Nuclear Waste Host Research Articles

Reliability of petrogenetic information obtained from the distribution of trace elements in zircon in light of the complex infusion by foreign-element-bearing low temperature solutions

The increasing application of minor elements such as REE, Ti, Fe in zircon as petrogenetic indicators depends on zircon remaining a closed chemical system over a history that can include metamorphic events, erosion, transport, and sedimentation. Whereas examples have been described of UPb loss and element exchange during metamorphic reactions, for over 50 years there have been reports of the accumulation of a range of non-formula elements such as Ca, Al, Fe, in zircons with no metamorphic history that have been tentatively related to hydrothermal action or some other process such as weathering. This open system behaviour of zircons needs further explanation and in this contribution we present data from three zircons from a late Archaean granite from Western Australia that confirm previous observations on the distribution of non-formula elements in radiation damaged zircon and support our earlier conclusion that elements are added from percolating low temperature weathering solutions and provide new insights into the mechanisms and processes controlling access of weathering fluids into the zircon. These include radiation damage accumulation over time from the decay of U and Th to produce domains of porous amorphous material that vary in porosity and the presence of weaknesses along the boundaries of oscillatory zones which allow the penetration of low temperature groundwater fluids carrying solute elements. Also important is the size of particles of the solute elements such as Al and Ca, which depends on Eh - pH of the groundwater and porous domains within the zircon structure. This low temperature fluid interaction is distinguished from hydrothermal activity by a lack of any reaction fronts or annealing of the radiation damage and a lack of any modification of existing zircon structures. Capillary action is proposed as the mechanism driving the penetration of weathering solutions into radiation damage expansion fractures and the radiation damaged zonal structure. The susceptibility of radiation damaged zircon to infusion by low temperature fluids and deposition of accompanying solute elements has implications for the application of zircon trace element chemistry as a petrogenetic and temperature indicator as well as a geochronometer and host for nuclear waste.

Interaction of lanthanide and actinide with BaHfO3 perovskite: A case study with cerium and uranium

AbstractSpeciation of actinide (An) and lanthanide (Ln) in technologically important ceramics is very important from both fundamental as well as technological aspects. The intrinsic structural flexibility of perovskite containing AO6 and BO12 polyhedra makes them suitable and rich hosts for An and Ln. In this work, emphasis was given to deciphering information such as oxidation state, local dopant site, charge compensating defects, excited state kinetics, and so forth in BaHfO3 (BHO) related to dopant uranium (BHO‐U) and cerium (BHO‐Ce). Several spectroscopic techniques namely, time‐resolved photoluminescence (TRPL), positron annihilation lifetime spectra (PALS), and thermoluminescence (TL) spectroscopy coupled with density functional theory (DFT) were employed to probe the same. Ce and U though are distributed at both Ba and Hf sites, Ce prefers the former, while U prefers the latter site. Uranium on the other hand stabilizes as U6+ in the form of octahedral uranate ion giving green emission. PALS suggested the formation of defects in BHO‐Ce and BHO‐U with oxygen vacancies predominating in the former whereas BHO‐U perovskites are loaded with cation vacancies and vacancy clusters. These cation vacancies are responsible for lower TL output in BHO‐U. TL measurements also suggested cerium doping leads to a higher density of deeper traps in BHO‐Ce compared to uranium doping in BHO‐U which is in concurrence with DFT results and may have implications in designing afterglow phosphors based on perovskite. We believe this work would have a long‐term impact on exploring the potential of perovskite for nuclear waste host and afterglow phosphors applications.

Controllable sites and high-capacity immobilization of uranium in Nd2Zr2O7 pyrochlore.

As potential nuclear waste host matrices, two series of uranium-doped Nd2Zr2O7 nanoparticles were successfully synthesized using an optimized molten salt method in an air atmosphere. Our combined X-ray diffraction, Raman and X-ray absorption fine-structure (XAFS) spectroscopy studies reveal that uranium ions can precisely substitute the Nd site to form an Nd2-xUxZr2O7+δ (0 ≤ x ≤ 0.2) system and the Zr site to form an Nd2Zr2-yUyO7+δ (0 ≤ y ≤ 0.4) system without any impurity phase. With increasing U concentration, there is a phase transition from pyrochlore (Fd3m) to defect fluorite (Fm3m) structures in both series of U-doped Nd2Zr2O7. The XAFS analysis indicates that uranium exists in the form of high-valent U6+ in all samples. To balance the extra charge for substituting Nd3+ or Zr4+ by U6+, additional oxygen is introduced accompanied by a large structural distortion; however, the Nd2Zr1.6U0.4O7+δ sample with high U loading (20 mol%) still maintains a regular fluorite structure, indicating the good solubility of the Nd2Zr2O7 host for uranium. This study is, to the best of our knowledge, the first systematic study on U-incorporated Nd2Zr2O7 synthesized via the molten salt method and provides convincing evidence for the feasibility of accurately immobilizing U at specific sites.

An Integrated Imaging Study of the Pore Structure of the Cobourg Limestone—A Potential Nuclear Waste Host Rock in Canada

With the development of imaging technology, tools to quantitatively describe pore structure, morphology, and connectivity have been widely applied on low permeable rocks; however, it is still questionable to what extent this information can be used to predict permeability. Applicability and comparability of different techniques are discussed here for the Middle Ordovician Cobourg limestone (Canada), a rock dominated by calcite grains of variable sizes (µm–cm) and heterogeneously distributed quartz, dolomite, pyrite, and meshy clay minerals. Absolute porosities determined by helium pycnometry (HP) in literature are approximately 1.6% (±0.9%), and gas permeabilities range from 10−20 to 10−19 m2. Porosities obtained from BIB-SEM are much smaller compared to those from HP (16–69% of HP). Pores found in clays are smaller, slit-shaped, and more densely spaced when compared to those in calcite minerals. Connectivity between pores could not be resolved with 3D micro-CT or FIB-SEM reconstructions, which have a resolution limit of 8 µm and 10 nm, respectively. However, assuming the pores to be connected, laboratory-derived permeability data could be fitted using a simple capillary bundle model, including information about the visible pore size distributions obtained from BIB-SEM images and a tortuosity range of 8 to 15.

Structural assessment and irradiation response of La2Zr2O7 pyrochlore: Impact of irradiation temperature and ion fluence

State of the art applications (nuclear waste host, scintillators, piezoelectric, SOFCs, etc.) of isometric pyrochlore, A2B2O7, are very sensitive to the structural assessment upon ion irradiation. In the present study, La2Zr2O7 is irradiated using 1 MeV Xe4+ ions with fluence of 1 × 1013, 5 × 1013, and 1 × 1014 ions/cm2 at ~ 88 K and 300 K. The impact of irradiation temperature and ion fluence on the structural properties of the La2Zr2O7 are investigated using the GIXRD, Raman spectroscopy and high resolution transmission electron microscopy (HR-TEM). The GIXRD and Raman results indicate that the degradation of the crystallinity (i.e., damage/amorphization) are significantly higher with enhanced fluence at ~ 88 K than that of 300 K. The induced lattice strain also increases with an increase of ion fluence and it is more pronounced at ~88 K. The HR-TEM results of the La2Zr2O7 samples exhibit that degradation of the atomic ordering are more pronounced at ~ 88 K. The prominent induced strain and degradation of crystallinity (i.e., damage/amorphization) at ~ 88 K appear to be ion fluence and irradiation temperature-dependent. This study is enlightening the effect of the ion fluence and irradiation temperature on the degradation of crystallinity (i.e., damage/amorphization).

Luminescence Studies of Pyrochlore Metal Oxide Nanomaterials

Designing lanthanide doped luminescent materials especially nanomaterials with multifunctional applications is highly challenging and demanding. The chemistry and structure of the host materials for lanthanide ions are critical in the selection of known phosphors and in the discovery of new phosphor materials for being used for the applications. Materials with A2M2O7 pyrochlore composition recently have displayed a variety of advanced applications in solid oxide fuel cells, photocatalysis, thermographic phosphor, thermal barrier coating, X-ray scintillator, photoluminescence, and nuclear waste host, etc. In the past few years, we have focused on the studies of pyrochlore AIII 2MIV 2O7 nanoparticles (NPs, where A = trivalent heavy element ions of both lanthanides and actinides, and M = Zr4+, Hf4+, etc.) useful for solid-state lighting, X-ray scintillators, thermometry, and bioimaging. We have achieved substantial tunability of their particle size, crystal phase, and more importantly, luminescence properties. We have gained a clear understanding of the influences of synthesis conditions, particle morphology and composition on their photoluminescence and radioluminescence. The tunable luminescence properties of these lanthanide doped pyrochlore NPs indicate their great application potentials in solid-state lighting and multicolor luminescence devices.

Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Abstract A2B2O7 type pyrochlores have been recently proposed as a potential nuclear waste host due to their many interesting properties. To assess and understand the performance of these compounds as nuclear waste hosts, the speciation and structural investigations on actinide-doped RE2Hf2O7 are needed since both are imperative from their application perspective. In this work, we investigated the effect of uranium doping at different concentrations in the range of 0–10% on the structural and optical properties of RE2Hf2O7:U (RE = Y, Gd, Nd, and Lu) nanoparticles (NPs). The Y2Hf2O7 NPs exist in slightly disordered pyrochlore structure and the extent of disordering increases as a function of uranium doping while the structure reaches a cotunnite phase at 10.0% doping level. The Nd2Hf2O7 NPs also exist in a distorted pyrochlore structure and their distortion increases with increasing uranium doping inducing a phase transition into a disordered fluorite structure at 10.0% uranium doping. Both Gd2Hf2O7 and Lu2Hf2O7 NPs exist in a disordered fluorite structure and transforms into cotunnite structure at higher U concentrations (≥5.0%). Photoluminescence spectroscopy showed that uranium ions are stabilized in +6 oxidation state in all samples: in the form of uranate ion UO66− in the Y2Hf2O7, Nd2Hf2O7 and Lu2Hf2O7 NPs while in the form of uranyl ion UO22+ in the Gd2Hf2O7 NPs. Therefore, this work deepens the understanding of the behavior of uranium ions doped in different RE2Hf2O7 host matrices in the perspective of their application as nuclear waste hosts.

Investigating the impact of gamma radiation on structural and optical properties of Eu3+ doped rare-earth hafnate pyrochlore nanocrystals

A proposed method for treating nuclear high level radioactive (NHLW) is to encapsulate it in a matrix made of complex oxide materials, such as pyrochlores with the general formula A2B2O7. Rare-earth hafnate pyrochlores have the potential to advance the current methods of NHLW disposal due to their robust chemical stability in radioactive environments, their high thermal stability, and their natural structural compatibility with radionuclide species. In this study, RE2Hf2O7:Eu3+ (RE = Y, La, Pr, Gd, Er, Lu) nanoparticles synthesized by a molten salt method were exposed to highly energetic gamma-ray irradiation. La2Hf2O7:Eu3+ and Lu2Hf2O7:Eu3+ underwent the order pyrochlore-disorder fluorite structural phase transition after exposure to gamma-ray irradiation. There was a change in the O-Eu charge transfer band (CTB) position as a function of gamma-ray dose. There was no change in the local symmetry of the Eu3+ dopant in Y2Hf2O7, Gd2Hf2O7 and Lu2Hf2O7, but in La2Hf2O7:Eu3+ gamma-ray dose reduced the symmetry around the Eu3+. For Y2Hf2O7:Eu3+ and La2Hf2O7:Eu3+, the lifetime and emission intensity were found to degrade possibly due to creation of gamma ray-induced defects which provides non-radiative pathways. Regarding Gd2Hf2O7:Eu3+, the concentration of oxygen vacancy defects predominated over other defects leading to enhanced emission and lifetime after gamma-ray irradiation. This study is of utmost importance for the design of a robust rare earth hafnate pyrochlore to be used as a nuclear waste host or gamma-ray based scintillator material.

On structure and phase transformation of uranium doped La2Hf2O7 nanoparticles as an efficient nuclear waste host

Doping induced phase transformation and speciation of the uranium ions in La2Hf2O7 nanoparticles as nuclear waste host were investigated.

DFT + U as a reliable method for efficient ab initio calculations of nuclear materials

Density functional theory (DFT)-based ab initio methods become standard research tools in various research fields, including nuclear materials science. However, having strongly correlated f-electrons, lanthanide- and actinide-bearing nuclear materials are computationally challenging for DFT methods and straightforward DFT calculations of these materials can easily produce false results. In this contribution we benchmark the DFT + U method, with the Hubbard U parameter derived ab initio, for prediction of structural and thermochemical parameters of nuclear materials, including various actinide-bearing molecular complexes and lanthanide-bearing monazite- and xenotime-type prospective ceramic nuclear waste host forms. Our studies show that the applied DFT + U method improves significantly prediction of DFT by producing results with uncertainties similar to those of the higher order, but computationally unfeasible ab initio methods, and the experimental data, and thus allows for reliable and feasible ab initio computation of even chemically complex nuclear materials.

New insights on pressure, temperature, and chemical stability of CsAlSi5O12, a potential host for nuclear waste

A Cs-bearing polyphase aggregate with composition (in wt%): 76(1)CsAlSi5O12 + 7(1)CsAlSi2O6 + 17(1)amorphous, was obtained from a clinoptilolite-rich epiclastic rock after a beneficiation process of the starting material (aimed to increase the fraction of zeolite to 90 wt%), cation exchange and then thermal treatment. CsAlSi5O12 is an open-framework compound with CAS topology; CsAlSi2O6 is a pollucite-like material with ANA topology. The thermal stability of this polyphase material was investigated by in situ high-T X-ray powder diffraction, the combined P–T effects by a series of runs with a single-stage piston cylinder apparatus, and its chemical stability following the “availability test” (“AVA test”) protocol. A series of additional investigations were performed by WDS–electron microprobe analysis in order to describe the P–T-induced modification of the material texture, and to chemically characterize the starting material and the run products. The “AVA tests” of the polyphase aggregate show an extremely modest release of Cs+: 0.05 mg/g. In response to applied temperature and at room P, CsAlSi5O12 experiences an unquenchable and displacive Ama2-to-Amam phase transition at about 770 K, and the Amam polymorph is stable in its crystalline form up to 1600 K; a crystalline-to-amorphous phase transition occurs between 1600 and 1650 K. In response to the applied P = 0.5 GPa, the crystalline-to-amorphous transition of CsAlSi5O12 occurs between 1670 and 1770 K. This leads to a positive Clapeyron slope (i.e., dP/dT > 0) of the crystalline-to-amorphous transition. When the polyphase aggregate is subjected at P = 0.5 GPa and T > 1770 K, CsAlSi5O12 melts and only CsAlSi2O6 (pollucite-like; dominant) and Cs-rich glass (subordinate) are observed in the quenched sample. Based on its thermo-elastic behavior, P–T phase stability fields, and Cs+ retention capacity, CsAlSi5O12 is a possible candidate for use in the immobilization of radioactive isotopes of Cs, or as potential solid hosts for 137Cs γ-radiation source in sterilization applications. More in general, even the CsAlSi5O12-rich aggregate obtained by a clinoptilolite-rich epiclastic rock appears to be suitable for this type of utilizations.

Thermal-Driven Fluorite–Pyrochlore–Fluorite Phase Transitions of Gd2Zr2O7 Ceramics Probed in Large Range of Sintering Temperature

Fluorite (F)–pyrochlore (P)–fluorite (F) phase transitions of Gd2Zr2O7 were investigated from 573 K to 1873 K (300 °C to 1600 °C), by means of X-ray diffraction, infrared spectra (IR), and Raman spectra. The low-temperature F phase can stably exist below 1523 K (1250 °C) and the F–P transition occurs at 1523 K to 1573 K (1250 °C to 1300 °C). The ordering process of P phase forms at 1573 K to 1773 K (1300 °C to 1500 °C) and the ordering degree increases with increasing sintering temperature and heat preservation period. High-temperature phase transition from P to F occurs between 1773 K and 1823 K (1300 °C and 1550 °C). IR spectra of samples with different ordering degrees show an interesting shift at 510 cm−1. Raman spectra show that only the A1g mode displays a significant change between F and P phases. This ordering degree and phase transition temperature studies would allow a more targeted experimental optimization of Gd2Zr2O7 to use in nuclear waste host, thermal barrier coatings, and solid oxide fuel cells.

Response of strontium titanate to electron irradiation for the immobilization of strontium

The radiation tolerance is a critical concern in nuclear waste host materials due to the decay of constituent radioactive elements. In order to evaluate the effect of beta irradiation on ceramic waste forms, the bulk SrTiO3 ceramic was subjected to 1.8 MeV of electron irradiation and characterized by X-ray diffraction, Raman spectroscopy and field scanning electron microscope, respectively. The dose rate was compared between electron irradiation and decay of 90Sr. The results indicated that the as-prepared ceramic exhibits excellent resistance to electron radiation. The pristine crystalline form was retained after 1.8 MeV electron radiation. Also no noticeable change was observed in the measured elements distribution of the irradiated sample. The results indicated that the irradiation damage via beta decay is imperceptible in ceramic solids.

Immobilizationof LanthanideOxides Waste fromPyrochemical Process

Lanthanideradioactive wastes are generated during the LiCl-KCl waste salt purification process developed in KAERI.The phosphate-based ceramics were studied for the immobilization of the wastes. The immobilization matrix developed in this study(ZITceramic wasteform)composed of zinc titanate(Zn2TiO4), CaHPO4, SiO2and B2O3. The lanthanide oxides were reacted with CaHPO4during solid phase sintering. The reaction producs of lanthanides phosphate(LnPO4), which havea composition similar to monazite, has been developed as a high-level nuclear waste host. The physico-chemical properties (XRD, SEM, density and thermal conductivity) and leach resistance of the borosilicate glass and ZIT wasteforms are discussedin this paper.

On the crystal chemistry of londonite [(Cs,K,Rb)Al4Be5B11O28]: A single-crystal neutron diffraction study at 300 and 20 K

The crystal chemistry of londonite, a rare Cs-bearing mineral [ideal chemical formula: (Cs,K)Al 4 Be 4 (B,Be) 12 O 28 ; a = 7.3098(2) A at 300 K, space group P 43 m ] from the granitic pegmatites of the Antsongombato Gem Mine, Betafo district, Madagascar, has been reinvestigated by means of wavelength dispersive X-ray spectroscopy, laser ablation-inductively coupled plasma-mass spectroscopy, inductively coupled plasma-atomic emission spectroscopy, X-ray powder diffraction, and single-crystal neutron diffraction at 300 and 20 K. Single-crystal anisotropic structural refinement at 300 K gave a final agreement index R 1 = 0.0479 for 32 refined parameters and 416 unique reflections with F o > 4σ ( F o ). The analysis of the difference-Fourier maps of the nuclear density discounts the presence of hydroxyl groups, as wrongly reported in some of the previous studies. The structural refinements and the chemical analyses suggested that: the tetrahedral 4 e site (at x ~ 0.258) is mainly occupied by Be, but a low amount of B (~3%) likely occurs; the tetrahedral 12 h site (at x ~ 0.248) is mainly occupied by B, but a significant fraction of Be (~12%) is present; the octahedral 4 e site (at x ~ 0.361) is fully occupied by Al; and the 1 a site (at x = 0, with CN = 12) is mainly occupied by Cs, with Rb and K. The significantly high amount of B 2 O 3 (~50 wt%) and Cs+Rb(CsO 2 +RbO 2 ≥ 8 wt%) makes the synthetic counterpart of londonite of interest as a potential neutron absorber or a potential host for nuclear waste.

Concerned public and the paralysis of decision‐making: nuclear waste management policy in Germany

Efforts to site a high level nuclear waste repository in Germany date back to the 1960s. Ten years later the site Gorleben was officially selected and designated as a host for nuclear waste. However, public opposition and maneuvering by the major political actors prevented the completion of the site selection process, resulting in decades of political paralysis. The main reasons for this failure were the polarization in advocates and opponents of nuclear energy, the neglect for due process and participatory procedures, the inability to integrate technical, political, and social rationales in designing a viable nuclear waste policy, and the confusing mix of responsibilities between and among political actors. In spite of the apparent failure to find a solution for high level waste, the German government succeeded in designating a site for radioactive waste with negligible heat generation (Konrad mine). It is assumed that further progress in waste management can be accomplished only if more deliberative elements are introduced into the policy arena.

Elastic behavior and phase stability of pollucite, a potential host for nuclear waste

The elastic behavior and the phase stability of natural pollucite, (Cs, Na) 16 Al 16 Si 32 O 96 · n H 2 O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P -induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic ( Ia 3 d ) to triclinic symmetry ( P 1). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch-Murnaghan Equation-of-State (BM-EoS), we obtain V 0 = 2558.3(4) A 3 , K T0 = 41(2) GPa, and K T ′ = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V 0 = 2577.5(40) A 3 , K T0 = 25.1(9) GPa, and K T ′ = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are a 0 = 13.699(12) A, K T0 ( a ) = 25.5(17) GPa, and K T ′ ( a ) = 6.8(6) for the a axis; b 0 = 13.728(12) A, K T0 ( b ) = 23.2(15)GPa, and K T ′ ( b ) = 7.7(7) for the b axis; c 0 = 13.710(7) A, K T0 ( c ) = 25.2(10) GPa, and K T ′ ( c ) = 6.8(4) for the c axis [ K T0 ( a ): K T0 ( b ): K T0 ( c ) = 1.10:1:1.09]. Brillouin light-scattering was used to investigate the single-crystal elastic properties of pollucite at ambient conditions. The aggregate adiabatic bulk modulus ( K s ) and shear modulus ( G ), calculated using the Voigt-Reuss-Hill averaging procedures, are K s = 52.1(10) GPa and G = 31.5(6) GPa. The elastic response of pollucite and other isotypic materials (e.g., analcime, leucite, and wairakite) is compared. The high thermo-elastic stability of pollucite, reflected by the preservation of crystallinity at least up to 9 GPa (at room T ) and 1470 K (at room P ) in elastic regime, the large amount of Cs hosted in this material (Cs 2 O ~ 30 wt%), the immobility of Cs at high-temperature and high-pressure conditions, and the extremely low leaching rate of Cs, make of this open-framework silicate a functional material with potential use for fixation and deposition of Cs radioisotopes in high-level nuclear waste.

Stability at high-pressure, elastic behaviour and pressure-induced structural evolution of CsAlSi5O12, a potential host for nuclear waste

The elastic and structural behaviour of the synthetic zeolite CsAlSi5O12 (a = 16.753(4), b = 13.797(3) and c = 5.0235(17) Å, space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch–Murnaghan equation-of-state gives: V 0 = 1,155(4) Å3, K T0 = 20(1) GPa and K′ = 6.5(7). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a 3, b 3, c 3) for the volume. The refined axial-EoS parameters are: a 0 = 16.701(44) Å, K T0a = 14(2) GPa (βa = 0.024(3) GPa−1), K′ a = 6.2(8) for the a-axis; b 0 = 13.778(20) Å, K T0b = 21(3) GPa (βb = 0.016(2) GPa−1), K′ b = 10(2) for the b-axis; c 0 = 5.018(7) Å, K T0c = 33(3) GPa (βc = 0.010(1) GPa−1), K′ c = 3.2(8) for the c-axis (K T0a:K T0b:K T0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4), 1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi5O12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000°C in elastic regime and the extremely low leaching rate of Cs from CsAlSi5O12 allow to consider this open-framework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes.

Evidence for the stability of the potential nuclear waste host, sphene, over geological time, from uranium-lead ages and uranium-series measurements

A sphene-based, glass-ceramic material is being investigated as a potential host for Canadian nuclear fuel recycle waste. The ability of sphene to retain certain radionuclides over geological time is examined using observed properties and age measurements of natural sphenes. The long-term stability of sphene is determined by comparing U Th Pb isotopic age data for sphene, from the literature, with ages determined for zircon and other minerals. Better internal agreement is found between U Th Pb sphene ages than for zircon ages indicating the resistance of sphene to daughter radionuclide loss over periods as long as 3000 Ma. Excellent agreement was observed between sphene ages and the estimated age of crystallization of the host rocks or vein infillings, except where subsequent metamorphim had reset the sphene age. Complete lead loss was generally found to accompany high-grade metamorphic conditions when temperatures exceeded ∼ 500°C. Stability over shorter periods of geological time was found for vein infilling and mineral separates of sphene using uranium-series disequilibrium analysis. Conditions of secular equilibrium in the decay chain 238U- 234U- 230Th- 226Ra were observed for most of the 21 sphenes studied indicating that no radionuclide migration had occurred over periods as long as the last 1 Ma. Exceptions to this were four sphenes that had 234U deficiencies between 6 and 24% and two sphenes that showed 226Ra/ 230Rh disequilibrium. No correlation was found between the extent of radiation damage suffered by the sphenes and the presence of uranium-series disequilibrium. The presence of alteration products in three sphenes did correlate, however, with measured disequilibrium, indicating that some 234U and 226Ra had been lost or gained by these samples in the last 1 Ma and 8 ka, respectively.