Uranium-plutonium-americium cation interdiffusion in polycrystalline (U,Pu,Am)O mixed oxides

Vauchy, R.; Matsumoto, Taku; Hirooka, Shun; Uno, Hiroki*; Tamura, Tetsuya*; Arima, Tatsumi*; Inagaki, Yaohiro*; Idemitsu, Kazuya*; Nakamura, Hiroki; Machida, Masahiko; et al.

Journal of Nuclear Materials, 588, p.154786_1 - 154786_13, 2024/01

https://doi.org/10.1016/j.jnucmat.2023.154786

Defect equilibria and thermophysical properties of CeO based on experimental data and density functional theory calculation result

Watanabe, Masashi; Nakamura, Hiroki; Suzuki, Kiichi; Machida, Masahiko; Kato, Masato

Journal of the American Ceramic Society, 105(3), p.2248 - 2257, 2022/03

https://doi.org/10.1111/jace.18249

Properties of CeO were evaluated by DFT simulation to determine band gap, Frenkel defect formation energy and defect migration energy. Band gap and Frenkel defect formation energy were used to analyze defect equilibria. Oxygen partial pressure dependence of defect equilibria was evaluated based on oxygen potential experimental data and DFT calculation, and a Brouwer diagram was derived. The defect formation energies, including Frenkel defect, electron-hole pair and so on, were determined and used to evaluate the properties, including oxygen diffusion coefficients, electrical conduction, heat capacity and thermal conductivity. Mechanisms of various properties were discussed for a deeper understanding based on defect chemistry, and the relationship among properties were systematically described.

Crystal structure change of katoite, CaAl(OD), with temperature at high pressure

Kyono, Atsushi*; Kato, Masato*; Sano, Asami; Machida, Shinichi*; Hattori, Takanori

Physics and Chemistry of Minerals, 46(5), p.459 - 469, 2019/05

https://doi.org/10.1007/s00269-018-1016-4

To reveal the decomposition mechanism with temperature under high-pressure, crystal structure of a hydrogrossular, katoite CaAl(OD) has been studied by in-situ neutron diffraction at 8 GPa. Although unusual expansion behavior was discerned at 200-400C, the unit cell was continuously expanded up to 850C. At 900C, katoite was decomposed, indicating that pressure strongly increases dehydration temperature from 300C to 900C. On release of pressure, the katoite reappear together with corundum and portlandite. At 8 GPa, CaO and AlO polyhedra expand with temperature up to 850C by about 8% and 13%, respectively. On the other hand, tetrahedral interstices are isotopically squeezed by about 10%: due to the expansion of above polyhedra. The neighboring D-D distance remains almost unchanged in this temperature range, while the O-D bond distance shrinks drastically just before decomposition. This finding suggests that the shortening of O-D distance caused by the D-D repulsion destabilizes the O-D bond, which induces the thermal decomposition of katoite.

First-principles calculation of phonon and Schottky heat capacities of plutonium dioxide

Nakamura, Hiroki; Machida, Masahiko; Kato, Masato

Journal of the Physical Society of Japan, 84(5), p.053602_1 - 053602_5, 2015/05

https://doi.org/10.7566/JPSJ.84.053602

Plutonium dioxide (PuO) is a key ingredient of mixed oxide (MOX) and advanced nuclear fuels. Its thermophysical data is crucial in understanding the high-temperature behaviors of nuclear fuels. In particular, the high-temperature heat capacity is of great importance for their safety and performance analyses. Here, we evaluate the main contributions to the heat capacity of PuO from 0 to 1400 K through suitable first-principles calculations. Consequently, we successfully obtain a temperature dependence in good agreement with experimental measurements. This success mainly results from accurate calculations of the Schottky heat capacity caused by the excited levels of -electrons of Pu. Our calculations resolve the mystery of why previous works failed to reproduce the measurement data. This study extends the possibility of performing simulation-based nuclear-fuel research instead of difficult measurements.

LDA+ study on plutonium dioxide with spin-orbit couplings

Nakamura, Hiroki; Machida, Masahiko; Kato, Masato

Progress in Nuclear Science and Technology (Internet), 2, p.16 - 19, 2011/10

https://doi.org/10.15669/pnst.2.16

Plutonium dioxide is a main component of mixed oxide fuels. We perform first-principles density functional calculations on plutonium dioxides considering proper relativistic effects and strong correlations. Though this material is a paramagnetic insulator, standard calculations predict metallic states. The reason of this failure is often considered lack of strong correlations in LDA. However, even if strong correlation effects on plutonium -orbital electrons are taken into account using the LDA+ method, an insulating state cannot be obtained. In this paper, we point out that not only strong correlations but also spin-orbit couplings as relativistic effects are necessary to obtain paramagnetic insulating states. Based on this proper method, we calculate several physical properties of plutonium dioxide and compare them with other calculations and experiments. We also discuss the effects of oxygen deficiency on the electronic structures and thermal properties.

Defect equilibria and oxygen potential of PuO and CeO

Kato, Masato; Nakamura, Hiroki; Watanabe, Masashi; Machida, Masahiko

no journal, , 

no abstracts in English

The Bredig transition and physical property change in CaF, 2; Machine-learning molecular dynamics study

Nakamura, Hiroki; Machida, Masahiko; Watanabe, Masashi; Kato, Masato

no journal, , 

It is difficult to measure the thermal properties of nuclear fuel materials such as uranium dioxide, at high temperature just below their melting points. CaF is sometimes employed as alternative nuclear fuel materials. In this paper, we evaluate the Bredig transition and its effect on the materials properties, using machine-learning molecular dynamics trained by first-principles calculations. In particular, we investigate the dynamical structure factor measured by inelastic neutron scattering, and we explore the characteristics of the Bredig transition. We also discuss application to nuclear materials such as uranium dioxide.

Evaluation of inelastic neutron scattering of (Ca,Sr)F with machine-learning molecular dynamics

Nakamura, Hiroki; Machida, Masahiko; Watanabe, Masashi; Kato, Masato

no journal, , 

It is difficult to measure the thermal properties of nuclear fuel materials, such as MOX fuel materials, at high temperature just below their melting points. Solid solution of calcium fluoride and strontium fluoride, (Ca,Sr)F, is sometimes employed as alternative MOX fuel materials. In this paper, we evaluate the (Ca,Sr)F dynamic structure factor measured by inelastic neutron scattering using machine-learning molecular dynamics trained by first-principles calculations. We investigate the behavior of the dynamic structure factor at the Bredig transition temperature, at which heat capacity rapidly increases. We also discuss application of the present method to MOX fuel materials.

Evaluation of fundamental properties of CaF, 2; DFT analysis of high-temperature properties

Nakamura, Hiroki; Machida, Masahiko; Kato, Masato

no journal, , 

It is difficult to measure thermal properties of nuclear fuel materials such as uranium dioxide, at high temperature just below their melting points. Employing calcium fluoride as alternative nuclear fuel materials, we aim to clarify its thermophysical properties and to understand the mechanism and behavior of nuclear fuel at high temperature. In this paper, we evaluate thermal properties of calcium fluoride using first-principles calculations and confirm the reliability of the calculations comparing with experimental data. Besides, we analyze the mechanism of the Bredig transition.

Defect equilibria, electrical conductivity and oxygen diffusion

Kato, Masato; Nakamura, Hiroki; Watanabe, Masashi; Machida, Masahiko

no journal, , 

Brouwer's diagrams for PuO and CeO were constructed based on results of oxygen potential measurement and DFT calculation. Defect concentration, oxygen diffusion coefficient and electrical conductivity were evaluated, and differences between PuO and CeO were discussed.