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Bulk properties such as glass transition temperature, density and thermal expansion of iron phosphate glass compositions, with replacement of Cs by Ba, are investigated as a surrogate for the transmutation of 137Cs to 137Ba, relevant to the immobilisation of Cs in glass. These studies are required to establish the appropriate incorporation rate of 137Cs in iron phosphate glass. Density and glass transition temperature increases with the addition of BaO indicating the shrinkage and reticulation of the iron phosphate glass network. The average thermal expansion coefficient reduces from 19.8 × 10−6 K−1 to 13.4 × 10−6 K−1, when 25 wt. % of Cs2O was replaced by 25 wt. % of BaO in caesium loaded iron phosphate glass. In addition to the above bulk properties, the role of Ba as a network modifier in the structure of iron phosphate glass is examined using various spectroscopic techniques. The FeII content and average coordination number of iron in the glass network was estimated using Mössbauer spectroscopy. The FeII content in the un-doped iron phosphate glass and barium doped iron phosphate glasses was 20, 21 and 22 ± 1% respectively and the average Fe coordination varied from 5.3 ± 0.2 to 5.7 ± 0.2 with increasing Ba content. The atomic scale structure was further probed by Fe K-edge X-ray absorption spectroscopy. The average coordination number provided by extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure was in good agreement with that given by the Mössbauer data.

In Ca doped CePO 4 monazite (viz., Ce 0.8 Ca 0.2 PO 4 ) cerium exhibits mixed valence state (+3 and +4) and therefore can accommodate cations of any valence (by converting Ce 3+ to Ce 4+ or vice versa) subject to the size and other restrictions. A simulated waste form prepared based on the versatile monazite phase, Ce 0.8 Ca 0.2 PO 4 , showed higher leaching for Cs and Mo indicating that these elements do not enter the monazite lattice. Iron phosphate glass (IPG) is an alternate glass waste form considered for HLW immobilization. A glass–ceramic composite of crystalline monazite and IPG could have synergistic effects and can improve the leaching resistance. The preparation and characterization of a versatile monazite–IPG glass–ceramic waste form with 20 wt.% simulated HLW oxides and its leaching behavior are reported in this paper. The monazite–IPG glass–ceramic simulated waste form is found to have excellent chemical durability.

Alkaline earth (Ca, Sr, Ba) molybdates were synthesized by solid state reaction route. The compounds were characterized by powder-XRD, TG–DTA techniques. The electrical conductivities of these compounds were measured by AC-impedance technique at 673–1073 K. The activation energies of electrical conduction of CaMoO 4 , SrMoO 4 and BaMoO 4 were found to be 1.29 ± 0.01 eV, 1.33 ± 0.01 eV and 1.31 ± 0.01 eV respectively. The linear thermal expansion of these molybdates was measured by dilatometry. The mean coefficients ( α m ) of thermal expansion for these compounds were found to be in the range of 9.38 ± 0.18 × 10 −6 –12.96 ± 0.25 × 10 −6 K −1 at 305–1005 K temperature range. The diffusion coefficient ( D ) values of oxide ion conduction for these molybdates were determined and found to be in the range of 9.48 ± 0.02 × 10 −14 –3.32 ± 0.01 × 10 −10 for CaMoO 4 , 5.86 ± 0.02 × 10 −14 –2.50 ± 0.01 × 10 −10 for SrMoO 4 and 3.46 ± 0.02 × 10 −14 –1.22 ± 0.01 × 10 −10 cm 2 s −1 for BaMoO 4 at 673–1073 K range of temperature.

Borosilicate glass (BSG) bonded composites of Sr-chloroapatite (SrApCl) and its simulated pyrochemical waste loaded compositions were synthesized by solid state reaction route. These compositions were characterized by XRD and SEM. Percentage (%) thermal expansion of the composites was measured by high temperature XRD (HT-XRD) and dilatometry. 10–16 wt.% waste loaded composites show minor variations in % linear expansion at 298–900 K. The composite with 10 wt.% waste loading and without BSG encapsulation exhibited slightly higher % of expansion compared to other compositions above 700 K. The coefficient of thermal expansion ( α m ) measured by HTXRD and dilatometry are found to be of the same order of magnitude. The % expansion measured by dilatometry shows higher values for waste loaded compositions compared to SrApCl; whereas values measured by HTXRD show a decreasing trend with respect to pristine SrApCl. The instantaneous coefficient of thermal expansion ( α i ) is used to determine the glass transition temperature (T g ) of the glass-bonded composites from the dilatometry data. The α i measured by HTXRD for the composites ranges from 2.40(1) × 10 −7 to 9.51(2) × 10 −6 K −1 along a -axis, 9.28(1) × 10 −8 to 7.68(1) × 10 −6 K −1 along c -axis and. α m measured by dilatometry ranges from 3.80(1) × 10 −7 to 13.14(1) × 10 −6 K −1 .

This article discusses the charge compensation mechanism in Ca and U - substituted monazite (CePO4). The monazite compositions Ce0.6Ca0.2U0.2PO4, Ce0.8Ca0.2PO4 and CePO4 were prepared by solution chemistry route. XRD and μ-Raman studies confirm that the compounds are single phase materials. XPS studies on Ce0.8Ca0.2PO4 shows the presence of Ce4+ in addition to Ce3+ whereas in Ce0.6Ca0.2U0.2PO4 cerium is in +3 state with uranium in +4 valence state. Partial substitution of Ca in CePO4 (i.e. Ce0.8Ca0.2PO4), leads to the generation of Ce4+ which subsequently reverts to Ce3+ in Ce0.6Ca0.2U0.2PO4 upon U substitution. The Ce3+⇌Ce4+ intrinsic charge compensation existing in monazite, essential for the incorporation of higher valence cations, is demonstrated. This mechanism assists the incorporation of M2+/4+ in the monazite lattice without requiring coupled substitution.

Four different glass compositions were used to encapsulate strontium chloroapatite (SrApCl) ceramics along with 10 mass/% simulated chloride waste to fabricate glass-bonded composites for nuclear waste immobilization applications. The glass compositions chosen were borosilicate glass (BSG), aluminoborosilicate glass (AlBSG), barium borosilicate glass (BaBSG) and lead borosilicate glass (PbBSG). The composites were fabricated by heat-treating the reaction mixtures at 1,073 K in air. The Products were characterised by powder XRD. The % linear thermal expansion of these composites was measured by dilatometry. The average thermal expansion coefficients (αav) of these composites were found to be in the range of 14.5 ± 0.29 × 10−6–16.4 ± 0.32 × 10−6 K−1 at 323–715 K range of measurement. The glass transition temperature (Tg) of the glass–ceramic composites were also measured by dilatometry. Tg and heat capacity (Cp) of SrApCl-10w20BSG, SrApCl-10w20AlBSG, SrApCl-10w20BaBSG and SrApCl-10w20PbBSG were also measured by differential scanning calorimetry (DSC), and the values obtained are 730 ± 22, 718 ± 21, 700 ± 20 and 700 ± 20 K, respectively. The Cp values for SrApCl-10w20BSG, SrApCl-10w20AlBSG, SrApCl-10w20BaBSG and SrApCl-10w20PbBSG are 0.57 ± 0.006, 0.60 ± 0.006, 0.52 ± 0.005 and 0.54 ± 0.005 J K−1 g−1, respectively.

Detailed characterization was carried out on an iron phosphate glass waste form containing 20 wt.% of a simulated nuclear waste. High temperature viscosity measurement was carried out by the rotating spindle method. The Fe 3+ /Fe ratio and structure of this waste loaded iron phosphate glass was investigated using Mossbauer and Raman spectroscopy respectively. Specific heat measurement was carried out in the temperature range of 300–700 K using differential scanning calorimeter. Isoconversional kinetic analysis was employed to understand the crystallization behavior of the waste loaded iron phosphate glass. The glass forming ability and glass stability of the waste loaded glass were also evaluated. All the measured properties of the waste loaded glass were compared with the characteristics of pristine iron phosphate glass.

Monazite is one of the candidate ceramic matrices for the immobilization of high level radioactive waste (HLW) from the reprocessing of spent nuclear fuel. The monazite phase, Ce0.8Ca0.2PO4, can accommodate cations of different valences due to the mixed valence state (+3 and +4) of Ce in this compound, by facilitating the oxidation and reduction of the Ce3+ and Ce4+ as required by the in-coming cation. This will assist in accommodating HLW of different compositions in the monazite crystal structure even if the average valence of the HLW elements is other than 3. Therefore, the monazite phase, Ce0.8Ca0.2PO4, can be a versatile host for the immobilization of HLW. The enthalpy increment and heat capacity of this versatile monazite phase and a simulated waste form based on it with 20 mass% HLW oxides were measured by drop calorimetry in the temperature range from 373 to 873 K, and the results are compared with those measured for CePO4.

This paper describes the melt viscosity behaviour and the crystallization mechanism of a series of iron phosphate glasses. High temperature viscosity measurements were carried out on pristine iron phosphate glass and a series of cesium loaded iron phosphate glasses in order to understand the effect of addition of Cs 2 O on viscosity of iron phosphate glasses. Activation energy of viscous flow was estimated from the experimental data by applying Arrhenius model of viscosity–temperature relationship. Activation energy of viscous flow is observed to be strongly correlated to glass transition temperature of these glasses. Fragility of iron phosphate and cesium loaded iron phosphate glass systems were also evaluated in region of high temperature. Crystallization of these glasses was studied using thermal analysis techniques. Temperature integral approximation free method was utilized to evaluate the kinetic parameters such as activation energy of crystallization ( E c ) and Avrami exponent ( n ). The value of Avrami exponent ‘ n ’ obtained showed that the glasses under present study crystallize via bulk crystallization mechanism, i.e., nucleation and three dimensional growth.

Various compositions of Sr1−x La x MoO4+δ (x = 0.05, 0.1, 0.2, and 0.3) compounds were prepared by solid state reaction route. The samples were characterized by powder-XRD, TG-DTA, and SEM-EDAX techniques. Formation of single crystalline phases of Sr1−x La x MoO4+δ was confirmed from powder-XRD patterns. The thermal stability of La-doped SrMoO4 compounds was investigated by TG-DTA. Uniform grain distribution was observed in the SEM image of 10-20 mol.% La-substituted compositions. Needle-shaped structures were observed in the SEM image of Sr0.3La0.1MoO4+δ and were confirmed to be La2Mo2O9 by XRD examination. The electrical conductivity of these compounds was measured by AC-impedance technique in the temperature range of 373-1073 K in air ambience and compared with that of pristine SrMoO4. The electrical conductivity was found to decrease for La-substituted SrMoO4 compared to pristine SrMoO4. The diffusion coefficient calculated from the electrical conductivity was found to be in the range of 1.94 ± 0.02 × 10−13 to 1.15 ± 0.01 × 10−11 cm2/S at 873-1173 K for substituted composition and 3.47 ± 0.02 × 10−13 to 2.48 ± 0.01 × 10−10 cm2/S for pristine SrMoO4 at 673-1073 K temperature range.

Synthetic analogues of monazite are considered as an alternate ceramic host for the immobilization of high level waste (HLW) generated in the nuclear fuel cycle. The Ca doped CePO 4 (Ce 0.8 Ca 0.2 PO 4 ) is expected to be a versatile monazite phase for this purpose due to the mixed valence state of cerium in this compound which assists in accommodating cations of any valence by initiating the internal redox reaction, viz., Ce 3+ ⇌ Ce 4+ + e − , as required by the incoming cation for charge neutrality of the lattice. The thermal expansion of this versatile monazite phase with 20 wt.% simulated HLW was measured using high temperature x-ray diffractometer (HTXRD) in the temperature range of 298–973 K and are reported here. The thermal expansion behaviour is found to be similar for CePO 4 , Ce 0.8 Ca 0.2 PO 4 and Ce 0.8 Ca 0.2 PO 4 with 20 wt.% simulated HLW with the average volume expansion coefficient of 27.4, 26.5 and 24.1 × 10 −6 K −1 respectively in the temperature range of 298–973 K.

Electrical conductivity and percentage linear thermal expansion of the borosilicate glass (BSG) and simulated waste-loaded borosilicate glass (BSGW) were measured in the temperature range of 300–780 K and compared. Pronounced increase in electrical conductivity was observed around glass transition temperature (T g) of BSG and BSGW. The activation energy (E a) of electrical conduction determined from the measured data for BSG and BSGW is 0.961 ± 0.005 and 0.960 ± 0.005 eV, respectively. The % average linear thermal expansion of BSGW showed a slight decreasing trend compared with pristine BSG. The average coefficient of thermal expansion determined from dilatometry data is 12.87 ± 0.24 × 10−6 and 11.94 ± 0.23 × 10−6 K−1 for BSG and BSGW, respectively. The T g measured by dilatometry is 806 ± 24 K for BSG and 790 ± 23 K for BSGW, respectively. The T g measured by DTA was found to be 820 ± 7 and 805 ± 5 K for BSG and BSGW, respectively, for heating cycle. The T g values obtained from DSC measurements are 805 ± 5 and 803 ± 5 K for BSG and BSGW, respectively. The T g of BSGW showed a slight decrease compared with that of BSG. The values obtained by DSC examination also showed the lowering of T g values for the waste-loaded composition. The lowering of T g may be attributed to the interaction of glass-forming agents and simulated waste elements.

Sodium titanium phosphate (NTP), a potential host for immobilization of high level nuclear waste resulting from the reprocessing of spent nuclear fuel was synthesized by two simple wet chemical methods and characterized. Based on the characterization of pristine NTP, one of these synthesis methods was employed for the preparation of simulated waste loaded NTP. The waste loading and the synthesis conditions were varied systematically to study phase behavior of the waste form. Up to 15 wt% simulated waste of fast reactor origin was immobilized and the final phase assemblage comprised of NTP as major phase and monazite like phase as secondary phase. The NTP waste form was characterized by XRD and SEM-EDS; elemental mapping showed the waste constituents to be homogenously distributed in the waste form. The leach resistance of the simulated waste form was studied and was found to exhibit excellent chemical durability.

High temperature thermal expansion and specific heat measurements were carried out on iron phosphate glass (IPG) and a series of cesium substituted iron phosphate glasses. Thermal expansion of the bare IPG and the effect of cesium loading on expansion behavior were determined by push rod dilatometry in the temperature range of 300–700 K in air. Specific heat was also measured in the same temperature range by differential scanning calorimetry (DSC) under flowing argon atmosphere. Enthalpy increment was computed using the specific heat data. The enthalpy increments were also measured using isoperibol drop calorimetry in air in the same temperature range. Specific heat was evaluated from the experimental enthalpy increment data. The experimentally measured enthalpy increment data were compared with those evaluated from DSC data for iron phosphate and cesium loaded iron phosphate glasses.

The activation energy of crystallization of iron phosphate glass (IPG) as a function of the fraction of crystallization was evaluated by Kissinger–Akahira–Sunose isoconversional method utilizing crystallization curves obtained from differential thermal analysis (DTA). The crystallization products were identified by using X-ray diffraction technique. Similar crystallization kinetic studies were carried out on IPG loaded with 5.5 mol% Cs2O. The glass stability and glass forming ability of IPG and various cesium loaded IPG were also estimated in terms of the characteristic temperatures obtained from DTA. The characteristic temperatures include glass transition, crystallization and liquidus temperature. The glass forming ability is correlated with the evaluated critical cooling rate of the glasses.

Ca-chloroapatite (CaApCl), glass-bonded CaApCl compositions loaded with 16–32 wt.% simulated pyrochemical chloride waste were prepared by mixing and heating (773–1023 K) apatite and borosilicate glass (BSG) forming reagents in appropriate ratios. The compositions were characterized by XRD, TGA/DTA, SEM, and EDAX. Among the products, 16–27 wt.% chloride waste loaded composition yielded phase pure Ca-chloroapatite and were resistant to leaching of Cl− and other ions. In case of 28–32 wt.% waste loaded compositions, even though formation of phase pure Ca-Chloroapatites was observed by XRD, the leaching of Cl− and other ions was found to be significant. Bulk thermal expansion behavior of the samples was studied by dilatometry. The 16 wt.% chloride waste loaded matrix showed nearly the same thermal expansion compared to pure Ca-Chloroapatites. The % linear thermal expansion of the matrices decrease on increasing the chloride waste loading; however, Ca-chloroapatite mixed with 20 wt.% BSG mixed matrix showed slightly higher thermal expansion. The coefficient of thermal expansion of borosilicate glass is the lowest among all the matrices measured. The coefficient of thermal expansion (CTE) is found to be 12.76 ± 0.64 × 10− 6 K− 1 for CaApl and 12.18 ± 0.63 × 10− 6 K− 1 for 16 wt.% waste loaded BSG-encapsulated CaApl in the temperature range of 298–780 K. The glass transition temperature of the waste loaded matrices is lower than that of the bare BSG and 20 wt.% BSG encapsulated Ca-chloroapatite.

Synroc waste forms containing a simulated nuclear waste of fast reactor origin were synthesized employing a simple and inexpensive wet chemical method by completely avoiding the use of organic reagents. Peroxytitanic acid prepared by dissolving titanium metal powder in an alkaline hydrogen peroxide solution was used as source for titania, the other constituents were taken as metal nitrates. A nano precursor with high surface area was obtained by co-precipitation, and it gave a dense synroc monolith on sintering. The precursor and final product were characterized by TG-DTA coupled with EG-MS, XRD, SEM and BET. � 2011 Elsevier B.V. All rights reserved.

The structure of cesium loaded iron phosphate glasses (IPG) was investigated using infrared and Raman spectroscopy. The spectra of the cesium doped samples revealed a structural modification of the parent glass owing to the incorporation of cesium. The structural changes could be correlated with the variation observed in the glass transition temperature of these glasses. Increased Cs-mediated cationic cross linking appears to be the reason for the initial rise in glass transition temperature up to 21 mol% Cs 2 O in IPG; while, breakdown of the phosphate network with increasing cesium content, brings down the glass transition temperature.

Alkaline earth chloroapatites, M10(PO4)6Cl2 (M = Ba, Sr, Ca) were prepared by solid state reaction route and characterized by XRD. The lattice constants of Ba10(PO4)6Cl2, Sr10(PO4)6Cl2, Ca10(PO4)6Cl2 were determined to be a = 10.27(1) A, c = 7.66(1) A; a = 9.87(2) A, c = 7.18(1) A and a = 9.52(1) A, c = 6.85(1) A respectively. The enthalpy increments were measured by inverse drop calorimetric method in the temperature range 523–1423 K using a high temperature calorimeter. Thermodynamic functions such as heat capacity, entropy and Gibbs energy functions in the temperature range 298–1500 K, were computed from the measured enthalpy increments.

Preliminary data were obtained on the immobilization of various heterovalent ions present in the two selected high-level nuclear waste compositions. The entire set of ions present in the waste compositions were immobilized into sodium zirconium phosphate (NZP) structure. The waste loading was in the range 5–25%. The two types of wasteforms loaded with the simulated high-level waste compositions were characterized by powder X-ray diffraction, FT-IR, TGA/DTA, and scanning electron microscopy. The difference in the compositions of the two simulated wastes was reflected in the waste loading percentage and the crystallization of the wasteforms into NZP structure. Up to 12% waste loading, single phase isostructural with NZP was obtained as a major product in the case of the first waste composition. An increase in the waste loading led to the segregation of ZrP2O7 as a secondary phase. With the second waste composition, an NZP-like phase was obtained as the major product even at 25% waste loading.

Separation of Pd from borosilicate glass-melt containing simulated high level radioactive waste (Pd, Mo, Cs, Sr, Zr, Nd, Ni, Fe) using Sn as the solvent metal was studied on lab scale. The separation of Pd was studied using varying amounts of Sn, and the percentage of extraction of Pd and other elements was investigated by varying the process temperature and the starting material for Pd. The process temperature was observed to have a profound influence on the separation of Pd from the glass melt to the metallic tin phase. The % extraction of Pd was found to be less for the batches heat-treated at temperatures below 900 °C. The decrease in the % extraction was due to the entrainment of Pd in the glass phase for the samples heat-treated at low temperatures. The entrainment of Pd leads to the formation of Pd-rose like structures in the glass phase as observed by SEM and EDAX. Similarly, Mo, Fe and Na also formed separate phases inside glass matrix when the samples were heated below 900 °C. However, entrainment was not seen when the samples were heat-treated above 1050 °C in Ar. At 1200 °C, a recovery of about 99% of Pd was obtained. The formation of a PdSn4 type intermetallic phase was noticed in tin metallic phase. The extraction behavior of Pd in borosilicate glass melt was also studied by using Pd(NO3)2 instead of Pd powder as staring material, and the results showed that the process temperature is the key determinant of the extraction behavior of Pd.

Ba10−x Cs x (PO4)6Cl2, (x = 0, 0.5) chloroapatite ceramics were prepared by sonochemical method of synthesis. The measured room temperature lattice parameters of Ba10 (PO4)6Cl2 and Ba9.5Cs0.5 (PO4)6Cl2−δ are practically the same; that is, a = 10.26 (8), c = 7.65 (7) and a = 10.27 (7), c = 7.65 (5), respectively. Heat capacity measurements were carried out on these materials by differential scanning calorimetry (DSC) in the temperature range 298–800 K. The heat capacity values of Ba9.5Cs0.5(PO4)6Cl2−δ are found to be slightly higher at all temperatures than those of Ba10(PO4)6Cl2. From the heat capacity data, other thermodynamic functions such as enthalpy and entropy increments were computed. The heat capacity values of Ba10(PO4)6Cl2 and Ba9.5Cs0.5(PO4)6Cl2−δ at 298 K are 0.3912 and 0.4310 J K−1 g−1, respectively. Thermal expansion property of the doped and undoped barium chloroapatites was measured by using a home built dilatometer which uses LVDT as displacement sensor. The bulk thermal expansion of Ba10(PO4)Cl2 and Ba9.5Cs0.5(PO4)Cl2−δ is observed to be about 0.9% in the temperature range of 298–973 K.

High level waste (HLW) originating from aqueous reprocessing of fast reactor fuels consists mainly of fission products, minor actinides and noble metals which are higher in concentration compared to that from thermal reactors. Borosilicate glass may not be an efficient matrix to fix this waste. Similarly, chloride HLW arising from the pyro-metallurgical reprocessing has poor solubility in borosilicate glass. The present study explores various alternate systems consisting of a glass (iron phosphate glass, IPG), ceramic (monazite) and glass -ceramic (apatite) for the immobilization of such HLW. Studies related to synthesis, thermal stability and waste loading, point to the potential of the three matrices to emerge as viable waste forms for different types of HLW

The crystallization kinetics of Cs 2 O–Fe 2 O 3 –P 2 O 5 glasses containing 12.5–27 mol% Cs 2 O were studied by using differential scanning calorimetry under nonisothermal conditions. Strong dependence of activation energy with temperature was observed, indicating the complex nature of the crystallization process. The various crystallization products were identified by X-ray diffraction technique. CsFeP 2 O 7 was found to be the major crystalline phase in all cases. The overall activation energy obtained by classical model-free kinetic method was compared with that of isoconversional method; and from the results, the dependence of activation energy on extent of reaction and average temperature was delineated.

Drop calorimetric studies were carried out on cubic BaTiO 3 and Ba 0.9 Nd 0.1 TiO 3 . Enthalpy increments were measured by inverse drop calorimetric method in the temperature range 573–1523 K using a multidetector high temperature calorimeter. Thermodynamic functions such as heat capacity, entropy, Gibbs energy functions in the temperature range 298–1600 K were computed from the measured enthalpy increments.

The high-temperature phase stability of Ca10−x Cs x (PO4)6(OH)2, (x = 0–3) compositions synthesized by various wet chemical methods was investigated. The thermal expansion property of Ca10(PO4)6(OH)2 (abbreviated as CaHAp) and Cs-substituted CaHAp was measured by high-temperature XRD and dilatometry. The average crystallite size of the powders synthesized by wet chemical methods was found to be 10–50 nm range as shown by XRD and TEM. Up to 30 mol% Cs loading was observed to show only the apatite phase by XRD when the apatite powder was nanocrystalline in nature. However, high-temperature stability of the Cs-substituted system is limited to ≤5 mol%. Cs3(PO4) is observed to be separated out on heating the material above 773 K for compositions substituted with more than 5 mol% of Cs in the Ca-sublattice. The coefficient of thermal expansion measured by HTXRD is αa = 12.42 × 10−6 K−1, αc = 14.98 × 10−6 K−1; and αa = 12.62 × 10−6 K−1, αc = 12.57 × 10−6 K−1 for CaHAp and Ca9.78Cs0.2(PO4)6(OH)1.96, respectively, in the temperature range of 298-1083 K. Bulk thermal expansion measurements are seen to be in agreement with the lattice expansion results.

Rare earth uranates Nd6UO12 ,G d 6UO12 and Eu6UO12 were prepared by combustion synthesis and characterized by XRD. Single-phase rhombohedral struc- ture was observed for all the above compounds. Heat capacity measurements were carried out on Nd6UO12 and Gd6UO12 with differential scanning calorimetry in the temperature range 298-800 K. Enthalpy, entropy and Gibbs energy functions were computed. Heat capacity values of Nd6UO12 and Gd6UO12 at 298 K are 436 and 400 J K -1 mol -1 , respectively. Thermal expansion char- acteristics were studied using high temperature X-ray dif- fraction (HTXRD) in the temperature range 298-873 K. The coefficients of thermal expansion measured for Eu6UO12 are 10.5 9 10 -6 and 7.3 9 10 -6 K -1 along a- and c-axis, respectively. Similarly, the coefficients of thermal expansion of Gd6UO12 along a-axis are 10.0 9 10 -6 K -1 and along c-axis is 9.7 9 10 -6 K -1 .

Les reacteurs surgenerateurs rapides a cycle du combustible ferme constituent la deuxieme des trois etapes du programme d'energie nucleaire indien. Un prototype de reacteur surgenerateur rapide est en cours de construction a Kalpakkam, et sa mise en service est attendue pour l'annee prochaine. Par la suite, une serie de reacteurs surgenerateurs rapides devraient etre construits et accompagnes d'une gamme complete d'operations du cycle du combustible associe. Pour garantir le fonctionnement economique des reacteurs au niveau industriel, des taux de combustion eleves et le retraitement des combustibles a court temps de refroidissement sont envisages. Afin de remplir les objectifs en matiere de production d'energie ainsi que les criteres de securite, de durabilite et de disponibilite elevee des centrales, plusieurs innovations destinees a etre integrees au cycle du combustible sont en cours d'etude. Ces innovations incluent notamment l'utilisation de differents types de combustibles nourriciers, de processus de fabrication du combustible, de schemas de retraitement et de matrices d'elimination des dechets. Le present article examine ces divers aspects.

The crystallisation kinetics of 36 mol% Cs 2 O–26 mol% Fe 2 O 3 –P 2 O 5 glass was studied by differential scanning calorimetry under non-isothermal conditions at various heating rates. X-ray diffraction technique was employed to understand the product of crystallisation. This glass crystallises to a mixture of CsFe(P 2 O 7 ), Cs 7 Fe 7 (PO 4 ) 8 O 2 and Cs 3 PO 4 . The activation energy of the crystallisation process was determined by classical multi heating rate methods like Kissinger, Mahadevan and Augis–Bennet. Heating rate dependence of the activation energy was observed when the data were analysed using Coats and Redfern method. This dependence of the activation energy was evaluated by the application of Kissinger–Akahira–Sunrose isoconversional method. It was observed that the activation energy of crystallisation was found to vary with the degree of conversion. The value of Avrami exponent ‘ n ’ obtained was 4, suggesting the mechanism of crystallisation to be bulk nucleation and three-dimensional growth.

Isotopes of cesium and strontium can be utilized as radiation source for various industrial and medical applications after their separation from high level nuclear waste. However, these elements need to be immobilized in a suitable matrix. In the present work, a systematic approach has been made to immobilize inactive cesium into iron phosphate glass. Up to 36 mol% of Cs 2 O has been loaded successfully without crystallization. The glass transition temperature of the cesium loaded glass was found to increase initially and then decrease as a function of Cs 2 O content. Mossbauer studies show that the concentration of Fe 3+ ions in the cesium loaded glasses is >95%. Volatilization experiments at 1263 K show that the weight loss is >0.5% for a period of 4 h. The 36 mol% of Cs 2 O loaded iron phosphate glass with high Fe 3+ content described in this paper is reported for the first time.

Strontium and calcium chloroapatites were synthesized by wet chemical method, characterized by X-ray diffraction and are found to be phase pure materials. The measured room temperature lattice parameter of Ca10(PO4)6Cl2 is a = 9.523 A, c = 6.855 A and for Sr10(PO4)6Cl2 is a = 9.876 A, c = 7.188 A. Heat capacity measurements were carried out on Ca10(PO4)6Cl2 and Sr10(PO4)6Cl2 by DSC in the temperature range 298–800 K. The heat capacity values of Sr10(PO4)6Cl2 is higher at all temperatures than Ca10(PO4)6Cl2. Enthalpy and entropy increments were computed. Heat capacity values of Ca10(PO4)6Cl2 and Sr10(PO4)6Cl2 at 298 K are 758 and 868 J K−1 mol−1, respectively.

X-ray photoelectron spectroscopic (XPS) studies were carried out on wet-chemically synthesized cubic BaTiO3, Ba0.9Nd0.1TiO3 and BaTi0.9Fe0.1O3−δ powders. The compounds were prepared by hydrothermal and gel to crystallite conversion technique; and phases formed readily at 420 K. The phase purity of the powders was confirmed from X-ray diffractometry. Chemical state and chemical environment of the constituent elements in the compositions were examined by XPS. Ba2+ was found to exist in two different chemical environments in these titanates. The Ti 2p3/2 photoelectron peak in BaTi0.9Fe0.1O3−δ was found to be broadened after Fe3+ substitution. Any resolvable broadening was not observed distinctly in the Ti 2p peak for Ba0.9Nd0.1TiO3, unsintered BaTiO3 and BaTiO3 annealed in hydrogen (8% H2 + Ar) at 1000 K. The prevalence of mixed-valent titanium and iron in BaTi0.9Fe0.1O3−δ composition was evident from the XPS results and was further supported by the enhanced electrical conductivity at 298–550 K for BaTi0.9Fe0.1O3−δ in comparison to BaTiO3 and Ba0.9Nd0.1TiO3. Hydroxyl incorporation was facilitated by substituting Nd3+ in Ba-sublattice. The presence of hydroxyls was observed from the broadening of the O 1s peak in XPS studies of the compounds.

Experiments were conducted to remove palladium (Pd) from molten borosilicate glass (BSG) containing simulated nuclear waste using lead (Pb) or aluminum (Al) as solvent metal. The removal of Pd from molten BSG could be carried out successfully when lead was used as the solvent metal, and the percentage extraction fraction was nearly 100%. Significant quantities of Ni, Mo, and Fe also settled into the solvent metal phase along with Pd. The metal phase found to contain intermetallic compounds of Pb2Pd and Pb9Pd13. However, when Al was used as the collecting metal separation of metal and glass phase did not occur. The results of the experiments are interpreted based on the thermodynamic stabilities of the oxides of the collecting metals and the constituent elements in the molten BSG. Even though the experiments were conducted in argon atmosphere, the traces of oxygen present in the Ar are sufficient to oxidize Al metal powder to its oxide, leading to the formation of alumino-silicates (mullites) in mo...

La0.5Li0.5TiO3 perovskite was synthesized by various wet chemical methods. By adopting low temperature methods of preparation lithium loss from the material is prevented. La0.5Li0.5TiO3 (LLTO) was formed with cubic symmetry at 1473 K. LLTO was formed at relatively lower temperature by using hydrothermal preparation method. PVA gel-decomposition route yield tetragonal LLTO on annealing the dried gel at 1473 K. By using gel-carbonate route LiTi2O4 minor phase was found to remain even after heat-treatment at 1473 K. The hydroxylation of LLTO was done in deionized water as well as in dilute acetic acid medium. By hydroxylation process incorporation of hydroxyls and leaching out of Li+ was observed from the material. The Li+ concentration of these compositions was examined by AAS. The electrical conductivities of these compositions were measured by dc and ac impedance techniques at elevated temperatures. The activation energies of electrical conduction for these compositions were estimated from the experimental results. The measured activation energy of Li+ conduction is 0.34 eV. Unhydroxylated samples exhibit only Li+ conduction, whereas, the hydroxylated LLTO show proton conductivity at 298–550 K in addition to Li+ conductivity. The effect of Zr or Ce substitution in place of Ti were attempted. La0.5Li0.5ZrO3 Perovskite was not formed; instead pyrochlore phase (La2Zr2O7) along with monoclinic ZrO2 phases was observed above 1173 K; below 1173 K cubic ZrO2 is stable. (La0.5Li0.5)2CeO4 solid solution was formed in the case of Ce substitution at Ti sublattice on heat-treatment up to 1673 K.

This paper describes the solid solubility of uranium in gadolinium zirconate (Gd2Zr2O7), a candidate crystalline matrix to immobilize actinide elements present in radioactive wastes. A series of uranium-containing gadolinium zirconate compositions were prepared by the citric acid–ethylene glycol resin precursor method. The final heat treatment was carried out at 1673 K in air. It was found that up to 40 at.% uranium could be incorporated at the Gd position. The pyrochlore ordering of the zirconate phase was lost above approximately 10 at.% uranium, and the structure transformed to fluorite. At the Zr position, the whole of Zr could be replaced by uranium, with its end member of mixed uranium–gadolinium oxide with fluorite structure. The oxidation state of uranium in the solid solutions was investigated by X-ray photoelectron spectroscopy.

Hydroxide perovskites of MSn(OH)6 (M = Ba, Ca, Mg, Co, Zn, Fe or Mn) were synthesized by co-precipitation and sonochemical reaction routes. Some of the compounds were also prepared by the hydrothermal method. These were characterized by XRD, TGA/DTA, TEM and IR studies. The hydroxide perovskites are stable up to ∼523 K and all the compounds stabilize in cubic crystal system (SG = Pn3m) at 298–523 K except BaSn(OH)6. The as-prepared powders of the above compositions show cubic square net in the reciprocal lattice of their electron diffraction patterns. Among these perovskites; MgSn(OH)6 and CoSn(OH)6 exhibit proton conduction at 298–500 K. Above ∼523 K, solid solutions of SnO2 and the corresponding MO components were observed. On heat-treatment at 850–950 K, M2−x(VM)xSnO4 (M-deficient spinel, x ≤ 1) along with SnO2 are observed. On further sintering to 1773 K for a duration of 6–7 h, M2−x(VM)xSnO4 phase is stabilized. The electrical properties of these compounds were measured by four-probe dc method. Co2−x(VCo)xSnO4 + SnO2 mixed phase showed oxide ion conductivity at 500–1273 K. The activation energy of oxide ion conduction is 0.93 eV. CaSn(OH)6does not show any type of electrical conduction at 298–850 K. The loss of proton conduction above 523 K may be ascribed to dehydroxylation and subsequent decomposition to SnO2 + M2−xSnO4 (defect spinel, M = Mg, Zn, Co) or MSnO3 (M = Ba, Ca).

LaGaO 3 , La 0.9 Ga 0.8 Mn 0.1 O 3− δ and La 0.9 Sr 0.05 Ga 0.8 Li 0.01 O 3− δ synthesized by the decomposition of metal nitrate–tartarate complex were found to yield the cubic phase on heat treatment at 1023–1273 K; above which they transformed to distorted cubic phase (orthorhombic). LaGaO 3 was also synthesized by the co-precipitation of metal hydroxides and form single phase above 1623 K. The nano-crystalline powders obtained from tartarate route were examined by TEM, electron diffraction, XRD, XPS, infra-red, TG/DTA and EGA–MS techniques. Electrical conductivity measurement was carried out by dc and ac methods on the sintered pellets at 298–1273 K in air. LaGaO 3 showed predominantly oxide ion conductivity above 650 K. The measured activation energy of conduction is ∼1.35 eV. La 0.9 Sr 0.05 Ga 0.8 Li 0.01 O 3− δ exhibited electrical conductivity above 550 K with diminished activation energy of ∼0.97 eV. Loss of lithium from the lattice on sintering the powders above 1173 K was observed. La 0.9 Ga 0.8 Mn 0.1 O 3− δ showed mixed ionic and electronic conduction in the temperature range 298–1273 K. The valence state of Mn in this compound was investigated by XPS.

Hydrothermally synthesised powders of BaTiO3 and its Fe- or Nd-doped analogues contain hydroxyl groups in the lattice substitutional to oxide ion, as confirmed from TGA/DTA, IR spectral analysis of D2O-treated powders, EGA-MS, the contraction in lattice constant with heat treatment by XRD and surface examination by XPS. Electrical resistivity measurements were carried out on the pellets from 298 to 1000 K by ac impedance spectroscopy and dc methods in dry or moist air and 8% H-2 + Ar environments. The electrical conductivity observed for unsintered pellets between 298 and 500 K, is in the range of 10(-3) to 10(-7) S/cm and can be attributed to the extrinsic hydroxyls in BaTiO3. The acceptor-doped composition, BaTi0.9Fe0.1O3-sigma: 2delta(OH) exhibits higher electrical conductivity than BaTiO3 or the donor-doped Ba0.9Nd0.1TiO3-delta: 2delta(OH) in moist air. The hydrothermally prepared powders heat treated below 1000 K having cubic symmetry at room temperature, possess higher proton conductivity and reabsorption capability for hydroxyls on exposure to moisture than the powders sintered at 1673 K (tetragonal symmetry). The conductivity at 298-500 K is due to the mobility of proton along O-H-O octahedra in the perovskite lattice. The conduction at 550-1000 K is a combined effect of proton as well as oxygen vacancy mobility in BaTiO3 and Ba0.9Nd0.1TiO3; electron hole (Ti4+, Ti3+, Fe3+, Fe2+) participation is the additional contribution in acceptor-doped composition in this temperature range.

Sodium zirconium phosphate, NZP, ideally NaZr 2 (PO 4 ) 3 , is a potential host matrix for nuclear waste. Thermophysical properties of these materials are necessary to assess the thermal stability of these compounds. This paper presents the synthesis and calorimetric measurements of some NZP-type compounds containing alkaline earth elements. It also describes the indigenously developed room temperature drop calorimeter for low temperature calorimetric measurements. Calibration of the instrument is done with standard reference materials. Enthalpy increment of these materials is measured using this setup in the temperature range 373–873 K, and specific heat data are derived. Temperature dependence of enthalpy increment and specific heat of these materials, and the influence of the chemical composition on these properties are discussed.

Powder X-ray diffraction data are reported for RE6UO12 (RE=Eu, Gd, and Dy). The powders were prepared by a solution combustion method using urea as fuel. All compositions exhibit a rhombohedral structure with hexagonal unit cell parameters of a=1.012 67 (9) nm, c=0.9601 (1) nm for Eu6UO12; a=1.008 78 (6) nm, c=0.954 24 (7) nm for Gd6UO12; and a=0.998 06 (7) nm, c=0.944 03 (8) nm for Dy6UO12. The diffraction patterns of all the compounds are indexed on the R3¯ space group with Z=3. The a and c values decrease with increasing atomic number of the rare earth ion.

Rare earth–uranium ternary oxides were synthesized by a solution combustion route. The starting materials were the corresponding metal nitrates and urea. In these preparations, the metal nitrates act as oxidizer and urea as fuel. Highly exothermic decomposition of the metal nitrate–urea complexes on heating at about 500 K leads to a combustion process yielding RE 6 UO 12 fine powders. Thermal expansion measurements of these compounds were carried out in the temperature range of 298–1173 K by high temperature X-ray powder diffractometry. The observed axial thermal expansion behaviour is explained on the basis of the crystal chemistry of the compounds.

Pyrochlore, A1–2B2O6(O,OH,F)0–1, is an actinide-bearing phase in Synroc, a polyphase ceramic proposed for the immobilization of high level nuclear waste. Structural damage due to alpha-decay events can significantly affect the chemical and physical stability of the nuclear waste form. Pyrochlore can effectively incorporate a variety of actinides into its structure. Four titanate pyrochlores were synthesized with compositions of Gd2Ti2O7, Sm2Ti2O7, Eu2Ti2O7 and Y2Ti2O2. These samples were irradiated with 1 MeV Kr+ in order to simulate alpha-decay damage and were observed by in situ electron microscopy. Irradiations were conducted from 25 K to 1023 K. At room temperature, Gd-, Sm- and Eu-pyrochlores amorphized at a dose of ∼2×1014 ions/cm2 (∼0.5 dpa) and Y-pyrochlore amorphized at 4×1014 ions/cm2 (∼0.8 dpa). The amorphization dose became higher at elevated temperatures with different rates of increase for each composition. The critical temperatures for amorphization are ∼1100 K for Gd-, Sm-, Eu-pyrochlore and ∼780 K for Y-pyrochlore. The rare-earth-pyrochlores are more susceptible to amorphization and have higher critical temperatures than Y-pyrochlore. The difference in amorphization dose and critical temperature is attributed to the different cascade sizes caused by the different cation masses of the target. Based on a model of cascade quenching, the larger cascade is related to a lower amorphization dose and higher critical temperature. The irradiated materials were studied by electron diffraction and high-resolution electron microscopy. All the pyrochlores transformed to a fluorite substructure prior to the completion of amorphization of the observed regions. This transformation was caused by the disordering between cations and between oxygen and oxygen vacancies. The concurrence of cation disordering with amorphization suggests the partial recrystallization of the displacement cascades. Isolated cascade damage regions were observed by high-resolution electron microscopy, and the cation disordering was associated with the damaged regions.

Oxide ion conductivity of the pure and aliovalent ion substituted rare-earth pyrohafnates in the series RE 2− x Sr x Hf 2 O 7 and RE 2 Hf 2− x Al x O 7 (RE=Gd and Nd; x =0–0.2) has been explored in the temperature range 400°C–700°C for the first time. It is seen that, conductivity is enhanced by doping 5 atom% Sr at the rare–earth site in these systems. Well defined impedance plots due to grain interior and grain boundary resistances were obtained in the Gd pyrohafnate with Sr substitution. The results of the conductivity variation for the pure, Sr and Al doped phases are explained on the basis of pyrochlore structure.

Thermal expansion of the sodium zirconium phosphate (NZP) family of compounds A1/2M2(PO4)3 (A = Ca or Sr; M = Ti, Zr, Hf or Sn) has been measured in the temperature range 298–1273 K by high-temperature X-ray powder diffractometry. Some of the compounds in the series (calcium zirconium phosphate and calcium hafnium phosphate) display the typical thermal expansion behaviour of NZP compounds, namely expansion along the hexagonal c axis and contraction along the a axis. The other compounds, depending on their interstitial and framework composition, behave differently. The observed axial thermal expansion and contraction behaviour is explained on the basis of the crystal chemistry of the compounds. Low-expansion compounds in this series are identified and their expansion anisotropy examined. Infared spectra of the compounds are reported. Differential scanning calorimetry measurements on the tin compounds indicate the occurrence of a diffuse phase transformation at high temperatures.

This paper reports oxide ion conductivity measurements on pyrozirconates of neodymium and gadolinium in which the rare earth elements were partially substituted by strontium, and zirconium partially substituted by aluminium. 5–10 atom% Sr and very little Al were found to enter the pyrochlore lattice, ac impedance measurements have shown an enhancement of conductivity as a result of the incorporation of small amounts of Sr in both the pyrochlores. In the Gd2Zr2O7 system, this could be traced to an improvement of the grain interior conductivity, by resolving the grain interior and grain boundary contributions to the total conductivity. Such a separation of the bulk and grain boundary conductivities could not be achieved in the Nd2Zr2O7-based compositions. The observed variations in conductivity are explained in terms of the pyrochlore crystal chemistry.

Two series of NZP compounds, NaM[sub 2](PO[sub 4])[sub 3] and NbM(PO[sub 4])[sub 3] [M = Ti, Zr, Hf, Sn and Th] have been synthesized and their thermal expansion coefficients in the temperature range 298--1,273 K determined by using high temperature x-ray powder diffractometry. In the first series, the influence of the M-O bond is clearly seen even in the presence of the large contribution of the Na-O bond to thermal expansion; the expansively of the lattice is found to vary with the ionic radius of the M atom. This trend is also reflected in the infra-red absorption frequencies of these compounds. In the second series, while the expansion behavior is found to be dependent on M, the variation of expansively with M is different from that found in the first series. The tin compounds in both the series display anomalous behavior. The authors also report here, for the first time, a phase transformation in NbSn(PO[sub 4])[sub 3] around 673 K characterized by a maximum and a minimum in the lattice parameters a and c respectively.

Enthalpy increments of the title compounds are measured in the temperature range 573—1523 K for the first time by inverse drop calorimetry.

Thermal expansion behaviour of sodium zirconium phosphate structure type phosphates of the formula AM3+SnP3O12 (A = Ca, Sr and Ba; M3+ = Cr and Fe) was studied by high temperature X-ray diffraction and dilatometry in the temperature range 298-1073K. The variation in the hexagonal lattice parameters of the Ca-containing compounds is in line with the 'sodium zirconium phosphate behaviour'. However, the strontium- and barium-containing compounds display an altogether different behaviour of axial expansion. The results are explained based on the crystal chemistry of these compounds. ? 2003 Elsevier Ltd. All rights reserved.

Structural transformation and ionic transport properties are investigated on wet-chemically synthesized La 1− x MnO 3 ( x =0.0–0.18) compositions. Powders annealed in oxygen/air at 1000–1080 K exhibit cubic symmetry and transform to rhombohedral on annealing at 1173–1573 K in air/oxygen. Annealing above 1773 K in air or in argon/helium at 1473 K stabilized distorted rhombohedral or orthorhombic symmetry. Structural transformations are confirmed from XRD and TEM studies. The total conductivity of sintered disks, measured by four-probe technique, ranges from 5 S cm −1 at 298 K to 105 S cm −1 at 1273 K. The ionic conductivity measured by blocking electrode technique ranges from 1.0×10 −6 S cm −1 at 700 K to 2.0×10 −3 S cm −1 at 1273 K. The ionic transference number of these compositions ranges from 3.0×10 −5 to 5.0×10 −5 at 1273 K. The activation energy deduced from experimental data for ionic conduction and ionic migration is 1.03–1.10 and 0.80–1.00 eV, respectively. The activation energy of formation, association and migration of vacancies ranges from 1.07 to 1.44 eV.

Pyrochlore is an important nuclear waste form phase for actinide immobilization. Two synthetic pyrochlores, Gd2Ti2O7and Gd2Zr2O7were irradiated at various temperatures (25 K to 1073 K) by different ion species (1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+). The titanate pyrochlore amorphized at relatively low doses (0.5 ∼ 0.6 dpa). Temperature dependence of the amorphization dose for titanate pyrochlore was measured, and the critical temperatures for amorphization were 1300 K, 1100 K and 950 K by 1.5 MeV Xe+, 1.0 MeV Kr+and 0.6 MeV Ar+, respectively. The higher critical temperature for the heavier ion irradiation is consistent with an amorphization mechanism by which the heavier ion produces a larger cascade. The zirconate pyrochlore, Gd2Zr2O7, showed a strong amorphization “resistance”. Gd2Zr2O7did not become amorphous under 1.0 MeV Kr+and 1.5 MeV Xe+irradiation. After prolonged irradiation (up to 7 dpa) even at a temperature of 25 K, no amorphization was observed. The irradiated zirconate pyrochlore showed abundant dislocations as observed by TEM. The pyrochlore structure of Gd2Zr2O7transformed to the fluorite structure after irradiation. The diffraction patterns of irradiated Gd2Zr2O7showed the existence of short-range ordering of cations. The large difference between these two pyrochlores emphasizes the strong effect of chemical composition on radiation-induced amorphization.