• Nuclear Engineering and Technology
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• v.36 no.1
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• pp.104-111
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• 2004

The melting temperatures of $UO_2,\;UO_2-6wt\%Gd_{2}O_3,\;UO_2-12wt\%Gd_{2}O_3,\;UO_2-2wt\%Er_{2}O_3,\;and\;UO_2-4wt\%Er_{2}O_3$ fuels were measured. Fuel materials were loaded in a tungsten capsule of which shape met the black body condition. The melting temperature was measured by the thermal arrest method during heating of the capsule in an induction furnace. The measured melting temperature of $UO_2$ fuel was $2815{\pm}20^{\circ}C$. The solidus and liquidus temperatures of $UO_2-Gd_{2}O_3\;and\;UO_2-Er_{2}O_3$ had also been measured, and it was observed that the solidus temperatures of them were lower than the liquidus temperature by $15{\sim}25^{\circ}C$. Measured melting temperatures of $UO_2,\;UO_2-Gd_{2}O_3\;and\;UO_2-Er_{2}O_3$ fuels were as follows:

• Nuclear Engineering and Technology
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• v.30 no.2
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• pp.128-139
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• 1998

The effect of UO$_2$ powder property and oxygen potential on characteristics of sintered UO$_2$-Gd$_2$O$_3$ fuel pellets has been investigated. Two types of powder, mixture of AUC-UO$_2$ and Gd$_2$O$_3$powders (type I) and mixture of ADU-UO$_2$ and Gd$_2$O$_3$powders (type II), have been prepared, pressed, and sintered at 168$0^{\circ}C$ for 4 hours. Four sintering atmospheres with different mixing ratios of $CO_2$to H$_2$ gas ranging from 0 to 0.3 have been used. UO$_2$-Gd$_2$O$_3$ fuel has lower sintered density than UO$_2$ fuel, and the density drop is larger for powder type I than for powder type II. As the oxygen potential increases, the sintered density of UO$_2$-2wt% Gd$_2$O$_3$pellets increases but that of UO$_2$-10wt% Gd$_2$O$_3$ pellets decreases. It is found that pores are newly formed in UO$_2$-10wt% Gd$_2$O$_3$ pellets in accordance with the decrease in density. The grain size of UO$_2$-Gd$_2$O$_3$ fuel increases and a short range G4 distribution becomes homogeneous as the oxygen potential increases. A long range ed distribution and grain structure are inhomogeneous for powder type II. The lattice parameter of (U,Gd)O$_2$solid solution decreases linearly with Gd$_2$O$_3$ content. The dependence of UO$_2$-Gd$_2$O$_3$fuel characteristics on powder type and sintering atmosphere have been discussed.

• Journal of the Korean Ceramic Society
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• v.39 no.2
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• pp.204-209
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• 2002

In this study, $UO_4{\cdot}2NH_4F$, the precipitates which has low solubility, was obtained by chemical precipitation method to recover and reuse the trace uranium from the liquid waste producing in AUC process and for this compound it was characterized by means of chemical analysis, TG-DTA, XRD and FT-IR analyses. This compound was analyzed as $UO_4{\cdot}2NH_4F$ and shape of this precipitate was hexagonal type, having the size of 2∼3 ${\mu}m$. Also, the intermediates were obtained as $UO_4F,\;UO_4,\;UO_3,\;and\;U_3O_8$ by the thermal decomposition over the temperature of 220, 310, 515 and 640$^{\circ}C$, respectively. It is concluded that under the condition of a constant heating rate of 5$^{\circ}C$/min in air atmosphere range of between room temperature and 800$^{\circ}C$, thermal decomposition reaction mechanism of $UO_4{\cdot}2NH_4F$ is as follow; $UO_4{\cdot}2NH_4F{\rightarrow}UO_4F{\rightarrow}UO_4{\rightarrow}UO_3{\rightarrow}U_3O_8$.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.05b
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• pp.74-79
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• 1997

$UO_2$ 및 $UO_2$-3.23wt%CeO$_2$가 128$0^{\circ}C$에서 소결되면 밀도는 각각 93%T.D., 91.2%T.D.이었으나, Li$_2$O가 0.lwt%씩 첨가된 경우에는 소결밀도가 95.5%T.D., 95.1%T.D.로 증가하였다. 소결온도가 증가함에 따라서 $UO_2$ 및 $UO_2$-3.23wt%CeO$_2$의 소결밀도는 현저하게 증가되는 반면에, Li$_2$O가 첨가된 경우에는 저온에서도 이미 치밀화가 많이 일어났기 때문에 소결밀도의 증가폭이 완만하였다. Li$_2$O가 첨가된 분말에서는 소결온도가 더 높아지면, 결정립성장이 주로 일어나게 되어 168$0^{\circ}C$에서 소결되었을 때, $UO_2$와 $UO_2$-0.1wt%Li$_2$O의 결정립크기가 각각 8.7$\mu\textrm{m}$, 120$\mu\textrm{m}$ 이고, $UO_2$-3.23wt%CeO$_2$와 $UO_2$-3.23wt%CeO$_2$-0.lwt%Li$_2$O는 각각 10.9$\mu\textrm{m}$, 34$\mu\textrm{m}$ 이었다. $UO_2$와 $UO_2$-3.23wt%CeO$_2$ 그리고, 두 조성에 Li$_2$O가 각각 첨가된 경우, Ar-4vol.%H$_2$ 분위기보다 H$_2$분위기에서 소결했을 때 밀도가 더 높았다. 그러나, 결정립은 $UO_2$와 $UO_2$-Li$_2$O의 경우, 수소분위기에서 소결했을 때, (U,Ce)O$_2$와 (U,Ce)O$_2$-Li$_2$O에서는 Ar-4vol.%H$_2$분위기에서 소결했을 때 더욱 성장하였다.

• Proceedings of the Korean Nuclear Society Conference
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• 2002.10a
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• pp.269.2-269
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• 2002

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05c
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• pp.46-51
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• 1996

본 연구는 MDD(modified direct denitration)공정의 주 우라늄염인 암노늄 우라닐 나이트레이트의 화학특성을 밝히고 이들 화합물의 열분해 및 환원반응의 반응기구에 대하여 조사되었다. 암모늄 우라닐 나이트레이트는 제조 조건에 따라 N $H_4$$UO_2$N $O_3$와 (N $H_4$)$_2$$UO_2$(N $O_3$)$_4$.2$H_2O$의 두가지 형태의 복염으로 존재함이 화학 및 원소분석, X산 회절 분석, 그리고 적외선 분광분석에 의하여 확인되었다. 암모늄 우라닐 나이트레이트는 질소분위기에서 N $H_4$$UO_2$(N $O_3$)$_3$$\longrightarrow$ Amorphous $UO_3$$\longrightarrow$ a-$UO_3$$\longrightarrow$ U$_3$ $O_{8}$$\longrightarrow$ $\alpha$-U$_3$ $O_{8}$의 경로를 따라서 열분해 되며, 수소분위기에서는 N $H_4$$UO_2$(N $O_3$)$_3$$\longrightarrow$ $UO_3$$\longrightarrow$ U$_3$ $O_{8}$$\longrightarrow$ U$_4$ $O_{9}$ $\longrightarrow$ $UO_2$의 경로로 환원되었다.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.05b
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• pp.171-176
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• 1997

UO$_2$-Gd$_2$O$_3$fuel has been sintered to study the effect of powder property and sintering atmospheres on densification and microstructure. Three types of powders have been used; AUC-UO$_2$ powder and ADU-UO$_2$ powder were mixed with Gd$_2$O$_3$ Powder, and co-milled AUC-UO$_2$ and Gd$_2$O$_3$ powder. UO$_2$-(2, 5, 10)wt% Gd$_2$O$_3$pellets have been sintered at 168$0^{\circ}C$ for 4 hours in the mixture of H$_2$ and $CO_2$ gases, of which oxygen potential has been controlled by the ratio of $CO_2$ to H$_2$ gas. Densities of UO$_2$-Gd$_2$O$_3$ fuel pellets are quite dependent on powder types, and UO$_2$-Gd$_2$O$_3$ fuel using co-milled UO$_2$ powder yields the highest density. A long range homogeneity of Gd is determined by powder mixing. As the oxygen potential of sintered atmosphere increases, the sintered densities of UO$_2$-Gd$_2$O$_3$ pellets decrease but grain size increases. In addition, (U, Gd)O$_2$ solid solution becomes more homogeneous. The UO$_2$-Gd$_2$O$_3$fuel having adequate density and homogeneous microstructure can be fabricated by co-milling powder and by high oxygen potential.

• Journal of the Korean Chemical Society
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• v.29 no.2
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• pp.121-128
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• 1985

The electrochemical reduction of uranyl ion in aqueous basic media has been examined by d. c. polarography, differential pulsed polarography and cyclic voltammetry. From voltammograms obtained in uranyl solutions containing 0.1M sodium bicarbonate, either with or without the same amounts of sodium tripolyphosphate it is concluded that the first wave corresponds to the reduction of $UO_2^{2+}$ to $UO_2^+$. It is assumed that the uranyl ion undergoes appreciable hydrolysis in these media. The hydrolysis product $UO_2OH^+$ from $UO_2^{2+}$ is not reduced at the first wave, but is reduced at the second wave together with $UO_2^+$. The diffusion current was found proportional to the uranyl concentration in a range between $7.5 {\times} 10^{-4}$ and $3.75 {\times}10^{-3}$M.

• Journal of Energy Engineering
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• v.15 no.1 s.45
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• pp.8-13
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• 2006

The oxidation behavior of $UO_2$, pellet was studied using an XRD and a thermogravimetric analyzer in the temperature range from 573 to 873 K and in the density range from 94.64 to 99.10% of theoretical density in air. It was found from the XRD study that $UO_2$ was completely converted to $U_3O_8$ in this experimental temperature range. The formation of $U_3O_8$ displays sigmoidal reaction kinetics. The oxidation rate was reduced with density. Induction time for the oxidation of $UO_2$ was delayed with density because of open pore formed in surface of $UO_2$ pellet. The activation energy for oxidation of $UO_2$ was determined to be 89.54 kJ/mol and 34.40 kJ/mol in the temperature range from 573 to 723 K and from 723 to 873 K, respectively.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05a
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• pp.650-655
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• 1995

AUC-$UO_2$, ADU-$UO_2$ 분말이 ball mill과 attritor mill에서 분쇄될 때, 기공도변화를 측정함으로서 분말의 특성변화와 분쇄거동을 관찰하였다. 분쇄전 ADU-$UO_2$ 분말은 0.3-7 $\mu\textrm{m}$ 범위의 기공들이 고르게 분포하였으며, ball mill에서 분쇄되어도 그 분포는 거의 변화가 없었다. 분쇄전 AUC-$UO_2$ 분말은 3-8 $\mu\textrm{m}$와 0.05-0.2 $\mu\textrm{m}$ 범위의 기공이 주로 생성되어 있는 bimodal 분포를 나타내었다. Ball mill에서 분쇄됨에 따라 3 $\mu\textrm{m}$이상의 큰 기공과 0.2 $\mu\textrm{m}$ 이하의 작은 기공이 소멸되고, 0.2-3 $\mu\textrm{m}$의 기공들이 고르게 분포하는 경향을 나타내었다. 반면에 AUC-$UO_2$가 attritor mill에서 분쇄될 경우에는 bimodal 분포는 그대로 유지하면서 3-8 $\mu\textrm{m}$의 큰 기공은 줄어들고 0.05-0.2 $\mu\textrm{m}$의 기공은 그 양이 증가하는 경향을 나타내었다. 이것은 ball mill에서는 주로 충격작용에 의해서, attritor mill에서는 전단작용에 의해서 분쇄가 진행됨으로 인한 분쇄기구의 차이인 것으로 사료된다.