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Browsing by Author "Begg, BD"

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Alternative Synroc formulations
(Cambridge University Press/Springer Nature, 2011-02-25) Vance, ER; Smith, KL; Thorogood, GJ; Begg, BD; Moricca, SA; Angel, PJ; Stewart, MWA; Ball, CJ
Perovskite is the least durable of the resistate minerals comprising Synroc-C and it is desirable to reduce its abundance in Synroc. Kinetic limitations and competition with Csapparently affect the incorporation of Sr into hollandite during hot-pressing at 1200°C/20 MPa so that ~ 10% of perovskite (a value below the percolation limit) is probably an optimum target. Zirconolite-rich Synroc formulations have been prepared for actinide-rich wastes. Background XRD and TEM studies have also been performed to study the crystal-chemical behaviour of Nd (a simulant of trivalent actinides) in zirconolite. Either rare-earth compensated perovskite or freudenbergite in Synroc can evidently be used to immobilise Na-bearing HLW. © 1992 Materials Research Society
Atomistic simulation of cation ordering and radiation damage in Sr1-3x/2LaxTiO3 defect perovskites
(Australian Institute of Physics, 2005-01-31) Thomas, BS; Marks, NA; Begg, BD; Corrales, LR; Devanathan, RL
Sr-1.3x/2LaxTiO3 perovskites are known to contain charge-compensating cation vacancies, which display one-dimensional ordering at high La concentrations. Recently, the radiation resistance of these perovskites has been measured, revealing an anomalously high radiation resistance at around x = 0.2. We use atomistic computer simulation techniques to study short-range cation and vacancy ordering as a function of La concentration and thermal history. Long-range electrostatic effects dominate the interactions, and ordering in one- and two-dimensions is observed. We also give preliminary results on the effects of La concentration and ordering on radiation resistance, including both primary damage creation and defect annealing. © 2005 Australian Institute of Physics
Capture and sequestration of radioactive iodine
(Materials Research Society, 2010-04-06) Westphal, BR; Cummings, DG; Giglio, JJ; Wahlquist, DL; Bateman, KJ; McCartin, WM; Park, JJ; Shin, JM; Begg, BD
Trapping experiments have been performed at the Idaho National Laboratory to assess the performance of AgX sorbent media in capturing volatile iodine during the oxidation of irradiated oxide fuel. The demonstration of iodine release and capture from the used fuel has been accomplished with laboratory-scale equipment in a hot cell environment. Iodine loadings as high as 6 ug/g media have been achieved via chemical adsorption with filter efficiencies in excess of 90%. In addition to iodine, significant quantities of tritium have also been collected on the AgX filter media. Filter media loaded with radioactive iodine has been sequestered in a tin matrix by hot isostatic pressing at 200oC. The placement and encapsulation of the sorbent media was examined by neutron radiography, thus confirming the sequestration of radioactive iodine.
Cell parameters and the orthorhombic to cubic transformation in strontium substituted perovskite (CaTiO3)
(Australian and New Zealand Institutes of Physics, 1994-02-10) Ball, CJ; Begg, BD; Vance, ER
The departure of the orthorhombic unit cell of perovskite (CaTiO3) from the cubic symmetry can be characterised by the angle of shear, ∅ of the pseudo-cubic sub-cell. We have determined ∅ for a series of strontium-substituted perovskites, of general formula Cai-xSrxTiO3, at room temperature. ∅ decreases linearly with increases in x and extrapolates to zero at x ≈ 0.4, in fair agreement with previous work. For x > 0.4 the structure is cubic, contrary to previous reports.
Charge compensation mechanisms for aliovalent impurities in perovskite and zirconolite
(Australian and New Zealand Institutes of Physics, 1994-02-10) Vance, ER; Day, RA; Begg, BD; Blackford, MG
As part of the chemical design of Synroc-type ceramics for the immobilisation of different high-level radioactive wastes from nuclear fuel reprocessing, it is necessary to understand the various possible charge compensation mechanisms which occur when up to tens of atomic percent of rare earths and actinides are incorporated in solid solution in perovskite (CaTiO3) and zirconolite (CaZrTi2O7). In particular the solid solution of Gd in the Ca site of perovskite and the incorporation of Nd, Ce, U, Np and Pu in the Ca and Zr sites of zirconolite have been studied by XRD, SEM, TEM, and XANES. The essential conclusions are that in formulations where charge compensating ions are made available, then this is the preferred mechanism for incorporation of these cations in Ca and Zr sites. However in formulations where such compensators are not made available, it is possible for charge compensation to take place via significant abundances of cation vacancies, or by the appearance of unexpected valence states stabilised by crystal-chemical forces. An example of the latter is the probable stabilisation of Ti3+ in Ti sites, and of trivalent Ce and actinides in Ca and Zr sites, even under quite oxidising conditions. A complication in these studies is the effect of prevailing or inherited redox conditions. Redox conditions influence phase abundances and compositions as they control the valencies of cations capable of more than one oxidation state. There is also an indication that reducing conditions can promote oxygen site deficiencies in some formulations. Other complicating factors relate to sample fabrication, arising from the need to make extremely chemically uniform phases having the desired composition. This requires prolonged heating at high temperatures to achieve complete solid-state reaction that may result in selective losses due to volatilisation. Incipient melting due to localised eutectic formation and the apparently straightforward task of efficient stoichiometric mixing on a 1 to 10 um scale are other problems which have had to be overcome in sample fabrication.
Flexible process options for the immobilisation of residues and wastes containing plutonium
(American Society of Mechanical Engineers (ASME), 2007-09-02) Stewart, MWA; Moricca, SA; Begg, BD; Day, RA; Scales, CR; Maddrell, ER; Eilbeck, AB
Residues and waste streams containing plutonium present unique technical, safety, regulatory, security, and sociopolitical challenges. In the UK these streams range from lightly plutonium contaminated materials (PCM) through to residues resulting directly from Pu processing operations. In addition there are potentially stocks of Pu oxide powders whose future designation may be either a waste or an asset, due to their levels of contamination making their reuse uneconomic, or to changes in nuclear policy. While waste management routes exist for PCM, an immobilisation process is required for streams containing higher levels of Pu. Such a process is being developed by Nexia Solutions and ANSTO to treat and immobilise Pu waste and residues currently stored on the Sellafield site. The characteristics of these Pu waste streams are highly variable. The physical form of the Pu waste ranges from liquids, sludges, powders/granules, to solid components (e.g., test fuels), with the Pu present as an ion in solution, as a salt, metal, oxide or other compound. The chemistry of the Pu waste streams also varies considerably with a variety of impurities present in many waste streams. Furthermore, with fissile isotopes present, criticality is an issue during operations and in the store or repository. Safeguards and security concerns must be assessed and controlled. The process under development, by using a combination of tailored waste form chemistry combined with flexible process technology aims to develop a process line to handle a broad range of Pu waste streams. It aims to be capable of dealing with not only current arisings but those anticipated to arise as a result of future operations or policy changes.
The incorporation of cerium in zirconolite
(Materials Research Society, 1997) Begg, BD; Vance, ER
Zirconolite (CaZrTi2O7) is the primary actinide-bearing Synroc phase for the immobilisation of high-level nuclear waste. Using X-ray absorption spectroscopy and microanalysis we have investigated the incorporation of cerium, as a simulant for plutonium, on both zirconolite's Ca and Zr sites under a range of redox conditions. The Ce valence state was found to vary between Ce3+ and Ce4+ depending on the both the sintering atmosphere and temperature. The existence of alternative charge compensation schemes, predominantly cation vacancies, in addition to those used in the sample design was inferred in many of the zirconolites and will be discussed in detail. © Materials Research Society 1997
The incorporation of neptunium and plutonium in thorutite (ThTi2O6)
(Elsevier, 2013-12-25) Zhang, YJ; Gregg, DJ; Lumpkin, GR; Begg, BD; Jovanovic, M
The incorporation of neptunium (Np) and plutonium (Pu) into the brannerite structured lattice was studied using thorutite (ThTi2O6) as host lattice and sintering in air. The uncompensated Np and Pu doped samples and the low Y-charge compensated Np and Pu doped samples showed main phases as designed together with trace amounts of rutile. Those samples with larger amounts of Y produced yttrium pyrochlores as an additional minor phase. XRD analyses reveal anisotropic changes of the cell parameters; the a-parameter contracts while b- and c-parameters expand with mean cationic radius. This is in reasonable agreement with previous experimental data on ThTi2O6 and Ce0.975Ti2O5.95. Attempts to form Np or Pu valences >4+ by adding Y as a charge compensator were unsuccessful, suggesting that tetravalent Np and Pu ions are favoured in air-fired thorutite. © 2013, Elsevier B.V.
Incorporation of uranium in zirconolite (CaZrTi2O7)
(John Wiley & Sons, Inc, 2002-07) Vance, ER; Lumpkin, GR; Carter, ML; Cassidy, DJ; Ball, CJ; Day, RA; Begg, BD
Up to 0.15 formula unit (f.u.) of U4+ incorporated in the Zr site of zirconolite by firing CaUxZr(1−x)Ti2O7 compositions in argon at 1400°C allows retention of the 2M polytype. Further U4+ substitution for Zr, up to 0.4 f.u., produces the 4M polytype (containing ∼0.4 f.u. of U) plus the 2M polytype containing 0.15 f.u. of U. The pyrochlore structure (containing 0.6 f.u. of U) forms in conjunction with the 4M polytype at U contents of 0.4 f.u. up to 0.7 f.u. Higher U contents give the pyrochlore structure, but the solid-state reactivity of even alkoxide-based preparations becomes increasingly poor for x > ∼0.7 and hot pressing in graphite dies at ∼1250°C is necessary to achieve near single-phase pyrochlore structures for x= 1. When samples of CaUxZr1−xTi2O7 stoichiometry (x= 0.1 and 0.2) are oxidized at 1400°C in air, diffuse reflectance spectroscopy (DRS) shows evidence for U5+ formation at the expense of U4+ via enhanced absorption bands, ∼50 nm in half-width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm. Weight gains consistent with complete oxidation of U4+ to U5+ are observed when finely powdered argon-fired samples with x= 0.1 and 0.4 are heated in air to 1200°C. Evidence for U valence states higher than +4 in both argon- and air-heated materials containing charge compensators to encourage U5+ or U6+ formation was also derived from DRS (showing U5+ in particular), and XANES. DRS shows weak absorption bands attributable to U4+ in zirconolites containing ∼0.2 f.u. of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U4+ in the Zr site. Zirconolite and pyrochlore compositions which are melted at 1500° or 1550°C in argon and furnace-cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials. © 2002 The American Ceramic Society
Leaching of Synroc in D2O
(Monash University, Department of Physics, 1996-02-03) Vance, ER; Dytlewski, N; Begg, BD; Prince, KE; Thorogood, GJ
The use of D2O rather than H2O for leaching Synroc and its constituent phases allows the study of hydration and hydroxylation reactions without the contaminating influence of atmospheric moisture. For Synroc leached at 190 deg C for 5-30 days, the penetration of D into the Synroc is found as ∼30 nm from both energy recoil analysis using 1.5 MeV He ions from a Van de Graaff accelerator and secondary ion mass spectroscopy near mass 18(OD-). Analogous studies have been carried out on single crystals of SrTiO3 which have the perovskite (a constituent phase of Synroc) structure, and augmented by additional atomic force and scanning electron microscope studies of secondary TiO2 alteration products. Alteration products on single crystals of CaTiO3 have also been studied.
Perovskite and tiranite crystallisation from titanosilcate melts
(Institute of Materials Engineering, Australasia Ltd, 2004-11-29) Vance, ER; Begg, BD; Jahn, C
On cooling alkali-calcia-titania-silica melts, both perovskite (CaTiO3) and titanite (CaTiSiO5) have been to observed to form [1-4]. Further work has now been carried out on two of these compositions (wt%): (a) SiO2 (48); Al2O3 (19); CaO (17); MgO (6); TiO2 (5); K2O (5) [1] and (b) SiO2 (53.0); Al2O3 (8.1); CaO (14.3); TiO2 (18.3); Na2O (6.6) [2,3]. For the former, it has been previously argued [1] that perovskite is preferred over titanite when fcc noble metal alloys are present, due to possible epitaxial relationships between the (010) and ((101) planes of perovskite and the (100) plane of the noble metal. The addition of simulated radioactive waste (which included noble metals) to the latter glass was argued previously [4] to affect the probabilities of titanite versus perovskite on cooling from the melt, but in the present work we are checking whether the presence of noble metals alone is the main determinant to encourage perovskite formation as against titanite (noting that the addition of rare earths in composition (b) can also promote perovskite crytallisation) . The amount of noble metal added was in the range of 0.1-1 wt%. Samples were prepared from mixed oxides and carbonates which were slurried with water to which the noble metals were added as soluble salts. After drying and calcining at 750oC in air, the mixtures were melted in alumina crucibles at ~1400oC and then cooled in the furnace or by pouring onto a metal plate. X-ray diffraction and scanning electron microscopy were carried out to characterise the materials. For melt (a), the omission of Ir from the starting preparation yielded diopside only on reheating glass for 1 hour at 1050oC, in contrast to results obtained previously [1] for the same composition when it contained 0.3 wt% of added Ir. The effect of Ag and Pd additions will also be reported. For melt (b) The effects of the inclusion of Pd and Ag in the starting composition will be measured
Plutonium and neptunium incorporation in zirconolite
(Materials Research Society, 1997) Begg, BD; Vance, ER; Day, RA; Hambley, M; Conradson, SD
The incorporation of Pu and Np in zirconolite (CaZrTi2O7) has been investigated over a range of redox conditions. Zirconolite formulations designed to favour either trivalent or tetravalent Pu and Np were prepared by limiting the amount of charge compensating additives available to maintain electroneutrality. From near-edge X-ray absorption spectroscopy the Pu valence state was found to vary with the processing atmosphere, from completely tetravalent when fired in air, and located on either the Ca or Zr sites, to trivalent, when substituted on the Ca site after annealing in 3.5% H2/N2. Np was predominantly tetravalent over the range of redox conditions examined and was readily incorporated on either of zirconolite's Ca or Zr sites. The charge compensation mechanisms at work in different zirconolites are also discussed. © Materials Research Society 1997
Pu, U, and Hf incorporation in Gd silicate apatite
(John Wiley & Sons, Inc, 2004-12-20) Vance, ER; Ball, CJ; Begg, BD; Carter, ML; Day, RA; Thorogood, GJ
Trivalent Pu can be incorporated in the silicate apatite structure to form Ca2Pu8(SiO4)6O2 by sintering under reducing conditions, while the incorporation of tetravalent Pu in the Ca/rare earth sites in oxidizing or neutral conditions is limited to only 0.6 formula units (f.u.). The d-spacings and intensities of the X-ray pattern of hexagonally structured Ca2Pu8(SiO4)6O2 after firing at 1250°C are given, and the a and c lattice parameters are 0.95611 and 0.70281 nm, respectively. The respective solid solubility limits of U and Hf in Ca2Gd8–x(U/Hf)x(SiO4)6O2 apatite samples were 0.3 and 0.2 f.u. © 2004 The American Ceramic Society
Radiation damage effects in pyrochlore and zirconolite ceramic matrices for the immobilization of actinide-rich wastes
(Commissariat à l'énergie atomique (CEA), 2000-07-01) Lumpkin, GR; Begg, BD; Smith, KL
Actinide-doping experiments using short-lived 238Pu and 244£m have demonstrated that pyrochlore and zirconolite become fully amorphous at a dose of 0.2-0.5 x 1016 cc/mg at ambient temperature and exhibit bulk swelling of 5-7%. Detailed studies of natural samples have included determination of the critical amorphization dose, long-term annealing rate, microstructural changes as a function of dose, and the thermal histories of the host rocks. Together, the laboratory based work and studies of natural samples indicate that the critical amorphization dose will increase by about a factor of 2-4 for samples stored at temperatures of 100-200 °C for up to 10 million years. These studies of alpha-decay damage have been complemented by heavy ion irradiation studies over the last ten years. Most of the irradiation work has concerned the critical amorphization dose as a function of temperature in thin films; however, some work has been carried out on bulk samples. The irradiation work indicates that most pyrochlore and zirconolite compositions will have similar critical amorphization doses at low temperatures (e.g., below 300-400 °C). Pyrochlores with Zr as the major B-site cation transform to a defect fluorite structure with increasing ion irradiation dose, but do not become amorphous.
The role of titanate ceramics in immobilisation of wastes arising from partitioning and weapons disposition
(Japan Atomic Energy Research Institute (JAERI), 2002-03-01) Hart, KP; Zhang, YJ; Begg, BD; Aly, Z; Brownscombe, AJ; Day, RA; Stewart, MWA; Lumpkin, GR; Vance, ER; Jotsons, A
The development of ceramics for the immobilisation of radioactive wastes has been on-going at ANSTO since 1980 and has involved collaboration with JAERI, the USDOE, LLNL and other international laboratories. This work has examined various processing routes, radiation damage effects and long-term aqueous durability for a range of titanate ceramics. Within these studies, information on the long-term behaviour of ceramics chosen to immobilities excess Pu and actinide-rich wastes arising from partitioning strategies have been obtained. Overall, the long-term durability, limitations on criticality under processing and disposal conditions and existence of viable natural analogues endorse the use of titanate ceramics for immobilisation of actinide-rich wastes. © 2002 Japan Atomic Energy Research Institute
Safe immobilization of high-level radioactive waste in waste forms for geological repositories
(Elsevier, 2011-10) Vance, ER; Stewart, MWA; Moricca, SA; Lumpkin, GR; Begg, BD
The idea that spent fuel and other hazardous radioactive high-level wastes (HLW) would need to be dealt with arose soon after the first experimental demonstration of nuclear reactors in 1942. HLW is spent nuclear power plant fuel or waste deriving directly from reprocessing or recycling of spent fuel. This latter waste consists of mainly fission products (FPs), as well as minor actinides such as Np, Pu, Am and Cm. Table 1 shows the main components and relevant half-lives of reprocessing waste from the well-known Purex process. Also, there are abundant wastes from the production of Pu for nuclear weapons, mainly in the US and Russia. These wastes (Table 2), although designated as HLW in the US, have only around 0.1- 1% of the radioactivity per unit volume of the Purex-type HLW, and if they were located in most other countries would be categorised as intermediate-level waste (ILW). Current HLW inventories around the world run into tens of millions of tonnes. The manageability of HLW impacts directly on the sustainability of nuclear power. © 2011 Elsevier Ltd.
Solid solubilities of Pu, U, Gd and Hf in candidate ceramic nuclear wasteforms
(Australian Nuclear Science and Technology Organisation, 2001-04-02) Vance, ER; Carter, ML; Lumpkin, GR; Day, RA; Begg, BD
The goal of this research project was to determine the solid solubility of Pu, U, Gd, and Hf in candidate ceramics for immobilization of high-level nuclear waste. The experimental approach was to saturate each phase by adding more than the solid solubility limit of the given cation, using a nominated substitution scheme, and then analyzing the candidate phase that formed to evaluate the solid solubility limit under firing conditions. Confirmation that the solid solution limit had been reached insofar as other phases rich in the cation of interest was also required. The candidate phases were monazite, titanite, zirconolite, perovskite, apatite, pyrochlore, and brannerite. The valences of Pu and U were typically deduced from the firing atmosphere, and charge balancing in the candidate phase composition as evaluated from electron microscopy, although in some cases it was measured directly by x-ray absorption and diffuse reflectance spectroscopies (for U). Tetravalent Pu and U have restricted (< 0.1 formula units) solid solubility in apatite, titanite, and perovskite. Trivalent Pu has a larger solubility in apatite and perovskite than Pu4+. U3+ appears to be a credible species in reduced perovskite with a solubility of {approximately} 0.25 f.u. as opposed to {approximately} 0.05 f.u. for U4+. Pu4+ is a viable species in monazite and is promoted at lower firing temperatures ({approximately} 800 C) in an air atmosphere. Hf solubility is restricted in apatite, monazite (< 0.1 f.u.), but is {approximately} 0.2 and 0.5 f.u. in brannerite and titanite, respectively. Gd solubility is extended in all phases except for titanite ({approximately} 0.3 f.u.). U5+ was identified by DRS observations of absorption bands in the visible/near infrared photon energy ranges in brannerite and zirconolite, and U4+ in zirconolite was similarly identified.
Structures in the system CaTiO3/SrTiO3
(Elsevier, 1998-09) Ball, CJ; Begg, BD; Cookson, DJ; Thorogood, GJ; Vance, ER
At room temperature the sequence of phases with increasing amounts of strontium in the system CaTiO3/SrTiO3is orthorhombic (Pnma), orthorhombic (Bmmb), tetragonal (I4/mcm), and cubic (Pm3m). All phase boundaries shift toward smaller strontium contents with increase of temperature. Volume changes resulting from phase transformations are small for all compositions. Shape changes are greatest (∼0.3%) for theBmmb/I4/mcmtransition, but would probably be accommodated by microtwinning and so are unlikely to affect the mechanical integrity of a specimen. ©1998 Academic Press
Synroc derivatives for the Hanford waste remediation task
(Materials Research Society, 1997) Vance, ER; Hart, KP; Day, RA; Carter, ML; Hambley, M; Blackford, MG; Begg, BD
Three wt% each of Cs and Tc were mixed with the standard Synroc precursor and the ceramic was formed by hot-pressing. Attempts were made to incorporate the Tc as either metal or Tc 4+, using different redox conditions in processing. Volatile losses of Tc during calcination were < 0.1% in all cases. Short-term Tc leach rates when the Tc was present as a metal alloy were in the order of 10−4 g/m /d at 90°C with frequently changed water, and decreased with increasing leaching time. The valence of the Tc was monitored by X-ray absorption spectroscopy at the drying and calcination stages of the production. The general viability of Synroc/glass composites for immobilising the Hanford HLW sludges is further demonstrated by using further refinements of additive schemes for the inactive “All-blend” formulation and initial studies using the U-containing “All-blend” waste formulation. © Materials Research Society 1997

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