Research On Manufacturing Of Gd-Bearing UO₂ Pellets And Their Industrial Application

Nowadays, light water power reactor is one of the most important nuclear reactors in the world. Power and life are the two most important parameters for such reactors. In order to increase the power and life, the starting nuclear fuel is usually many times as much as the critical amount, leading to a superhigh initial reactivity in the reactor core. The optimal technique to control the reactivity is to introduce a burnable poison. (The operation mode of high burnup, long cycle and low leak is adopted in the up-to-date development of advanced reactors.Few meaning here, it can be omitted.) In the nuclear power plants which have to be shutted down to refuel, the long-term reactivity of reactors and temperature coefficient of moderator can not be controlled effectively using the dismountable burnable (associated) assemblies, and thereby the desired flexable fuel cycle life can not be achieved. It is a more effective technique to use the integrative burnable poison fuel assemblies. UO₂-Gd₂O₃ burnable poison pellet is an integrative burnable poison adopted widely in the world. It reduces the initial reactivity of the over–loaded fuel and equilibrates the distribution of core neutron flux and power, and hence improves the burnup, power and safety.In China, the industrial production of Gadolinium (Gd)-containing UO₂ pellets has not been performed. For this reason, it is necessary to research on the industrial application of Gd-containing UO₂ pellets as integrative burnable poisons, so as to meet the demand for fuels by plant customers. In this dissertation, systematic researches were conducted in the following four aspects. Some creationary achievements were reached, which can be guideances for the domestic production of the Gd-containing UO₂ pellets.â‘ Research on IDR process. An adaptation was implemented for the miniature IDR converter and renewed development was carried out for the control system, the filter of metallic uranium and the disposal device for tail gases. Using the converter, the IDR conversion process was researched and the UO₂ powders applied to high-performance UO₂ pellets were produced. An IDR recycling device for UO₂ powders with high fluorine content was developed, and the recycling process was established. And the high fluorine-containing powder was recycled. The disposing technique for hydrofluoric acid (HF) in tail liquid was also established and thus the waste quantity in the production was reduced. The methods of analyzing and testing the uranium content in HF were developed, which ensured the quality of HF and the effluence of waste water meeting the requirement of the standards. It is the first time in China to achieve the industrial production of UO₂ powder by IDR process. Improving the yield of uranium metal, reducing the cost of production, decreasing the discharges of three wastes (liquor waste, gaseous waste and solid waste) and lightening the labor intensity of operators, the IDR process has good economic effectiveness and social benefits.â‘¡Research on Gd-bearing pellets. Since Gd-containing pellets are difficult to manufacture due to high technological requirement, especially the uniform distribution of gadolinium and the aggregated free Gd₂O₃ as well as Gd₂O₃ have great effect on the pressing and sintering performance, the research on Gd-containing pellet manufacturing processes are conducted in China. 1) Blending process. The high efficient blender and the practical and feasible blending process are succeeded in using, in order to homogeneously mix Gd₂O₃ powder with UO₂ powder, which are quite different in density. 2) Pressing technology. Ceramic Gd₂O₃ powder is brittle. When it is added into UO₂ powder, the pressing performance of the powder can not be improved, and it is difficult to eject the pellets from dies and to store them. The green pellets finally will cracks transversally due to the uneven stress release. However, these problems were successfully solved by decreasing the pressing pressure to a proper extent. 3) Sintering process. In comparison to normal UO₂ pellets, on the one hand, the pre-sintering and sintering time is much longer to get the adequate solid-solution UO₂-Gd₂O₃ sintered pellets; on the other hand, the final sintered density can not be out of the criteria. Therefore, the sintering temperature can not be too high. Moreover, the wet hydrogen sintering described herein has higher oxidation tendency, which is helpful to the migration of high-valence uranium ion and repair of the defects resulting from the change of oxidation tendency in Gd₂O₃, thus it is helpful for the densification. 4) The homogeneity of gadolinium in the pellets. By means of colored metallography analysis, the free Gd₂O₃ and UO₂-Gd₂O₃ solid solution are distinguished, and thus the homogeneity of gadolinium as well as the solid solubility of Gd₂O₃ are determined. Through tests for process conditions and industrial tests of batch productions, we have succeeded in determine the above-mentioned critical technology and accomplished the stable and reliable production, meeting the demands of the national nuclear power plants.â‘¢Research on great grain-size Gd-containing pellets. If the grain size in the UO₂ pellets sintered by normal process is 10 um or more, they are called great grain-size pellets. But for UO₂-Gd₂O₃ pellets, the grain size of Gd-containing pellets is generally smaller than 7 um, if they are sintered with the same process as that applied to UO₂ pellets, due to the solute Gd₂O₃ in UO₂ restraining the growth of pellet grain. In this subject, tests developing the great grain size Gd-containing UO₂ pellets were implemented with the grain growth additives. In the case of a given quantity of additives, the Al₂O₃/SiO₂ ratio has great effect on pellet grain size. Therefore, the grain size in Gd-containing UO₂ pellets is larger than 10 um with the addition of Al₂O₃ and/or SiO₂.â‘£Theoretical calculation and analysis of Gd-containing fuel rod. The fuel rod cladding is the first shield against radioactive material, which can accept more than 90% of fission product. So, the fuel rod soundness must be provided in design and operation. 1) Calculation and analysis show that the burnup of AFA 3G fuel rods reach 62 GWd/t(U) and the corresponding fuel assemblies up to 57 GWd/t(U). The fuel rod performance fully meets the design requirements for pellet temperature, internal pressure of fuel rod, cladding stress and strain, uniform corrosion rate on the outer cladding. Furthermore, enough tolerance has also been provided so as to be compatible with the mechanical design of fuel assemblies. 2) From the standpoint of reactor core fuel management, no matter low burnable poison content with more Gd-containing fuel rods or 6%-8% of Gd₂O₃ content with less Gd-containing fuel rods are loaded in the core, a long burnup core can be achieved. And thereby the corresponding technological criteria of nuclear power plants are met.