Takeo Fujino

Analyses of the oxygen potential of the solid solutions MyU1−yO2+x (M = M3+andM2+) by statistics of defects and defect complexes

The oxygen potential, ΔGO2, of the solid solutions My3U1−yO2+x and My2+U1−yO2+x was analysed by calculating the number of ways of arranging the cationic defects as a function of x. The intra-cation complex (M3+ U5+) was assumed to be formed in My3+U1−yO2+x, while the presence of two kinds of complexes, i.e. (M2+U5+) and (M2+2U5+), was assumed in My2+U1−yO2+x with an average composition, (M2+αU5+). According to the experimental results reported, the x values at which the steepest change in ΔGO2 takes place seem to be in the range of x < 0 for My2+U1−yO2+x, which was seen to be well explained by the present model as x = −(1 − α/2)y with α < 2 by calculating the number of ways of the arrangement of the above complexes. The model also makes it possible to explain the fact that the x value for the steepest change of ΔGO2 is zero for My3+U1−yO2+x. By introducing a factor β (0 < β < 1), which is multiplied to the logarithmic term in ΔSO2, the oxygen potentials of the exemplified La, Mg and Eu solid solutions were found to be represented satisfactorily by the respective theoretical curves in a wide range of the x values. Calculations revealed that nearly x-independent ΔHO2 values, as expected, could be obtained by using β.

Solubility of magnesium in uranium dioxide

Abstract The solubility of magnesium in uranium dioxide under low oxygen pressures was studied at 1200°C. Magnesium was found to dissolve up to y > 0.1 (and below y = 0.15) of the apparent formula, MgyU1−yO2 + x ( x ⪋ 0 ) on heating at po2 = 10−15 and ≤ 10−19 atm. The formed solid solution in such a low po2 region was of the type (MgaU1−a){Mgb}O2 + c, in which the magnesium atoms partly occupy the interstitial sites together with the substitutional sites for uranium atoms. The ration of interstitial atoms to the total magnesium atoms increased from 0.23 (y = 0.05) or 0.39 (y = 0.1) at po2 = 10−15 atm with decreasing oxygen partial pressure to 0.62–0.63 (y = 0.05 and 0.1) at po2 ≤ 10−19 atm. The lattice parameter of the (MgaU1−a){Mgb}O2 + c solid solutions was represented as a linear equation of a, b and c. The interstitial magnesium caused an increase in the lattice parameter, in contrast to the substitutional magnesium which largely decreases the lattice parameter. It is possible that the uranium atoms in the solid solutions prepared at low oxygen partial pressures (≤ 10−19 atm) were reduced to slightly less than the tetravalent state.

Synthesis, crystal structure refinement and electrical properties of uranium oxysulfide, UOS

Abstract Gas–solid reactions of UO2SO4 with H2, H2S and CS2 to form uranium (IV) oxysulfide, UOS, as a single phase were studied on the basis of thermodynamical analysis of these reactions. H2S was proved to be the best reaction agent. The reaction of UO2SO4 with CS2 resulted in the formation of UOS and US2 mixtures. A refinement of the crystal structure of UOS was made giving isotropic temperature factors and a smallest R value of 1.71%. The obtained UOS showed a p-type semi-conductivity with activation energy 67.5 meV.

A refined analysis of oxygen potential of MyU1−yO2+x (M = M3+and M2+) by lattice statistics based on the grand partition function and the Flory methods

Partial molar thermodynamic quantities (ΔG O2 , ΔS O2 and ΔH O2 ) of solid solutions M y U 1-y O 2+x (M=M 3+ and M 2+ , x ≤ ≥ 0) were calculated using a cation-cation complex model in combination with the lattice statistics based on the grand partition function method or the Flory method. With these refined methods, configurational entropies were obtained without any discrepancies between the numbers of lattice sites and the sum of ions and complexes. The cation-cation complex model assumes the formation of (M 3+ U 5+ ) complex for M y 3+ U 1-y O 2+x while two kinds of complexes, i.e. (M 2+ U 5+ ) and (M 2+ U 5+ ) for M y 2+ U 1-y O 2+x . The experimental results that the steepest change of ΔG O2 arises at x<0 FOR M y 2+ U 1-y O 2+x were well explained by the relation of this model, i.e. x=-(1-α/2)y with 1<α<2. THE CALCULATED ΔG O2 values were in good agreement with the experimental results for lanthanum, magnesium and europium solid solutions

Synthesis of BaZrS3 by short time reaction at lower temperatures

Abstract The reaction to form a mixed sulfide, BaZrS3, was studied using the mixture 0.9BaS+0.1BaCl2+ZrS2+αS (α=0.5, 1.0, 1.5 and 2.0) in evacuated glass ampoules with the reaction time ranging from 10 min to 168 h. At 723 K, the reaction was slow, and the mole fraction of BaZrS3 started to increase after 12 h. At 773 K, the rapid reaction took place based on the different reaction mechanism. Namely, BaZrS3 was formed with a high mole fraction of 0.8 after 1 h heating for α=0.5. At 823 K, the rate of reaction was still more increased, and a mole fraction of 0.92 was obtained for α=2.0 after a very short heating time of 10 min. The maximum BaZrS3 yield attained 0.96 for α=2.0 when the sample was heated at 823 K for 1 h.

Preparation of neodymium sulphides by the reaction of Nd2(So4)3 with carbon disulphide

Abstract The reaction to synthetize neodymium sulphides from neodymium sulphate octahydrate in a stream of carbon disulphide gas was studied. The dehydration of the octahydrate in vacuum was finished at 300 °C. At 1050–1100 °C in air neodymium oxysulphide, Nd 2 O 2 SO 4 , was formed. Neodymium oxysulphide, Nd 2 O 2 S, was formed upon heating with a reducing agent such as annealed carbon. The reaction of neodymium sulphate with carbon disulphide commenced at 500–600 °C, resulting in formation of the disulphide, NdS 2 . The crystal structure of NdS 2 heated at 500 °C was, however, different from that of the sample heated at 600 °C. In the temperature range 800–900 °C α-Nd 2 S 3 was obtained as a single phase after heating for at least 3 h in high flow rates of gas mixtures of nitrogen and high concentrations of carbon disulphide. The sesquisulphide, γ-Nd 2 S 3 (or Nd 3 S 4 ), was formed at temperatures as high as 1100 °C. The reaction conditions for the compounds mentioned above are discussed together with the analysis of their crystal structures by X-ray powder diffractometry.

Post-irradiation examination of high burnup mg doped UO2 in comparison with undoped UO2, Mg-Nb doped UO2 and Ti doped UO2

Abstract The pellets of UO 2 , magnesium doped UO 2 (Mg–UO 2 ), magnesium and niobium doped UO 2 (Mg–Nb–UO 2 ) and titanium doped UO 2 (Ti–UO 2 ) were irradiated to burnups ranging from 19 to 94 GWd/tU at temperatures 550–930°C. The solubility of magnesium in UO 2 was low around 2 mol%. The addition of magnesium and titanium caused to form large grain sized pellet on sintering. The swelling of pellets during irradiation was unchanged by magnesium addition below 60 GWd/tU in agreement with the literature rate for UO 2 . The thermal conductivity of unirradiated Mg–UO 2 was higher than that of undoped UO 2 , which seemed to also hold for irradiated specimens. Pellet fracturing occurred by irradiation mainly by thermal stress. The undoped and metal doped UO 2 pellets in the 84–94 GWd/tU range at the irradiation temperatures of 560–640°C showed large bubbles and sub-divided grains of sub-micron size and the rim structure formation all over the surface. The xenon release from the pellets during irradiation increased with increasing burnup. In the fuels of close burnups, the xenon release increased rapidly with increasing temperature above about 600°C. At high burnups, the effect of metal addition seemed to recede unclear perhaps due to the formation of heavily damaged fuel matrix.

Synthesis and crystal structure of alkali metal uranium sulfides, Li2US3 and Na2US3

Abstract New mixed uranium sulfides, A 2 US 3 (A=Li, Na), in which uranium is in a tetravalent state, have been synthesized. In the disordered state, the compounds are written as A(A 1/3 ,U 2/3 )S 2 which have a hexagonal ( R 3 m ) structure the same as the lanthanide homologue, ALnS 2 (Ln=trivalent lanthanides). In the ordered state, the compounds take on a monoclinic ( C 2/ m ) structure in which the atom arrangement is very close to the above hexagonal structure. The partial ordering is realized by the coexistence of the two phases. The lattice parameters of hexagonal Li 2 US 3 are a =3.898 and c =18.391 A, while those of monoclinic Li 2 US 3 are a =6.747, b =11.679, c =6.537 A and β =110.2°. The lattice parameters of hexagonal Na 2 US 3 are a =4.036 and c =19.780 A. Those of monoclinic Na 2 US 3 are a =6.990, b =12.105, c =6.992 A and β =109.5°. The molar ratios of the hexagonal and monoclinic phases are 52.2:47.8 for Li 2 US 3 and 68.0:32.0 for Na 2 US 3 , respectively. The atom parameters of uranium and sulfur were obtained by Rietveld calculation of the observed X-ray peaks. The atom separations are discussed in relation to the crystal radii of the component ions.

High temperature electrical conductivity and conduction mechanism of (U, Pu)O2±x at low oxygen partial pressures

Abstract The electrical conductivities (σ) of U 1− y Pu y O 2 ± x were measured in the oxygen partial pressure range from p (O 2 ) = 10 −15 to 10 −1.5 Pa at 1273 K. Below about 10 −5 Pa, the electrical conductivity was independent of p (O 2 ) and the σ versus y curve showed a maximum at y = ∼ 0.5. The conduction mechanism in this region was analyzed on the basis of the hopping or small polaron conduction formalism. The maximum was found to be reasonably explained by the disproportionation reaction Pu 4+ + U 4+ = Pu 3+ + U 5+ . The activation energy for electric conduction was measured by quenching the U 1− y Pu y O 2± x specimens from 1273 K. The observed (apparent) activation energy steadily increased with increasing y from 0.52 eV at y =0.05 to 0.75 eV at y =0.9. This change was well followed by assuming that the activation energy for electric conduction between uranium ions is different from that between plutonium ions. Discussion was made on the rate of the disproportionation reactions, which is low enough to yield the temperature independent concentrations of U 5+ and/or Pu 3+ when quenching.

Synthesis of BaZrS3 in the presence of excess sulfur

Abstract Synthetic reaction of barium zirconium sulfide, BaZrS 3 , was studied in a large excess amount of sulfur melt at temperatures of 623–723 K and in the presence of a small amount of excess sulfur at temperatures of 623–823 K, respectively. The results showed that BaZrS 3 was formed with high yields by heating at temperatures ranging from 723 to 873 K, if a proper amount of excess sulfur and 10 mole% of BaCl 2 were added in the starting materials. The compound could be obtained in almost a single phase with only 2–3 mole% ZrO 2 impurity after the 8 min water treatment of the products obtained at 873 K. The modified method studied in this paper, i.e. reaction in the medium temperature range of 723–873 K, was shown to be applicable for synthesizing the mixed sulfides of some kinds including BaZrS 3 .