T. S. Livshits

Synthetic minerals with the pyrochlore and garnet structures for immobilization of actinide-containing wastes

Complex oxides of the pyrochlore (space groups Fd3m, [8]A2[6]B2O7) and garnet (Ia3d, [8]A3[6]B2[4]T3O12) structures (“A” = Ca2+, Ln3+/4+, An3+/4+; “B” = (Ti, Sn, Hf, and Zr)4+ in pyrochlore, and Al3+, Ga3+, and Fe3+ in garnet alone; “T” = (Al3+, Ga3+, and Fe3+) are promising matrices for actinide-bearing wastes. In order to identify optimal compositions of these phases, their isomorphic capacity with respect to REE, actinides, and other components of wastes was examined. The long-term behavior of the matrix at a repository was predicted based on data obtained on the behavior of pyrochlores and garnets under ion irradiation and 244Cm decay and on the determined leaching rates of REE from the matrices because of their interaction with aqueous solutions, including that after amorphization. In order to propose efficient synthesis techniques, samples prepared with the use of various methods were studied. The possibility of incorporating long-lived decay products of 99Tc into the crystalline matrices was analyzed.

Amorphization of rare earth aluminate garnets under ion irradiation and decay of 244Cm admixture

The stability of synthetic REE-aluminate garnets irradiated by accelerated Kr2+ ions and affected by alpha decay of 244Cm (T1/2 = 18.1 yr) has been studied. The dose of irradiation sufficient for the complete disordering of the aluminate garnet structure is 0.40–0.55 displacements per atom. This value increases with rising temperature due to the increasing intensity of recovery from radiation damage to the lattice by heating. The critical temperature above which the structure of REE-aluminate is not damaged by radiation is 550°C. The amorphization dose for aluminates with garnet structure is two to three times higher than of that previously studied ferrites; the critical temperature of both is similar. In resistance to radiation, aluminate garnets do not yield to zirconolite and exceed titanate pyrochlore. Heating to 250°C does not lead to substantial recovery from radiation defects in the garnet structure. The radiation impact on matrices of real actinide (An) wastes is lower than that related to ion irradiation and 244Cm doping, and this facilitates a higher radiation resistance of garnets containing HLW.

Radiation and chemical resistance of synthetic ceramics based on ferritic garnet

Critical dose of X-ray amorphization of the garnet phase Ca1.5Gd0.908Cm0.092Th0.5ZrFe4O12, which is the base of the synthetic ceramics doped with 241Cm (∼2 wt %), was determined. Amorphization occurs at a dose of 1.6 × 1018 α-decay g−1 or 0.17 dpa (displacements per atom). The chemical resistance of the amorphized ceramics to leaching (MCC-1 test) is comparable to that of the initial ceramics. The rates of Cm leaching from the freshly prepared and amorphized ceramics decrease with time and after storage for two weeks amount to 1.4 × 10−3 and 5.8 × 10−3 g m−2 day−1, respectively. Data on thermal restoration of the crystal structure of the ceramics are also presented.

Radiation resistance and chemical stability of yttrium aluminum garnet

The radiation resistance and chemical stability of yttrium aluminum garnet doped with short-lived 244Cm for accelerated accumulation of radiation damages was studied. Two garnet samples of the stoichiometry Y2.8853Cm0.1024Pu0.0092Al5O12, containing ∼4 wt % Cm with 70% content of 244Cm, were synthesized. The ceramics consist of the target phase of the garnet structure and a small amount of corundum. The chemical stability of the freshly prepared sample in water was studied (MCC-1 test, 90°C). The Cm leach rate is ∼10−2 g m−2 day−1, and that of Al and Y, ∼10−3 g m−2 day−1. The garnet became fully X-ray amorphous owing to self-irradiation in 530 days at the accumulated dose of 4.0 × 1018 α-disintegrations per gram, or 0.3 displacement per atom. After the destruction of the garnet structure, the Cs leach rate from the matrix 14 days after the start of the experiment increased by a factor of 10, and that of Y, by a factor of 60 relative to the freshly prepared ceramics.

Natural and Synthetic Minerals — Matrices (Forms) for Actinide Waste Immobilization

The reprocessing of irradiated fuel of nuclear power plants results in the formation of a great amount of radioactive wastes, including high-level radioactive wastes (HLW). Selection of suitable immobilizing materials is a key part of safe HLW management in the nuclear fuel cycle. The search for confinement matrices began in the 1950s with study of various glassy and crystalline materials containing silicates, phosphates, and titanates (Ewing and Lutze 1988). Only borosilicate and alumophosphate glasses are used for this purpose on an industrial scale (Hench et al. 1984; Vashman et al. 1997). These glasses are not capable of incorporating sufficient amounts of actinides (particularly, Pu) and have low resistance to chemical corrosion by water (Matzke and van Geel 1996; Laverov et al. 1997). Interaction of the vitreous matrices with underground waters will result in the formation of colloidal particles (Glass as a waste form ... 1996), which can carry actinides for very long distances. The glasses also easily crystallize on aging (Vashman et al. 1997), which significantly decreases stability of the waste forms due to appearance of various soluble phases, such as alkali and alkaline-earth silicates, phosphates, and molybdates.

The behavior of rare-earth pyrochlores and perovskites under ion irradiation

The behavior of samples of A2B2O7 composition under irradiation with 1-MeV Kr2+ ions was studied (A is a simulator of the REE-actinide fraction of the wastes of the treatment of used nuclear fuel and B is a quadrivalent cation of Zr, Sn, or Ti). Depending on the B elements, the samples are crystallized either in pyrochlore (Zr and Sn) or in the perovskite structural type (Ti). The matrices of the pyrochlore structure are radiation-resistant, which is shown by their high critical doses and low critical temperatures of amorphization. The phases of monocline REE titanate structure are characterized by low irradiation resistance and should be amorphized even within centuries of storage. To characterize the possibilities of their usage as matrices for waste immobilization, synthesis of materials containing short-living actinides and studies of the degree of the amorphization effect on their stability in aqueous solutions are required.

Chemical and radiation stability of 244Cm-doped aluminate perovskite

Aluminate perovskite with a 75% simulator of actinide-REE (Nd, Sm, Ce) fraction of high-level radioactive wastes (HLW) from reprocessing of spent nuclear fuel (SNF) has been synthesized and studied. The radiation stability of perovskite in the process of 244Cm decay (T1/2 = 18 yr) was investigated. Its structure has been amorphized at accumulated dose of 2.3 × 1018 α-decays/g, or 0.26 displacements per atom (dpa). The critical temperature above which amorphization does not occur at any dose is estimated to be 500°C. Radiation resistance of aluminate perovskite is close to previously studied titanate pyrochlore and ferrite garnet. The stability of perovskite in water before and after amorphization has been studied as well. The leach rate of Cm by water (90°C) from crystalline perovskite in runs 3–14 days long was 10−2–10−3 g/m2. This value is close to the stability of titanate pyrochlore and aluminate garnet. The intensity of element leaching from perovskite after amorphization of its structure increases 10–100 times and thus is higher than for other previously studied actinide phases.

Curium-doped stannate pyrochlore: Durability under radiation and leaching in water

The radiation resistance of the phase (Gd,Cm)2Sn2O7 with a pyrochlore-type structure containing 3.0 wt % 244Cm was studied. It was established that amorphization occurs at a dose of 1019 α-decay/g (1.52 displacements per atom), which is 2–5 times higher than that needed for amorphization of titanate and titanate–zirconate pyrochlore phases with a similar structure. The heating of the amorphous ceramics restores the structure of the pyrochlore. The restoration process begins in the temperature interval of 600–700°C. This allows us to estimate the critical amorphization temperature as 650°C. On the 14th day, the rate of Cm leaching from the initial sample in water at 90°C is 10–1; Gd, 10–2; and Sn, 10–3 g/(m2 day). After amorphization the leaching rate increases by an order of magnitude (Cm) and two orders of magnitude (Gd), but it does not change for Sn. Compared to the zirconate and titanate–zirconate phases, stannate pyrochlore is markedly less resistant in water and cannot be regarded as a matrix for the immobilization of REE-actinide fraction wastes.

Radiation Response of Synthetic U-Containing Sn-Zr Based Pyrochlore

A closed nuclear fuel cycles will generate plutonium and minor actinides (Np, Am, Cm) that are required to be immobilized into durable nuclear waste forms for long-term geologic disposal. Isometric pyrochlore, A2B2O7, is a promising host phase for the incorporation of actinides. Many studies have simulated the alpha-decay damage in a wide range of compositions using ion beam irradiation. In the present study, the response to the radiation damage of U-doped Sn-Zr based pyrochlore has been studied by 1-MeV Kr ion irradiation and characterized by in situ transmission electron microscopy (TEM).

New Actinide Waste Forms with Pyrochlore and Garnet Structures

Cubic oxides with pyrochlore and garnet structures are promising matrices for long-lived actinides immobilization. Their isomorphic capacity with respect to An and REE was determined. To predict the long-term behavior of these matrices under their underground disposal radiation stability of synthetic pyrochlores and garnets was studied. Most of titanate phases have the critical (amorphization) doses close to 0.2 displacements per atom at 298 K. This value is significantly higher for Sn- and Zr-rich pyrochlores. Corrosion behavior of the pyrochlore- and garnet-composed matrices was investigated. The lowest actinides leach rates were observed in water and alkaline solutions most typical for underground waste repositories. Amorphization of the phases has a low influence on their corrosion behavior in solutions. Possibility for joint incorporation of actinides and Tc into zirconate- and titanate-based matrices with the pyrochlore structure is discussed.