Pilot plant development of processes for the production of ammonium diuranate

Nuclear grade ammonium diuranate (ADU) and UO2 powders were produced on a pilot plant scale by the continuous single-stage precipitation of ADU with ammonium hydroxide, dewatering with a rotary drum vacuum filter or a solid bowl centrifuge, and batch-tray drying and calcination-reduction to UO2 powder. Precipitation at 500C and pH values in the range 7.2 to 8.0 produced ADU materials which could be converted to UO2 powder by calcination and reduction at temperatures of 600 to 7300C, and fabricated into sintered pellets with densities of 10.37 to 10.77 g cm-3. The lower the pH of precipitation the lower was the reduction temperature required to achieve a specified pellet density. Precipitation with ammonium hydroxide at 800C and with ammonia gas at 500C offered no advantages over precipitation with ammonium hydroxide at 500C. The UO2 powders and sintered pellets produced from ADU powders precipitated by the three methods were similar. Precipitation at pH 7.5 and 500C is recommended since a reasonably filterable precipitate can be produced reproducibly without a need for stringent control, and considerable flexibility is available in the subsequent production of a sinterable UO2 powder. Dewatering of ADU slurries was carried out more efficiently using a solid bowl centrifuge rather than a rotary drum vacuum filter. Clearer discharge liquids were produced at a higher rate of throughput in the solid bowl centrifuge. (author)