Hot-pressed phosphate glass–ceramic matrix composites containing calcium phosphate particles for nuclear waste encapsulation

Sodium aluminium phosphate (NaAlP) glass–ceramic composites were produced as potential wasteforms for the immobilization of special categories of halide-containing radioactive waste. Sintering conditions for encapsulating a simulated waste (a calcinated mixture of calcium phosphate host and various oxides) in the cold-pressed NaAlP glass–ceramic were first determined and the results were compared with similar samples prepared by hot pressing. In both cases, the conditions aimed to provide a very high-density material, via as low production temperatures as possible, in conjunction with a high waste loading (75 wt.% simulated waste to 25 wt.% glass). It was found that by hot pressing and using a NaAlP glass–ceramic containing 2 mol% B₂O₃, significantly lower temperatures could be employed compared to the cold pressing and sintering route. The lowest temperature at which a sufficiently dense hot-pressed product was achieved (86% theoretical density), that exhibited mechanical properties similar to those of borosilicate glass (e.g. Young’s modulus 67 ± 2 GPa), was 550 °C. This processing temperature is considerably lower than values reported in the literature for similar systems. As such, hot pressing can be considered as a convenient technique for the fabrication of this type of composite for waste encapsulation.     

Long-term field and laboratory leaching tests of cemented radioactive wastes

Experiments with real and simulated radioactive cementitious wasteforms were set up to compare the leaching behaviour of cementitious wasteforms containing nuclear power plant operational waste in field and laboratory test conditions. Experiments revealed that the average annual (137)Cs leach rate in deionised water was about thirty-five times greater compared with the measured average value for the 1st year of the field test. Cumulative leached fraction of (137)Cs for 1st year (3.74%) was close to values reported in literature for similar laboratory experiments in deionised water, however more than two orders of magnitude higher than the 1st year leached fraction of (137)Cs in the repository test (0.01%). Therefore, to compare field and laboratory test results, a scaling factor is required in order to account for surface to volume factor difference, multiplied by a temperature factor and a leach rate decrease coefficient related to the ground water composition.     

The crystal-chemical principle in designing mineral-like phosphate ceramics for immobilization of radioactive waste

The crystal-chemical principle of designing multicomponent compounds of the desired structure was applied to the development of solid matrices for immobilization of radioactive waste components. The approach to designing mineral-like crystalline materials was based on the structural features and isomorphism concept. The structural types of monazite, kosnarite (NZP), and langbeinite were considered as matrices for the incorporation of simulated wastes containing f elements and certain uni-, bi-, and trivalent elements involved in radiochemical processes. Phosphates of complex cationic compositions were prepared and studied by X-ray phase analysis and scanning electron microscopy. The main phases of the expected structures (monazite, kosnarite, langbeinite) and additional phases (Al and Fe phosphates, Zr pyrophosphate, Pd metal) were identified. The phase formation in the temperature range 80–1300°C was analyzed.     

Extraction of Long-Lived Radionuclides from High-Level Alkaline Waste with <Emphasis Type="Italic">p</Emphasis>-Alkylcalix[8]arene

p-Alkylcalix[8]arene containing six tert-butyl and two isononyl groups on the upper rim of the macrocycle was suggested for extraction recovery of long-lived β- and α-emitting nuclides from high-level alkaline waste. The influence of the compositions of the organic and aqueous phases on the extraction efficiency was examined. After four contacts of the extractant solution with the real high-level alkaline solution, the activity of the major dose-producing radionuclide, ¹³⁷Cs, decreases by two orders of magnitude, and that of α-emitting radionuclides, by a factor of 3.5–40 depending on the initial solution pretreatment. The degree of stripping of the radionuclides after the first contact of the loaded organic phase with the acid exceeds 99%. p-Alkylcalix[8]arenes show promise for treatment of high-level alkaline waste at the Mayak Production Association.     

Isomorphism in Crystalline Phosphates of the NaZr2(PO4)3 Structural Type and Radiochemical Problems

The problems of isomorphism in phosphates that are structural analogs of sodium zirconiumorthophosphate NaZr₂(PO₄)₃ (NZP) are discussed. Data on the crystal chemistry of orthophosphates of theframework structure are systematized according to the suggested classification of their formula types. Thetheoretically possible, actually occurring isomorphism of cations in phosphates, in particular, isomorphisminvolving f-element cations, is discussed. The isomorphous capacity of the NZP structure toward variouscations was evaluated. Data on isomorphism in phosphates are necessary for choosing a suitable single-phase ceramic matrix for incorporation of radionuclides and adjusting its composition to actual nuclear waste.     

Crystal chemistry of the NaZr2(PO4)3, NZP or CTP,structure family

The NaZr₂(PO₄)₃ type structure (abbreviated as NZP or CTP, CaTi₄(PO₄)₆), has emerged as a new family, which has extraordinary technological utility in three fields: fast-ion conductors, radwaste solidification and zero expansion ceramics. NZP or CTP is formed by an extraordinary range of discrete compositions and crystalline solutions. In this paper these compositions are classified according to their crystal chemical substitution scheme, and some uncommon trends in the systematic variation of their lattice parameters are shown. Some of the major trends are explained by correlation with the rotation of polyhedra in the structure.     

Crystal chemistry of sodium zirconium phosphate based simulated ceramic waste forms of effluent cations (Ba(2+), Sn(4+), Fe(3+), Cr(3+), Ni(2+) and Si(4+)) from light water reactor fuel reprocessing plants

A novel concept of immobilization of light water reactor (LWR) fuel reprocessing waste effluent through interaction with sodium zirconium phosphate (NZP) has been established. Such conversion utilizes waste materials like zirconium and nickel alloys, stainless steel, spent solvent tri-butyl phosphate and concentrated solution of NaNO(3). The resultant multi component NZP material is a physically and chemically stable single phase crystalline product having good mechanical strength. The NZP matrix can also incorporate all types of fission product cations in a stable crystalline lattice structure; therefore, the resultant solid solutions deserve quantification of crystallographic data. In this communication, crystal chemistry of the two types of simulated waste forms (type I-Na(1.49)Zr(1.56)Sn(0.02)Fe(0).(28)Cr(0.07)Ni(0.07)P(3)O(12) and type II-Na(1.35)Ba(0.14)Zr(1.56)Sn(0.02)Fe(0).(28)Cr(0.07)Ni(0.07)P(2.86)Si(0.14)O(12)) has been investigated using General Structure Analysis System (GSAS) programming of the X-ray powder diffraction data. About 4001 data points of each have been subjected to Rietveld analysis to arrive at a satisfactory structural convergence of Rietveld parameters; R-pattern (R(p))=0.0821, R-weighted pattern (R(wp))=0.1266 for type I and R(p)=0.0686, R(wp)=0.0910 for type II. The structure of type I and type II waste forms consist of ZrO(6) octahedra and PO(4) tetrahedra linked by the corners to form a three-dimensional network. Each phosphate group is on a two-fold rotation axis and is linked to four ZrO(6) octahedra while zirconium octahedra lies on a three-fold rotation axis and is connected to six PO(4) tetrahedra. Though the expansion along c-axis and shrinkage along a-axis with slight distortion of bond angles in the synthesized crystal indicate the flexibility of the structure, the waste forms are basically of NZP structure. Morphological examination by SEM reveals that the size of almost rectangular parallelepiped crystallites varies between 0.5 and 1.5 microm. The EDX analysis provides the analytical evidence of immobilization of effluent cations in the matrix. The particle size distributions of the material along selected reflecting planes have been calculated by Scherrer's formula.     

Studies of structural and magnetic properties of sol–gel synthesized cobalt substituted Zn–Mn chromites

Cobalt substituted Zn–Mn chromites Zn_1?xCo_xMnCrO₄ (0 ≤ x ≤ 1) were prepared by sol–gel method. The synthesized material was characterized by various physico-chemical methods. The temperature of decomposition, crystallization and phase transformation of the as-prepared powder was studied using TGA–DTA measurement. X-ray diffraction patterns revealed that all compositions are in single phase cubic structure. An infra-red spectral study shows two strong bands, which is attributed to the intrinsic vibrations of tetrahedral sites and the octahedral sites. Formation of spherical particles was revealed by scanning electron microscopy analysis. The elemental analysis as obtained from energy dispersive analysis of X-ray is in close agreement with the expected composition from the stoichiometry of reactant solutions used. Magnetic properties were measured by vibrating sample magnetometer. All the compositions indicated ferrimagnetic nature. The substitution of cobalt ions in the lattice affected the structural as well as magnetic properties of spinel.     

Molybdenum Fixation in Crystalline NZP Matrices

A possibility of fixation of molybdenum present in spent nuclear fuel in ceramic matrices with the structure of the NaZr₂(PO₄)₃ (NZP) type was studied. The crystallochemical features of molybdenum incorporation into various crystallographic NZP structures depending on the synthesis conditions were considered. New molybdate-phosphates of variable composition Na _1-x ZrMo_xO₁₂ (0 x 0.6), crystallizing in the NZP crystal type, were prepared and characterized by X-ray diffraction and IR spectroscopy. The synthesis conditions and the concentration and temperature fields of stability of molybdate-phosphates in the system Na₂O-ZrO₂-MoO₂-P₂O₅ were studied. The crystallographic parameters of single-phase samples were evaluated. The results obtained suggest that the basic factors of formation of chemically stable single-phase NZP ceramics incorporating MoO₄ ^{2 -} anions are the composition of wastes and oxidative synthesis conditions.     

制备条件对NZP族磷酸盐粉体材料物性的影响

用化学共沉淀法配合高温煅烧合成了几种不同化学组成的NZP族磷酸盐粉体材料,从实验结果和对共沉淀反应过程的理论分析两方面入手,着重研究了反应体系的pH值与形成单相NZP族晶体化合物的关系,以及煅烧温度对NZP族粉体比表面积的影响规律.结果表明:控制液相反应过程的pH值=8.5有利于形成单相的NZP族晶体化合物,结晶形态完整的NZP族化合物的比表面积＜30m2/g,在低于晶化温度下煅烧则可以制备大比表面积的NZP族粉体.此外,本研究所合成的NZP族化合物在pH=3～9的水溶液体系中表面带负电荷.     

Measurements of enthalpy and phase equilibrium for simulated natural gas mixtures and correlation of the results by a modified Starling equation

New direct measurements of isobaric enthalpy change are reported for a number of 6-component simulated natural gas mixtures containing from 70 to 97% CH₄, 4 to 28% N₂ and higher hydrocarbons to C-4. The pressures explored were in the range of 1.3–5.7 MPa and the temperatures embraced a range starting from about 370K down to a minimum exit temperature of 153K. When phase separation occurred the measured compositions of the co-existing phases are reported.A modified version of the generalised Starling equation was developed in which the parameter γ is made composition dependent since it exerts a major influence on liquid phase behaviour. The resulting interaction parameter was evaluated for 17 binary mixtures of hydrocarbons from methane to n-decane and was expressed as a function of reduced pressure and temperature. Using the appropriate combination rules to go from binary to multicomponent behaviour, this version of the Starling equation was tested against the new experimental data.     

[NZP], a new radiophase for ceramic nuclear waste forms

The NZP structure is a candidate for immobilization of certain types of nuclear waste. It will incorporate ¹³⁷Cs, ⁹⁰Sr, and a range of other nuclides. The leach resistance of CsZr₂(PO₄)₃ appears to be comparable with that of other phases under consideration for radio-Cs immobilization. This phase can be formed by sintering at ～850°C; it is reasonably refractory, and it is compatible with monazite, a favored immobilizing agent for waste actinides.     

A family of phosphates of langbeinite structure. Crystal-chemical aspect of radioactive waste immobilization

A systematic analysis of orthophosphates of framework (tetrahedral-octahedral) structure, belonging to langbeinite structural type (cubic cell, space group P2₁3), is performed. The langbeinite structure is considered as favorable for environmentally safe crystalline forms of radioactive waste solidification. Experimental data on the compositions of known phosphates of such structure, on isomorphism of cations in various crystallographic positions, and on specific features of distribution of various cations in them are summarized. Possible formula compositions of isostructural phosphates are calculated. Some of them have already been synthesized (data given in this paper), whereas other phosphates are yet unknown. The structural-chemical aspect was also applied to langbeinite-like phosphates of f elements, some of which were prepared in accordance with the “crystal-chemical prediction.” Phosphates of the rhombohedral [NaZr₂(PO₄)₃ type, NZP] and cubic (langbeinite type) tetrahedral-octahedral framework modifications, their cationic compositions, and structural features are compared. Properties of phosphates of langbeinite structure, useful in the development of new materials for radiochemical applications, and examples of possible use of crystal-chemical principles in solution of these problems are discussed.     

Synthesis of castable sodium zirconium phosphate monoliths employing reactions between zirconyl nitrate hydrate and condensed phosphates

Reactions between zirconyl nitrate hydrate and condensed phosphates can be used to produce castable low CTE sodium zirconium phosphate (NZP) monoliths. Reaction between sodium nitrate, zirconyl nitrate hydrate and condensed phosphoric acid at room temperature (alkali nitrate method) produces monoliths having a heterogeneous microstructure, which are multiphasic in appearance. Except for the presence of crystalline sodium nitrate, they are X-ray amorphous. Differential thermal analysis revealed two distinct exothermic crystallization events when these materials are heated. The first event, with an onset temperature of 650°C, is the result of NZP and ZrO₂crystallization. The second is the result of ZrP₂O₇ crystallization. Reaction between zirconyl nitrate hydrate and condensed sodium phosphate (condensed alkali phosphate method) results in a more homogeneous microstructure in which crystalline zirconium hydrogen phosphate hydrate and sodium nitrate are present. Two exothermic peaks, with onset temperatures of approximately 570 and 860°C, are observed. The first exotherm is the result of NZP, ZrO₂ and ZrP₂O₇ crystallization; the second exotherm is the result of a further NZP formation. After heating materials made by these two methods at 940°C for 24 h, the condensed-alkali-phosphate-method-derived material converted to phase-pure NZP, while the alkali-nitrate-method-derived material contained ZrP₂O₇. The differences in phase evolution between the materials prepared by these two methods are attributable to the differences in chemical and microstructural homogeneity that result from the reactants used. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation of surface layers in leaching of borosilicate glasses incorporating different amounts of simulated HLW

Borosilicate glass specimens incorporating 10, 15, and 20 wt % simulated high-level waste (HLW) were leached at 20, 90, and 120°C in deionized water following the procedure accepted in Russia, GOST (State Standard) R 52 126-2003. At 90°C, the altered surface layer is formed already from the first days of leaching. The main interaction in the system occurs at the phase boundary between the altered surface layer and water, rather than at the phase boundary between the glass and water through the porous surface layer as it is commonly believed. The altered surface layer acts as a natural barrier preventing transfer of simulated waste components to the liquid phase. The barrier properties of the altered surface layer are manifested to the greatest extent at 20 wt % content of simulated HLW in the initial glass and leaching temperature of 120°C.     

Ion beam mixing to study the metallic glass formation of the Cu-Zr system

An ion beam mixing experiment for Cu-Zr system was conducted and two supersaturated solid solutions were observed with compositions of 16 atom% Zr in Cu and 17 atom% Cu in Zr, respectively, which are much greater than almost nil found from the equilibrium phase diagram of Cu-Zr system. The observation indicates that Cu-Zr metallic glasses could possibly be obtained in composition range bounded by the two observed solid solubilities, i.e. 16-83 atom% Zr. Besides, a unique Cu₆₅Zr₃₅ metallic glass was obtained by ion beam mixing and its composition is very close to the so-called best composition referred in the literature.     

Matrices for immobilization of the rare earth–actinide waste fraction,synthesized by cold crucible induction melting

The structure of eight samples containing simulated rare earth–actinide fraction of high-level waste was studied. Samples of weight from 0.2 to 6 kg were prepared by cold crucible induction melting followed by crystallization of the melt. The target phases (britholite, pyrochlore, zirconolite, rhombic and monoclinic rare earth titanates) prevail in all the matrices; glass, zirconolite, and rutile were detected as impurities, sometimes in significant amounts. These phases do not contain waste components (rutile) or are stable in solutions (zirconolite); therefore, their presence should not impair the properties of the matrix. The possibility of controlling the phase composition of the matrix by introducing zirconium or aluminum oxide into the charge was demonstrated.     

Effect of mechanical depoling on piezoelectric properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–<Emphasis Type="Italic">x</Emphasis>BaTiO<Subscript>3</Subscript> in the morphotropic phase boundary region

The effect of mechanical stress on the direct piezoelectric properties of pre-poled (1 ? x)(Na_0.5Bi_0.5)TiO₃–xBaTiO₃ (NBT–xBT) in the range 4% ≤ x ≤ 13% was studied in situ using a mechanical load frame. Prior to mechanical loading, compositions near the morphotropic phase boundary (MPB, x = 6–7% BT) exhibited enhanced ferroelectric and piezoelectric properties compared to compositions further from the MPB. Specifically, the lowest ferroelectric coercive field and highest piezoelectric coefficient within this composition range occur at x = 7% BT. During mechanical compression, the MPB compositions exhibited the lowest depoling stress. The results demonstrate that, while favorable piezoelectric and ferroelectric properties can be obtained at compositions near the MPB, these compositions are also the most susceptible to the effects of mechanical depoling. Ferroelastic domain wall motion is suggested as the primary factor that may be responsible for these behaviors.     

Interaction of plutonium with iron- and chromium-containing precipitates under the conditions of reservoir bed for liquid radioactive waste

The behavior of Pu under the conditions of liquid radioactive waste (LRW) disposal in a reservoir bed was studied. The pH dependence of the Pu sorption is the same in the cases when the radionuclide is introduced together with the simulated LRW solution or after the precipitate formation. The composition and morphology of Fe,Cr-containing precipitates formed under the conditions simulating the radioactive waste disposal were determined. The solid phase was characterized by X-ray diffraction analysis and by transmission and scanning electron microscopy. The phase composition of the precipitates was determined in relation to the presence of acetate ions, Fe concentration in the solution, and storage time and temperature. The main phases formed are ferrihydrite, goethite, hematite, and grimaldiite.     

Thermal stability and radiation resistance of trifluoromethyl phenyl sulfone in the presence of nitric acid

The effect of HNO₃ on the thermal stability and radiation resistance of trifluoromethyl phenyl sulfone (FS-13) used as extractant diluent in high-level waste partitioning was studied. Heating of FS-13 in the presence of 14 M HNO₃ is not accompanied by exothermic effects up to 160°С. The amount of gaseous products released in the course of heating the system consisting of FS-13 and HNO₃ and the composition of the major radiolysis products of the system components at irradiation doses of up to 0.5 MGy were determined.