarily due to hydration. Zirconolite contains up to 18 wt% ThO2 and 24 wt% UO2, and exhibits strong compositional zoning, but no internal cracking due to differential (and anisotropic) volume expansion and no visible alteration. The available evidence demonstrates that both oxycalciobetafite and zirconolite retained actinides for approximately 40 million years after the final stage of vein formation. During this time, oxycalciobetafite and zirconolite accumulated a total alpha-decay dose of 3.0–3.6 × 1016 and 0.2–2.0 × 1016 α/mg, respectively.

Trapping of helium in nano-bubbles in euxenite: Positive identification and implications

The (Y,REE,U,Th)–(Nb,Ta,Ti) oxides, like euxenite, fergusonite, pyrochlore, zirconolite, are known to contain nanometric spherical cavities or bubbles, interpreted to contain radiogenic helium. In-situ analyses by Scanning Transmission Electron Microscopy (STEM) coupled with Electron Energy Loss Spectroscopy (EELS) inside nano-bubbles from an euxenite crystal, sampled in its host c. 920 Ma old pegmatite in Norway, deliver, for the first time, a positive identification of helium and an estimation of helium pressure in such bubbles. The chemically unaltered euxenite crystal proves amorphous and homogeneously speckled with bubbles ranging from 5 to 68 nm in diameter, around a log-normal distribution centered at 19 nm. The euxenite contains 9.87 wt% UO2 and 3.15 wt% ThO2. It accumulated a theoretical alpha-decay dose of 3.46×1020 α/g (i.e. 170 He/nm3), at a dose rate of 11926 α/g/s. This corresponds to production of 0.23 wt% He. The density of helium inside the bubbles, estimated from EELS data, ranges from 2 to 45 He/nm3, leading to a pressure of 8 to 500 MPa. The proportion of produced helium trapped in bubbles is about 10%. Helium bubbles clearly influence helium diffusion. They may contribute to the swelling of euxenite during amorphization and to the fracturing of the host rock. Our results suggest that dose, dose rate and structural state seem to be important parameters for the nucleation, growth and coalescence of helium bubbles but also demonstrate the crucial need of experimental studies to be able to develop a predictive model of the long term behavior of materials in response to helium irradiation. Furthermore, chemical alteration of euxenite, here materialized by fluid driven dissolution–precipitation towards silica bearing euxenite, removes the bubbles and mobilizes helium into the rock via cracks and grain boundaries. It is then suggested that helium-rich fluid released from such U–Th rich sources may percolate into surrounding rock units, inducing perturbation of the (U–Th)/He systematics of apatite and zircon in these units.

Evolution of spent nuclear fuel in dry storage conditions for millennia and beyond

Significant amounts of spent uranium dioxide nuclear fuel are accumulating worldwide from decades of commercial nuclear power production. While such spent fuel is intended to be reprocessed or disposed in geologic repositories, out-of-reactor radiation damage from alpha decay can be detrimental to its structural stability. Here we report on an experimental study in which radiation damage in plutonium dioxide, uranium dioxide samples doped with short-lived alpha-emitters and urano-thorianite minerals have been characterized by XRD, transmission electron microscopy, thermal desorption spectrometry and hardness measurements to assess the long-term stability of spent nuclear fuel to substantial alpha-decay doses. Defect accumulation is predicted to result in swelling of the atomic structure and decrease in fracture toughness; whereas, the accumulation of helium will produce bubbles that result in much larger gaseous-induced swelling that substantially increases the stresses in the constrained spent fuel. Based on these results, the radiation-ageing of highly-aged spent nuclear fuel over more than 10,000years is predicted.

Specific outcomes of the research on the radiation stability of the French nuclear glass towards alpha decay accumulation

This paper presents an overview of the main results of the French research on the long-term behavior of SON68 nuclear glass towards alpha decay accumulation. The effect of the radiation damage induced by alpha decay and also helium build-up were investigated by examining glass specimens, doped with a short-lived actinide 244Cm, irradiated by light and heavy ions. Additionally, atomistic simulations by molecular dynamics have provided further information on the atomic-scale effects of the macroscopic phenomena observed.These studies have shown that some macroscopic properties vary with the accumulation of alpha decay, but then stabilize after integrated doses of the order of 4×1018αg−1. For example, the glass density diminishes by about 0.6%, its Young’s modulus by about 15%, and its hardness by about 30%, while its fracture toughness increases by around 50%. The SEM and TEM characterization showed that the glass is still homogeneous. No phase separation, crystallization or bubbles formation was noticed up to an alpha decay dose corresponding to several thousand years of disposal of nuclear glass canister. Moreover the initial alteration rate of the glass is not significantly affected by the glass damage induced by alpha decays or heavy ions irradiations.The comparison of the macroscopic evolutions of the Cm doped glass with those obtained for glasses irradiated with light or heavy ions (from either experimental and molecular dynamic studies) suggests that the macroscopic evolutions are induced by the nuclear interactions induced by the recoil nuclei of alpha decay.The analysis of the behavior of the glass structure subjected to ballistic effects with various spectroscopic studies, together with the results of atomistic modeling by molecular dynamics, have identified some slight changes in the local order around some cations. Moreover a modification of the medium-range order has also been demonstrated through changes in the bond angles between network formers and broadening of the ring size distributions, indicating increasing disorder of the glass structure. This structural evolution induced by alpha decays would be driven by the reconstruction of the glass disorganized by displacement cascades of the recoil nuclei, freezing a glass structure with a higher fictive temperature. This “ballistic disordering (BD) fast quenching” event induces a new glassy state characterized by a higher enthalpy state. Accumulation of α decays induce similar phenomena of “BD-fast quenching”, increasing the fraction of the sample volume characterized by a “high enthalpy state”. At dose around 4×1018αg−1 the entire sample volume has been affected by “BD-fast quenching” events at least once, which explain the stabilization of the evolutions of glass structure and properties.Helium behavior was also studied by measuring the helium solubility constants and diffusion coefficients. Helium atoms are incorporated into the glass free volume with a solubility constant that varies less than 10% around a value of about 1011atcm−3Pa−1 and a density of solubility sites accessible for helium around 2×1021 sitescm−3 which is larger than helium production in a glass package. Helium diffusion experiments performed on infused and Cm doped SON68 glasses indicate that helium migration is controlled by a classical thermally activated diffusion process, whose activation energy (e.g. 0.6±0.03eV) is not affected by an alpha decay dose of around 1019αg−1.Helium implantation studies suggest that helium trapping could exist in nanometer size bubbles. SEM and TEM analysis performed on a Cm doped glass damaged by an alpha decay dose of around 1019αg−1, showed a homogeneous glass without crystallization, phase separation or bubbles with a spatial resolution limit of 10nm. Bubbles of significant size seem very unlikely to form at room temperatures. But, the ability to form helium bubbles of nanometer size, at temperature below the glass vitreous transition temperature cannot be excluded. However, all these studies agree on one point, the absence of macroscopic consequence on the glass integrity of accumulation of high helium concentration in the glass.

Chemistry, microstructure, and alpha decay damage of natural brannerite

To investigate the long-term stability of brannerite with respect to alpha decay damage and interaction with aqueous fluids, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually located within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. Based on a total of 3 metal cations, the Ti and U contents of unaltered brannerite range from 1.8 to 2.1 and 0.4 to 0.9 atoms per formula unit (apfu), respectively. Maximum amounts of the other major cations on the A-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. Maximum values of other cations on the B-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from an aqueous fluid phase, and up to 40–90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO 2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nm, indicating that brannerite is rendered completely amorphous by alpha decay damage. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have accumulated alpha decay doses in the range of 0.7–200 × 10 16 α mg −1. Although imprecise, the U–Th–Pb chemical ages determined by microanalysis are generally consistent with the known ages and geological histories of the brannerite host rocks. To a first approximation, it is possible to understand the results of chemical and isotopic dating of brannerite by treating each sample as a complex system composed of nominally unaltered, altered, and recrystallised areas. The dominant effects are U loss from altered areas and Pb loss from unaltered brannerite and, to a lesser extent, altered brannerite, the structure of which approximates to a Ti–Si–O glass-like network wherein Pb is more comfortable as a network modifier.

Characterisation of 20 Year Old Pu238 -Doped Synroc C

AbstractIn 1987 samples of Pu238 and Cm244 doped Synroc C were prepared in the UKAEA Laboratories at Harwell. They were studied for five years before being archived. During decommissioning of the Harwell laboratories the samples were transferred to Sellafield and the opportunity was taken to conduct further studies. Given the age of the samples, they offer a unique insight into the long term radiation stability of Synroc. To date, the three Pu238 samples have been examined. The alpha decay dose experienced by the samples is estimated to be 3 × 1019 alphas per gram.The sample allowed to accumulate alpha decay damage (10587) was slightly heterogeneous, with an apparent grain size in the range 3–15νm. EDX analysis confirmed the phases present to be those expected in Synroc C and that the Pu had partitioned predominantly into the perovskite and zirconolite. Microcracking was observed in the hollandite and rutile but cracks arrested once they reached a zirconolite or perovskite grain.Sample 10588 was annealed after five years, at temperatures up to 1200°C, this sample was microstructurally similar to 10587 at the 20-50νm scale but differs at small scales. A major difference is the presence of both intergranular and intragranular porosity.Sample 10589 was annealed at the same time as 10588 but differed significantly probably due to actual fabrication temperatures being higher than those recorded.Information on these samples will be presented, along with a discussion of the implications for the expected long term stability of titanate ceramic wasteforms.

Effect of alpha radiation on the leaching behaviour of nuclear glass

The behaviour of nuclear glass subjected to the stress loading expected in a repository must be investigated to demonstrate that it is capable of durably confining radioactive elements. Because of the minor actinides (Np,Am,Cm) contained in the glass, alpha radiation is one of the stresses that could affect the glass properties. This study focuses on the effect of alpha radiation on the chemical reactivity of R7T7 glass with pure water. Various glass samples doped with 237Np, 238Pu, 239Pu, 241Am or 244Cm were fabricated in the CEA facilities at Marcoule. The content of each actinide was adjusted to cover a range of alpha activity varying from 105 to 1011Bqg−1 and a range of alpha decay doses up to 4×1018g−1. Inactive glass samples were also subjected to multiple-energy external irradiation by heavy ions to simulate the impact of alpha damage. The initial glass alteration rates, which reflect the chemical reactivity between the glass matrix and pure water, were determined by standard Soxhlet tests together with analysis of the leaching solution by ICP-AES. Comparing the glass leaching rates is indicative of the impact of alpha radiation. The experimental data show that neither the alpha activity nor the alpha decay dose has a significant impact on the initial alteration rate of R7T7 glass.

Irradiation stability of R7T7-type borosilicate glass

High-level nuclear waste containment glass is subjected to irradiation-induced stresses whose consequences must be assessed to guarantee the material behavior over time. Alpha decay from the minor actinides confined in the glass structure is responsible for most of the atom displacements. Minor actinide concentrations in the glass exceeding current levels are under consideration for future vitrification scenarios. Four R7T7-type borosilicate glass samples doped with 0.04, 0.4, 1.2 and 3.25 wt% 244CmO 2 were fabricated to study not only the cumulative effect of high alpha decay doses (>10 19 α/g) but also of the dose rate. The homogeneity of the materials and their chemical compositions were characterized to ensure they were suitably representative for the experimental program. The macroscopic behavior of the glasses was characterized for doses up to 3 × 10 18 α/g. No significant effect on the initial alteration rate was detected which means that the chemical reactivity of the glass with pure water is not affected by such levels of alpha doses. The glass swelled slightly depending on the dose, saturating at about 0.5% after receiving a dose of about 2 × 10 18 alpha disintegrations per gram of glass. The mechanical properties were observed to improve appreciably, with lower hardness but greater fracture toughness. A stabilization phenomenon comparable to that of the glass density was also observed. Comparing the experimental data with other findings obtained by atomistic modeling and external irradiation suggests that nuclear interactions caused by alpha decay recoil nuclei could be responsible for the experimental variations observed. The possible origins of the observed variations in macroscopic properties are also discussed.

Determination of the uranium valence state in the brannerite structure using EELS, XPS, and EDX

In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO2, CaUO4, SrCa2UO6, UTi2O6, and Y0.5U0.5Ti2O6) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M5/(M4 +M5), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimise the effects of electron beam reduction of U6+ and U5+. Test samples consisted of three additional synthetic brannerites (Th0.7U0.3Ti2O6, Ca0.2U0.8Ti2O6, and Th0.55U0.3Ca0.15Ti2O6) and three natural brannerites from different localities. The natural brannerite samples are all completely amorphous, due to cumulative alpha decay events over geological time periods (24–508 Ma). Our U valence calibration results are in reasonable agreement with previous work, suggesting possibly a non-linear relationship between the branching ratio and the number of f electrons (and hence the average valence state) of U in solids. We found excellent agreement between the nominal valence states of U and the average valence states determined directly by EELS and estimated by EDX analysis (with assumptions regarding stoichiometry) in two of the three synthetic brannerite test samples. The average U oxidation states of the five synthetic brannerite samples, as derived from XPS analyses, are also in good agreement with those determined by other techniques. The average valence states of U in three amorphous (metamict) natural brannerite samples with alpha decay doses ranging from 3.6×1016 to 6.9×1017 α/mg were found to be 4.4, 4.7, and 4.8, consistent with the presence of U5+ and/or U6+ as well as U4+ in these samples. These results are in general agreement with previous wet chemical analyses of natural brannerite. However, the average valence states inferred by SEM-EDX for two of the natural brannerite samples do not show satisfactory agreement with the EELS determined valence. This may be due to the occurrence of OH− groups, cation vacancies, anion vacancies, or excess oxygen in the radiation-damaged structure of natural brannerite.

Plutonium-238 Alpha-Decay Damage Study of A Glass-Bonded Sodalite Ceramic Waste Form

Abstract An accelerated alpha-decay damage study of a glass-bonded sodalite ceramic waste form has been completed recently. The study was designed to investigate the physical and chemical durability of the waste form after exposure to 238Pu alpha decay. The alpha-decay dose at the end of the four year study was approximately 1.0 × 1018 decays/gram of material. The ceramic waste form (CWF) was developed to immobilize fission products accumulated during the treatment of spent nuclear fuel from the Experimental Breeder Reactor II performed at Argonne National Laboratory-West in Idaho. Small quantities of actinide elements are also found in the waste form. The CWF is currently undergoing qualification testing and characterization for acceptance by the Office of Civilian Radioactive Waste Management for geologic disposal. Methods used to monitor the 238Pu-loaded CWF material in this study included: immersion density determination to measure possible macroscopic swelling, chemical durability by leach testing, microstructural analysis by scanning and transmission electron microscopy, and phase composition and stability by powder X-ray and electron diffraction. After four years of exposure to alpha decay, the investigation observed little alteration to the CWF. Specifically, the 238Pu-loaded CWF maintained its physical integrity, the density remained constant, no cracking or phase debonding was observed by microscopy, and the material's chemical durability did not change significantly over the duration of the study. The only significant measured change was an increase of the unit-cell lattice parameters of the plutonium oxide and sodalite phases of the material, but these were very small and did not lead to any loss of waste form durability.

Effect of Alpha Decay on Nuclear Borosilicate Glass Properties

AbstractThe properties of actinide glasses are studied in the context of high-level waste management programs. Reprocessing high burnup fuels in particular will increase the minor actinide content in the glass package, resulting in higher cumulative alpha decay doses in the glass, and raising the question of the glass matrix behavior and especially its containment properties. The effect of alpha self-irradiation on the glass behavior is evaluated by doping the glass with a short-lived actinide (244Cm) to reach in several years the alpha dose received by the future glass packages over several thousand years. “R7T7” borosilicate glasses were doped with 3 different curium contents (0.04 wt%, 0.4 wt% and 1.2 wt% 244CmO2). The glass homogeneity was characterized by SEM and XRD. Chemical analysis after dissolution of the glass confirmed the chemical composition. The curium content and its distribution in the glass specimens were checked by calorimetry measurements and gamma spectrometry on 243Cm and 245Cm isotopes. The density and mechanical properties of the curium-doped glasses were characterized up to 2 × 1018 α/g, revealing only a slight evolution of the macroscopic behavior of R7T7 glass in this range. The leaching behavior of curium-doped glass was also studied by Soxhlet tests. The results do not show any significant evolution of the initial alteration rate with the alpha dose.

Alpha-Decay Radiation Damage Study of a Glass-Bonded Sodalite Ceramicwaste form

ABSTRACTEffects of alpha decay on the ceramic waste form used to immobilize fission products and actinides accumulated from electrometallurgical treatment of Department of Energy metallic spent nuclear fuel is being studied. This involved the preparation of a surrogate ceramic waste form containing two-weight percent plutonium-238 and monitoring changes to the waste form resulting from alpha-decay damage. The phase assembly of the ceramic waste form consists of glass-bonded sodalite with small quantities of nepheline, halite and plutonium dioxide. The waste form has been monitored for three years and has acquired an internal alpha-decay dose of 1 × 1018 disintegrations per gram of material. Methods used to monitor the material include: density determination, chemical durability measured by immersion testing, microscopy and powder xray diffraction. The investigation has, to date, found little alteration to the waste form due to alpha-decay damage. X-ray diffraction analysis has detected a unit cell volume increase of 0.7 percent of the plutonium dioxide phase. Furthermore, bubbles and/or voids have been observed by transmission electron microscopy in the sodalite and glass phase. The origin of these defects is unclear and under investigation.