Preparation and characterisation of Tl2Pu(MoO4)3 and Tl4Pu(MoO4)4 in Tl-Pu-Mo-O system by X-ray and thermal methods

The quaternary Tl-Pu-Mo-O system was investigated for the first time and two new quaternary compounds Tl₂Pu(MoO₄)₃ and Tl₄Pu(MoO₄)₄ were synthesized by solid state reactions of Tl₂MoO₄ with Pu(MoO₄)₂ in 1:1 and 2:1 molar proportions at 773 K and 823 K, respectively. X-ray powder diffraction data of these compounds were indexed on orthorhombic system. Thermogravimetric (TG) curves of Tl₂Pu(MoO₄)₃ and Tl₄Pu(MoO₄)₄ were recorded in air and no significant weight changes were observed up to 973 K and 1033 K, respectively. Differential thermal analysis (DTA) curves of Tl₂Pu(MoO₄)₃ and Tl₄Pu(MoO₄)₄ showed endothermic peaks due to the melting of the compounds at 838 K and 1013 K, respectively. Non-isothermal kinetics of decomposition of Tl₂Pu(MoO₄)₃ and Tl₄Pu(MoO₄)₄ in air was studied using thermogravimetric technique. Tl₂Pu(MoO₄)₃ and Tl₄Pu(MoO₄)₄ when heated up to 1673 K decomposed to Tl₂O and MoO₃ in the gaseous form, giving solid PuO₂ as an end product. The reactions followed unimolecular nucleation and growth mechanism with activation energies of 106 kJ/mol and 157 kJ/mol, respectively.     

Synthesis and characterization of thallium-thorium molybdates and thallium-uranium (IV) molybdates

In quaternary Tl-Th-Mo-O and Tl-U(IV)-Mo-O systems, Tl₂Th(MoO₄)₃, Tl₄Th(MoO₄)₄, Tl₂U(MoO₄)₃ and Tl₄U(MoO₄)₄ were synthesized by reacting Tl₂MoO₄ with molybdates of thorium and uranium in stoichiometric molar proportions at 773 K. Thallium U(IV) molybdates were prepared in evacuated sealed quartz ampoules. These compounds were characterized by X-ray, thermal and infrared techniques. Powder X-ray diffraction data of all four compounds were indexed on orthorhombic system. When heated in thermoanalyzer in He atmosphere, Tl₂U(MoO₄)₃ and Tl₄U(MoO₄)₄ showed melting at 792 K and 853 K, respectively, whereas heating of both the compounds in air oxidized them to Tl₂UO₂(MoO₄)₂. Both Tl₂Th(MoO₄)₃ and Tl₄Th(MoO₄)₄, when heated in air, showed a reversible phase transition followed by melting of the compounds. Isothermal heating of all thallium-thorium and thallium-uranium molybdates at 1573 K led to the formation of ThO₂ and U₃O₈, respectively, as end products.     

Calorimetric measurements on (U,Th)O2 solid solutions

Enthalpy increments of urania - thoria solid solutions, (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ were measured by drop calorimetry in the temperature range 479 - 1805 K. Heat capacity, entropy and Gibbs energy function were computed. The heat capacity measurements were carried out also with differential scanning calorimetry in the temperature range 298 - 800 K. The heat capacity values of (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ at 298 K are 59.62, 61.02, 63.56 J K⁻¹ mol⁻¹, respectively. The results were compared with the data available in the literature. From the study, the heat capacity of (U,Th)O₂ solid solutions was shown to obey the Neumann - Kopp’s rule.     

Rapid synthesis of Pb5(VO4)3I, for the immobilisation of iodine radioisotopes, by microwave dielectric heating

Rapid synthesis of Pb₅(VO₄)₃I, a potential immobilisation host for iodine radioisotopes, was achieved in an open container by microwave dielectric heating of a mixture of PbO, PbI₂, and V₂O₅ at a power of 800 W for 180 s (at 2.45 GHz). The resulting ceramic bodies exhibited a zoned microstructure, differentiated by inter-granular porosity and phase assemblage, as a consequence of the inverse temperature gradient characteristic of microwave dielectric heating. Liquid PbI₂ within the interior of microwave processed ceramics assisted formation of Pb₅(VO₄)₃I, and reduced inter-granular porosity. In contrast, the exterior of microwave processed ceramics comprised poorly sintered Pb₅(VO₄)₃I with the presence of minor reagent relics. Quantitative microanalysis, electron diffraction and Rietveld analysis, confirmed the synthesis of stoichiometric Pb₅(VO₄)₃I within precision. The crystal structure of Pb₅(VO₄)₃I was found to adopt space group P6₃/m with a = 10.4429(3) Å and c = 7.4865(2) Å.     

The standard molar enthalpy of formation of CdMoO4

The molar enthalpies of solution of CdMoO₄(s), CdO(s), Na₂ MoO₄(s) and NaF(s) in (10 mol HF(aq) + 4.41 mol H₂O₂(aq)) dm⁻³ have been measured using an isoperibol type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of CdMoO₄(s) has been calculated to be Δ_fH°(298.15 K) = −(1034.3 ± 5.7) kJ mol⁻¹. This value of enthalpy of formation of CdMoO₄(s) agrees well with the estimated enthalpy of formation of this compound. There is no other report on the thermodynamic property measurements on this compound.     

Modeling of postulated accident release of MoO3 powder from ETRR-2 Stack

The postulated accident modeled in this study simulates the release of ⁹⁹MoO₃ powder, activated by neutrons, to the environment through the ventilation system of the universal cell of ETRR-2 (Egyptian second nuclear research reactor). The simulation was carried out in both the normal and abnormal situation cases. The ⁹⁸MoO₃ powder was contained in an ampoule of quartz surrounded with a tight Aluminum can. The can is purposed to be irradiated in the ETRR-2 irradiation grid with a neutron flux of 1.4 × 1014 n/cm² s to produce ⁹⁹MoO₃. The Aluminum can was delivered after irradiation to the universal cell to remove the quartz ampoule to the outside the Aluminum can. During the process of removing the quartz ampoule from the Aluminum can, the ampoule may be broken due to a human error and the ⁹⁹MoO₃ powder released in the universal cell that is connected to the hot cell ventilation system. The postulated radiation doses to the public at various downwind distances were calculated using the health physics computer code HotSpot 2.06 developed at the Lawrence Livermore National Laboratory, USA. This study is a complementary study for ⁹⁹MoO₃ production safety document. The results indicated that the persons who are within downwind distances for all metrological conditions (A–F classes) would receive a committed effective dose (CED) less than the permissible dose for both the normal and abnormal cases.     

Low silicon U(Al,Si)3 stabilization by Zr addition

Previous knowledge states that (U,Zr)Al₃ and U(Al,Si)₃ phases with Zr and Si content higher than 6 at.% (7.7 wt%) and 4 at.% (1.4 wt%), respectively, does not partially transform to UAl₄ at 600 °C. In this work, four alloys within the quaternary system U-Al-Si-Zr were made with a fixed nominal 0.18 at.% (0.1 wt%) Si content in order to assess the synergetic effect of both Zr and Si alloying elements to the thermodynamic stability of the (U,Zr)(Al,Si)₃ phase. Heat treatments at 600 °C were undertaken and samples were analyzed by means of XRD, EPMA and EDS techniques. A remarkable conclusion is that addition of 0.3 at.% Si in the (U,Zr)(Al,Si)₃ phase reduces in 2.7 at.% the necessary Zr content to inhibit its transformation to U(Al,Si)₄.     

Thermal conductivities of irradiated UO2 and (U, Gd)O2 pellets

Thermal diffusivities of UO₂ and (U, Gd)O₂ pellets irradiated in a commercial reactor (maximum burnups: 60 GWd/t for UO₂ and 50 GWd/t for (U, Gd)O₂) were measured up to about 2000 K by using a laser flash method. The thermal diffusivities of irradiated UO₂ and (U, Gd)O₂ pellets showed hysteresis phenomena: the thermal diffusivities of irradiated pellets began to recover above 750 K and almost completely recovered after annealing above 1400 K. The thermal diffusivities after recovery were close to those of simulated soluble fission products (FPs)-doped UO₂ and (U, Gd)O₂ pellets, which corresponded with the recovery behaviors of irradiation defects for UO₂ and (U, Gd)O₂ pellets. The thermal conductivities for irradiated UO₂ and (U, Gd)O₂ pellets were evaluated from measured thermal diffusivities, specific heat capacities of unirradiated UO₂ pellets and measured sample densities. The difference in relative thermal conductivities between irradiated UO₂ and (U, Gd)O₂ pellets tended to become insignificant with increasing burnups of samples.     

Separation of uranium from (U, Th)O2 and (U, Pu)O2 by solid state reactions route

Solid state reactions of UO₂, ThO₂, PuO₂ and their mixed oxides (U, Th)O₂ and (U, Pu)O₂ were carried out with sodium nitrate upto 900 °C, to study the formation of various phases at different temperatures, which are amenable for easy dissolution and separation of the actinide elements in dilute acid. Products formed by reacting unsintered as well as sintered UO₂ with NaNO₃ above 500 °C were readily soluble in 2 M HNO₃, whereas ThO₂ and PuO₂ did not react with NaNO₃ to form any soluble products. Thus reactions of mixed oxides (U, Th)O₂ and (U, Pu)O₂ with NaNO₃ were carried out to study the quantitative separation of U from (U, Th)O₂ and (U, Pu)O₂. X-ray diffraction, X-ray fluorescence, thermal analysis and chemical analysis techniques were used for the characterization of the products formed during the reactions.     

Enthalpy measurements of La2Te3O9 and La2Te4O11

Enthalpy increment measurements on La₂Te₃O₉(s) and La₂Te₄O₁₁(s) were carried out using a Calvet micro-calorimeter. The enthalpy values were analyzed using the non-linear curve fitting method. The dependence of enthalpy increments with temperature was given as: H°(T) − H°(298.15 K) (J mol⁻¹) = 360.70T + 0.00409T² + 133.568 × 10⁵/T − 149 923 (373 ? T (K) ? 936) for La₂Te₃O₉ and H°(T) − H°(298.15 K) (J mol⁻¹) = 331.927T + 0.0549T² + 29.3623 × 10⁵/T − 114 587 (373 ? T (K) ? 936) for La₂Te₄O₁₁.     

Combustion of Na2B4O7 + Mg + C to synthesis B4C powders

Boron carbide powder was fabricated by combustion synthesis (CS) method directly from mixed powders of borax (Na₂B₄O₇), magnesium (Mg) and carbon. The adiabatic temperature of the combustion reaction of Na₂B₄O₇ + 6 Mg + C was calculated. The control of the reactions was achieved by selecting reactant composition, relative density of powder compact and gas pressure in CS reactor. The effects of these different influential factors on the composition and morphologies of combustion products were investigated. The results show that, it is advantageous for more Mg/Na₂B₄O₇ than stoichiometric ratio in Na₂B₄O₇ + Mg + C system and high atmosphere pressure in the CS reactor to increase the conversion degree of reactants to end product. The final product with the minimal impurities’ content could be fabricated at appropriate relative density of powder compact. At last, boron carbide without impurities could be obtained after the acid enrichment and distilled water washing.     

In situ studies of ion irradiated inverse spinel compound magnesium stannate (Mg2SnO4)

Magnesium stannate spinel (Mg₂SnO₄) was synthesized through conventional solid state processing and then irradiated with 1.0 MeV Kr²⁺ ions at low temperatures 50 and 150 K. Structural evolutions during irradiation were monitored and recorded through bright field images and selected-area electron diffraction patterns using in situ transmission electron microscopy. The amorphization of Mg₂SnO₄ was achieved at an ion dose of 5 × 10¹⁹ Kr ions/m² at 50 K and 10²⁰ Kr ions/m² at 150 K, which is equivalent to an atomic displacement damage of 5.5 and 11.0 dpa, respectively. The spinel crystal structure was thermally recovered at room temperature from the amorphous phase caused by irradiation at 50 K. The calculated electronic and nuclear stopping powers suggest that the radiation damage caused by 1 MeV Kr²⁺ ions in Mg₂SnO₄ is mainly due to atomic displacement induced defect accumulation. The radiation tolerance of Mg₂SnO₄ was finally compared with normal spinel MgAl₂O₄.     

Thermal conductivity of (U,Pu,Np)O2 solid solutions

The thermal conductivities of (U_,Pu_,Np)O₂ solid solutions were studied at temperatures from 900 to 1770 K. Thermal conductivities were obtained from the thermal diffusivity measured by the laser flash method. The thermal conductivities obtained below 1400 K were analyzed with the data of (U,Pu,Am)O₂ obtained previously, assuming that the B-value was constant, and could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(z₁, z₂) = 3.583 × 10⁻¹ × z₁ + 6.317 × 10⁻² × z₂ + 1.595 × 10⁻² (m K/W) and B = 2.493 × 10⁻⁴ (m/W), where z₁ and z₂ are the contents of Am- and Np-oxides. It was found that the A-values increased linearly with increasing Np- and Am-oxide contents slightly, and the effect of Np-oxide content on A-values was smaller than that of Am-oxide content. The results obtained from the theoretical calculation based on the classical phonon transport model showed good agreement with the experimental results.     

Thermoluminescence properties of nanocrystalline K2Ca2(SO4)3:Eu irradiated with gamma rays and proton beam

Thermoluminescence properties of nanocrystalline K₂Ca₂(SO₄)₃:Eu prepared by ball milling technique have been studied and the nanophosphor’s suitability as an effective gamma radiation and proton beam dosimeter material has been examined. It is found that the nanophosphor is suitable for dosimetry over a very wide range of doses ∼1 Gy to 1 kGy for gamma radiation. And for proton beam the same nanophosphor shows a more or less linear response for the dose range 0.1-100 Gy. A comparative study of this nanophosphor with its corresponding microcrystalline form (prepared by solid-state diffusion method) as well as the nanocrystalline form prepared by (the more conventional) co-precipitation technique has shown that the nanophosphor prepared by the ball milling technique is in almost all respects better than the other two forms reported earlier.     

Reactivity of hydrogen peroxide towards Fe3O4, Fe2CoO4 and Fe2NiO4

The kinetics of CRUD oxidation by H₂O₂ has been studied using aqueous suspensions of metal oxide powder. Fe₃O₄, Fe₂CoO₄ and Fe₂NiO₄ were used as model compounds for CRUD. In addition, the activation energies for the reaction between H₂O₂ and the three CRUD models were determined. The rate constants at room temperature were determined to 6.6 (±0.4) × 10⁻⁹, 3.4 (±0.4) × 10⁻⁸ and 1.6 × 10⁻¹⁰ m min⁻¹ for Fe₃O₄, Fe₂CoO₄ and Fe₂NiO₄, respectively. The corresponding activation energies are 52 ± 4, 44 ± 5 and 57 ± 7 kJ mol⁻¹, respectively. The mechanism of the reaction is briefly discussed indicating that the final solid product in all three cases is Fe₂O₃. In addition to the experimental studies, the theoretical grounds for kinetics of reactions in particle suspensions are discussed. The theoretical discussion is also used to explain the somewhat unexpected trends in reactivity observed experimentally.     

The high-temperature behaviour of PuPO4 monazite and some other related compounds

Thermal behaviour and lattice parameters of monazites MPO₄ (M³⁺ = Ce³⁺, Nd³⁺ and Pu³⁺) and cheralite CaTh(PO₄)₂ were studied using high-temperature X-ray diffraction. Heat treatment under inert atmosphere caused the decomposition of PuPO₄ and CaTh(PO₄)₂ into the corresponding oxides above 1473 K. The influence of the cation type within the crystallographic structure on the thermal expansion coefficient and the possible cation substitutions are discussed in the frame of nuclear waste management.     

Thermal and mechanical properties of (U,Er)O2

Erbium is considered as a slow burnable poison suitable for use in light water reactors (LWRs). Addition of a small amount of Er₂O₃ to all UO₂ pellets will make it possible to develop super high burnup fuels in Japanese nuclear facilities which are now under the restriction of the upper limit of ²³⁵U enrichment. When utilizing the (U,Er)O₂ fuels, it is very important to understand the thermal and mechanical properties. Here we show the characterization results of (U_1−xEr_x)O₂ (0 ? x ? 0.1). We measured their thermal and mechanical properties and investigated the effect of Er addition on these properties of (U,Er)O₂. All Er completely dissolved in UO₂, and the lattice parameter decreased linearly with the Er content. Both the thermal conductivity and Young’s modulus of (U,Er)O₂ decreased with the Er content. These results would be useful for us in evaluating the performance of the (U,Er)O₂ fuels in LWRs.     

Thermal conductivities of hypostoichiometric (U, Pu, Am)O2−x oxide

The thermal conductivities of (U_0.68Pu_0.30Am_0.02)O_2.00−x solid solutions (x = 0.00-0.08) were studied at temperatures from 900 to 1773 K. The thermal conductivities were obtained from the thermal diffusivities measured by the laser flash method. The thermal conductivities obtained experimentally up to about 1400 K could be expressed by a classical phonon transport model, λ = (A + BT)⁻¹, A(x) = 3.31 × x + 9.92 × 10⁻³ (mK/W) and B(x) = (−6.68 × x + 2.46) × 10⁻⁴ (m/W). The experimental A values showed a good agreement with theoretical predictions, but the experimental B values showed not so good agreement with the theoretical ones in the low O/M ratio region. From the comparison of A and B values obtained in this study with the ones of (U,Pu)O_2−x obtained by Duriez et al. [C. Duriez, J.P. Alessandri, T. Gervais, Y. Philipponneau, J. Nucl. Mater. 277 (2000) 143], the addition of Am into (U, Pu)O_2−x gave no significant effect on the O/M dependency of A and B values.     

Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies

High molybdenum concentration in glass compositions may lead to alkali and alkaline-earth molybdates crystallization during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy waste forms. To understand the effect of molybdenum addition on the structure of a simplified nuclear glass and to know how composition changes can affect molybdates crystallization tendency, the structure of two glass series belonging to the SiO₂-B₂O₃-Na₂O-CaO-MoO₃ system was studied by ²⁹Si, ¹¹B, ²³Na MAS NMR and Raman spectroscopies by increasing MoO₃ or B₂O₃ concentrations. Increasing MoO₃ amount induced an increase of the silicate network reticulation but no significant effect was observed on the proportion of units and on the distribution of Na⁺ cations in glass structure. By increasing B₂O₃ concentration, a strong evolution of the distribution of Na⁺ cations was observed that could explain the evolution of the nature of molybdate crystals (CaMoO₄ or Na₂MoO₄) formed during melt cooling.     

Thermoelectric power studies of ferromagnet U2ScB6C3

The thermoelectric power (TEP) of a ferromagnet U₂ScB₆C₃ (T_C = 61 K) has been measured in the temperature range 5-300 K. The TEP is positive over the whole measured temperature range and reaches a relatively large value at room temperature of 29 μV/K. Below 30 K and above 200 K the TEP follows a straight line S(T) ∼AT, with slope of 0.23 and 0.085 μV/K², respectively. The change in the slope can be explained by the electron-phonon interaction renormalization effects or spin-reorientation associated with a change in the electronic structure. Analysing the temperature dependence of the ratio [S(T)/T]/[S_300 K/300] and taking into account the specific heat data, we suggest that spin fluctuations are another important factor in determining the thermoelectric power behaviour of U₂ScB₆C₃.