Sodalites have been investigated experimentally for the capture and long-term containment of 129I, a significant and hazardous waste product of the nuclear fuel cycle. Sodalites are zeolite-type structures commonly occurring in nature in alkaline igneous rocks and having the prototype formula Na8(AlSiO4)6Cl2. The crystal structure is based around β-cages consisting of corner-sharing SiO4 and AlO4 tetrahedra. In the centre of the β-cage is an anion X. Iodine captured by sodalites sits in the centre of the β-cages as iodide anions. Silver iodide (AgI) plays an important role in the capture and subsequent processing of 129I in the nuclear fuel cycle. Using ab initio density functional theory (DFT) modelling, we investigate the energetics and feasibility of iodine capture and containment in mixed cation sodalites Na8-xAgx(AlSiO4)6I2, and compare the results with experimental observations.

EURO-GANEX aims to recycle both major and minor actinides. As the final waste composition is free from actinides, adapted immobilization matrices should be developed. Synroc is a potential wasteform that has proven itself to be efficient in immobilizing high-level wastes (HLW). In this study, a new composition of Synroc, Synroc-Z, is designed and characterized. The key modification is in decreasing the amount of zirconolite phase, which is the main host phase for actinides and increasing the amount of other phases (hollandite and perovskite). As designed the obtained amount of zirconolite is lower than in Synroc-C compositions. Synroc-Z samples were synthesized with a waste loading of 20 wt.% at various temperatures and pressures via hot-pressing to determine the optimum process parameters, which were determined to be 1150-1200°C and 20 MPa, respectively.

Two Bax CsyMnzTi8-zO16 hollandite samples, containing excess rutile, have been prepared, targeting the Mn valence as 3+ by sintering at 1300oC in air (z = 2x +y) and as 2+ (z = x + y/2) in argon. SEM showed the sample sintered in air to contain major hollandite and minor Mn titanate and rutile. The sample sintered in argon contain major hollandite, and minor Ba titanate and rutile. X-ray absorption near edge spectroscopy (XANES), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) were used to determine the oxidation state of the Mn in the samples. Transmission manganese K-edge X-ray absorption spectra of the samples were similar but the precise ratio of oxidation states could not be calculated from these spectra.

Zirconates and titanates, based on the nominal baseline composition developed for the Plutonium Immobilization Project (PIP), have been prepared with and without process impurities. The titanates form pyrochlore as the major phase and the zirconates form a defectfluorite. Little, if any, of each impurity is accommodated in the defect-fluorite and powellite, kimzeyite, a spinel and a silicate glass appear as extra phases in this ceramic. In the titanates the pyrochlore incorporates more impurities, with the remainder being accomodated in zirconolite and a small amount of silicate glass. At extremly high levels of impurities, traces of magnetoplumbite, perovskite, and loveringite were found. The defect-fluorite zirconate phase is more radiation damage resistant than the titanate pyrochlore, though the secondary phases in the zirconate will reduce the radiation damage resistance of zirconate monoliths. To produce a dense product the oxide-route zirconate required sintering temperatures of about 1550°C, 200°C higher than that required for the titanate. Silicate impurities reduce the sintering temperatures.

Baseline formulation titanate ceramics for surplus Pu disposition are based upon a target mineralogy of 95 wt.% pyrochlore (Ca0.89Gd0.22Hf0.23Pu0.22U0.44Ti2O7) plus 5 wt.% Hf-doped rutile (∼ Ti0.9Hf0.1O2), where Ce is used as an analogue for Pu and U, and Th for Pu. Typically, Pu/U, Th/U and Ce/U-baseline samples form major pyrochlore, plus minor brannerite (AnTi2O6) and rutile. Ce/Ce-baseline ceramics were similar but did not form brannerite. Sintering in air produced more U5+ in the ceramics than sintering in Ar. In the pyrochlore the charge compensation for U5+ principally occurred by an increase in Ca2+ and decrease in Ti4+ and Th4+. In the matrix these phase compositional changes result in an increase in brannerite content of the ceramic at the expense of pyrochlore in the air sintered ceramic relative to the Ar sintered sample. Sintering in reducing 3.7% H2 in Ar atmospheres eliminates the brannerite and rutile and results in 2M-zirconolite and perovskite in addition to the major pyrochlore phase. Varying the sintering temperature between 1250°C and 1400°C had little effect on the phase chemistry other than slightly incomplete reactivity of coarse actinide oxide at the lower sintering temperatures and a general increase in density (to ∼ 1350°C) and grain size with increased sintering temperatures. Th4+ appears to be a good analogue for Pu4+ from a crystal chemistry aspect.

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, Al2O3, TiO2 and SiO2 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 < 0. 1 g/m2 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/m2/day at 70°C), and can be improved by alloying with iron group metals. With a choice of “neutral” (P(O2) ∼ 10−4 atm., near the Ni/NiO buffer) hot-pressing conditions, Tc can also be incorporated as Tc4+, substituting for Ti4+ in the ceramic phases, and in this form, it should be highly leach resistant also.

The incorporation of cerium, as a simulant for plutonium, has been investigated in both zirconolite and perovskite under oxidising and reducing conditions. The Ce valence in zirconolite was very sensitive to the processing atmosphere, whilst in perovskite the valence of the Ce was determined essentially by the chemical design of the perovskite. Charge compensation in zirconolite and perovskite samples displaying an apparent excess of positive charge was inferred to occur by the formation of cation vacancies, in samples prepared under oxidising conditions, and by trivalent Ti being produced in samples subject to a reducing atmosphere. Oxygen vacancies are inferred to be present in samples exhibiting an apparent excess of negative charge.

The decomposition of topaz to mullite and other siliceous phases on heating above about 1100 °C was found to depend on sample size and the presence of water vapor in the heating atmosphere. The principal experimental technique employed was scanning electron microscopy, but the data were supported by x-ray diffraction, thermal analysis, mass spectroscopy of volatile emissions, and solid-state nuclear magnetic resonance. In relatively large samples, the transformation to mullite evidently takes place by a vapor phase mechanism within the bulk. The surface reaction that took place for samples heated in a wet atmosphere allowed the formation of high-silica glass, as well as mullite. The use of a hydrogenous heating atmosphere resulted in the sublimation and reformation of mullite whiskers, well outside the boundary of the original topaz.

Synroc, a ceramic made from a reactive mixture of Al, Ba, Ca, Ti, and Zr oxides, is proving to be a suitable and effective medium for immobilizing nuclear wastes.

Synroc-C, a titanate-based ceramic variant, was initially developed in 1978 by Ringwood et al. for immobilizing high-level nuclear waste (HLW) from nuclear power reactor fuel reprocessing. HLW is essentially a solution of radioactive fission products, actinides, and process contaminants in ~3 mol/L nitric acid. The developers of Synroc-C aimed to immobilize radioactive waste ions by incorporating them in a ceramic. They accomplished this by mixing the HLW solution (liquid waste) with a ceramic precursor, then forming the ceramic by drying, calcining, and hot-pressing the mixture in a metal container for two hours at 1200°C/20 MPa. The result, Synroc-C, is composed of hol-landite, zirconolite, perovskite, and rutile, together with a few percent of minor phases and metal alloys. The Synroc-C precursor has the following composition (wt%): Al203(5.4); BaO(5.6); CaO(11); TiO2(71.4); and ZrO2(6.6). Since 1984, it has been prepared by hydrolyzing a mixture of Al, Ti, and Zr alkoxides with an aqueous slurry of Ba and Ca hydroxide. The abundances of the phases, and the radionuclides contained in them in dilute solid solution, are identified in Table I.

27Al MAS nmr spectra from alkoxide-derived Al2O3-BaO-CaO-TiO2-ZrO2 Synroc precursors were studied after isochronal annealing in air for 1 h at temperatures between 200 and 1300 °C. At temperatures below 1000 °C, some tetrahedral and a trace of 5-coordinated Al were observed together with a majority of octahedral Al. The amount of nonoctahedral Al reached a maximum at 400–700 °C, beyond which it diminished and disappeared; essentially, complete conversion to octahedral Al took place on full crystallization at 1200–1300 °C, with the Al being contained in barium hollandite and minor α-alumina. The hollandite nmr signal was absent in Synroc which was hot-pressed at 1200 °C/20 MPa, presumably because of coincorporation of paramagnetic Ti3+ in the hollandite phase, but a signal from α-alumina was observed. Samples containing simulated waste fission products gave results similar to their waste-free counterparts when calcined at 750 °C in 3.5% H2/N2 and when hot-pressed.

Synroc C waste form specimens prepared using the Australian-developed technology are uniaxially pressed in stainless steel bellows at 1200°C and 20MPa. This produces a material with high chemical and physical durability and with the radioactivity enclosed inside both the waste form and the bellows. An alternative method of producing the ceramic product is to use cold pressing of pellets followed by reactive sintering to provide densification and mineralization. Depending on the scale of waste form preparation required and on the activity level and nature of the waste streams, the cold press and sinter method may have advantages. To evaluate the effects of production method on waste form characteristics, especially resistance to dissolution or leaching of waste elements, we have prepared two simulated waste samples for evaluation. Both samples were prepared from liquid precursor materials (alkoxides, nitrates, and colloidal silica) and then doped with waste elements. The precursor material in each case corresponded to a basic phase assemblage of 60% zirconolite, 15% nepheline, 10% spinel, 10% perovskite, and 5% rutile. One sample was doped with 25% by weight of U; the other with 10% by weight each of U and Gd. Each sample was calcined at 750°C for 1 hr. in a 3.5% H2 in N2 atmosphere. Then one portion of each sample was hot pressed at temperatures ranging from 1120 to 1250°C and 20MPa pressure in steel bellows. A separate portion of each sample was formed into pellets, cold pressed, and sintered in various atmospheres at 1200°C to produce final products about 2/3 cm in diameter. Samples were then examined to determine density of the product, grain sizes of the phases, phase assemblage, and the location of the U and Gd in the final phases. Density data indicate that sintering gives good results provided that the samples are held at 200°C for long enough to allow trapped gases to escape.

Synroc is a multi-component titanate ceramic, designed to immobilise High Level Waste (HLW) from nuclear fuel reprocessing plants [1]. Synroc precursor powders have been previously produced by various methods, such as oxide and alkoxide-hydrolysis routes [2]. However, various technological aspects of HLW processing make the use of free-flowing, dustfree, highly sinterable precursor powders desirable. Such powders have been produced by spray-drying colloidal precursors, yielding microspherical particles with controlled porosity.

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 1200ºC/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.

Cs dissolution kinetics for Cszr2(PO4)3 in a variety of aqueous media at 70°C approximately obeyed a first-order law. However in 0.1 M NaHCO3,at1/2 law was appropriate. The Cs extraction was a minimum under neutral conditions. In the pH range of 1–4, the Cs extraction varied approximately as [H+]0.3 and was a stronger function of pH in alkaline media. The survival of α-Zr phosphate gel in water at 300°C was essentially confirmed. ZrP2O7, reacted sluggishly with water at 200–300°C to produce an apparently fully-protonated analogue of NaZr2(PO4)3

It has been shown that between 10 and 20% of a simulated PW–4b radwaste composition can be incorporated into a single nhase with the NZP (= ‘MaZr2 P3 o12’) structure. By changing the P/Na and Zr/Na molar ratios (i.e., adjusting the crystal chemical model of where each ion is located in the structure) it has been possible to outline a very ‘forgiving’ compositional regime both at the 10% and the 20% waste loading level within which one obtains one ([NZP]) or two ([NZP] and [monazite]) phases. We have also succeeded in substituting Tio2 for Zro2 in making a TiO2-rich [NTP] waste form analogous to the [NZ]] materials.

Thus we have succeeded in creating monophasic and diphasic ceramic waste forms which can be sintered below 1000° C. Only preliminary leach data have been obtained at 25° and 300°C, and they place this material with good ceramic forms.

Pollucite (CsAlSi206) has been suggested [1,2] as a suitable phase for the incorporation and immobilization of 137Cs, on the basis of its refractory nature and leach resistance. The phase also occurs in tailored ceramic formulations designed for the disposal of commercial reprocessing waste [3,4] and zeolites which are heavily loaded with Cs [5,6].

The immobilization of non-radioactive Na in nuclear waste has not been given much attention. However since Na compounds are normally very soluble, Nabearing phases may render a waste form more susceptiible to leaching of radionuclides. The Na-phases may incorporate radionuclides and also, leaching of soluble Na phases will increase the surface area of the waste form. The crystalline Na-bearing phases which might occur in tailored ceramic formulations for nuclear waste are nepheline [1,2], magnetoplumbite [3] and sodalitetype materials [4]. Other Na phases which might occur in titanate-tailored [2,5] or titanate-encapsulated [6,7] forms are perovskite [8,9], Na2Ti3O7 (especially in titanate-encapsulated materials) and Na2TiSiO5 or Na2Ti2Si2O9, etc., if the waste contained sand or zeolites [10].