Comprehensive nuclear materials 5 22 minerals and natural analogues Comprehensive nuclear materials 5 22 minerals and natural analogues Comprehensive nuclear materials 5 22 minerals and natural analogues Comprehensive nuclear materials 5 22 minerals and natural analogues Comprehensive nuclear materials 5 22 minerals and natural analogues Comprehensive nuclear materials 5 22 minerals and natural analogues
5.22 Minerals and Natural Analogues G R Lumpkin Australian Nuclear Science and Technology Organisation, Kirrawee, NSW, Australia T Geisler-Wierwille Universitaăt Bonn, Bonn, Germany Crown Copyright ß 2012 Published by Elsevier Ltd All rights reserved 5.22.1 5.22.2 5.22.3 5.22.3.1 5.22.3.2 5.22.3.3 5.22.3.4 5.22.3.5 5.22.3.6 5.22.3.7 5.22.3.8 5.22.4 5.22.4.1 5.22.4.2 5.22.4.3 5.22.4.4 5.22.5 5.22.5.1 5.22.5.2 5.22.5.3 5.22.5.4 5.22.6 5.22.6.1 5.22.6.2 5.22.6.3 5.22.6.4 5.22.7 References Appendix: Introduction a-Decay Damage in Minerals Oxides Pyrochlore Group Zirconolite Brannerite Perovskite Baddeleyite Crichtonite ABO4 and AB2O6 Minerals (B ¼ Nb, Ta, and Ti) Hollandite Silicates Zircon Thorite Titanite (Sphene) Allanite Phosphates Monazite Apatite Group Kosnarite and Related NZP Materials Xenotime Ore Deposits: Analogs for Spent Fuel Preamble General Aspects of Uraninite Alteration in Natural Systems Natural Fission Reactors in Gabon Uranium Migration in the Koongarra Ore Deposit Conclusions List of Mineral Names and Compositions Oxides Silicates Phosphates, Arsenates, Vanadates Carbonates, Fluorocarbonates, Fluorides Sulfides Uranyl Minerals (Section 5.22.6) Abbreviations apfu AES AFM Atoms per formula unit Auger electron spectroscopy Atomic force microscopy BSE DTA 564 565 565 565 570 574 575 578 578 579 580 581 581 585 585 586 587 587 588 590 590 591 591 591 592 593 594 595 599 599 599 599 599 600 600 Backscattered electron, an imaging mode in SEM, sensitive to mean atomic number Differential thermal analysis 563 564 Minerals and Natural Analogues EDX EELS EPMA EXAFS HLW ICP-OES IMF IR Ln M–M M–O M–O–M MOX NZP RDF SEM Synroc Synroc-C Synroc-F TEM TOF-SIMS Urf6 vpfu XANES XPS XRD Energy dispersive X-ray analysis Electron energy loss spectroscopy Electron probe microanalysis, quantitative X-ray analysis with crystal spectrometers Extended X-ray absorption fine structure High level waste Inductively coupled plasma-optical emission spectrometry Inert matrix fuel Infrared, a type of vibrational spectroscopy complementary to Raman spectroscopy Lanthanide series elements, La–Lu Metal–metal distance in a crystal or amorphous material Metal–oxygen distance in a crystal or amorphous material Metal–oxygen–metal angle Mixed oxide fuel, composed of uranium and plutonium oxide Sodium zirconium phosphate Radial distribution function, a way of describing M–O and M–M distances Scanning electron microscopy Synthetic rock Hollandite + perovskite + rutile + zirconolite-based material for HLW Pyrochlore + uraninite ceramic for partially reprocessed fuel Transmission electron microscopy Time-of-flight-secondary ion mass spectrometry A uranyl group with six equatorial oxygen atoms Vacancies per formula unit X-ray absorption near edge structure X-ray photoelectron spectroscopy X-ray diffraction 5.22.1 Introduction The immobilization and long-term disposal of nuclear waste is one of the greatest challenges that modern society faces today Various types of high level waste (HLW) have been generated from nuclear operations around the world; for example, spent fuel from commercial nuclear power stations, liquid waste from the reprocessing of spent fuel, and waste from the production of nuclear weapons and weapons grade plutonium resulting from nuclear disarmament treaties between the United States and Russia Some plutonium, particularly in France, has been used in mixed oxide fuel (MOX, composed of uranium and plutonium oxide) in place of the standard uranium oxide fuel Previous US policy adopted the strategy of a once-through fuel cycle followed by direct disposal of the spent fuel; however, recent changes have seen a move toward more effective use of uranium-based fuels in programs that combine reprocessing, transmutation, and separations technology in advanced fuel cycles (e.g., Generation IV nuclear power systems) Borosilicate glass is the currently accepted waste form of choice for many countries that reprocess their commercial spent fuel (see Chapter 5.18, Waste Glass), but there exists a significant fraction of ‘legacy’ waste and other nuclear materials that are very complex in physical form and chemical composition (e.g., the Na-, Al-, and Zr-rich waste stored in tanks at sites in the United States) These complex waste materials, together with impure plutonium, and the separated fission products and actinides generated from the various partitioning strategies may be better suited for existing or new types of high-performance crystalline waste forms or glass-ceramics (see Chapter 5.19, Ceramic Waste Forms) Some of these materials, for example, inert matrix fuels (IMFs), are being designed for recycling of reactor-grade plutonium and minor actinides in commercial power stations, followed by geological disposal, an attractive option that does not generate new plutonium.1 As envisaged by G.J McCarthy, A.E Ringwood, and others in the 1970s, there exist alternative crystalline waste forms that may be capable of providing a much higher level of chemical durability than borosilicate glass or directly disposed spent fuel Many of these materials have been extensively developed over the previous 20–25 years, while others are relatively new Materials such as tailored ceramics,2 the synthetic rock (Synroc) polyphase titanate waste forms,3,4 and related special purpose waste forms are reasonably well developed and have been the subject of extensive leach testing and radiation damage studies Pyrochlore is the major component of Synroc-F, a polyphase ceramic designed for partially reprocessed nuclear fuel5 and later appeared as the principal host phase for excess weapons Pu and U in a crystalline titanate ceramic form Zirconolite has also been proposed as an ideal host phase for actinides due to a combination of crystal chemical flexibility and very high durability in aqueous fluids,6 and hollandite Minerals and Natural Analogues may provide an excellent host material for separated long-lived radioactive Cs for similar reasons.7 Additional special purpose waste forms for actinides include zircon,8 monazite,9 and zirconium-based materials having the fluorite, defect fluorite, or pyrochlore structures.10,11 Except for zircon, none of these materials has been studied to the same extent as the titanate waste forms Nevertheless, monazite and Zr-based materials are promising in view of their resistance to amorphization and excellent chemical durability Nuclear waste form materials must meet several requirements in order to reach final consideration for use in a repository, including a high level of durability in aqueous fluids, crystal chemical flexibility allowing the material to cope with variations in the composition of the waste stream, reasonably high waste loadings, volume reduction, and reliable and cost-effective processing technologies Information derived from minerals can be used to assess all but the latter criterion Furthermore, studies of U ore deposits are useful in the assessment of the performance of spent fuel, including transport of U away from the repository The purpose of this chapter is to summarize the performance of minerals in terms of their response to a-decay damage and their interactions with natural aqueous fluids in geological environments For comparison, we also discuss some of the relevant literature results involving accelerated radiation damage of synthetic compounds doped with short-lived actinides, controlled laboratory experiments on the dissolution of synthetic materials with and without short-lived actinides, and dissolution of radiation-damaged natural samples Following a brief introduction to uraninite and its alteration products in natural systems, we conclude with an overview that uranium ore deposits as analogs for spent fuel under repository conditions, including aspects of the natural fission reactors in Gabon and uranium migration around the Koongarra ore body, Northern Territory, Australia 5.22.2 a-Decay Damage in Minerals In this section, we briefly summarize the effects of a-decay damage on the structures of some of the more important natural analogs Here, it is important to point out that a-decay involves two concurrent processes, the release of a high energy ($4–5 MeV) a-particle together with a low energy ($70–100 keV) recoil atom The process can be expressed in the following general way: A nỵ ZP n2ịỵ !A4 ỵ2 He2ỵ Z2 R ẵI 565 In this expression, P is the parent isotope, R is the recoiling daughter isotope, and the charged He atom is the emitted a-particle The symbols A, Z, and n represent the mass, atomic number, and nuclear charge, respectively The massive recoil nucleus has a range of 20–25 nm and typically displaces on the order of 1000 atoms primarily by nuclear stopping processes; whereas, the a-particle has a range of about 10–15 mm and loses most of its energy through electronic interactions before displacing on the order of 100 atoms near the end of its track The valence states of the a-particle and recoil atom are rapidly reconfigured in the solid to produce 4He and to return the recoil atom to a more stable state This charge reconfiguration process may be complex in the case of U that can exist as the U4+, U5+, and U6+ ions in solids or in general if there are other elements present with variable valence states such as the transition metals (see Section 5.22.4.3) The important parent isotopes in minerals are 238 U, 235U, and 232Th These isotopes decay to the stable isotopes 206Pb, 207Pb, and 208Pb through their respective decay series Based on each decay series, the total a-decay dose D can be calculated using the following equation: D ¼ 8N238 ðel238 t À 1ị ỵ 7N235 el235 t 1ị ỵ 6N232 el232 t 1ị ẵ1 In this equation, t is the geological age, N represents the present-day concentration of the parent isotope, and l is the decay constant This equation is strictly applicable to samples wherein the isotopic composition of the U has been determined In situations where only the Th and U elemental concentrations have been determined, one may assume that the U isotopic composition consists of 99.28% 238U and 0.72% 235U Alternatively, the second term in the equation may be ignored without major consequence, as the associated error is usually smaller than the uncertainty in the geological age In this chapter, we give all dose values in units of 1016 a-decays per milligram (this is because  1016 a-decays per milligram is approximately equivalent to one displacement per atom in minerals) 5.22.3 Oxides 5.22.3.1 Pyrochlore Group Pyrochlore is an anion-deficient derivative of the fluorite structure type with a doubled a cell 566 Minerals and Natural Analogues parameter and change in space group from Fm3¯ m to Fd3¯ m.12–14 Minerals of the pyrochlore group conform to the general formula A2ÀmB2X6ÀwY1ÀnÁpH2O, where A represents cations in eightfold coordination, B represents cations in sixfold coordination, and X and Y are anion sites The basic structural element of pyrochlore is the framework of corner-sharing octahedra Within this framework, continuous tunnels exist parallel to the h110i directions Both the A-site cations and Y-site anions are located in these tunnels In synthetic systems, some A-site cation exchange capacity has been demonstrated in defect pyrochlores, in which the values of m in the general formula can be quite large Most natural pyrochlores form under magmatic conditions in granitic pegmatites, nepheline syenite pegmatites, and carbonatites, or late-stage veins associated with these rock types The composition of common pyrochlore usually approaches the stoichiometric form (Na, Ca, Ln, U)2(Nb, Ta, Ti)2O6(F, OH, O), but the structure type is extremely flexible in terms of the sheer number of elements that can be incorporated and is particularly amenable to the incorporation of actinides Natural samples are known to contain up to 30 wt% UO2, wt% ThO2, and 16 wt% Ln2O3, an important consideration for the issue of nuclear criticality However, as shown by the general formula, the crystal chemistry of pyrochlore is complicated by the potential for vacancies at the A-, X-, and Y-sites (m ¼ 0.0–1.7, w ¼ 0.0–0.7, and n ¼ 0.0–1.0) and the incorporation of water molecules (p ¼ 0–2) in the vacant tunnel sites The total water content of the natural defect pyrochlores may be as high as 10–15 wt% H2O (with speciation as both water molecules and OH groups) In a little known but classic paper, Krivokoneva and Sidorenko15 examined a suite of Russian pyrochlores using X-ray diffraction (XRD) methods An analysis of the line broadening showed that strain increased from 0.0009 to 0.0035 as crystallite dimensions decreased from 100–120 nm down to 35–40 nm in the initial stages of damage, a decrease to 15 nm was observed in the latter stages of damage These authors also carried out an analysis of the radial distribution function (RDF) of an amorphous sample and showed that there was no long-range order present beyond the second coordination sphere However, peaks in the RDF representing the major M–O and M–M distances showed that the fundamental structural units (e.g., the coordination polyhedra) still existed in the amorphous state Lumpkin and Ewing16 also used XRD to determine both the beginning (Di) of the crystalline–amorphous transformation and the critical amorphization dose (Dc) for a large suite of pyrochlores from different localities They showed that the transformation zone increased in dose as a function of the geological age of the samples Both dose curves are well-described by an equation of the form: Di;c ẳ D0 etK ẵ2 In this expression, D0 is the intercept dose for Di or Dc and K is a rate constant Analysis of the dose-age data gives a value of D0 ¼ 1.4  1016 a per milligram for the amorphization dose curve and K ¼ 1.7  10À9 yearÀ1.17 The Bragg peak intensities of a subset of these samples were fitted to an equation of the form: I =I0 ẳ eBD ẵ3 Here, I/I0 represents the total intensity of all observable Bragg peaks divided by the total intensity obtained from an undamaged sample of similar composition and B is a constant related to the amount of material damaged by each a-decay event Equation [3] gives an excellent fit to the data with B ¼ 2.6  10À16 mg per a-particle, corresponding to an average cascade radius of 2.3 nm in which a maximum of 2600 atoms are displaced An analysis of line broadening in these samples showed that crystallite dimensions decreased from about 500 to 15 nm with increasing dose Strain initially increased with dose and reached a maximum of $0.003 before falling to values below 0.0005 at higher dose levels, consistent with a description of the crystalline–amorphous transformation as a type of ‘percolation’ transition.18 With increasing a-decay dose, transmission electron microscopy (TEM) images reveal mottled image contrast due to strain, followed by the appearance of local amorphous domains that increase in volume and begin to overlap to produce larger amorphous areas until they are connected throughout the material This is the first percolation transition With further increases in dose, the crystalline areas diminish in volume until they become isolated, giving way to a microstructure dominated by amorphous pyrochlore.16 This is the second percolation transition During the 1980s, Greegor and coworkers19–22 carried out several studies of the local structure and bonding around Ti, Nb, Ta, and U atoms in pyrochlore using EXAFS–XANES (extended X-ray absorption fine structure–X-ray absorption near edge structure) Results of these studies demonstrated that the M–O coordination polyhedra of amorphous pyrochlore exhibit reduced bond Minerals and Natural Analogues Zircon Apatite Pyrochlore Zirconolite 14 12 DV/ V0 (%) distances, reduced coordination number, and increased distortion relative to the undamaged crystalline structure Furthermore, there was no periodicity in evidence beyond the second coordination sphere, with some disruption of the M–M distances From these studies, it was realized that only a slight increase in the mean M–M distance was required in order to explain the overall increase in volume caused by a-decay damage and that this could be facilitated by increased M–O–M angles The thermal behavior of radiation-damaged natural pyrochlore was investigated using differential thermal analysis (DTA) and XRD.23,24 Results of this study indicated that the samples recrystallized in the range of 400–700 C, depending upon the composition and degree of crystallinity Measured values of the recrystallization energy are 125–200 J gÀ1 and are inversely correlated with the level of crystallinity In the early to mid-1980s, Clinard and his colleagues conducted an extensive set of experiments on ‘cubic zirconolite’ CaPuTi2O7 in which the Zr is completely replaced by 238Pu (t1/2 ¼ 87.7 years) This material is actually a pyrochlore compound similar to synthetic CaUTi2O7 In their first publication, Clinard et al.25 reported that CaPuTi2O7 has a total volume expansion of 4.7%, an ‘apparent’ lattice volume expansion of 2.2%, and a critical amorphization dose of 0.3  1016 a per milligram based on XRD analysis Further analysis of the data showed that CaPuTi2O7 exhibits a bulk volume expansion of 5.4% at ambient temperature and becomes amorphous at a dose of 0.5  1016 a per milligram based on bulk swelling curves For samples held at 302 C, the bulk swelling saturates at 4.3%, and the material becomes amorphous at a dose of  1016 a per milligram as estimated from the swelling data This is a very significant result, as it indicates that the critical dose for an experiment lasting $3 years at 302 C is roughly equivalent to what nature produces in 107–109 years When stored at a temperature of 602 C, CaPuTi2O7 did not become amorphous; however, the material showed a bulk expansion of 0.4% consistent with accumulation of lattice point defects.26 In retrospect, this is a stunning result and represents the first and only realistic ‘bracket’ for the critical temperature for amorphization of a nuclear waste form material Also during the 1980s, Weber et al.27 investigated synthetic Gd2Ti2O7 doped with wt% 244 Cm (t1/2 ¼ 18.1 years) and determined the amorphization dose of $0.4  1016 a per milligram with B ¼ 4.4  10À16 mg per a-particle, a total volume expansion of $5.1% at saturation (Figure 1), and 567 10 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Dose (1016 a per milligram) Figure Plot showing total volume expansion of synthetic pyrochlore Gd2Ti2O7 doped with 244Cm, zirconolite CaZrTi2O7 doped with 244Cm, apatite (e.g., britholite) CaNd4(SiO4)3O doped with 244Cm, and zircon (ZrSiO4 doped with 238Pu) as a function of increasing a-decay dose an increase in fracture toughness together with a decrease in hardness and elastic modulus Changes in microstructure with increasing dose mimic the results for natural pyrochlores described earlier Based on the results of DTA experiments, an activation energy of Ea ¼ 3.8 eV was determined for recrystallization of this pyrochlore A recrystallization temperature of 700–800 C was determined by isochronal annealing The authors also performed leach tests on single-phase Cm-doped Gd2Ti2O7 pyrochlore samples In this work, the leach tests were limited to annealed, fully crystalline and fully amorphous samples, and were exercised at 90 C in pure water for 14 days The experiments revealed weight losses of 0.02% and 0.05% for the crystalline and amorphous pyrochlore samples, respectively The results of this study also indicated that the leach rate of Cm increased by a factor of 17 as a consequence of amorphization More recently, Strachan and coworkers28 investigated the effect of 238Pu on the structure of four synthetic pyrochlore samples with variable amounts of Al, Gd, Hf, and U The results of detailed XRD and bulk swelling measurements indicate that the critical dose for amorphization is $(0.2–0.4)  1016 a per milligram and is associated with a total volume expansion of