Kohta Yamada

Oxide electrodes in molten carbonates Part 1. Electrochemical behaviour of nickel in molten Li + K and Na + K carbonate eutectics

Abstract Spontaneous electrochemical processes occurring on Co and CoO in molten Li + K and Na + K carbonate eutectics saturated with a 0.9 O 2 + 0.1 CO 2 atmosphere were investigated at 1000 K. It was shown that oxidation of Co to CoO proceeds via formation of an unstable compound in Li 2 CO 3 + K 2 CO 3 . There is no indication that such a compound is formed in Na 2 CO 3 + K 2 CO 3 . CoO undergoes further oxidation in molten carbonates producing LiCoO 2 in Li 2 CO 3 + K 2 CO 3 and NaCoO 2 (presumably) in Na 2 CO 3 + K 2 CO 3 . The oxidation processes increase the active area of the electrode as indicated by the measurements of exchange current densities, double-layer capacitances and impedances of electrodes.

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.

Solubility of in-situ oxidized NiO in (62 + 38)mol.% (Li + K)CO3 melt under pressurized conditions

Abstract The solubilities of lithiated NiO produced by in-situ oxidation of Ni porous plaques have been measured in (62 + 38)mol.%(Li + K)CO 3 melt under pressurized conditions as functions of partial pressures of CO 2 and O 2 and of temperatures. The high performance liquid chromatography technique was applied to the solubility measurements. It was found that the NiO solubility was proportional to the CO 2 partial pressure (0.1 ⩽ P CO 2 ⩽ 3.6 atm, P O 2 = 0.9 atm) and independent of the O 2 partial pressure (0.1 ⩽ P O 2 ⩽ 3.6 atm, P CO 2 = 0.9 atm), and decreased with increasing temperature from 823 to 973 K under a total pressure of 5 atm of air +CO 2 (0.7 : 0.3 oxidant gas). The results are explained in terms of the acidic dissolution mechanism at the acidic side ( P CO 2 ⩾ 0.1 atm). However, it is noted that the solubility obtained under 5 atm of air +CO 2 (0.9:0.1) was independent of temperature.

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.

Water effect on oxygen reduction in molten (Li + K)CO3 eutectic

Abstract The effect of water on electrode kinetics and mechanism of the oxygen reduction at the fully immersed flag-type Au electrode in (62 + 38)mol% (Li + K)CO 3 melt at 650°C have been investigated using the cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and the digital simulation of the single scan voltammograms. The cyclic voltammograms of the oxygen reduction under P O 2 : P CO 2 = 0.9:0.1 atm with 2.9–15.4 Torr H 2 O vapor showed more reversible waves than those obtained under a dry atmosphere. The EIS spectra at the open circuit potential for the oxygen reduction also showed distinctly reduced impedance values in the presence of water. From the analysis of both the data, it was found that the apparent mass transfer parameter C√D was increased by the presence of water, indicating that water can act as an acid species in the neutralization reaction of the oxide ion according to the reaction H 2 O + O 2− ⇆ 2OH − .

In situ NiO dissolution behavior in (Li + Na)CO3 melts under pressurized oxidant gas atmospheres

Abstract The dissolution behavior of in situ oxidized NiO is investigated under pressurized oxidant gas conditions as a function of pressure, temperature and lithium content in (Li + Na)CO 3 melts. For the measurement of NiO solubility, a high performance liquid chromatography (HPLC) method was employed. It is found that NiO solubility is proportional to CO 2 partial pressure (0.1 ≤ p CO 2 ≤ 3.6 atm at p O 2 = 0.9 atm) but not to the O 2 partial pressure (0.1 ≤ p O 2 ≤ 3.6 atm at p CO 2 = 0.9 atm). This indicates that NiO dissolution under pressurized condition follows the acidic dissolution mechanism. On increasing temperature from 823 to 973 K under a total pressure of 5 atm with an air/CO 2 mixture results in decreasing NiO dissolution because the melts shifted to more basic media. Similarly, increasing the lithium content from 40 to 70 mol% increases the basicity of the carbonate melt. This also resulted in decreasing NiO solubility.

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.

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 .

Unusual variation of temperature factor of uranium dioxide at high temperature

Abstract High temperature behavior of UO 2 was examined by X-ray diffraction. The temperature factor of UO 2 was evaluated at room temperature and at high temperatures from 1003 to 1573 K. Two types of factors were calculated: the overall temperature factor of U and O atoms, designated as B ; and the temperature factor of U atom, B U . In both cases, the value of the factors increased with increasing temperature up to ca. 1400 K. However, an anomalous decrease in these factors was detected above 1400 K. X-ray Debye temperature, Θ , was derived from the temperature factors as a function of temperature. Corresponding to the anomaly of the temperature factor, Θ increased atypically at the temperature higher than ca. 1400 K.