• Title/Summary/Keyword: $UO_2$ pellet

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Microstructural Changes of AlOOH Doped $UO_2$ Pellet during the Annealing Process

  • Hosik Yoo;Lee, Shinyoung;Lee, Seungjae;Kwenho Kang;Kim, Hyoungsu
    • The Korean Journal of Ceramics
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    • v.6 no.3
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    • pp.209-213
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    • 2000
  • Microstructural changes of AlOOH doped UO$_2$pellet after annealing up to 216h have been observed and they were compared with those of the standard pellet. Grain and pore size of UO$_2$pellet increased with the addition of AlOOH and its effect was still validated during annealing. Densification rate was reduced by the addition of AlOOH and it was attributed to coarsened pores with spherical shape. Grain and pore growth was stopped and density increase was the least after 144h of annealing. The variation of pore size resulting from annealing has a linear relationship with that of grain size.

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A Study on Dissolution Characteristics of UO2 Pellet in Nitric Acid (질산용액에 UO2 Pellet의 용해특성에 관한 연구)

  • Choung, Won-Myung;Park, Jin-Ho;Lee, Kue-Il;Yoo, Jae-Hyung;Jeong, Kyung-Chai
    • Applied Chemistry for Engineering
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    • v.9 no.3
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    • pp.388-393
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    • 1998
  • The effects of surface area of $UO_2$ pellets($s:0.034{\sim}0.282cm^2/g-UO_2$), concentration of nitric acid(1.5~10N) and temperature($40{\sim}105^{\circ}C$) on the dissolution rate were investigated in this experiment. The results showed that dissolution rate of $UO_2$ pellets was in direct proportion to effective surface area(s) and well fit to linear plot in high surface area. The dissolution rate was expressed as dc/dt = 10.6s at $90^{\circ}C$, [$HNO_3$]=8N, and proportioned to the power 1.42 of $HNO_3$ concentration. And also, the results showed that dissolution rate was sharply increased according to temperature increase in temperature below $90^{\circ}C$, but decreased in temperature above $90^{\circ}C$. Activation energy(E) was evaluated to be 36.3KJ/mol.

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Thermodynamic and experimental analyses of the oxidation behavior of UO2 pellets in damaged fuel rods of pressurized water reactors

  • Jung, Tae-Sik;Na, Yeon-Soo;Joo, Min-Jae;Lim, Kwang-Young;Kim, Yoon-Ho;Lee, Seung-Jae
    • Nuclear Engineering and Technology
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    • v.52 no.12
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    • pp.2880-2886
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    • 2020
  • A small leak occurring on the surface of a fuel rod due to damage exposes UO2 to a steam atmosphere. During this time, fission gas trapped inside the fuel rod leaks out, and the gas leakage can be increased due to UO2 oxidation. Numerous studies have focused on the steam oxidation and its thermodynamic calculation in UO2. However, the thermodynamic calculation of the UO2 oxidation in a pressurized water reactor (PWR) environment has not been studied extensively. Moreover, the kinetics of the oxidation of UO2 pellet also has not been investigated. Therefore, in this study, the thermodynamics of UO2 oxidation under steam injection due to a damaged fuel rod in a PWR environment is studied. In addition, the diminishing radius of the UO2 pellet with time in the PWR environment was calculated through an experiment simulating the initial time of steam injection at the puncture.

Development of thermal conductivity model with use of a thermal resistance circuit for metallic UO2 microcell nuclear fuel pellets

  • Heung Soo Lee;Dong Seok Kim;Dong-Joo Kim;Jae Ho Yang;Ji-Hae Yoon;Ji Hwan Lee
    • Nuclear Engineering and Technology
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    • v.55 no.10
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    • pp.3860-3865
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    • 2023
  • A metallic microcell UO2 pellet has a microstructure where a metal wall is connected to overcome the low thermal conductivity of the UO2 fuel pellet. It has been verified that metallic microcell fuel pellets provide an impressive reduction of the fuel centerline temperature through a Halden irradiation test. However, it is difficult to predict the effective thermal conductivity of these pellets and researchers have had to rely on measurement and use of the finite element method. In this study, we designed a unit microcell model using a thermal resistance circuit to calculate the effective thermal conductivity on the basis of the microstructure characteristics by using the aspect ratio and compared the results with those of reported metallic UO2 microcell pellets. In particular, using the thermal conductivity calculated by our model, the fuel centerline temperature of Cr microcell pellets on the 5th day of the Halden irradiation test was predicted within 6% error from the measured value.

The Influence of Sintering Atmosphere on the Reduction Behaviour of Refractory Bricks and the Basic Properties of $UO_{2}$ Pellet

  • Lee, Seung-Jae;Kim, Kyu-Tae;Chung, Bum-Jin
    • The Korean Journal of Ceramics
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    • v.4 no.4
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    • pp.279-285
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    • 1998
  • The $UO_2$ pellets are usually sintered under hydrogen gas atmosphere. Hydrogen gas may cause unexpected early failure of the refractory bricks in the sintering furnace. In this work, nitrogen was mixed with hydrogen to investigate the effect of nitrogen gas on a failure machanism of the refractory bricks and on the microstructure of the $UO_2$ pellet. The hydrogen-nitrogen mixed gas experiments show that the larger nitrogen the mixed gas contains, the less the refractory materials are reduced by hydrogen. The weight loss measurements at $1400^{\circ}C$ for fire clay and chamotte refractories containing high content of $SiO_2$ indicate that the weight loss rate for the mixed gas is about half of that for the hydrogen gas. Based on the thermochemical analyses, it is proposed that the weight loss is caused by hydrogen-induced reduction of free $SiO_2$ and/or $SiO_2$ bonded to $Al_2O_3$ in the fire clay and chamotte refractories. However, the retardation of the hydrogen-induced $SiO_2$ reduction rate under the mixed gas atmosphere may be due to the reduction of the surface reaction rate between hydrogen gas and refractory materials in proportion to volume fraction of nitrogen gas in the mixed gas. On the other hand, the mixed gas experiments show that the test data for $UO_2$ pellet still meet the related specification values, even if there exists a slight difference in the pellet microstructural parameters between the cases of the mixed gas and the hydrogen gas.

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Effect of $U_3O_8$-seed on the grain growth of uranium dioxide ($U_3O_8$ 종자가 $UO_2$ 핵연료 소결체의 입자성장에 미치는 영향)

  • Rhee, Young-Woo;Kim, Dong-Joo;Kim, Keon-Sik
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.17 no.2
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    • pp.75-81
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    • 2007
  • Densification and grain growth have been investigated in 5 wt% $U_3O_8$ seeded $UO_2$ and compared with those of the common $UO_2$ pellet. $UO_2$ compacts and $U_3O_8$ seeded $UO_2$ compacts were sintered at $1300{\sim}1700^{\circ}C$ for 0 h to 4 h. Density and grain size of the sintered pellets were measured by the water immersion method and the image analyzer, respectively. The seeded pellet has a slightly lower density during the intermediate sintering stage. However, the difference of density between two pellets decreases to about 0.5%TD with increasing the sintering temperature. The grain size of the two pellets is similar until $1600^{\circ}C$ but that of the seeded pellet rapidly increases with increasing the sintering temperature.

[ $(Th,U)O_2$ ] Pellets: Fabrication and Thermal Properties

  • Kang Ki Won;Yang Jae Ho;Kim Keon Sik;Song Kun Woo;Lee Chan Bock;Jung Youn Ho
    • Nuclear Engineering and Technology
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    • v.35 no.4
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    • pp.299-308
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    • 2003
  • Fabrication technique of $(Th,U)O_2$ pellets has been investigated. Powder mixtures of $ThO_2\;and\;UO_2$ were milled in two different ways-dry and wet milling. Milled powder was compacted and sintered to $(Th,U)O_2$ pellets. The wet-milled powder leads to a $(Th,U)O_2$ pellet having a high sintered density and uniform distribution of U and Th, compared to the dry-milled powder. The sintered density of a $(Th,U)O_2$ pellet tends to decrease by increasing the content of $ThO_2$. The thermal conductivity of $ThO_2\;and\;(Th,U)O_2$ pellets was measured by the laser flash method. The thermal conductivity of the $ThO_2$ pellet is higher than that of the $UO_2$ pellet, and the thermal conductivities of $(Th,U)O_2$ pellets containing $65wt\%\;and\;35wt\%\;ThO_2$ pellets are lower than that of the $UO_2$ pellet.

EBSD studies on microstructure and crystallographic orientation of UO2-Mo composite fuels

  • Tummalapalli, Murali Krishna;Szpunar, Jerzy A.;Prasad, Anil;Bichler, Lukas
    • Nuclear Engineering and Technology
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    • v.53 no.12
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    • pp.4052-4059
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    • 2021
  • The microstructure of the fuel pellet plays an essential role in fission gas buildup and release and is critical for the safe and continued operation of nuclear power stations. Structural analysis of uranium dioxide (UO2)-molybdenum (Mo) composite fuel pellets prepared at a range of sintering temperatures from 1300 to 1800 ℃ was performed. Mo micro and nanoparticles were used in making the composite pellets. A systematic investigation into the influence of processing parameters during Spark Plasma Sintering (SPS) of the pellets on the microstructure, texture, grain size, and grain boundary characters of UO2-Mo is presented. UO2-Mo composite show significant differences in the fraction of general boundaries and also special/coincident site lattice (CSL) boundaries. EBSD orientation maps demonstrated that <111> texturing was observed in the pellets fabricated at 1500 ℃. The experimental investigations suggest that UO2-Mo composite pellets have favorable microstructural features compared to the UO2 pellet.