Segregation of components of molten core oxidic materials using cold crucible induction heating technique (Joint research)

Sudo, Ayako; Mszros, B.*; Sato, Takumi; Nagae, Yuji

JAEA-Research 2023-007, 31 Pages, 2023/11

JAEA-Research-2023-007.pdf:3.61MB

For the criticality assessment of fuel debris generated by the accident in Fukushima Daiichi Nuclear Power Station, understanding of the elemental localization in fuel debris is important. Especially, the distribution of Fe and Gd, which may behave as potential neutron absorber materials in the fuel debris, is of particular important from the viewpoint of nuclear criticality safety. To investigate the localization tendency of Gd and Fe in molten core materials during solidification progress, liquefaction/solidification tests on core materials containing UO, ZrO, FeO, GdO, and simulated fission products (MoO, NdO, SrO, and RuO) and concrete (SiO, AlO, and CaO) were performed using cold crucible induction heating technique. During the test, the molten core materials gradually subsided and solidified from the bottom to the top of the melt. Elemental analysis showed that Fe content in the inner region increased approximately up to 3.4 times that in the bottom region. The concentration of Fe into the inner region was observed in all the samples regardless of the initial FeO composition, cooling rates, and phase separation. This suggests that FeO may be concentrated into the low temperature region, where the melt solidified later. In contrast, Gd content in the bottom region increased approximately up to 2.6 times that in the inner region. The concentration of Gd into the bottom region was observed when the initial GdO content was higher than 1 at.%. This suggests that GdO may be concentrated into the earlier solidified region. On the other hand, no significant localization was observed on the simulated fission products.

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

https://doi.org/10.1080/00223131.2021.1879690

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 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 is predicted to solidify, in advance, in the concrete-based melt around 2177 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 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.

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

https://doi.org/10.3327/taesj.J18.029

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, ZrO, FeO, BC and sim-FP oxides) were performed. Simulated corium was heated up to 2600C under Ar atmosphere and then cooled down with two different cooling processes; furnace cooling (average cooling rate is approximately 744C/min) and slow cooling (cooling rate in 2600C2300C is 5C/min and in 2300C1120C is approximately 788C/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 125at% in both samples, which is in reasonable accordance with the value estimated from UO-ZrO-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.

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

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(B,C) in the database. The present results give useful information on improvement of iron-boron-carbon phase diagram.

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

Reaction products at interface region between concrete and core melt

Sudo, Ayako

no journal, , 

no abstracts in English

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 (UZr)O (No.1) and (UZr)O/Zr/SUS316L/BC (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 particle and silicate glass containing U and Zr. Ca, Fe and Mg oxide were dissolved in the particle. UO and ZrO were dissolved in the silicate glass. No.2 sample had same phases, but Fe-Cr oxide deposits from SS was identified.

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

Takano, Masahide; Onozawa, Atsushi; Sudo, Ayako

no journal, , 

no abstracts in English

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, Zr, (U,Zr)O, stainless steel (SS), and BC 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 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. GdO, 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 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.