Toshihiko Ohmichi

On the relation between lattice parameter and O/M ratio for uranium dioxide-trivalent rare earth oxide solid solution

Abstract The lattice parameters and the O/M ratios of the solid solutions between UO2 and trivalent rare earth oxides (RO1.5, R= Y, Gd, Eu or Nd) were investigated. The lattice parameter behavior for the types of solid solutions of U1−yRyO2.00 and U 1−y R y O 2− y 2 was discussed using the radii of ions composing the fluorite lattice. It was found that, in the former type of solid solution, the oxidation state of uranium was pentavalent. The analysis of the lattice parameter for the latter type of solid solution showed that the radius of the oxygen vacancies, rov, was approximately 10% larger than that of the O2− ion. Aside from rov, an apparent hypothetical radius of the oxygen vacancies, r ∗ ov , which was smaller than that of the O2− ion was defined for the hypostoichiometric solid solution. With this r ∗ ov , an explanation for the lattice parameter change with x for U1−yPuyO2−x was made.

The effect of gadolinium content on the thermal conductivity of near-stoichiometric (U,Gd)O2 solid solutions

Abstract The thermal conductivities of near-stoichiometric (U, Gd)O2 solid solutions containing CdO1.5 up to 15 mol% were determined in the temperature range 700 to 2000 K from thermal diffusivities measured by the laser flash method. Temperature dependence of the thermal conductivities up to around 1600 K could be expressed by the phonon conduction equation K = (A + BT) −1 . The thermal conductivity decreased gradually with an increase of gadolinium content. Thermal resistivities caused by lattice defects were calculated from a theoretical model considering U4+, U5+ and Gd3+ ions as phonon scattering centers. It was found that this model was in good agreement with the experimental results. The calculation based on this model indicates that the lattice strain effect on the lattice defect thermal resistivity is much larger than the mass effect.

Thermophysical properties of rock-like oxide fuel with spinel-yttria stabilized zirconia system

Abstract Thermal expansion, thermal diffusivity, melting temperature, Vickers hardness and creep rate of the rock-like oxide (ROX) fuel were measured with the MgAl 2 O 4 (spinel)–ZrO 2 (Y,Gd) (YSZ: stabilized zirconia) system and the MgAl 2 O 4 –YSZ–UO 2 system in the temperature range between room temperature and 1800 K in order to evaluate thermophysical properties. Thermal expansion coefficients of MgAl 2 O 4 –YSZ composites increased with increasing YSZ content and the values were well represented by the Turners equation. Addition of UO 2 to MgAl 2 O 4 –YSZ composite resulted in an increase of thermal expansion. Thermal conductivity values of the MgAl 2 O 4 –YSZ composites decreased with increasing YSZ content and agreed with predictions of the Maxwell–Eucken equation. The eutectic temperature of MgAl 2 O 4 –YSZ and MgAl 2 O 4 –YSZ–UO 2 systems was found to be 2200 K. High temperature hardness of the composites was higher than that of YSZ or MgAl 2 O 4 . The hardness of ROX fuel was considerably higher than that of UO 2 . The creep rate of MgAl 2 O 4 –YSZ composite was controlled by the lattice diffusion of YSZ.

Dependence of the thermal conductivity of (U,Pu)N on porosity and plutonium content

Abstract The dependence of the thermal conductivity of (U, Pu)N on porosity and plutonium content was investigated from 680 to 1600 K. The modified Maxwell-Eucken equation was applied to the experimental results for (U0.8Pu0.2)N pellets with 82–95% of theoretical density. The coefficients β in the equation were separately estimated for the pellets sintered at different temperatures and for those sintered with pore former particles. The plutonium content dependence was examined by use of (U1−xPux)N pellets with x = 0, 0.2, 0.35, 0.6, 0.8 and 1.0. Prominent decrease in thermal conductivity with plutonium content was found in the UN-rich region and the temperature dependence diminished with the increase of plutonium content.

Thermal conductivity of near-stoichiometric (U, Pu, Nd)O2 and (U, Pu, Eu)O2 solid solutions

Abstract The thermal conductivities of near-stoichiometric [(U0.8Pu0.2),R]O2 solid solutions containing RO1.5(R = Nd or Eu) up to 10 mol% were determined by the laser flash method in the temperature range 700–1900 K. The thermal conductivities for the solid solutions up to about 1550 K. satisfied the phonon conduction equation K = (A + BT) −1 . The thermal conductivity decreased gradually with the increase of the rare earth content. This decrease was mainly caused by the lattice defect thermal resistance. The measured defect thermal resistivities ( = A ) were in good agreement with the calculated results based on the lattice defect model in which U4+, U5+, Pu4+ and R3+ ions were considered as phonon scattering centers. The lattice strain parameter ϵ = 97 and 103 were obtained for (U, Pu, Nd)O2 and (U, Pu, Eu)O2 solid solutions, respectively. The lattice strain effect on the thermal resistivity was about 15 times larger than the mass difference one.

Rock-Like Oxide Fuels and Their Burning in LWRs

Research on the plutonium rock-like oxide (ROX) fuels and their once-through burning in light water reactors has been performed to establish an option for utilizing and disposing effectively the excess plutonium. The ROX fuel is a sort of the inert matrix fuels and consists of mineral-like compounds such as yttria stabilized zirconia, spinel and corundum. A particle-dispersed fuel was devised to reduce damage by heavy fission fragments. Some preliminary results on swelling, fractional gas release and microstructure change for five ROX fuels were obtained from the irradiation test and successive post-irradiation examinations. Inherent disadvantages of the Pu-ROX fuel cores could be improved by adding 238U or 232Th as resonant materials, and all improved cores showed a nearly the same characteristics as the conventional UO2 core during transient conditions. The threshold enthalpy of the ROX fuel rod failure was found to be comparable to the fresh UO2 rod by pulse-irradiation tests simulating reactivity initiated accident conditions.

Vaporization behaviour of (Pu,Am)N

Abstract An unusual vaporization behaviour of plutonium and americium from a reactor grade PuN sample was found in a Knudsen cell effusion mass spectrometric analysis, where initially the mass-239 signal was much lower than the thermodynamic expectation while the mass-241 signal was significantly large. The observation was adequately explained with a thermodynamic model of (Pu,Am)Ni 1− x . The analysis was based on a working hypothesis that the Gibbs free energy of formation of AmN is not so different from those of UN, PuN and LaN. The agreement of the calculation and observation supported this hypothesis. The second-law enthalpy of formation of AmN was estimated to be −294 kJ mol −1 at 1600 K from the measured vapour pressure of Am over the PuN sample.

The effect of oxygen impurity on the characteristics of uranium and uranium-plutonium mixed nitride fuels

Abstract The effect of oxygen impurity on the typical characteristics of UN and (U, Pu)N fuels was investigated. The oxygen content of the samples was adjusted to ∼0.3, ∼0.6 and ∼1.0 wt%. It was confirmed from chemical, X-ray and ceramographic analyses that oxygen added to the nitrides existed in the oxide precipitates after sintering. While sinterability was observed. Furthermore, the addition of ∼1.0wt% of oxygen impurity decreased the thermal conductivities of UN and (U, Pu)N by 9–10 and 12–13% at 1000 and 1500 K, respectively.

Thermal conductivity of near-stoichiometric (U, Nd)O2, (U, Sm)O2 and (U, Eu)O2 solid solutions

Abstract The thermal conductivities of near-stoichiometric (U, R)O 2 solid solutions (R = Nd, Sm and Eu) containing RO 1.5 up to 15 mol% were determined in the temperature range 700–2000 K by the measurement of thermal diffusivity. The thermal conductivities satisfied the phonon conduction equation K = ( A + BT ) −1 within ± 5%. The constant A corresponding to the lattice defect thermal resistivity increased linearly with the rare earth element content, while the temperature coefficient B was almost independent of it. The change in A with the rare earth element content increased in order of (U, Eu)O 2 , (U, Sm)O 2 and (U, Nd)O 2 solid solutions. The increase of A was explained reasonably by the lattice defect model considering U 4+ , U 5+ and R 3+ ions in the solid solutions as phonon scattering centers, using a common value for the strain parameter ( ϵ = 110). For all solid solutions, the lattice strain effect on the lattice defect thermal resistivities was much larger than the mass effect. In addition, the effect of U 5+ ions on the lattice defect thermal resistivity caused by the lattice strain effect was larger than that of R 3+ ions.

Vaporization behavior of uranium-plutonium mixed nitride

The vaporization behavior of UN, PuN and uranium-plutonium mixed nitride, (U, Pu)N, was observed by a Knudsen-effusion mass spectrometry. The mixed nitride samples with the wide PuN composition range were prepared by the homogenization of the mixtures of UN and PuN synthesized by the carbothermic reduction. The results of evaporation for UN and PuN agree reasonably with those reported previously. The partial pressures of U and Pu over mixed nitride were depressed by the formation of the solid solution of UN and PuN. The estimation of the activities and activity coefficients of UN and PuN in mixed nitride revealed that mixed nitride does not behave as an ideal solution. It is also suggested that mixed nitride evaporates congruently in the temperature range of the present study without the formation of any molten phase in a condition of Knudsen effusion.