Hiroyuki Serizawa

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.

X-ray Debye temperature and Grüneisen constant of NpO2

Abstract The Debye temperature, Θ D , and the Gruneisen constant, γ , are useful parameters to estimate thermal properties of ceramics. In the present study, those for NpO 2 were determined through a high-temperature X-ray diffraction analysis. The measurements were carried out in the temperature range from room temperature up to 1174 K. The temperature factor of Np at each measured temperature was refined by Rietveld analysis to obtain Θ D , assuming that the factor of O is the same as in UO 2 . The calculated Θ D values tended to decrease with increasing temperature, which was attributed to the anharmonicity of lattice vibration. From the decrease, we determined γ using Paskins temperature calibration which corrects the effect of anharmonicity. The values of Θ D and γ obtained in this study were 435±2 K and 1.93±0.03, respectively.

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

The evaluation of thermal conductivity of irradiated fuel is very important since it directly affects the fuel operating temperature. The disk-shaped UO2 and UO2-10 wt%Gd2O3 samples were prepared and irradiated to about 4%FIMA to measure the thermal diffusivities by the laser flash method. The burnup was almost uniform within each sample. The irradiation temperature was almost constant and uniform within each sample except the temperature escalation that occurred during the irradiation. The thermal conductivity, determined from the thermal diffusivity, density and specific heat capacity, decreased by irradiation, while it partly recovered after the thermal diffusivity measurement at temperatures up to about 1800 K. The thermal conductivity reduction attributable to the irradiation-induced point defects was small in the samples which experienced higher temperature than 1273 K during the temperature escalation. The present results were compared with the reported models

Crystallographic study on the phase transition of α-U2N3

Abstract X-ray and neutron diffraction studies were performed on nonstoichiometric α - U 2 N 3 + x having a composition range 1.68 N / U α - U 2 N 3 + x in this composition range was a distorted Mn 2 O 3 -type structure. Structure parameters of U and N atoms were determined. Results showed that the positions of U atoms varied continuously with increasing nitrogen content. No evidence of the modification from bcc to fcc could be obtained. Interatomic distances of U-U and U-N were calculated. The position parameter of N atoms showed that the position of N atoms slightly deviated from the tetrahedral site.

Simultaneous determination of X-ray Debye temperature and Grüneisen constant for actinide dioxides : PuO2 and ThO2

The lattice vibrations of PuO2 and ThO2 were examined between room temperature and 1274 K using a high temperature X-ray diffractometer. The temperature factors for the metal atom, BPu and BTh in the dioxides were evaluated by the Rietveld analysis. Debye temperature was calculated using the temperature factor. It was confirmed that the Debye temperature decreased with increasing temperature. From the temperature dependence of the Debye temperature, the Gruneisen constants were evaluated to be 1.62 for PuO2 and 1.51 for ThO2. The Debye temperatures modified for the thermal expansion of PuO2 and ThO2 were 429 and 463 K, respectively.

Formation of a nitrogen-rich α-U2N3+x phase by the reaction of uranium with a stream of ammonia

Abstract The reactions U+NH 3 (stream) and U+N 2 (stream) were performed in the temperature range from 400 to 800 °C. The results show that the use of a stream of NH 3 leads to the formation of nitrogen-rich uranium sesquinitrides, which cannot be obtained by the reactions of uranium with N 2 under normal pressures, suggesting that a stream of NH 3 possesses higher nitrogen activity compared with N 2 at 1 atm. The experimental results were analysed on the basis of thermodynamic considerations. It was found that enormously high nitrogen pressures would be required to obtain nitrogen-rich uranium sesquinitride as prepared by the reaction of uranium with a stream of NH 3 if one tried to prepare this material by the reaction of uranium with N 2 .

Thermodynamics of carbothermic synthesis of actinide mononitrides

Abstract Carbothermic synthesis will be further applied to the fabrication of nitride fuels containing minor actinides (MA) such as neptunium, americium and curium. A thorough understanding of the carbothermic synthesis of UN will be beneficial in the development of the MA-containing fuels. Thermodynamic analysis was carried out for conditions of practical interest in order to better understand the recent fabrication experiences. Two types of solution phases, oxynitride and carbonitride phases, were taken into account. The PuNO ternary isotherm was assessed for the modelling of M(C, N, O). With the understanding of the UN synthesis, the fabrication problems of Am-containing nitrides are discussed.

An examination of the estimation method for the specific heat of TRU dioxides : evaluation with PuO2

Abstract This work set out to study the estimation method of the specific heat, C p , for the dioxides of the transuranic elements. C p was evaluated as a sum of three terms, contributions of phonon vibration, C ph , dilation, C d , and Schottky specific heat, C s , C ph and C d were calculated using the Debye temperature and Gruneisen constant obtained by high-temperature X-ray diffractometry. The method was applied to PuO 2 . The estimated C p was in good accordance with the reported one measured using a calorimeter. The error in the estimation was small compared to that which arises from using the conventional method based on C p (298) and the melting temperature.

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.

Some problems in nonstoichiometry of α-uranium sesquinitride

Abstract To confirm the existence of stoichiometric UN2, experiments of the reaction of uranium with flowing NH3 have been carried out. Attempts to prepare UN2 were unsuccessful. Stoichiometric UN2 may not exist. The maximum N/U ratio, which can be attained by action of high nitrogen activity, may be close to 1.90. Also, some merits of NH3 gas as nitriding agent compared with N2 gas were demonstrated.