[en] This study was carried out to investigate the radiation dose-response of micronucleus frequencies in Tradescantia pollen mother cells. The number of micronuclei increased in the tetrads as a result of chromosome deletion after irradiation. The maximal frequency of micronucleus showed a good dose-response relationship in the range of dose 0∼50 cGy. On the basis of the relationship, a dose of 1 cGy resulted in two additional micronuclei in 100 tetrads. The radiation dose-response relationship of micronucleus occurrence is prerequisite to biological monitoring of radiation and can be modified for biological risk assessment of toxicants, and to safety test of water or soil integrity

[en] Technologies for the fuel Kernel preparation for HTGR by wet process have been reviewed. Kernel with good sphericity, uniform size and chemical composition can be prepared by wet process. Sol-gel preparation is based on four major steps: preparation of a sol or of a special solution(broth), gelation of droplets of sol or broth to give semi- rigid spheres, and washing and drying of gel droplets, calcination and sintering of these spheres to a high density. After sol drop is formed, gelation may be accomplished by water extraction or ammonia gelation. Ammonia gelation can be accomplished either externally, via ammonia gas and ammonium hydroxide solution, or internally, via an added ammonia donor such as hexamethylenetetramine. Review of general sol-gel process and of specific process condition in the three different processes have been conducted, to establish the basic knowledge and to compare the advantages and disadvantages of the processes

[en] The studies have shown that crystalline ceramics are better hosts compared to glass matrix due to their superior chemical durability and higher density. Synroc, atitanate based ceramic containing hollandite, perovskite, zirconolite and rutile phases, is considered as the most effective and durable means of immobilizing various forms of high-level radioactive wastes for disposal. The oxide-route (solid state reaction) is the most known process to form a solid solution. However, the synthesis of nano crystalline powders using an exothermic redox reaction between nitrate and organics in an aqueous solution has been reported. In this study, the SCS and oxide route was compared to form the solid solution of Synroc-B nano crystalline powders. TEM mapping results on combustion synthesized powder has been confirmed that it contains the fully solid solution occurred on constituent elements for the composition Synroc-B in all powders. TEM analyses also show that particle size is very homogeneous and evenly distributed in 20257-258⁓30 nm. By a combustion synthesis method Synroc-B nano powder of a multi-phase is could be produced in a relatively easy way. Solidified in the Synroc-B complex, radioactive elements contained in waste can easily be immobilized

[en] The management of nuclear HLW is the biggest challenge faced by the nuclear industry world wide. Borosilicate glass has been considered as an HLW immobilization matrix; however, researches have shown that crystalline ceramics are a better matrix compared to glass owing to their chemical durability and higher density. The solution combustion synthesis process, which uses metal nitrates as the reactant materials, is applied to immobilize the Sr- and Lns-based radioactive wastes elements. During the combustion process, an externally initiated reaction is self-sustained owing to the exothermic reaction. A significant volume of gas evolved during the combustion reaction and led to loosely agglomerated particles. A high temperature of up to 1200 degree C, inherent to the highly exothermic nature of the redox reaction, leads to well-crystallized powder in a short reaction time. In this study, the combustion process was applied to form a solid solution of Sr doped CaTiO₃. Glycine and other organic compounds and their mixtures consisting of carboxylic acid and an amine group were used as fuel in order to investigate the effects of fuel on the combustion reaction. The synthesized powders are characterized by XRD, SEM, and TEM, and CaTio₃ phase formation was also investigated.

[en] TRISO-coated fuel particles for high-temperature gas-cooled reactors consist of UO₂ microspheres coated with multilayers of porous pyrolytic carbon (PyC), inner dense PyC (IPyC), silicon carbide (SiC), and outer dense PyC (OPyC). For a uniform coating of the microspherical particles, a fluidized-bed chemical vapor deposition (FBCVD) method is utilized. Among the TRISO coating layers, the SiC layer is particularly important because it acts as a diffusion barrier to gaseous/metallic fission products and a miniaturized pressure vessel of the fuel particle. In order to insure the integrity of the SiC layer after a fabrication, the microstructure, mechanical properties, and chemical composition of the SiC layer should be properly controlled. Properties of the coating layer depend largely on the FBCVD conditions such as a flow rate, concentration of the coating gas, coating temperature, etc. In this study, we investigated the influence of deposition temperatures on the property of SiC layer. Microstructure, chemical composition, and other properties of the SiC layer were characterized by various techniques. TRISO coatings on ZrO₂ surrogate kernels were conducted using a FBCVD method. The influence of the deposition temperature on the properties of the SiC layer were investigated. The SiC layers coated at 1350 .deg. C contained a small amount of excessive carbon, while the stoichiometric composition of SiC coating layer was obtained at 1400 .deg. C~1500 .deg. C. Moreover, it was verified that the observed acoustic SiC bands in Raman spectra was due to an increased grain boundary caused by fine grains and stacking faults.

[en] The VHTR (Very High Temperature Gas Reactor) is one of the reactor concepts in the Gen IV International Collaboration. The nuclear fuel of a VHTR in the US is based on microspheres containing a mixture of UO₂ and UC₂ coated with multi carbon layers and a SiC layer. This mixture is called a 'UCO (uranium oxi carbide)' kernel. The fabrication process of this kernel was based on the sol-gel method between an ADUN and HMTA and urea, a process referred to as internal gelation. UCO kernel microspheres were first prepared at ORNL in the late 1970s. CB(Carbon Black) as a carbon source in the final UCO kernel is added during the broth solution preparation, in the processing of UCO kernel fabrication. The preparation of a good quality UCO kernel is very difficult due to the homogeneous distribution of carbon in a UCO kernel. The key requirement to obtain a good quality kernel is a uniform distribution of carbon in the ADU gel sphere forming process before the thermal treatment, i.e., during the gel formation step. The internal gelation concept was adapted in ADU gel sphere fabrication in the ORNL process of the US. Generally, UO₂ kernel microspheres are prepared by an internal gelation method (USA, India) or external gelation method (Germany, China, Japan). The UCO kernel microspheres prepared only in the US, use an internal gelation method. A material flow chart on the preparation of the microsphere kernel is simply shown in Fig. 1. The broth solution preparation, the raw material, additives, and thermal steps such as calcining and sintering processes were different to compared with the external gelation and internal gelation methods. In this study, we first carried out the matching CB selection experiments among the various kinds of CBs in a broth solution, for UCO kernel preparation using an external gelation method.

[en] TRISO-coated fuel particles for high-temperature gas-cooled reactors consist of UO₂ microspheres coated with layers of porous pyrolytic carbon (porous PyC), inner dense PyC (IPyC), silicon carbide (SiC), and outer dense PyC (OPyC). The porous PyC coating layer, the so-called buffer layer, attenuates fission recoils and provides a void volume for gaseous fission products and carbon monoxide. The IPyC layer acts as containment for gaseous products. The OPyC layer protects the SiC coating layer by inducing a compressive stress along with the IPyC layer and provides chemical compatibility with the graphite matrix in a fuel compact. Among the TRISO coating layers the SiC layer is particularly important because it acts as a diffusion barrier to gaseous and metallic fission products and as a miniature pressure vessel for the particle. In order to insure the integrity of the SiC layer after fabrication and in use, the microstructure, mechanical properties, and chemical composition of the SiC layer should be controlled properly. For a uniform coating of the microspherical particles, the TRISO coating is performed using a fluidized-bed chemical vapor deposition (FBCVD) method. In the method, the process conditions such as the gas flow rate, concentration of the coating gas, coating temperature, etc., largely affect the characteristics of the coating layer. Among the deposition parameters the gas flow rate mainly determines the fluidization behavior of microspherical particles. In this study, we investigated the effect of the gas flow rate on the microstructure and properties of the SiC layer while fixing the other deposition parameters

[en] A computer code, COPA-FPREL, has been developed to estimate the releases of fission products from a high temperature gas-cooled reactor (HTGR) fuel under normal and accident conditions. The COPA-FPREL code treats the migration of fission product in a coated fuel particle and a fuel element, and the leakage into a coolant using a finite difference method. In the finite difference method, the kernel, buffer and the coating layers of a coated fuel particle are divided into small intervals, respectively. A Fickian diffusion equation including birth rates is applied to the intervals. The fuel element is also divided into various small intervals, respectively. A Fickian diffusion equation including source terms is applied to the intervals. For the isotopes Cs-137, Sr-90 and Ag-110m, the fractional releases from a TRISO-coated fuel particle, a pebble and a fuel block under heating and irradiation were calculated using the COPA-FPREL

[en] The HTGR(High Temperature Gas Reactor) will play a dominant role in the worldwide fleet of nuclear reactors of the next decade for electricity production and high temperature heat for hydrogen gas production. HTGR have two reactor types, pebble or prismatic, which use the basic fuel concept based on the dispersion of TRISO coated particles in graphite powder. The inner part of this TRISO coated particle has a UO₂ sphere of various sizes, which was prepared with a sol-gel method. An external gelation method for UO₂ spheres was designed and developed from a modified GSP(gelation supported precipitation) process as shown in Fig.1

[en] These minerals have the capacity to accept nearly all of the elements present in the high-level nuclear waste (radwaste) produced during the reprocessing of spent nuclear fuel rods of nuclear reactors. Synroc minerals can accommodate up to 20 wt% (as oxide) of radwaste in their crystal lattices as dilute solid solutions. Synroc-B refers to the waste free composition, proposed for the immobilization of nuclear wastes generated in the commercial nuclear power plants, while the waste-loaded synroc is called synroc-C. The oxide-route (solid state reaction) with high temperatures and long sintering times is the most known process to form a solid solution. However, the synthesis of nano powders using an exothermic redox reaction between nitrate and organics in an aqueous solution has been reported. Most of the high-level radioactive wastes forms were dissolved in nitric acid, and therefore the solution combustion synthesis (hereafter called SCS) which uses all of the metal nitrates as reactant materials is a very promising process to immobilize the radioactive metal element wastes in the form of solid solutions. During the combustion, a significant volume of gas evolved and the high temperature inherent to the highly exothermic nature led to fine and homogeneous well-crystallized powder within a short reaction time. The following conclusions were obtained by comparing the combustion synthesis with the oxide route synthesized Synroc-B powders. With Oxide route synthesized synthesis through a wet ball milling and with a calcination temperature at 1100 .deg. C, the synthesized particles do not match the Synroc-B composition. It was determined to be a heterogeneous particle size showed about 1μm. However, Synroc-B particles prepared by combustion synthesis showed all Hollandite, Zirconolite, Perovskite and Rutile structures having a configuration of the complete Synroc-B at a calcination temperature of 1100 .deg. C