| With the energy crisis and strengthening of environmental protection and energy-saving consciousness, on purpose to reduce fuel manufacturing cost and produce excellent uranium dioxide pellet, material science researchers began to study low temperature sintering process of uranium dioxide pellet energetically. Low temperature sintering is activation sintering essentially. In hyperstoichiometric UO2+x , diffusion rate of uranium atom increases by form of exponent with the increase of superfluous oxygen. Former study indicates that when the value of superfluous oxygen component x of hyperstoichiometric UO2+x is about 0.25,diffusion activation energy of uranium atom is low and diffusion coefficient is high, this promotes low temperature sintering. Therefor, it is necessary to explore a process to modify UO2 powder in order to obtain the powder whose O/U ration of surface is approximate 2.25,while core is UO2.In this research on purpose to explore the optimum modification process, atmosphere oxidation orthogonal test, thermal analysis test, XRD test and oxidation increase mass test at 240℃and 360℃were carried out. Original UO2 powder was modified and oxidation mechanism of UO2 powder is analyzed. By thermal expansion sintering test sintering characteristic between pellet compacted by original uranium dioxide powder and pellet compacted by modified uranium dioxide powder is compared. The research results show that:1. Under this test condition, in order to realize low temperature sintering and obtain 2.25 O/U atom ratio, the optimal surface modification process for UO2 powder is keeping temperature for 8 hours in air at 240℃and cooling in furnace.2. In 99.999% Ar atmosphere the beginning sintering shrinkage temperature of green pellet compacted by modified UO2 powder is about 570℃(572~576℃),while that of green pellet compacted by original UO2 powder is about 1100~1170℃(1096-1171℃). The beginning sintering shrinkage temperature of the former is lower 530-600℃than that of the latter. The average linear expansion coefficient of green pellet compacted by modified uranium dioxide powder(from -308.2275×10-6/K to -328.7439×10-6/K ) is much larger than that ( from -151.5266×10-6/K to -203.121×10-6/K) of green pellet compacted by original UO2 powder during shrinkage period.3.Density of green pellet compacted by modified UO2 powder after RT-1773K thermal expansion test is larger than that of green pellet compacted by original UO2 powder. Compared with green pellet compacted by original UO2 powder, densification extent of green pellet compacted by modified UO2 powder is higher.4. Surface modification atmosphere is the most significant factor that affects the value of specific surface of UO2 powder, oxidation temperature is the second, increase temperature rate is the third and keeping temperature time is the fourth. The effect of increase temperature rate and keeping temperature time on specific surface of UO2 powder is almost the same. Under this test condition, the optimal process is increase temperature at speed of 3℃per minute at 360℃in flowing air for 8 hours. The specific surface of UO2 powder increased from 2.65 m2/g to 4.30 m2/g, if grind the powder the specific surface will increase to 4.40 m2/g, increased by 66%.5. Thermal analysis test and XRD test demonstrates that the UO2 powder loses planar water between 100 and 120℃approximately. A small quantity of U3O7 forms when the UO2 powder is kept in furnace at 240℃for 8 h,while U3O8 forms when the UO2 powder is kept in furnace at 382 or 815℃respectively for 8 hours.6.Oxidization course of UO2 powder at 360℃can be divided into two stages. In the first stage(0-4h) mass of UO2 powder increases dramatically, while in the second stage(5-12h) mass of UO2 powder keeps stable. The product oxidized at 360℃for 100 to 300 minutes is U3O8. The product of UO2 powder oxidized at 240℃for 100 to 480 minutes is mainly UO2 and a small amount of U3O7,the diffraction peak of U3O7 increases with keeping temperature time increase. The volume of UO2 powder oxidized at 360℃increased obviously, but increased slightly at 240℃. |
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