Wet granulation of mixed oxide powders de-nitrated by the microwave heating

A performance in a wet granulation of mixed oxide (MOX) powders prepared by the microwave heating de-nitration method was examined to fabricate MOX fuel pellets for fast breeder reactor at an extremely simplified process. An agitating granulator equipped with three blades and a chopper was used and its performance was evaluated. Characteristics and physical properties of the raw powder and the granules were measured together with observations by scanning electron microscopy before/after granulation. We could obtain the granules of 120–140 μm in diameter within a narrow range of water addition ratio 12.5–13.5 wt% combined with a flow-ability >73 and a product yield >90%. Specific surface area that corresponds to sintering performance was almost the same as that of raw powder. When the ratio was <9 wt%, no granule was observed, and flow-ability did not change from that of raw powder <30. It was estimated from an additional experiment that the reason was capillaries (or voids) in the raw powder. When the ratio was more than 14 wt%, the flow-ability saturated and the product yield decreased. This narrow range of water addition ratio and strong binding force were successfully understood based on the standard theory of granulation supposing the Pendular state and the Gorge method.