Oxygen potentials of mixed oxide fuels for fast reactors

Oxygen potentials of homogenous (Pu_0.2U_0.8)O_2−x and (Am_0.02Pu_0.30Np_0.02U_0.66)O_2−x which have been developed as fuels for fast breeder reactors were measured at temperatures of 1473-1623 K by a gas equilibrium method using an (Ar, H₂, H₂O) gas mixture. The measured oxygen potentials of (Pu_0.2U_0.8)O_2−x were about 25 kJ mol⁻¹ lower than those of (Pu_0.3U_0.7)O_2−x measured previously and were consistent with the values calculated by Besmann and Lindemer’s model. The measured oxygen potentials of (Am_0.02Pu_0.30Np_0.02U_0.66)O_2−x were slightly higher than those of MOX without minor actinides. No fuel-cladding chemical interaction is affected significantly by adding their minor actinides.     

Analysis of oxygen potential of (U0.7Pu0.3)O2±x and (U0.8Pu0.2)O2±x based on point defect chemistry

Stoichiometries in (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x were analyzed with the experimental data of oxygen potential based on point defect chemistry. The relationship between the deviation x of stoichiometric composition and the oxygen partial pressure P_O2 was evaluated using a Kröger-Vink diagram. The concentrations of the point defects in uranium and plutonium mixed oxide (MOX) were estimated from the measurement data of oxygen potentials as functions of temperature and P_O2. The analysis results showed that x was proportional to near the stoichiometric region of both (U_0.7Pu_0.3)O_2±x and (U_0.8Pu_0.2)O_2±x, which suggested that intrinsic ionization was the dominant defect. A model to calculate oxygen potential was derived and it represented the experimental data accurately. Further, the model estimated the thermodynamic data, and , of stoichiometric (U_0.7Pu_0.3)O_2.00 and (U_0.8Pu_0.2)O_2.00 as −552.5 kJ·mol⁻¹ and −149.7 J·mol⁻¹, and −674.0 kJ · mol^-1 and −219.4 J · mol⁻¹, respectively.     

Structural study of Pu1−xAmxO2 (x = 0.2; 0.5; 0.8) obtained by oxalate co-conversion

This study describes the synthesis and the characterisation of Pu_1−xAm_xO₂ (x = 0.2; 0.5; 0.8) mixed oxides obtained by oxalate co-conversion. We studied the self-irradiation effect in these compounds at the structural scale. We determined, for each composition, the initial lattice parameter and the equation describing its variation versus time and displacements per atom. Similarly to other α emitting compounds, it was observed a fast lattice parameter expansion rate, followed by a stabilisation at a maximum value. The observations also showed that the initial expansion rate varies according to the Am content and the maximum value to the Pu content. However, for all compositions, the lattice parameter relative variations are the same.     

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.     

Oxygen potential and thermal conductivity of (U, Pu) mixed oxides

(U, Pu) mixed oxides, (U_1−yPu_y)O_2−x, with y = 0.21 and 0.28 are being considered as fuels for the Prototype Fast Breeder Reactor (PFBR) in India. The use of urania-plutonia solid solutions in PFBR calls for accurate measurement of physicochemical properties of these materials. Hence, in the present study, oxygen potentials of (U_1−yPu_y)O_2−x, with y = 0.21 and 0.28 were measured over the temperature range 1073-1473 K covering an oxygen potential range of −550 to −300 kJ mol⁻¹ (O/M ratio from 1.96 to 2.000) by employing a H₂/H₂O gas equilibration technique followed by solid electrolyte EMFmeasurement. (U_1−yPu_y)O_2−x, with y = 0.40 is being used in the Fast Breeder Test Reactor (FBTR) in India to test the behaviour of fuels with high plutonium content. However, data on the oxygen potential as well as thermal conductivity of the mixed oxides with high plutonium content are scanty. Hence, the thermal diffusivity of (U_1−yPu_y)O₂, with y = 0.21, 0.28 and 0.40 was measured and the results of the measurements are reported.     

Oxygen potential of (U0.88Pu0.12)O2±x and (U0.7Pu0.3)O2±x at high temperatures of 1673-1873 K

The oxygen potential of (U_0.88Pu_0.12)O_2±x (−0.0119 < x < 0.0408) and (U_0.7Pu_0.3)O_2±x (−0.0363 < x < 0.0288) was measured at high temperatures of 1673-1873 K using gas equilibrium method with thermo gravimeter. The measured data were analyzed by a defect chemistry model. Expressions were derived to represent the oxygen potential based on defect chemistry as functions of temperature and oxygen-to-metal ratio. The thermodynamic data, and , at stoichiometric composition were obtained. The expressions can be used for in situ determination of the oxygen-to-metal ratio by the gas-equilibration method. The calculation results were consistent with measured data. It was estimated that addition of 1 wt.% Pu content increased oxygen potential of uranium and plutonium mixed oxide by 2-5 kJ/mol.     

Thermal expansion of TRU nitride solid solutions as fuel materials for transmutation of minor actinides

The lattice thermal expansion of the transuranium nitride solid solutions was measured to investigate the composition dependence. The single-phase solid solution samples of (Np_0.55Am_0.45)N, (Pu_0.59Am_0.41)N, (Np_0.21Pu_0.52Am_0.22Cm_0.05)N and (Pu_0.21Am_0.18Zr_0.61)N were prepared by carbothermic nitridation of the respective transuranium dioxides and nitridation of Zr metal through hydride. The lattice parameters were measured by the high temperature X-ray diffraction method from room temperature up to 1478 K. The linear thermal expansion of each sample was determined as a function of temperature. The average thermal expansion coefficients over the temperature range of 293-1273 K for the solid solution samples were 10.1, 11.5, 10.8 and 8.8 × 10⁻⁶ K⁻¹, respectively. Comparison of these values with those for the constituent nitrides showed that the average thermal expansion coefficients of the solid solution samples could be approximated by the linear mixture rule within the error of 2-3%.     

Light ion irradiation effects on stuffed Lu2(Ti2−xLux)O7−x/2 (x = 0, 0.4 and 0.67) structures

We have recently synthesized “stuffed” (i.e., excess Lu) Lu₂(Ti_2−xLu_x)O_7−x/2 (x = 0, 0.4 and 0.67) compounds using conventional ceramic processing. X-ray diffraction measurements indicate that stuffing more Lu³⁺ cations into the oxide structure leads eventually to an order-to-disorder (O-D) transition, from an ordered pyrochlore to a disordered fluorite crystal structure. At the maximum deviation in stoichiometry (x = 0.67), the Lu³⁺ and Ti⁴⁺ ions become completely randomized on the cation sublattices, and the oxygen “vacancies” are randomized on the anion sublattice. Samples were irradiated with 400 keV Ne²⁺ ions to fluences ranging from 1 × 10¹⁵ to 1 × 10¹⁶ ions/cm² at cryogenic temperatures (∼77 K). Ion irradiation effects in these samples were examined by using grazing incident X-ray diffraction. The results show that the ion irradiation tolerance increases with disordering extent in the non-stoichiometric Lu₂(Ti_2−xLu_x)O_7−x/2.     

Oxygen chemical diffusion in hypo-stoichiometric MOX

Kinetics of the oxygen-to-metal ratio change in (U_0.8Pu_0.2)O_2−x and (U_0.7Pu_0.3)O_2−x was evaluated in the temperature range of 1523-1623 K using a thermo-gravimetric technique. The oxygen chemical diffusion coefficients were decided as a function of temperature from the kinetics of the reduction process under a hypo-stoichiometric composition. The diffusion coefficient of (U_0.7Pu_0.3)O_2−x was smaller than that of (U_0.8Pu_0.2)O_2−x. No strong dependence was observed for the diffusion coefficient on the O/M variation of samples.     

Lattice parameters of (U, Pu, Am, Np)O2−x

The solid solutions of (U_{1−z−y’−y”}Pu_zAm_y’Np_y”)O_2−x (z = 0-1, y’ = 0-0.12, y” = 0-0.07) were investigated by X-ray diffraction measurements, and a database for the lattice parameters was updated. A model to calculate the lattice parameters was derived from the database. The radii of the ions present in the fluorite structure of (U, Pu, Am, Np)O_2−x were estimated from the lattice parameters measured in this work. The model represented the experimental data within a standard deviation of σ = ±0.025%.     

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.     

Melting behavior of MgO-based inert matrix fuels containing (Pu,Am)O2−x

The melting behavior of MgO-based inert matrix fuels containing (Pu,Am)O_2−x ((Pu,Am)O_2−x-MgO fuels) was experimentally investigated. Heat-treatment tests were carried out at 2173 K, 2373 K and 2573 K each. The fuel melted at about 2573 K in the eutectic reaction of the Pu-Am-Mg-O system. The (Pu,Am)O_2−x grains, MgO grains and pores grew with increasing temperature. In addition, Am-rich oxide phases were formed in the (Pu,Am)O_2−x phase by heat-treatment at high temperatures. The melting behavior was compared with behaviors of PuO_2−x-MgO and AmO_2−x-MgO fuels.     

Influence on the structural characteristic of defect solid solution GdxZr1−xO2−x/2 with 0.18 ? x ? 0.62 under longer term annealing studied by Gd Mössbauer spectroscopy

Three kinds of defect solid solution Gd_xZr_1−xO_2−x/2 with 0.18 ? x ? 0.62, including the three single crystal samples with x = 0.21, 0.26 and 0.30, were investigated by ¹⁵⁵Gd Mössbauer spectroscopy at 12 K. Difference in the structural characteristic under longer term annealing were confirmed by comparing the ¹⁵⁵Gd Mössbauer parameters of the polycrystalline samples sintered one time and twice at 1773 K for 16 h in air, respectively. The results indicated that the polycrystalline samples sintered twice have relatively equilibrated structure by comparing with the three single crystal samples. After being sintered twice, basically the local structure around the Gd³⁺ ions does not change, but the degree of the displacements of the six 48f oxygen ions from positions of cubic symmetry becomes slightly smaller, and distribution of the Gd³⁺ ions in the system becomes more homogeneous.     

Oxygen non-stoichiometries in (Th0.7Ce0.3)O2−x

Oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x oxide solid solutions was investigated from the viewpoint of Ce reduction. The oxygen non-stoichiometry was experimentally determined by means of thermogravimetric analysis as a function of oxygen potential at 1173, 1273 and 1373 K. Features of the isotherms of oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x similar to those in oxygen non-stoichiometric actinide and lanthanide dioxides were observed. The oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x was compared with those of CeO_2−x and (U_0.7Ce_0.3)O_2−x. It was concluded that the Ce reduction has some relation to defect forms and their transformations in the solid solutions.     

Thermodynamic investigations of ThO2-UO2 solid solutions

Heat capacities and enthalpy increments of solid solutions Th_1−yU_yO₂(s) (y = 0.0196, 0.0392, 0.0588, 0.098, 0.1964) and Simfuel (y = 0.0196) were measured by using a differential scanning calorimeter and a high temperature drop calorimeter. The heat capacities were measured in two temperature ranges: 127-305 K and 305-845 K and enthalpy increments were determined in the temperature range 891-1698 K. A heat capacity expression as a function of uranium content y and temperature and a set of self-consistent thermodynamic functions for Th_1−yU_yO₂(s) were computed from present work and the literature data. The oxygen potentials of Th_1−yU_yO_2+x(s) have been calculated and expressed as a polynomial functions of uranium content y, excess oxygen x and temperature T. The phase diagram, oxygen potential diagram of thorium-uranium-oxygen system and major vapour species over urania thoria mixed oxide have been computed using FactSage code.     

Thermal studies on fluorite type ZryU1−yO2 solid solutions

Solid state reactions of UO₂ and ZrO₂ in mild oxidizing condition followed by reduction at 1673 K showed enhanced solubility up to 35 mol% of zirconium in UO₂ forming cubic fluorite type Zr_yU_1−yO₂ solid solution. The lattice parameters and O/M (M = U + Zr) ratios of the solid solutions, Zr_yU_1−yO_2+x, prepared in different gas streams were investigated. The lattice parameters of these solid solutions were expressed as a linear equation of x and y: a₀ (nm) = 0.54704 − 0.021x - 0.030y. The oxidation of these solid solutions for 0.1 ? y ? 0.2 resulted in cubic phase MO_2+x up to700 K and single orthorhombic zirconium substituted α-U₃O₈ phase at 1000 K. The kinetics of oxidation of Zr_yU_1−yO₂ in air for y = 0-0.35 were also studied using thermogravimetry. The specific heat capacities of Zr_yU_1−yO₂ (y = 0-0.35) were measured using heat flux differential scanning calorimetry in the temperature range of 334-860 K.     

Modeling the thermochemical behavior of AmO2−x

A thermochemical representation of the fluorite structure AmO_2−x phase was developed using the compound energy formalism approach assuming constituents of (Am⁴⁺)₁(O²⁻)₂, (Am⁴⁺)₁(Va)₂, (Am³⁺)₁(O²⁻)₂, and (Am³⁺)₁(Va)₂. The Gibbs free energies for the constituents and a set of interaction parameters were determined using reported oxygen potential-temperature-composition data. A good fit to the experimental information was obtained which well-reproduces the behavior. The representation is also in a format that will allow incorporation of other dissolved metals and thus will be useful in generating multi-component compound energy formalism representations for complex oxide nuclear fuel and waste systems. A full assessment relating the fluorite structure phase to the phase equilibria for Am-O, however, must await adequate data for the remainder of the system.     

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.     

Effect of oxygen potential on the sintering behavior of MgO-based heterogeneous fuels containing (Pu, Am)O2−x

The effect of oxygen potential on the sintering behavior of MgO-based heterogeneous fuels containing (Pu, Am)O_2−x was experimentally investigated. Sintering tests in various atmospheres, i.e. air, moisturized 4%H₂-Ar, and 4%H₂-Ar atmosphere, were carried out. The sintering behavior was found to be significantly affected by the oxygen potential in the sintering atmosphere. The sintered density decreased with decreasing oxygen potential. The (Pu, Am)O_2−x phase sintered in a reductive atmosphere had hypostoichiometry. The aggregates of the (Pu, Am)O_2−x phase sintered in the reductive atmosphere grew, in comparison with those in the oxidizing one. The sintering mechanism was discussed in terms of the difference in sintering behavior of (Pu, Am)O_2−x and MgO.