Development of commercial manufacturing technology for large grain UO2 pellets

To increase economic benefit and safety margins of nuclear plant, development of nuclear fuel for high burn-up and long-term cycle is important. Research on fuel pellets focuses on increasing the pellet density and grain size to increase the uranium contents and the high burn-up safety margins for LWRs. Large grain pellet can reduce the corrosive fission gas release at high burn up. Large grain pellet also has viscoplasticity and viscoplastic soft pellets can lower the pressure to a cladding caused by a thermal expansion of a pellet at an elevated temperature during transient operations. Those advantages can provide room for additional power uprates and high burnup limits. Large grain UO2 pellets could be obtained by applying the additive doping technology. However, introduction of foreign element in UO2 fuel could alternate the in-reactor fuel performance. So in order to commercially use the doped UO2 fuel in reactor, long-term verification of the pellet is required. Besides the using of additives, there are other methods to promote grain growth of the UO2 pellet through the pretreatment of UO2 and/or U3O8 component powders. Because these methods do not change the fuel composition, technology can be more easily applied in the commercial. Among these, KAERI had been developed the large grain UO2 pellet fabrication process by optimizing the recycle process of defective UO2 pellets. The characteristic of that technology is lowering the oxidation temperature of defective UO2 pellet to obtain sinter active U3O8 powder. The powder properties of BET surface area and particle size of a recycled-U3O8 powder was enhanced by lowering the oxidation temperature. A low temperature oxidized U3O8 powder with a high BET surface area and a small particle size was effective in mitigating the density drop and in promoting a grain growth of UO2 pellets. This developed process only requires lowering the oxidation temperature so it has good compatibility with conventional commercial UO2 pellet fabrication process. KEPCO NF has conducted various qualification tests for the KAERI developed large grain UO2 pellet technology to commercialize that during the last two years. To enlarge the lab-scale technology to mass production scale, KEPCO NF has developed various technologies, and optimized several process parameters. In this study, the qualification test results of large grain UO2 pellets fabrication which were performed on the mass product line of commercial UO2 pellets in KEPCO NF is introduced. KEPCO NF developed commercial scale technologies are also presented