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Journal Articles

Direct ${it in-situ}$ temperature measurement for lamp-based heating device

Sumita, Takehiro; Sudo, Ayako; Takano, Masahide; Ikeda, Atsushi

Science and Technology of Advanced Materials; Methods (Internet), 2(1), p.50 - 54, 2022/02

Journal Articles

Experimental evaluation of Sr and Ba distribution in ex-vessel debris under a temperature gradient

Sudo, Ayako; Sato, Takumi; Ogi, Hiroshi; Takano, Masahide

Journal of Nuclear Science and Technology, 58(4), p.473 - 481, 2021/04

 Times Cited Count:0 Percentile:0.01(Nuclear Science & Technology)

Dissolution behavior of Sr and Ba is crucial for evaluating secondary source terms via coolant water from ex-vessel debris accumulated at Fukushima Daiichi Nuclear Power Plant. To understand the mechanism, knowing the distribution of Sr and Ba in the ex-vessel debris is necessary. As a result of reaction tests between simulated corium and concrete materials, two layered structures were observed in the solidified sample, (A) a silicate glass-based ((Si-Al-Ca-Fe-Zr-Cr-U-Sr-Ba)-O) phase-rich layer in the upper surface region and (B) a (U,Zr)O$$_{2}$$ particle-rich layer at the inner region. Measurable concentrations of Sr and Ba were observed in layer (A) (approximately 1.7 times that in the layer (B)). According to thermodynamic analysis, (U,Zr)O$$_{2}$$ is predicted to solidify, in advance, in the concrete-based melt around 2177 $$^{circ}$$C. Then, the residual melt is solidified as a silicate glass, and Sr and Ba are preferentially dissolved into the silicate glass. During the tests, (U,Zr)O$$_{2}$$ particles sank, in advance, in the melt because of its higher density, and the silicate glass phase relocated to the surface layer. On the other hand, silicate glass containing Sr and Ba is predicted to be hardly soluble in water and chemically stable.

Journal Articles

Segregation behavior of Fe and Gd in molten corium during solidification progress

Sudo, Ayako; Meszaros, B.*; Poznyak, I.*; Sato, Takumi; Nagae, Yuji; Kurata, Masaki

Journal of Nuclear Materials, 533, p.152093_1 - 152093_8, 2020/05

 Times Cited Count:1 Percentile:39.17(Materials Science, Multidisciplinary)

Journal Articles

Fundamental study on segregation behavior in U-Zr-Fe-O system during solidification process

Sudo, Ayako; Mizusako, Fumiki*; Hoshino, Kuniyoshi*; Sato, Takumi; Nagae, Yuji; Kurata, Masaki

Nihon Genshiryoku Gakkai Wabun Rombunshi, 18(3), p.111 - 118, 2019/08

Cooling rate of molten core materials during solidification significantly affects the segregation of major constituents of fuel debris. To understand general tendency of the segregation, liquefaction/solidification tests of simulated corium (UO$$_{2}$$, ZrO$$_{2}$$, FeO, B$$_{4}$$C and sim-FP oxides) were performed. Simulated corium was heated up to 2600$$^{circ}$$C under Ar atmosphere and then cooled down with two different cooling processes; furnace cooling (average cooling rate is approximately 744$$^{circ}$$C/min) and slow cooling (cooling rate in 2600$$^{circ}$$C$$sim$$2300$$^{circ}$$C is 5$$^{circ}$$C/min and in 2300$$^{circ}$$C$$sim$$1120$$^{circ}$$C is approximately 788$$^{circ}$$C/min). Element analysis detected three oxide phases with different composition and one metal phase in both solidified samples. Solubility of FeO in these oxide phases was mostly fixed to be 12$$pm$$5at% in both samples, which is in reasonable accordance with the value estimated from UO$$_{2}$$-ZrO$$_{2}$$-FeO phase diagrams. However, a significant grain-growth of one oxide phase, rich in Zr-oxide, was detected only in the slowly cooled sample. The composition of this particular oxide phase is comparable to the initial average composition. The condensation is considered to be caused by the connection of remaining liquid agglomerates during slow solidification.

Journal Articles

Fundamental experiments on phase stabilities of Fe-B-C ternary systems

Sudo, Ayako; Nishi, Tsuyoshi; Shirasu, Noriko; Takano, Masahide; Kurata, Masaki

Journal of Nuclear Science and Technology, 52(10), p.1308 - 1312, 2015/10

 Times Cited Count:9 Percentile:69.75(Nuclear Science & Technology)

For understanding the control blade degradation mechanism of BWR, the thermodynamic database for the fuel assembly materials is a useful tool. Although iron, boron, and carbon ternary system is a dominant phase diagram, phase relation data is not sufficient for the region in which the boron and carbon compositions are richer than the eutectic composition. The phase relations of three samples were analyzed by X-ray diffraction, scanning electron microscope and energy dispersed X-ray spectrometry. The results indicate that Fe$$_{3}$$(B,C) phase only exists in the intermediate region at 1273 K and that the solidus temperature widely maintains at about 1400 K for all three samples, which are different from the calculated data using previous thermodynamic database. The difference might be originated from the over-estimations of the interaction parameter between boron and carbon in Fe$$_{3}$$(B,C).

Oral presentation

Fundamental experiment on phase stability of Fe-B-C ternary system

Sudo, Ayako; Shirasu, Noriko; Nishi, Tsuyoshi; Kurata, Masaki

no journal, , 

no abstracts in English

Oral presentation

Fundamental experiment on phase stability of Fe-B-C ternary system

Sudo, Ayako; Nishi, Tsuyoshi; Shirasu, Noriko; Kurata, Masaki

no journal, , 

no abstracts in English

Oral presentation

Fundamental experiments on phase stabilities of Fe-B-C ternary systems

Sudo, Ayako; Nishi, Tsuyoshi; Shirasu, Noriko; Takano, Masahide; Kurata, Masaki

no journal, , 

Although Fe-B-C ternary system is a dominant phase diagram when considering the control blade degradation, phase relation data around eutectic composition were not sufficient. The phase relations of three Fe-B-C samples were analyzed by XRD and SEM/EDX and solidus were determined by DTA. The solidus detected in the present study was maintained at about 1400 K for all three samples, although preliminary calculation using conventional thermodynamic database estimated that solidus varies with roughly 50 K difference. The difference might be originated from the insufficient evaluation on the phase stability of Fe$$_{3}$$(B,C) in the database. The present results give useful information on improvement of iron-boron-carbon phase diagram.

Oral presentation

Oxidation behavior and some material properties of sintered (U,Zr)O$$_{2}$$ specimens

Takano, Masahide; Nishi, Tsuyoshi; Sudo, Ayako

no journal, , 

For the removal operation of fuel debris of the Fukushima Daiichi Nuclear Power Plant, some material properties of sintered (U,Zr)O$$_{2}$$ solid solution specimens were measured. Their heat capacity and thermal conductivity were determined in the temperature range from room temperature to 1073 K. Micro Vickers hardness and electrical resistivity were also measured at room temperature as a function of U fraction. In addition to these material properties, the correlation between phases and O/M ratios of U-Zr-O system under oxidizing conditions was investigated. In the (U,Zr)$$_{2+y}$$ hyperstoichiometric region, the hypothetical O/U ratio in the solid solution was found to be independent of either U fraction and crystal structure. Further, three orthorhombic phases in the U-Zr-O system under more oxidizing conditions were identified, and their phase relationship was successfully defined.

Oral presentation

Preparation and characterization of simulated MCCI products, 1; Phases and microhardness of solidified samples prepared by arc melting

Takano, Masahide; Onozawa, Atsushi; Sudo, Ayako

no journal, , 

no abstracts in English

Oral presentation

Preparation and characterization of simulated MCCI products, 2; Characterization of interface between concrete and core melt

Sudo, Ayako; Onozawa, Atsushi; Takano, Masahide

no journal, , 

no abstracts in English

Oral presentation

Reaction products at interface region between concrete and core melt

Sudo, Ayako

no journal, , 

no abstracts in English

Oral presentation

Reaction products between concrete and core materials under temperature gradient

Sudo, Ayako

no journal, , 

no abstracts in English

Oral presentation

Reaction experiment on core melt concrete interface under temperature gradient

Sudo, Ayako; Onozawa, Atsushi; Takano, Masahide

no journal, , 

To characterize the layer structure and respective temperatures of MCCI products, MCCI experiments using a light-concentrating furnace were performed. As the core-melt constituents, a powder mixture of (U$$_{0.5}$$Zr$$_{0.5}$$)O$$_{2}$$ (No.1) and (U$$_{0.5}$$Zr$$_{0.5}$$)O$$_{2}$$/Zr/SUS316L/B$$_{4}$$C (No.2) were compacted into tablets (10 mm in diameter) respectively. The tablet was placed on a cylindrical piece of basaltic concrete (25 mm in diameter). The light was concentrated on the tablet under argon gas flow and samples were heated. After heating, the vertical cross-section of the solidified sample piece was subjected to phase identification and elemental analysis by XRD and SEM/EDX. No.1 sample seemed to maintain original shape, but EDX analysis showed the inner region melted. This melt was identified as-melted (U,Zr)O$$_{2}$$ particle and silicate glass containing U and Zr. Ca, Fe and Mg oxide were dissolved in the particle. UO$$_{2}$$ and ZrO$$_{2}$$ were dissolved in the silicate glass. No.2 sample had same phases, but Fe-Cr oxide deposits from SS was identified.

Oral presentation

Chemical state of sea salt and fission products in MCCI products

Sudo, Ayako

no journal, , 

no abstracts in English

Oral presentation

Characterization of fuel debris (28'A), 3; Influence of minor additives on the phase relationships of (U,Zr)O $$_{2}$$ simulated fuel debris

Takano, Masahide; Onozawa, Atsushi; Sudo, Ayako

no journal, , 

no abstracts in English

Oral presentation

Characterization of fuel debris (28'A), 4; Characterization of simulated MCCI products by light-concentrating heating

Sudo, Ayako; Takano, Masahide; Onozawa, Atsushi

no journal, , 

To characterize the reaction layers with respective temperatures around core melt/concrete interface, we have performed MCCI experiments in laboratory scale by using a light-concentrating technique. As minor constituents, Gd$$_{2}$$O$$_{3}$$ (burnable poison and FP), Mo-Ru-Rh-Pd alloy (FP), and sea salt were added in the simulated MCCI debris. The analyses of the top part of the sample identified as-melted (U,Zr)O$$_{2}$$ particles precipitated in silicate glass containing U, Zr, Gd, Fe and Cr. Gd was included in both (U,Zr)O$$_{2}$$ and silicate glass and Mo and the platinum group elements formed alloys with Fe-Ni-Mo-Ru-Rh-Pd. Although almost sea salt evaporated and deposited on the belljar, some S originating from sea salt resulted in precipitation of FeS-type sulfide in the alloy.

Oral presentation

Characterization of core melt concrete interface region examined by light concentrating heating technique

Sudo, Ayako; Takano, Masahide; Onozawa, Atsushi

no journal, , 

To characterize the reaction layers around core melt/concrete interface, MCCI experiments by using a light-concentrating technique was performed. As the main constituents of the core-melt, powder mixtures of ZrO$$_{2}$$, Zr, (U,Zr)O$$_{2}$$, stainless steel (SS), and B$$_{4}$$C with various compositions were compacted into tablets. The tablet was placed on a concrete. Light from a lamp was concentrated on the tablet, and the vertical cross-section of the solidified sample was determined by XRD and SEM/EDX. The analyses identified 4 layers from top to bottom; (a) as-melted (U,Zr)O$$_{2}$$ particles and silicate glass with U, (b) the silicate glass with U, (c) imperfectly melted concrete, and (d) dehydrated concrete. Unoxidized metal particles (Fe-Ni-Cr) also precipitated. Gd$$_{2}$$O$$_{3}$$, Mo-Ru-Rh-Pd alloy, and sea salt were also added in the tablet. In this case Gd was included in both (U,Zr)O$$_{2}$$ and silicate glass, Mo and the platinum group elements formed alloys with Fe-Ni-Cr, and S originating from sea salt resulted in precipitation of FeS-type sulfide in the alloy.

Oral presentation

Phases and morphology in the simulated MCCI products prepared by arc melting method

Takano, Masahide; Onozawa, Atsushi; Sudo, Ayako

no journal, , 

To understand the characteristics of MCCI products in Fukushima Daiichi Nuclear Power Station, the simulated MCCI products in laboratory scale were prepared by arc melting of compacted powder mixtures of core materials and concrete. Stainless steel, boron carbide, metallic zirconium, (U,Zr)O$$_{2}$$, GdO$$_{1.5}$$, and platinum group elements were selected as the core materials. Phases, morphology, and micro hardness were analyzed on cross-section of the solidified specimens. The specimens consisted of oxide part (MO$$_{2}$$ corium and silicate glass or Al-Ca-O) and metallic part (alloys and borides). The phase relationships in the MCCI products were found to be dominated by the initial concrete/Zr mixing ratio, because the dehydration of concrete is the main oxidation factor and the metallic zirconium acts as a strong reductant. Micro hardness of main phases are 7 GPa for silicate glass, 13-15 GPa for (U,Zr,Gd,Ca)O$$_{2}$$ corium, and 25 GPa for ZrB$$_{2}$$ and ferrous borides, respectively.

Oral presentation

Solidification tests on the U-Zr-Gd-O molten corium

Sudo, Ayako

no journal, , 

In order to obtain the knowledge on the re-distribution of various elements under the solidification process of molten corium, the fundamental experiment on the U-Zr-Gd-O melt was performed. As the core-melt constituents, a powder mixture of wt.70%UO$$_{2}$$+30%ZrO$$_{2}$$ with adding of wt.2%Gd$$_{2}$$O$$_{3}$$ was prepared with mass of 655.5g. These powders were melted in a cold crucible using induction heating technique in the air atmosphere with metallic zirconium (6g) as an initiator. For the corium microstructure study, sample pieces from solidified ingot were subjected to phase identification and elemental analysis by XRD and SEM/EDX. For the evaluation of the characteristics of corium debris under the various solidification processes, two heating tests with different cooling rates were performed. During quick-solidification test (J1) ingot of 4.8cm was obtained. The near-bottom region of J1 sample was crystallized and not melted. The structure of slow-solidification sample (J2) was similar to J1, however, it was formed big cavity under the upper crust of ingot. This cavity was formed during slow-solidification process due to volume shrinkage. Both of samples were found that Gd was concentrated in the inner region where the later crystallization of the melt was occurred.

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