K.P. Hart

The durability of synroc

Abstract Polished specimens of well characterised Synroc (a polyphase titanate ceramic designed to immobilise high-level nuclear waste) containing 10 wt% simulated (PW-4b-D) waste were subjected to MCC-2 type dissolution tests at 150°C in doubly deionised water (surface area to volume ratio0.01 ± 0.0005 mm −1 ) for periods of up to 532 d. Scanning electron microscopy (SEM) of the polished surfaces and transverse sections confirmed that Synroc is highly durable and suffers little corrosion even after 532 d in doubly deionised water at 150°C (corrosion of the least resistant major phase, perovskite, was

Kinetics of uranium release from Synroc phases

This paper presents experimental studies on the kinetics of U release from near single-phase zirconolite, pyrochlore, brannerite and pyrochlore-rich titanate ceramic materials. The dissolution tests were conducted at 20–75°C with initial pHs from 2 to 12, and flow rates from 10 to 80 ml d−1 in the open atmosphere. The U releases from these titanate materials are controlled by initial fast process and then followed by linear kinetics. The close-to-stoichiometric U release from zirconolite and pyrochlore-rich materials and preferential U release from brannerite are consistent with the alterations observed for the natural samples. The rate constants for U releases were determined and the effects of pH and temperature were examined. For each material, the U release vs. pH exhibits a V-shape with a minimum near pH 8. The measured activation energies suggest surface reaction controlled dissolution mechanism. Pyrochlore-rich materials and zirconolite demonstrated higher chemical durability and more resistance to aqueous attack than brannerite. However, impurities and minor brannerite inclusions do not appear to have a detrimental effect on U releases from pyrochlore-rich multi-phase ceramics.

Retention of Actinides in Natural Pyrochlores and Zirconolites

Natural pyrochlore and zirconolite undergo a crystalline-aperiodic transformation caused by alpha-decay of Th and U at dose levels between 2X10 and 3X10 a!mg. The principal effects of the transformation are volume expansion and microfracturing, providing potential pathways for fluids. Geochemical alteration of the minerals may occur under hydrothermal conditions or in low temperature, near surface environments, but Th and U usually remain immobile and can be retained for time scales up to ΙΟ years. However, the Th-U isotope systematics of a zirconolite-bearing vein and dolomite host rock may provide evidence for disequilibrium between Th, U and U.

Solubility study of Ti,Zr-based ceramics designed to immobilize long-lived radionuclides

Abstract Aqueous alteration of five Ti-Zr-oxide-based ceramics containing elements simulating long-lived radionuclides was studied experimentally by leaching at 90 °C in deionized water for more than one year under conditions of high solid/liquid ratios. Four of these ceramics were synthesized by coldcrucible melting (two Synroc-like materials, one zirconia, and one aluminotitanate) and the fifth ceramic was a hot-pressed Synroc. Melted Synroc-like ceramics have the same major constitutive phases as hot-pressed Synroc, but crystal sizes are very different, millimetric as opposed to micrometric, respectively. After the first seven days of leaching, the alteration appeared to cease as solution concentrations for all of the constituent elements attained constant values. The altered mass percentages determined from the release of Ca and Mo were less than 0.2% of the initial mass. Thermodynamic equilibrium calculations using data or estimations for pure phases, or using a model of ideal solid solutions, showed that the cessation of the alteration cannot be explained by the solubility limit of the primary phases of these ceramics. But, the data could be interpreted by the development of a passivating layer of secondary phases, e.g., hydroxides. Examination of the altered surfaces was carried out using SEM, XRD and XPS; however, the thickness of the alteration layer, estimated as 3-5 nm, was below the resolution limit of these techniques. Finally, despite the differences in the crystal size and therefore the amount of grain boundaries, and in the synthesis redox conditions, the leaching behaviors of melted and hot-pressed Synroc are the same for the present experiments.

Gadolinium Borosilicate Glass-Bonded Gd-Silicate Apatite: A Glass-Ceramic Nuclear Waste Form for Actinides

A Gd-rich crystalline phase precipitated in a sodium gadolinium alumino-borosilicate glass during synthesis. The glass has a chemical composition of 45.39-31.13 wt% Gd2O3, 28.80-34.04 wt% SiO2, 10.75-14.02 wt% Na2O, 4.30-5.89 wt% Al2O3, and 10.75-14.91 wt% B2O3. Backscattered electron images revealed that the crystals are hexagonal, elongated, acicular, prismatic, skeletal or dendritic, tens of mm in size, some reaching 200 mm in length. Electron microprobe analysis confirmed that the crystals are chemically homogeneous and have a formula of NaGd9(SiO4)6O2 with minor B substitution for Si. The X-ray diffraction pattern of this phase is similar to that of lithium gadolinium silicate apatite. Thus, this hexagonal phase is a rare earth silicate with the apatite structure. We suggest that this Gd-silicate apatite in a Gd-borosilicate glass is a potential glass-ceramic nuclear waste form for actinide disposition. Am, Cm and other actinides can easily occupy the Gd-sites. The potential advantages of this glass-ceramic waste form include: (1) both the glass and apatite can be used to immobilize actinides, (2) silicate apatite is thermodynamically more stable than the glass, (3) borosilicate glass-bonded Gd-silicate apatite is easily fabricated, and (4) the Gd is an effective neutron absorber.

Alteration of Cold Crucible Melter Titanate-Based Ceramics: Comparison with Hot-Pressed Titanate-Based Ceramic

Synroc ceramics were synthesized in an induction-heated cold crucible at laboratory scale (1 kg) from an oxide mixture, and at industrial prototype scale (45 kg) from Synroc previously produced by sintering under load at high temperature. After melting, both materials contained the major phases of Synroc-C. The chemical durability of both melted materials, as determined by static leaching of powder samples in initially pure water at 90°C with an SA/V ratio of 20000m −1 , was equivalent to that of conventional hot-pressed Synroc-C. Cerium, used in this investigation to simulate the presence of tri-and tetravalent actinides, was found in steady-state concentrations on the order of 1 ppb (i.e. NL(Ce) ≤ 10 −6 g·m −2 ). The concentration in the leachates was independent of the initial CeO 2 content of the Synroc (at least up to 10 wt%); moreover, it is similar to the results obtained with hot-pressed Synroc-C specifically formulated for conditioning long-lived actinides.

Leaching Behavior of Zirconolite-Rich Synroc Used to Immobilize High-Fired Plutonium Oxide

This study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ Pu0 2 . The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≃ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low ( −4 g m −2 d −1 ) and comparable rates from the waste form.

pH dependence of the aqueous dissolution rates of perovskite and zirconolite at 90 C

Perovskite and zirconolite are two of the major phases of the Synroc titanate mineral assemblage. Their aqueous durability under a range of pH conditions at 90 C has been examined. Solution analysis, electron microscopy and X-ray diffraction have been used to investigate the dissolution behavior of these phases, and a perovskite phase doped with Nd, Sr and Al, using buffered solutions at pH levels of 2.1, 3.7, 6.1, 7.9 and 12.9. After 43 days of leaching, Ca and Ti extractions from perovskite and zirconolite show only a weak pH-dependence. SEM investigation of the samples leached at pH 2.1, 6.1 and 12.9 showed that a titanaceous surface layer formed on the perovskite specimens. XRD analysis of the perovskite samples showed that anatase formed on the leached surface at acidic and neutral pHs, but not under alkaline conditions, and that minor amounts of rutile also formed. In the leached perovskite specimens doped with Nd, Sr and Al, no rule was found by XRD and anatase was only detected in the sample leached at pH 2.1. There were no detectable changes in the leached zirconolite samples examined by SEM and XRD.

Durabilities of Pyrochlore-Rich Titanate Ceramics Designed for Immobilization of Surplus Plutonium

The chemical durabilities of two Pu-doped pyrochlore samples were studied by Single-Pass-Flow-Through (SPFT) tests at 70°C. The dissolution of pyrochlore is incongruent with preferential releases of Ca and Gd over Ti, close to stoichiometric releases of U and Ti, and lower releases of Hf and Pu than Ti. Altered pyrochlore and polymorphs of TiO 2 (brookite and probably anatase) have been identified on the surface of the leached sample and the principal secondary phase is an unknown polymorph of TiO 2 containing Hf and varying amounts of Gd and Pu. These surface alteration phases are consistent with reported studies of natural samples. The releases of U, Gd, Ca and Ti into solution follow linear kinetics, whereas the releases of Pu and Hf exhibit non-linear behavior. The presence of ∼5% PuO 2 and trace amounts of glass does not appear to have an effect on the overall durability of the material. Further, the low Pu release rate and the similar kinetics for Pu and Hf releases limit the possibility of nuclear criticality under repository conditions. Overall, this study provides useful information on the lower bounds of durabilities of the materials.

Further Studies of Synroc Immobilization of HLW Sludges and Tc for Hanford Tank Waste Remediation

Synroc/glass composites were designed for simulated Hanford HLW sludges containing U (the current “All-Blend” formulation). The composite contained ∼ 50 wt% of simulated HLW (oxide equivalent), to which ∼ 6, 10, 10, and 24 wt% of CaO, Al 2 O 3 , TiO 2 and SiO 2 were added, and melted under an argon atmosphere at 1350°C. The phase assemblage consisted of zirconolite, perovskite, spinel, nepheline, whitlockite and glass as major phases. Seven-day PCT tests yielded values of 2 for all elements studied. The PCT results were tolerant to changes of ∼ 20% of the inventories of the additives, and to variations in redox conditions. Technetium separated out during decontamination of liquid Hanford wastes can be incorporated as metal in hot-pressed Synroc prepared under reducing conditions, and its leach resistance is good (∼ 10 ∼3 g/m 2 /day at 70°C), and can be improved by alloying with iron group metals. With a choice of “neutral” (P(O 2 ) ∼ 10 −4 atm., near the Ni/NiO buffer) hot-pressing conditions, Tc can also be incorporated as Tc 4+ , substituting for Ti 4+ in the ceramic phases, and in this form, it should be highly leach resistant also.