Masatoshi Akashi - Academia.edu (original) (raw)

Papers by Masatoshi Akashi

Journal of Nuclear Materials, 2017

Oxygen potential of (U,Pu)O2 ± x was evaluated based on defect chemistry using an updated experim... more Oxygen potential of (U,Pu)O2 ± x was evaluated based on defect chemistry using an updated experimental data set. The relationship between oxygen partial pressure and deviation x in (U,Pu)O2 ± x was analyzed, and equilibrium constants of defect formation were determined as functions of Pu content and temperature. Brouwer's diagrams were constructed using the determined equilibrium constants, and a relational equation to determine O/M ratio was derived as functions of O/M ratio, Pu content and temperature. In addition, relationship between oxygen potential and oxygen diffusion coefficients were described.

The Proceedings of The Computational Mechanics Conference, 2007

Journal of the Society of Powder Technology, Japan, 2011

Journal of Nuclear Science and Technology, 2015

Specimens of (U, Pu, Zr)O 2 were prepared as simulated corium debris that were assumed like debri... more Specimens of (U, Pu, Zr)O 2 were prepared as simulated corium debris that were assumed like debris generated in the severe accident of the Fukushima Daiichi Nuclear Power Plant and their melting temperatures were measured by the thermal arrest technique in order to evaluate the influence of plutonium and zirconium content on the melting temperature of the corium debris. From the evaluation, it was found that the influence of zirconium on the melting temperatures of both (U, Pu, Zr)O 2 and (U, Zr)O 2 was similar and that the melting temperature of (U, Pu, Zr)O 2 had a local maximum value in the Pu-content between 0 and 20 mol%. The UO 2-PuO 2-ZrO 2 pseudo-ternary phase diagram at 2900 and 3000 K was evaluated from the present experimental results and previously reported results.

MRS Proceedings, 2012

ABSTRACTCorium which simulates the molten core of a Boiling Water Reactor was prepared as a param... more ABSTRACTCorium which simulates the molten core of a Boiling Water Reactor was prepared as a parameter of Zr content, and melting temperatures and thermal conductivities were measured. The melting temperatures were measured by the thermal arrest method and were 2622 oC, 2509 oC and 2540 oC, respectively, in the specimens of 24.3 at%, 49.0 at% and 73.5 at% Zr content. Thermal conductivities had low values of 2.0-3.5 W/m oC at temperatures of 400 to 1600 oC.

MRS Proceedings, 2012

ABSTRACTIn this study, sintered pellets were prepared from Zircaloy-2 oxide and UO2 as a paramete... more ABSTRACTIn this study, sintered pellets were prepared from Zircaloy-2 oxide and UO2 as a parameter of content ratio (Zr contents were 0, 24.3, 49.0, 73.4, and 97.9 at% in metal). The sintered pellets were heated in 5%H2/Ar gas. UO2 pellets underwent simple thermal expansion caused by thermal vibration while Zircaloy-2 oxide pellets underwent thermal expansion and volume change with phase transformation. Finally, the 24.3, 49.0, and 73.5 at%Zr-UO2 pellet specimens showed both phenomena. However, phase transformation temperatures were lower than that of Zircaloy-2 oxide, and volume changes were much smaller. X-ray diffraction patterns obtained after thermal expansion measurements showed that the 24.3 at%Zr-UO2 specimen contained tetragonal and cubic (Zr, U)O2 while the 73.5 at%Zr-UO2 specimen contained mainly monoclinic ZrO2.

KAGAKU KOGAKU RONBUNSHU, 2010

粉体挙動に大きな影響を与える粒子形状を離散要素法（DEM）シミュレーションに取り込むために，転がり摩擦に着目し，球形要素を用いて経済的に粒子形状の効果を表現する方法を提案した．まず，傾斜部を持つ... more 粉体挙動に大きな影響を与える粒子形状を離散要素法（DEM）シミュレーションに取り込むために，転がり摩擦に着目し，球形要素を用いて経済的に粒子形状の効果を表現する方法を提案した．まず，傾斜部を持つ平板上を転動する多角柱の挙動から転がり摩擦係数が粒子形状に関する係数と回転速度の関数で与えられることを明らかにし，円柱要素を用いたDEMシミュレーションにより多角柱の挙動を再現した．また，この粒子形状と転がり摩擦係数の関係を任意の物質へ適用する換算法を明らかにした．つづいて，輪郭フーリエ変換を用いて不規則形状粒子の形状の等価多角形数を求め，転がり摩擦係数を得た．傾斜樋上における不規則形状粒子群の流動挙動の実験およびシミュレーションの結果は良好な一致を示し，提案した形状表現法およびその形状から求める粒子群の等価転がり摩擦係数の信頼性を確認した．

Journal of Nuclear Materials, 2013

The UO 2 and zircaloy system has been studied in severe accident research because these materials... more The UO 2 and zircaloy system has been studied in severe accident research because these materials form most of a molten corium. In addition, during core meltdown, it is expected that water vapor and hydrogen which is produced in the H 2 O-zircaloy reaction fill the reactor core space. In this study, simulated corium specimens were prepared considering the ambient atmosphere in an accident, and adjustment of the oxygen-to-metal (O/M) ratio. Phase observations by XRD and EPMA and measure ment of thermal conductivity were then done on the specimens. It was confirmed that the prepared O/M ratio had hardly any effe ct on melting temperature although UO 2+x has obvious dep endency on the O/M ratio. Thermal conductivity of the molten specimens showed little dependency on the O/M ratio and temperature. It appeared that the thermal conductivity had already been significantly decreased by the solution of U and Zr. Microstr ucture information as lattice constant and phase segregation was obtained by XRD and EPMA observation. These properties will be basic data for the development of condit ioning techniques for an actual corium.

ISIJ International, 2008

In this paper, the particle size segregation of sintered ore during flowing through a laboratory-... more In this paper, the particle size segregation of sintered ore during flowing through a laboratory-scale chute was investigated to validate the simulated results. The chute angle, installation of a damper at the outlet of the chute or the particle mixing condition were changed. The particles were segregated during flowing through the chute. The smaller particles were at the bottom wall of the chute, and the larger ones were on the smaller particles. The particle discharging velocity decreased with increasing the rolling friction in DEM, and the velocity also became uniformly. The particle discharging behavior under the large rolling friction was not spread, this phenomenon was not comparable with the experimental one. The distributed coefficient of rolling friction was determined by the distribution of rolled distance of sintered ore particle, and every particle in DEM had the different value according to the distribution of rolled distance. This method was effective for the sintered ores' flow very much, and the simulated particle size segregation agreed with those of experimental very well, irrespective of chute angle, installation of a damper or particle conditions. Therefore, this simulation has been validated for the analysis of the granular flow in an ironmaking process.

ISIJ International, 2010

The objective of this paper is to analyze the particle behavior in bell-type charging process of ... more The objective of this paper is to analyze the particle behavior in bell-type charging process of actual blast furnace by using Discrete Element Method (DEM). The circumferential balance of charged mass in the quad-hopper and the effect of the nut coke particle position in the quad-hopper on the traveling behavior and the segregation were discussed. The mass flow rate discharged from the rotating chute was fluctuated with the time, and the peak values gave when the directions of the chute movement became same as the one of the conveyor. It leads to the unbalance of charged particles in each part of the quad-hopper. The nut coke moved to the upward in the sintered ore particle layer during traveling to large bell from the quad-hopper due to the particle segregation, when they were segregated at the bottom of hopper. However their relative positions moved downward when they were segregated at the top, because the time for starting discharging of nut coke became faster. The installation of the damper at the way to the small bell from the hopper affects on the circumferential balance of mass of nut coke. Most of nut coke particles were charged at near the wall of blast furnace, and the small peak of the distribution of its specific charged mass was seen around 3.0 m in the radial distance from the center, it caused by the particle segregation during flowing on the particle layer. The relative radial distribution of the nut coke particles wasn't affected by their positions in the large bell and the total mass. Thus, keeping the circumferential balance of nut coke mass in the large bell is very important. The position of nut coke particle didn't affect on the segregation of sintered ore. The radial distributions of relative charged mass for all conditions were quite similar.

ISIJ International, 2009

The objective of this paper is to analyze the particle behavior at a bell-less top of blast furna... more The objective of this paper is to analyze the particle behavior at a bell-less top of blast furnace by using Discrete Element Method. The effect of chute angle on the flowing behavior or the particle segregation was discussed. The particles in the chute are centrifuged by the chute rotation with moving toward the outlet. The most of the smallest particles are pressed up against the chute side wall, while the larger ones stay at the outside of clustered particles due to the particle size segregation. The specific charged mass at charging area increases with increasing the number of charges, and the distribution of charged mass spreads toward the center of blast furnace, because the particles slide downward along with the slope of heap. The coke particles are pressed toward the center of blast furnace by the subsequently charged sintered ore particles with increasing the number of charges. The large collapse of coke layer is seen after 16 times charging under 36.9°and 43.1°in chute angle. It is found that the chute angle affects not only the particle segregation but also the collapse of coke layer strongly.

Chemical Engineering Science, 2009

The objective of this paper is to improve the computing time for numerical analysis of particle c... more The objective of this paper is to improve the computing time for numerical analysis of particle charging process by using Discrete Element Method. The rule for ignoring the calculations of contact forces and updating trajectories of unmoved particles were discussed. When the relative displacement of a particle within certain calculation steps became less than 0.1% of particle radius, this particle was determined to be unmoved and the calculations of this particle were ignored. The computing time was improved significantly when this new method was used, and its calculation speed was more than two times faster than that of original. It was found that this speed-up method is more useful for the cases that the particle becomes unmoved in short time or the height of charged bed is large. The simulation of charging process in an industrial-scale surge hopper was studied by using new method, the calculation speed became 2.88 times faster than that of original, and the quite similar particle size segregation between original and new methods was given. This new method for speed-up of the charging process in DEM is very useful, and the charging processes of the industrial scale storages can be simulated by using this method.

Advanced Powder Technology, 2010

A particle size distribution (PSD) estimation method based on light-scattering properties was val... more A particle size distribution (PSD) estimation method based on light-scattering properties was validated on experimental visible/near-infrared scattering spectra of polystyrene suspensions, with a nominal particle size ranging from 0.1 to 12 μm in diameter. On the basis of μ s and g spectra extracted from double integrating sphere measurements, good PSD estimates were obtained for particles ≥1 μm. The particle volume fraction estimates in the case of μ s were close to the target concentrations, although influenced by small baseline fluctuations on the spectra. For submicrometer particles, on the other hand, the non-oscillating μ s spectra lack discriminating power, resulting in erroneous PSD estimates. The reduced scattering coefficient spectra (μ s ′) were found less useful for particle size estimation as they lack a characteristic shape, causing an over-or underestimation of the distribution width. In summary, the estimation routine proved to deliver PSD estimates in line with the reference measurements for micrometer-sized or larger particles based on their μ s and g scattering spectra. Additional validation on more polydisperse samples forms the next step before going to bimodal PSD estimates.

Journal of Nuclear Materials, 2017

Oxygen potential of (U,Pu)O2 ± x was evaluated based on defect chemistry using an updated experim... more Oxygen potential of (U,Pu)O2 ± x was evaluated based on defect chemistry using an updated experimental data set. The relationship between oxygen partial pressure and deviation x in (U,Pu)O2 ± x was analyzed, and equilibrium constants of defect formation were determined as functions of Pu content and temperature. Brouwer's diagrams were constructed using the determined equilibrium constants, and a relational equation to determine O/M ratio was derived as functions of O/M ratio, Pu content and temperature. In addition, relationship between oxygen potential and oxygen diffusion coefficients were described.

The Proceedings of The Computational Mechanics Conference, 2007

Journal of the Society of Powder Technology, Japan, 2011

Journal of Nuclear Science and Technology, 2015

Specimens of (U, Pu, Zr)O 2 were prepared as simulated corium debris that were assumed like debri... more Specimens of (U, Pu, Zr)O 2 were prepared as simulated corium debris that were assumed like debris generated in the severe accident of the Fukushima Daiichi Nuclear Power Plant and their melting temperatures were measured by the thermal arrest technique in order to evaluate the influence of plutonium and zirconium content on the melting temperature of the corium debris. From the evaluation, it was found that the influence of zirconium on the melting temperatures of both (U, Pu, Zr)O 2 and (U, Zr)O 2 was similar and that the melting temperature of (U, Pu, Zr)O 2 had a local maximum value in the Pu-content between 0 and 20 mol%. The UO 2-PuO 2-ZrO 2 pseudo-ternary phase diagram at 2900 and 3000 K was evaluated from the present experimental results and previously reported results.

MRS Proceedings, 2012

ABSTRACTCorium which simulates the molten core of a Boiling Water Reactor was prepared as a param... more ABSTRACTCorium which simulates the molten core of a Boiling Water Reactor was prepared as a parameter of Zr content, and melting temperatures and thermal conductivities were measured. The melting temperatures were measured by the thermal arrest method and were 2622 oC, 2509 oC and 2540 oC, respectively, in the specimens of 24.3 at%, 49.0 at% and 73.5 at% Zr content. Thermal conductivities had low values of 2.0-3.5 W/m oC at temperatures of 400 to 1600 oC.

MRS Proceedings, 2012

ABSTRACTIn this study, sintered pellets were prepared from Zircaloy-2 oxide and UO2 as a paramete... more ABSTRACTIn this study, sintered pellets were prepared from Zircaloy-2 oxide and UO2 as a parameter of content ratio (Zr contents were 0, 24.3, 49.0, 73.4, and 97.9 at% in metal). The sintered pellets were heated in 5%H2/Ar gas. UO2 pellets underwent simple thermal expansion caused by thermal vibration while Zircaloy-2 oxide pellets underwent thermal expansion and volume change with phase transformation. Finally, the 24.3, 49.0, and 73.5 at%Zr-UO2 pellet specimens showed both phenomena. However, phase transformation temperatures were lower than that of Zircaloy-2 oxide, and volume changes were much smaller. X-ray diffraction patterns obtained after thermal expansion measurements showed that the 24.3 at%Zr-UO2 specimen contained tetragonal and cubic (Zr, U)O2 while the 73.5 at%Zr-UO2 specimen contained mainly monoclinic ZrO2.

KAGAKU KOGAKU RONBUNSHU, 2010

粉体挙動に大きな影響を与える粒子形状を離散要素法（DEM）シミュレーションに取り込むために，転がり摩擦に着目し，球形要素を用いて経済的に粒子形状の効果を表現する方法を提案した．まず，傾斜部を持つ... more 粉体挙動に大きな影響を与える粒子形状を離散要素法（DEM）シミュレーションに取り込むために，転がり摩擦に着目し，球形要素を用いて経済的に粒子形状の効果を表現する方法を提案した．まず，傾斜部を持つ平板上を転動する多角柱の挙動から転がり摩擦係数が粒子形状に関する係数と回転速度の関数で与えられることを明らかにし，円柱要素を用いたDEMシミュレーションにより多角柱の挙動を再現した．また，この粒子形状と転がり摩擦係数の関係を任意の物質へ適用する換算法を明らかにした．つづいて，輪郭フーリエ変換を用いて不規則形状粒子の形状の等価多角形数を求め，転がり摩擦係数を得た．傾斜樋上における不規則形状粒子群の流動挙動の実験およびシミュレーションの結果は良好な一致を示し，提案した形状表現法およびその形状から求める粒子群の等価転がり摩擦係数の信頼性を確認した．

Journal of Nuclear Materials, 2013

The UO 2 and zircaloy system has been studied in severe accident research because these materials... more The UO 2 and zircaloy system has been studied in severe accident research because these materials form most of a molten corium. In addition, during core meltdown, it is expected that water vapor and hydrogen which is produced in the H 2 O-zircaloy reaction fill the reactor core space. In this study, simulated corium specimens were prepared considering the ambient atmosphere in an accident, and adjustment of the oxygen-to-metal (O/M) ratio. Phase observations by XRD and EPMA and measure ment of thermal conductivity were then done on the specimens. It was confirmed that the prepared O/M ratio had hardly any effe ct on melting temperature although UO 2+x has obvious dep endency on the O/M ratio. Thermal conductivity of the molten specimens showed little dependency on the O/M ratio and temperature. It appeared that the thermal conductivity had already been significantly decreased by the solution of U and Zr. Microstr ucture information as lattice constant and phase segregation was obtained by XRD and EPMA observation. These properties will be basic data for the development of condit ioning techniques for an actual corium.

ISIJ International, 2008

In this paper, the particle size segregation of sintered ore during flowing through a laboratory-... more In this paper, the particle size segregation of sintered ore during flowing through a laboratory-scale chute was investigated to validate the simulated results. The chute angle, installation of a damper at the outlet of the chute or the particle mixing condition were changed. The particles were segregated during flowing through the chute. The smaller particles were at the bottom wall of the chute, and the larger ones were on the smaller particles. The particle discharging velocity decreased with increasing the rolling friction in DEM, and the velocity also became uniformly. The particle discharging behavior under the large rolling friction was not spread, this phenomenon was not comparable with the experimental one. The distributed coefficient of rolling friction was determined by the distribution of rolled distance of sintered ore particle, and every particle in DEM had the different value according to the distribution of rolled distance. This method was effective for the sintered ores' flow very much, and the simulated particle size segregation agreed with those of experimental very well, irrespective of chute angle, installation of a damper or particle conditions. Therefore, this simulation has been validated for the analysis of the granular flow in an ironmaking process.

ISIJ International, 2010

The objective of this paper is to analyze the particle behavior in bell-type charging process of ... more The objective of this paper is to analyze the particle behavior in bell-type charging process of actual blast furnace by using Discrete Element Method (DEM). The circumferential balance of charged mass in the quad-hopper and the effect of the nut coke particle position in the quad-hopper on the traveling behavior and the segregation were discussed. The mass flow rate discharged from the rotating chute was fluctuated with the time, and the peak values gave when the directions of the chute movement became same as the one of the conveyor. It leads to the unbalance of charged particles in each part of the quad-hopper. The nut coke moved to the upward in the sintered ore particle layer during traveling to large bell from the quad-hopper due to the particle segregation, when they were segregated at the bottom of hopper. However their relative positions moved downward when they were segregated at the top, because the time for starting discharging of nut coke became faster. The installation of the damper at the way to the small bell from the hopper affects on the circumferential balance of mass of nut coke. Most of nut coke particles were charged at near the wall of blast furnace, and the small peak of the distribution of its specific charged mass was seen around 3.0 m in the radial distance from the center, it caused by the particle segregation during flowing on the particle layer. The relative radial distribution of the nut coke particles wasn't affected by their positions in the large bell and the total mass. Thus, keeping the circumferential balance of nut coke mass in the large bell is very important. The position of nut coke particle didn't affect on the segregation of sintered ore. The radial distributions of relative charged mass for all conditions were quite similar.

ISIJ International, 2009

The objective of this paper is to analyze the particle behavior at a bell-less top of blast furna... more The objective of this paper is to analyze the particle behavior at a bell-less top of blast furnace by using Discrete Element Method. The effect of chute angle on the flowing behavior or the particle segregation was discussed. The particles in the chute are centrifuged by the chute rotation with moving toward the outlet. The most of the smallest particles are pressed up against the chute side wall, while the larger ones stay at the outside of clustered particles due to the particle size segregation. The specific charged mass at charging area increases with increasing the number of charges, and the distribution of charged mass spreads toward the center of blast furnace, because the particles slide downward along with the slope of heap. The coke particles are pressed toward the center of blast furnace by the subsequently charged sintered ore particles with increasing the number of charges. The large collapse of coke layer is seen after 16 times charging under 36.9°and 43.1°in chute angle. It is found that the chute angle affects not only the particle segregation but also the collapse of coke layer strongly.

Chemical Engineering Science, 2009

The objective of this paper is to improve the computing time for numerical analysis of particle c... more The objective of this paper is to improve the computing time for numerical analysis of particle charging process by using Discrete Element Method. The rule for ignoring the calculations of contact forces and updating trajectories of unmoved particles were discussed. When the relative displacement of a particle within certain calculation steps became less than 0.1% of particle radius, this particle was determined to be unmoved and the calculations of this particle were ignored. The computing time was improved significantly when this new method was used, and its calculation speed was more than two times faster than that of original. It was found that this speed-up method is more useful for the cases that the particle becomes unmoved in short time or the height of charged bed is large. The simulation of charging process in an industrial-scale surge hopper was studied by using new method, the calculation speed became 2.88 times faster than that of original, and the quite similar particle size segregation between original and new methods was given. This new method for speed-up of the charging process in DEM is very useful, and the charging processes of the industrial scale storages can be simulated by using this method.

Advanced Powder Technology, 2010

A particle size distribution (PSD) estimation method based on light-scattering properties was val... more A particle size distribution (PSD) estimation method based on light-scattering properties was validated on experimental visible/near-infrared scattering spectra of polystyrene suspensions, with a nominal particle size ranging from 0.1 to 12 μm in diameter. On the basis of μ s and g spectra extracted from double integrating sphere measurements, good PSD estimates were obtained for particles ≥1 μm. The particle volume fraction estimates in the case of μ s were close to the target concentrations, although influenced by small baseline fluctuations on the spectra. For submicrometer particles, on the other hand, the non-oscillating μ s spectra lack discriminating power, resulting in erroneous PSD estimates. The reduced scattering coefficient spectra (μ s ′) were found less useful for particle size estimation as they lack a characteristic shape, causing an over-or underestimation of the distribution width. In summary, the estimation routine proved to deliver PSD estimates in line with the reference measurements for micrometer-sized or larger particles based on their μ s and g scattering spectra. Additional validation on more polydisperse samples forms the next step before going to bimodal PSD estimates.