S. V. Tomilin

Americium and plutonium in trigonal phosphates (NZP type) Am1/3[Zr2(PO4)3] and Pu1/4[Zr2(PO4)3]

Phosphates Am1/3[Zr2(PO4)3] and Pu1/4[Zr2(PO4)3] were synthesized and studied by X-ray diffraction. The X-ray patterns were indexed in trigonal crystal system, and the unit cell parameters were evaluated. The possibility of accommodation of actinides in the framework cavities of the structure of sodium zirconium phosphate was demonstrated for the first time. It was suggested that incorporation of highly charged Pu and Am cations in the framework cavities decreases the symmetry of the crystal structure to simple trigonal. The rates of Pu leaching from a ceramic material based on phosphate Pu1/4[Zr2(PO4)3] were measured.

Effect of amorphization on isolation properties of actinide pyrochlore matrix

The effect of 244Cm decay (T1/2 = 18 years) on the radiation and chemical stability of the pyrochlore Gd1.935Cm0.065TiZrO7 was examined. Irradiation leads to degradation of its structure. The amorphization dose at room temperature is 4.60 × 1018 α-decay g−1 (or 0.60 displacements per atom, dpa). It is close to the corresponding value at ion irradiation of phases of similar composition and three times higher as compared to titanium pyrochlores with 238Pu or 244Cm. The rate of Cm leaching from the initial ceramics is 1.7 × 10−2 g m−2 day−1, and after amorphization it increases to 4.7 × 10−2 g m−2 day−1. The critical temperature (330 K) corresponds to probable conditions in sites of underground waste disposal. Therefore, such matrix will not undergo amorphization at any irradiation doses and will preserve its isolation properties in a repository.

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.

On the possibility of realization of monazite and langbeinite structural types in complex americium and plutonium phosphates. Synthesis and X-ray diffraction studies

The existence of phosphates containing Am + Pu with monazite-type structure and of those containing Am + Zr with langbeinite-type structure was substantiated on the basis of the main principles of isomorphism. The compounds were synthesized and examined by X-ray diffraction. Samples of the composition CdAmPu(PO4)3 crystallized in the monazite structural type. In the sample containing Am + Zr, in which the formation of a langbeinite-type compound was expected by analogy with lanthanides and Zr [isoformula phosphates K2LnZr(PO4)3, where Ln = Ce-Yb, are known], a mixture of monazite and kosnarite phases was obtained from the chosen reactants under the chosen conditions.

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.

Orthophosphates of Langbeinite Structure for Immobilization of Alkali Metal Cations of Salt Wastes from Pyrochemical Processes

Mixtures of alkali metal chlorides used as electrolytes in pyrochemical processes for nuclear fuel production and spent nuclear fuel reprocessing were subjected to conversion into orthophosphate ceramic materials based on langbeinite mineral. The results of the solid-state synthesis of the phosphates are described. The phase and chemical composition, chemical stability, and heat resistance of the orthophosphates were studied by X-ray phase analysis, X-ray spectrum microanalysis, emission spectrum analysis, and scanning electron microscopy. Quantitative incorporation of alkali metal cations, including Cs, into such materials was proved. The use of a binder (89% Bi2O3 + 11% NaF mixture) considerably increases the density of the ceramic prepared and substantially enhances its chemical stability.