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Journal Articles

A Science-based mixed oxide property model for developing advanced oxide nuclear fuels

Kato, Masato; Oki, Takumi; Watanabe, Masashi; Hirooka, Shun; Vauchy, R.; Ozawa, Takayuki; Uwaba, Tomoyuki; Ikusawa, Yoshihisa; Nakamura, Hiroki; Machida, Masahiko

Journal of the American Ceramic Society, 107(5), p.2998 - 3011, 2024/05

Journal Articles

Uranium-plutonium-americium cation interdiffusion in polycrystalline (U,Pu,Am)O$$_{2 pm x}$$ 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

Journal Articles

Machine learning molecular dynamics reveals the structural origin of the first sharp diffraction peak in high-density silica glasses

Kobayashi, Keita; Okumura, Masahiko; Nakamura, Hiroki; Itakura, Mitsuhiro; Machida, Masahiko; Urata, Shingo*; Suzuya, Kentaro

Scientific Reports (Internet), 13, p.18721_1 - 18721_12, 2023/11

The first sharp peak diffraction peak (FSDP) in the structure factor of amorphous materials is thought to reflect the medium-range order structure in amorphous materials, and the structural origin of the FSDP has been a subject of ongoing debate. In this study, we employed machine learning molecular dynamics (MLMD) with nearly first-principles calculation accuracy to investigate the structural origin of the FSDP in high-density silica glass. First, we successfully reproduced various experimental data of high-density silica glass using MLMD. Furthermore, we revealed that the development (or reduction) of the FSDP in high-density silica glass is characterized by the deformation behavior of ring structures in Si-O covalent bond networks under compression.

Journal Articles

Level structures of $$^{56,58}$$Ca cast doubt on a doubly magic $$^{60}$$Ca

Chen, S.*; Browne, F.*; Doornenbal, P.*; Lee, J.*; Obertelli, A.*; Tsunoda, Yusuke*; Otsuka, Takaharu*; Chazono, Yoshiki*; Hagen, G.*; Holt, J. D.*; et al.

Physics Letters B, 843, p.138025_1 - 138025_7, 2023/08

Gamma decays were observed in $$^{56}$$Ca and $$^{58}$$Ca following quasi-free one-proton knockout reactions from $$^{57,59}$$Sc. For $$^{56}$$Ca, a $$gamma$$ ray transition was measured to be 1456(12) keV, while for $$^{58}$$Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the $$2^{+}_{1} rightarrow 0^{+}_{gs}$$ decays. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for $$2^{+}_{1}$$ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the N = 34 shell. Its constituents, the $$0_{f5/2}$$ and $$0_{g9/2}$$ orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic $$^{60}$$Ca and potentially drives the dripline of Ca isotopes to $$^{70}$$Ca or even beyond.

Journal Articles

Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe$$_2$$

Kinjo, Katsuki*; Fujibayashi, Hiroki*; Matsumura, Hiroki*; Hori, Fumiya*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; et al.

Science Advances (Internet), 9(30), p.2736_1 - 2736_6, 2023/07

Journal Articles

Low-temperature magnetic fluctuations investigated by $$^{125}$$Te-NMR on the uranium-based superconductor UTe$$_2$$

Fujibayashi, Hiroki*; Kinjo, Katsuki*; Nakamine, Genki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.

Journal of the Physical Society of Japan, 92(5), p.053702_1 - 053702_5, 2023/05

 Times Cited Count:0 Percentile:0(Physics, Multidisciplinary)

Journal Articles

Large reduction in the $$a$$-axis knight shift on UTe$$_2$$ with $$T_c$$ = 2.1 K

Matsumura, Hiroki*; Fujibayashi, Hiroki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.

Journal of the Physical Society of Japan, 92(6), p.063701_1 - 063701_5, 2023/05

Journal Articles

Machine learning molecular dynamics simulations for evaluation of high-temperature properties of nuclear fuel materials

Kobayashi, Keita; Nakamura, Hiroki; Itakura, Mitsuhiro; Machida, Masahiko; Okumura, Masahiko

Materia, 62(3), p.175 - 181, 2023/03

no abstracts in English

Journal Articles

Change of superconducting character in UTe$$_{2}$$ induced by magnetic field

Kinjo, Katsuki*; Fujibayashi, Hiroki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; Homma, Yoshiya*; et al.

Physical Review B, 107(6), p.L060502_1 - L060502_5, 2023/02

 Times Cited Count:2 Percentile:92.2(Materials Science, Multidisciplinary)

Journal Articles

Correlations for the specific heat capacity of (U$$_x$$Pu$$_{1-x}$$)$$_{1-y}$$Gd$$_y$$O$$_{2-z}$$ derived from molecular dynamics

Galvin, C. O. T.*; Machida, Masahiko; Nakamura, Hiroki; Andersson, D. A.*; Cooper, M. W. D.*

Journal of Nuclear Materials, 572, p.154028_1 - 154028_8, 2022/12

 Times Cited Count:0 Percentile:0.01(Materials Science, Multidisciplinary)

UO$$_2$$ is the primary conventional fuel used in most nuclear reactors with Gd$$_2$$O$$_3$$ commonly added as a burnable absorber to produce a more level power distribution in the reactor core at the beginning of operation. It can also be mixed with other actinide oxides to produce mixed oxide (MOx) fuel. In this study, molecular dynamics simulations were used to predict the specific heat capacity of Gd-doped PuO$$_2$$, UO$$_2$$ and (U,Pu)O$$_2$$ MOx accommodating Gd$$^{3+}$$ substituted at cation sites via two charge compensation mechanisms - oxygen vacancy formation and the oxidation of U$$^{4+}$$ to U$$^{5+}$$. The specific heat capacity values for PuO$$_2$$ and UO$$_2$$ are in good agreement with other studies showing a distinct peak at high temperatures - above 1800 K. As Gd$$^{3+}$$ is added, the peak height reduces for each composition considered. An analytical fit was applied to the data where Gd$$^{3+}$$ was fully charge compensated by either oxygen vacancies or U$$^{5+}$$. The expression was then validated by predicting the specific heat capacity for three compositions of (U$$_x$$Pu$$_{1-x}$$)$$_{1-y}$$Gd$$_y$$O$$_{2-z}$$ containing both oxygen vacancies and U$$^{5+}$$, and compared to molecular dynamics data.

Journal Articles

"Southwestern" boundary of the $$N = 40$$ island of inversion; First study of low-lying bound excited states in $$^{59}$$V and $$^{61}$$V

Elekes, Z.*; Juh$'a$sz, M. M.*; Sohler, D.*; Sieja, K.*; Yoshida, Kazuki; Ogata, Kazuyuki*; Doornenbal, P.*; Obertelli, A.*; Achouri, N. L.*; Baba, Hidetada*; et al.

Physical Review C, 106(6), p.064321_1 - 064321_10, 2022/12

 Times Cited Count:0 Percentile:0.02(Physics, Nuclear)

The low-lying level structure of $$^{59}$$V and $$^{61}$$V was investigated for the first time. The neutron knockout reaction and inelastic proton scattering were applied for $$^{61}$$V while the neutron knock-out reaction provided the data for $$^{59}$$V. Four and five new transitions were determined for $$^{59}$$V and $$^{61}$$V, respectively. Based on the comparison to our shell-model calculations using the Lenzi-Nowacki-Poves-Sieja (LNPS) interaction, three of the observed $$gamma$$ rays for each isotope could be placed in the level scheme and assigned to the decay of the first 11/2$$^{-}$$ and 9/2$$^{-}$$ levels. The ($$p$$,$$p'$$) excitation cross sections for $$^{61}$$V were analyzed by the coupled-channels formalism assuming quadrupole plus hexadecapole deformations. Due to the role of the hexadecapole deformation, $$^{61}$$V could not be unambiguously placed on the island of inversion.

Journal Articles

Extended $$p_{3/2}$$ neutron orbital and the $$N = 32$$ shell closure in $$^{52}$$Ca

Enciu, M.*; Liu, H. N.*; Obertelli, A.*; Doornenbal, P.*; Nowacki, F.*; Ogata, Kazuyuki*; Poves, A.*; Yoshida, Kazuki; Achouri, N. L.*; Baba, Hidetada*; et al.

Physical Review Letters, 129(26), p.262501_1 - 262501_7, 2022/12

 Times Cited Count:0 Percentile:29.72(Physics, Multidisciplinary)

The one-neutron knockout from $$^{52}$$Ca was performed at $$sim$$230 MeV/nucleon combined with prompt $$gamma$$ spectroscopy. The momentum distributions corresponding to the removal of $$1f_{7/2}$$ and $$2p_{3/2}$$ neutrons were measured. The cross sections are consistent with a shell closure at the neutron number $$N = 32$$, found as strong as at $$N = 28$$ and $$N = 34$$ in Ca isotopes from the same observables. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron $$1_{f7/2}$$ and $$2p_{3/2}$$ orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square radius of the $$2p_{3/2}$$ orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

Journal Articles

Materials science and fuel technologies of uranium and plutonium mixed oxide

Kato, Masato; Machida, Masahiko; Hirooka, Shun; Nakamichi, Shinya; Ikusawa, Yoshihisa; Nakamura, Hiroki; Kobayashi, Keita; Ozawa, Takayuki; Maeda, Koji; Sasaki, Shinji; et al.

Materials Science and Fuel Technologies of Uranium and Plutonium mixed Oxide, 171 Pages, 2022/10

Innovative and advanced nuclear reactors using plutonium fuel has been developed in each country. In order to develop a new nuclear fuel, irradiation tests are indispensable, and it is necessary to demonstrate the performance and safety of nuclear fuels. If we can develop a technology that accurately simulates irradiation behavior as a technology that complements the irradiation test, the cost, time, and labor involved in nuclear fuel research and development will be greatly reduced. And safety and reliability can be significantly improved through simulation of nuclear fuel irradiation behavior. In order to evaluate the performance of nuclear fuel, it is necessary to know the physical and chemical properties of the fuel at high temperatures. And it is indispensable to develop a behavior model that describes various phenomena that occur during irradiation. In previous research and development, empirical methods with fitting parameters have been used in many parts of model development. However, empirical techniques can give very different results in areas where there is no data. Therefore, the purpose of this study is to construct a scientific descriptive model that can extrapolate the basic characteristics of fuel to the composition and temperature, and to develop an irradiation behavior analysis code to which the model is applied.

Journal Articles

Machine learning molecular dynamics simulations toward exploration of high-temperature properties of nuclear fuel materials; Case study of thorium dioxide

Kobayashi, Keita; Okumura, Masahiko; Nakamura, Hiroki; Itakura, Mitsuhiro; Machida, Masahiko; Cooper, M. W. D.*

Scientific Reports (Internet), 12(1), p.9808_1 - 9808_11, 2022/06

 Times Cited Count:1 Percentile:73.76(Multidisciplinary Sciences)

no abstracts in English

Journal Articles

Status of decommissioning for prototype ATR Fugen and FBR Monju

Ito, Kenji; Kondo, Tetsuo; Nakamura, Yasuyuki; Matsuno, Hiroki; Nagaoki, Yoshihiro; Sakuma, Yuichi

Dekomisshoningu Giho, (63), p.1 - 26, 2022/05

The prototype advanced thermal reactor Fugen entered into the decommissioning phases with the approval of the decommissioning plan in February 2008. The prototype fast breeder reactor Monju entered into the decommissioning stage with the approval of the decommissioning plan in March 2018. In April 2018, the head office of Tsuruga decommissioning demonstration was newly established to oversee the decommissioning operations in Tsuruga area, and decommissioning projects for two unique reactors have progressed safely and steadily.

Journal Articles

Drastic change in magnetic anisotropy of UTe$$_2$$ under pressure revealed by $$^{125}$$Te-NMR

Kinjo, Katsuki*; Fujibayashi, Hiroki*; Nakamine, Genki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; et al.

Physical Review B, 105(14), p.L140502_1 - L140502_5, 2022/04

 Times Cited Count:3 Percentile:69.66(Materials Science, Multidisciplinary)

Journal Articles

A First glimpse at the shell structure beyond $$^{54}$$Ca; Spectroscopy of $$^{55}$$K, $$^{55}$$Ca, and $$^{57}$$Ca

Koiwai, Takuma*; Wimmer, K.*; Doornenbal, P.*; Obertelli, A.*; Barbieri, C.*; Duguet, T.*; Holt, J. D.*; Miyagi, Takayuki*; Navr$'a$til, P.*; Ogata, Kazuyuki*; et al.

Physics Letters B, 827, p.136953_1 - 136953_7, 2022/04

 Times Cited Count:2 Percentile:54.36(Astronomy & Astrophysics)

no abstracts in English

Journal Articles

Impressions of the 3rd Joint Meeting of Japanese Society of Radiation Safety Management and Japan Health Physics Society

Watanabe, Yuki; Tsuji, Tomoya; Hirota, Seiko*; Hokama, Tomonori; Nakajima, Junya; Tsujiguchi, Takakiyo*; Kimura, Tatsuki*; Koike, Hiromi*; Nakamura, Kaori*; Kuwata, Haruka*; et al.

Hoken Butsuri (Internet), 57(1), p.54 - 64, 2022/03

no abstracts in English

Journal Articles

Defect equilibria and thermophysical properties of CeO$$_{2-x}$$ 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

 Times Cited Count:1 Percentile:7.37(Materials Science, Ceramics)

Properties of CeO$$_{2}$$ 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.

Journal Articles

Slow electronic dynamics in the paramagnetic state of UTe$$_2$$

Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Haga, Yoshinori; Tokiwa, Yoshifumi; Opletal, P.; Fujibayashi, Hiroki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; et al.

Journal of the Physical Society of Japan, 91(2), p.023707_1 - 023707_5, 2022/02

 Times Cited Count:11 Percentile:92.14(Physics, Multidisciplinary)

$$^{125}$$Te NMR experiments in field ($$H$$) applied along the easy magnetization axis (the $$a$$-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe$$_2$$. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30$$-$$40 K, where the spin susceptibility along the hard magnetization axis (the $$b$$-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in $$H|a$$. We suggest that UTe$$_2$$ would be located on the paramagnetic side near an electronic phase boundary, where either the magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.

386 (Records 1-20 displayed on this page)