Young Woo Rhee

Recycling Process for Sinter-Active U3O8 Powders

The effect of the oxidation temperature of UO2 pellets on the powder properties of oxidized U3O8 powders has been investigated. The powder morphology, crystalline size, and BET surface area of recycled U3O8 could be modified by controlling the oxidation temperature of the UO2 pellets. Recycled U3O8 powder with a high BET surface area and a small particle size could be obtained by lowering the oxidation temperature. It was also found that this low-temperature-oxidized sinter-active U3O8 powder is effective in mitigating the density drop of the UO2 pellets when the recycled U3O8 powder is added. Moreover, the recycled powder acted as a grain-size-enlarging promoter in the UO2 pellets.

Improvement of UO2 Pellet Properties by Controlling the Powder Morphology of Recycled U3O8 Powder

Powder morphology evolution of recycled U3O8 according to the thermal treatments has been studied. The defective UO2 pellets are oxidized to U3O8 powders at a conventional temperature of 350 or 450°C in air. Those powders are pressed into green pellets and then sintered at 1,500 and 1,730°C in H2 gas flow. Final reoxidized U3O8 powers are obtained by reoxidizing those sintered pellets at 450°C in air. This paper shows that the reoxidized U3O8 powder morphology and the BET surface areas are greatly dependent on the density of sintered UO2 pellets before reoxidation. Reoxidized U3O8 powders are added to virgin UO2 powders to fabricate UO2 pellets and the effect of such addition on the UO2 pellet properties is investigated. The reoxidized U3O8 powders having a certain range of BET surface area significantly promote the grain growth of UO2 pellets.

Fabrication of Sintered Annular Fuel Pellet for HANARO Irradiation Test

The fabrication method of an annular pellet with highly precise diametric tolerances, same dimensions, and various sintered densities has been investigated. To examine the in-pile densification and swelling of the annular pellet, 5 different types of annular pellet were prepared for a HANARO irradiation test. In order to obtain annular fuel pellets with the same dimensions and various sintered densities, we control the green density of an annular compact, the sintering temperatures, and the sintering time. For a diametric tolerance control, we have introduced a new compaction process that combines the usual double-acting pressing and cold isostatic pressing. Annular fuel pellets with the same dimensions and various sintered densities were fabricated successfully, and all the pellets satisfied the pellet specification of the HANARO irradiation test. Sintered annular pellets show an excellent inner diametric tolerance of less than ±12 μm without an inner surface grinding.

Effects of MnO-Al2O3 on the Grain Growth and High-Temperature Deformation Strain of UO2 Fuel Pellets

The fabrication and high-temperature deformation strain of MnO-Al2O3-doped UO2 pelletswere studied. The effects of additive composition and amount on the microstructure evolution of a UO2 pellet were investigated. The compressive creep behaviors of MnO-Al2O3-doped UO2 pellets were examined. The results indicated that a MnO-Al2O3 binary additive can effectively promote the grain growth of UO2 pellets. In addition, the high-temperature deformation strain of the UO2 pellet can be improved significantly with 1,000 ppm 95MnO-5Al2O3 (mol%). The developed MnO-Al2O3-additive-containing UO2 pellets can be a potential candidate for a high-burn-up fuel and a pellet-cladding interaction (PCI)remedy.

RECYCLING PROCESS OF U3O8 POWDER IN MnO-Al2O3 DOPED LARGE GRAIN UO2 PELLETS

The effect of various process variables on the powder properties of recycled U₃O 8 from MnO-Al₂O₃ doped large grain UO₂ pellets and the effect of those recycled U₃O 8 powders on the sintered density and grain size of MnO-Al₂O₃ doped large grain UO2 pellets have been investigated. The evolution of morphology, size, and BET surface area of the recycled U₃O 8 powders according to the respective variation of the thermo-mechanical treatment variables of oxidation temperature, powder milling, and sequential cyclic heat treatment of oxidation and then reduction was examined. The correlation between the BET surface area of recycled U₃O 8 powder and the sintered pellet properties of MnO-Al₂O₃ doped pellets showed that the pellet density and grain size of doped pellets were increased and then saturated by increasing the BET surface area of the recycled U₃O 8 powder. The density and grain size of the pellets were maximized when the BET surface area of the recycled U₃O 8 powder was in the vicinity of 3㎡/g. Among the process variables applied in this study, the cyclic heat treatment followed by low temperature oxidation was a potential process combination to obtain the sinter-active U₃O 8 powder.

Rod-Inserted Sintering of Annular Pellets for Dual-Cooled Annular Fuel

A heat flux split is one of the important technical issues in dual-cooled annular fuel. The inner and outer diameters of an annular pellet should be carefully controlled because they determine the inner and outer gap sizes and thereby influence the balance in a heat flux split. The outer diameter of a sintered annular pellet can be controlled to a final uniform size by a centerless grinding. However, it is difficult and unproductive to grind the inner surface of all annular pellets. To obtain a uniform inner diameter among annular pellets and to minimize a diametric tolerance without inner surface grinding, we applied a rigid rod-inserted sintering process to the annular pellet fabrication. An annular compact was first compacted with a double-acting press and then sintered with a precisely machined rigid rod inserted. The rigid rod can prevent an inhomogeneous deformation of the inner surface during sintering, and thus it controls the inner diameter of the sintered annular pellets and reduces the inner diametric tolerance of a sintered annular pellet without inner surface grinding.