UO2 kernels, a key component of fuel elements for high temperature gas cooled reactors, have usually been prepared by sol-gel methods. Sol-gel processes have a number of advantages, such as simple processes and facilities, and higher sphericity and density. In this study, to produce 900 μm-sized UO2 particles using an external gelation process, contact length extension of the NH3 gas of the broth droplets pass and the improvement of the gelation device capable of spraying 14 MNH4OH solution are used to form 3,000 μm-sized liquid droplets. To produce high-sphericity and high-density UO2 particles, HMTA, which promotes the gelation reaction in the uranium broth solution, is added to diffuse ammonium ions from the outside of the gelation solution during the aging process and generate ammonium ions from the inside of the ADU gel particles. Sufficient gelation inside of ADU gel particles is achieved, and the density of the UO2 spheres that undergo the subsequent treatment is 10.78 g/cm3; the sphericity is analyzed and found to be 0.948, indicating good experimental results.

Tri-isotropic (TRISO) coatings on zirconia surrogate beads are deposited using a fluidized-bed vapor deposition (FB-CVD) method. The silicon carbide layer is particularly important among the coated layers because it acts as a miniature pressure vessel and a diffusion barrier to gaseous and metallic fission products in the TRISO-coated particles. In this study, we obtain a nearly stoichiometric composition in the SiC layer coated at 1400oC, 1500oC, and 1400oC with 20 vol.% methyltrichlorosilane (MTS), However, the composition of the SiC layer coated at 1300-1350oC shows a difference from the stoichiometric ratio (1:1). The density decreases remarkably with decreasing SiC deposition temperature because of the nanosized pores. The high density of the SiC layer (≥ 3.19 g/cm2) easily obtained at 1500oC and 1400oC with 20 vol.% MTS did not change at an annealing temperature of 1900°C, simulating the reactor operating temperature. The evaluation of the mechanical properties is limited because of the inaccurate values of hardness and Young’s modulus measured by the nano-indentation method.

Silicon carbide(SiC) layer is particularly important tri-isotropic (TRISO) coating layers because it acts as a miniature pressure vessel and a diffusion barrier to gaseous and metallic fission products in the TRISO coated particle. The high temperature deposition of SiC layer normally performed at 1500-1650˚C has a negative effect on the property of IPyC layer by increasing its anisotropy. To investigate the feasibility of lower temperature SiC deposition, the influence of deposition temperature on the property of SiC layer are examined in this study. While the SiC layer coated at 1500˚C obtains nearly stoichiometric composition, the composition of the SiC layer coated at 1300-1400˚C shows discrepancy from stoichiometric ratio(1:1). 3-7μm grain size of SiC layer coated at 1500˚C is decreased to sub-micrometer (＜ 1μm) -2μm grain size when coated at 1400˚C, and further decreased to nano grain size when coated at 1300-1350˚C. Moreover, the high density of SiC layer (≥3.19g/cm3) which is easily obtained at 1500˚C coating is difficult to achieve at lower temperature owing to nano size pores. the density is remarkably decreased with decreasing SiC deposition temperature.

This paper describes the spherical ammonium diuranate gel particles which are the intermediated material of the microsphere for an VHTR(very high temperature reactor) nuclear fuel. The characteristics of the intermediate-ADU gel particles prepared by AWD(ageing, washing, and drying) and FB(fluidized-bed) apparatus were examined and compared in a sol-gel fabrication process. The electrical conductivity of washing filtrate from the FB treating and the surface area of dried-ADU gel particles were higher than those of AWD treating. Also, an internal pore volume in dried-ADU gel particles showed a more decrease in AWD treatment than FB treatment because of decomposition of PVA affected by the washing time. However, the internal microstructures of ADU gel particles were similar regardless of the process variation.

In this study, we investigated the unit process parameters in spherical kernel preparation. Nearly perfect spherical microspheres were obtained from the 0.6M of U-concentration in the broth solution, and the microstructure of the kernel appeared the good results in the calcining, reducing, and sintering processes. For good sphericity, high density, suitable microstructure, and no-crack final microspheres, the temperature control range in calcination process was , and the microstructure, the pore structure, and the density of kernel could be controlled in this temperature range. Also, the concentration changes of the ageing solution in aging step were not effective factor in the gelation of the liquid droplets, but the temperature change of the ageing solution was very sensitive for the final ADU gel particles

The effects of thermal treatment conditions on ADU (ammonium diuranate) prepared by SOL-GEL method, so-called GSP (Gel supported precipitation) process, were investigated for kernel preparation. In this study, ADU compound particles were calcined to particles in air and Ar atmospheres, and these particles were reduced and sintered in 4%-/Ar. During the thermal calcining treatment in air, ADU compound was slightly decomposed, and then converted to phases at . At , the phase appeared together with . After sintering of theses particles, the uranium oxide phases were reduced to a stoichiometric . As a result of the calcining treatment in Ar, more reduced-form of uranium oxide was observed than that treated in air atmosphere by XRD analysis. The final phases of these particles were estimated as a mixture of and .