Kinji Ouchi

Composition and oxygen potential of cubic fluorite-type solid solution EuyU1−yO2+x () and rhombohedral Eu6UO12+x' (x'< 0)

A fluorite-type solid solution, EuyU1−yO2+x, was formed in single phase up to y = 0.51 when heated in vacuum at 1400°C. Between y = 0.51 and 0.80, a two-phase mixture of fluorite and rhombohedral phases exists. The rhombohedral phase was of single phase in a range y = 0.8–0.9. The change of lattice parameter and x value in the fcc solid solution were determined as a function of y value in EuyU1−yO2+x. The crystal structure of the nonstoichiometric Eu6UO11.41 is the same as RE6UO12 (R3, Z = 3) with Bartrams atom parameters. The measured oxygen potentials (ΔḠO2) for fcc EuyU1−yO2+x (y = 0.1 and 0.3) are not only much higher than those of the solid solutions of the other rare-earth elements, but are also shifted to the lower x side yielding a nearly vertical change of ΔḠO2 in the hypostoichiometric range (x < 0). As a result, the partial molar entropy of oxygen (ΔSO2) and enthalphy (ΔHO2) are significantly different from those of the solid solutions of the other rare-earth elements. ΔḠO2, ΔSO2 and ΔHO2 for rhombohedral Eu0.8571U0.1429O1.7143+x (x < 0) were measured and discussed.

Reaction of lithium and sodium nitrates and carbonates with uranium oxides

Abstract The reactivity and reaction conditions to form lithium and sodium uranates were studied in an attempt to grope some useful head-end processes in nuclear fuel reprocessing. In the reactions between alkali metal carbonates and U 3 O 8 in air at 800°C, the products with Na/U ratios 0.8 and 0.857 gave the same X-ray diffraction patterns in which the peaks of α-U 3 O 8 were almost not detected. The observed peaks for uranates with Li/U = 1.205, 2, 4 and Na/U = 1 are well consistent with the values reported previously. No indication of formation of Li 2 U 6 O 19 , Li 6 UO 6 and Na 4 UO 5 was observed. Thermogravimetric observations on the reactions between the carbonates and U 3 O 8 revealed that they consisted of two processes, i.e. (1) the formation of uranates and (2) the oxidation of the uranates formed. The rate of reaction (1) was higher than that of reaction (2) when the M/U ( M = Li, Na ) ratio was 0.5. When the ratio was 1, the rate of reaction (1) lowered and became rate determining. The reactions between alkali metal nitrates and UO 2 showed that the minimum M/U ratios for obtaining the uranates without U 3 O 8 were 0.667 and 0.8 for lithium and sodium uranates, respectively. They were formed by heating at 600°C for 3 h in oxygen or air. Mixing process of initial materials is not required for these reactions. The uranates formed were found to be dissolved in 1 M HNO 3 within 1 min.

The variation of lattice parameter with hydrogen content of non-stoichiometric plutonium dihydride

Abstract The relation between composition and lattice parameter of non-stoichiometric plutonium dihydride (cub.) has been studied. The samples were prepared by two different methods, namely, thermal decomposition of PuH3.0 and hydrogenation reaction of the lower hydrides. As the H/Pu mole ratio increased from 2·0 to 2·7, the lattice parameter of PuH2+x lineally decreased from 5·360 to 5·340A˚. The influence of the oxidation on the lattice parameter was also given.

Phase relation and thermodynamic properties of cubic fluorite-type solid solution, Bay2Yy2U1−yO2 + x (x 0)

Abstract The solubility of barium (large crystal radius) in UO2 was studied under the coexistence of yttrium (smaller crystal radius). After the phase relation and the change of lattice parameter were examined on the fcc fluorite phase of the solid solution, oxygen potentials were measured for Ba0.05Y0.05U0.9O2 + x. It was found that the Ba y 2 Y y 2 U1−yO2 + x solid solution was formed in single phase in a range 0 ≦ y ≦ 0.1 by heating in helium, vacuum and hydrogen at temperatures between 1000 and 1400°C. The change rate of the lattice parameter of the solid solution with barium concentration was found to be markedly smaller than the calculated value in contrast to the case of yttrium for which agreement between the observed and calculated rates was reasonably good. Oxygen potential measurements of this solid solution revealed higher Δ G O 2 than those of the solid solutions containing tri- or tetravalent foreign metals. The ratio of oxygen to metal atom giving a very steep change in Δ G O 2 was seen to exist at O/(Ba + Y + U) = 1.917. This O M value is significantly lower than 2.0 for most other solid solutions. Moreover, Δ G O 2 of the present solid solution does not vary linearly with temperature between 900 and 1300°C leading to give temperature dependent Δ S O 2 and Δ H O 2 . Thus, the Δ S O 2 and Δ H O 2 values at 912°C were found to be much lower than those at higher temperatures.

Electrical conductivity anomaly in near-stoichiometric plutonium dioxide

The variation of the electrical conductivity with oxygen pressure has been measured for plutonium dioxide at temperatures between 950 and 1100°C and in the oxygen partial pressure range from 2.1 × 104to 10−11 Pa. Minima in electrical conductivity and corresponding transitions from n- to p-type conduction as function of the oxygen partial pressure were observed. These minima could occur by the presence of some impurities, but the possibility of the existence of hyperstoichiometric PuO2 also could not be excluded by considering the concentration of the impurities. An intrinsic band-gap energy of 2.5 eV was calculated from the temperature dependence of the minimum electrical conductivity. The slopes of the plot of log o against log PO2 for the n-type region were −14.99, −14.72, −14.77 and −14.81 at 950,1000, 1050 and 1100°C, respectively, which are in good agreement with the values reported previously. The ionized oxygen vacancy model seems to be reasonable in the n-type region.

Kinetic study of the nitrogenation of plutonium hydride with nitrogen

Abstract The nitrogenation of plutonium hydride powder (200–325 mesh) with nitrogen has been studied in the temperature range of 200–300°C. It was found that the nitrogenation obeyed the so-called linear law during an early period of the reaction and that the nitrogenation rate was not affected by gas flow velocity. It was also found that the rate increased with decreasing partial pressure of nitrogen, and was independent of the partial pressure of hydrogen. The activation energy was about 36 kcal/mol for PuH 3 and 47 kcal/mol for PuH 2 . X-ray diffraction study showed the product to be PuN, of which lattice parameter was 4·902 ± 0·002 A. From these results, it is considered that the nitrogenation is controlled by the reaction at the hydride-nitride interface. On the basis of the Langmuir isotherm, the dependence of the nitrogenation rate on the partial pressure of nitrogen is discussed.

Kinetic study of the nitrogenation of plutonium hydride with ammonia

The nitrogenation of plutonium hydride with ammonia has been studied in the temperature range of 120–250°C under 20–500 torr ammonia. The nitrogenation of PuH3.0 begins at about 180°C and become rapid above 210°C, as follows: PuH3.0+NH3→PuN+3H2· The reaction follows rate equation 1 − (1 − α)13 = kLt, the reaction rate not influenced by ammonia pressure; and the activation energy is 22.6 kcal/mol. The nitrogenation of PuH2.0 proceeds in two stages. In the first stage, the reaction corresponds to the formation of PuN and PuH3.0: 6PuH2.0+NH3→PuN+5PuH3.0· The reaction rate follows the same rate equation, the reaction rate increasing with ammonia pressure; the activation energy is 18.8 kcal/mol. In the second stage, the reaction proceeds above 210°C. It seems that the formation of PuN occurs from NH3 and PuH3.0 produced in the first stage. Both the reactions are thus determined by the phase boundary reaction at hydride-solid products interface. The rate-determining step is discussed on the basis of the dependence of reaction rate on ammonia pressure and the activation energy. The hydrogenation of PuH2.0 is also described.

Calculation of oxygen potential change of irradiated UO2 and UO2—PuO2 mixed oxide fuels using the intra-cation complex model

Abstract The oxygen potential change of irradiated uranium dioxide and urania-plutonia mixed oxide fuels, which is caused by the formation of solid solutions of some fission product metals with the fuel oxides, was calculated using the intra-cation complex model. It assumes weak-bound complexes between the cations of relatively positive and negative charges with respect to U4+. Partial molar entropy of oxygen, Δ S O 2 , was derived by differentiating the logarithm of the number of ways of arranging the cationic defects and complexes over the cation sublattices. The fission product metals which dissolve into the fuel matrix were classified into +2, +3 and +4 valence state ion groups, and their effect on Δ G O 2 was clarified in the equations of Δ G O 2 representation. Comparison of the calculated results with the Δ G O 2 values for irradiated fuels as well as for the fuels containing some fission product simulant cations revealed that agreement was good, showing that the Δ G O 2 change on irradiation can be expressed as a function of M3+ fraction.

An improved gravimetric method for determining oxygen in binary and ternary uranium oxides by addition of alkali or alkaline earth metal nitrate

Abstract A known amount of lithium or calcium nitrate solution is added to the sample powder which is then heated in air or oxygen. The oxygen in test samples of UO 2+ x , Sr 0.1 U 0.9 O 2 + x and Sr 0.2 U 0.5 O 2 + x was determined with an estimated standard deviation of ± 0.002.

A study of ammonium plutonium(IV) fluorides—II: The thermal decomposition of ammonium plutonium(IV) fluorides, ammonium cerium(IV) fluoride and ammonium uranium(IV) fluoride

Abstract Thermal decomposition of PuF 4 · 7 6 NH 4 F (or PuF4·NH4F), PuF4·2NH4F, CeF 4 · 7 6 NH 4 F (or CeF4·NH4F) and UF 4 · 7 6 NH 4 F (or UF4·NH4F) have been studied under vacuum and in inert gas flow, using a microthermobalance. The anhydrous PuF3, CeF3 and UF4 were obtained as the decomposition products. The following reactions appeared to occur in thermal decomposition of these double salts. 1 M F 4 · 7 6 NH 4 F = M F 3 + 5 6 NH 4 F + 8 6 HF + 1 6 N 2 (M = Pu and Ce ) 2 UF 4 · 7 6 NH 4 F = UF 4 + 7 6 NH 4 F .