The flow sheet for UO2 pellet production is shown in Figure 10. The UO2 pellet fabrication process con­sists of mixing the UO2 powder with additives such as binder, lubricant and pore former materials, gran­ulating to form free-flowing particles, compaction in an automatic press, heating to remove the additives, sintering in a controlled atmosphere, and grinding to a final diameter. The process varies slightly according to the nature of the starting UO2 powder.

2.15.4.2.1 Powder preparation

In the pelletizing process, UO2 powder must be filled easily and consistently into dies. UO2 powder from the AUC process is free-flowing and can be pressed

without granulation. Usually it is mixed with a small amount of U3O8 to control the density and pore distribution of the pellets. The fine particle size of the integrated dry route (IDR) powders prevents them from being free-flowing when produced; these powders are therefore prepressed into briquettes, fractured, sieved to produce granules, and a dry

lubricant added. ADU powder is slurried with a sol­vent and a volatile binder such as polyethylene glycol or polyvinyl alcohol, spray dried and sieved to size. The obtained material flows freely and will con­sistently fill pellet dies but an extra operation is required to remove the binder. Additives known as pore formers are often included to give uniform final density: 95-97% TD for LWR UO2 and MOX, and 85-95% TD for FBR MOX fuel pellets. The pore former will decompose in the dewaxing process to leave closed pores that are stable in-reactor.