**4.1 Concept of crystallization apparatus**

The crystallizer is designed for continuous operation adoption high throughput and equipment scale-up and is developed for the U crystallization in the NEXT (Washiya et al., 2010). Figure 9 shows a schematic diagram of annular type continuous crystallizer. A rotarydriven cylinder has a screw blade to transfer UNH crystal slurry and annular shaped space is formed as crystallization section in between the rotary cylinder and outer cylinder. A dissolver solution of irradiated fast neutron reactor fuel is fed into the annular section from the lower part of the equipment, and the coolant is supplied into the cooling jacket located on the outside cylinder. The dissolver solution is cooled down gradually and is transferred to the outlet in the upper side of the equipment. The UNH slurry is obtained in the annular section and is discharged by the guide blade attached to the rotary cylinder. The mother liquor is separated from the UNH crystal and is discharged from the nozzle located in the more upper side than solution level. The discharged UNH slurry is still accompanied with a little solution. Hence, it needs to be dried by the crystal separator as centrifugal dewatering process.

#### **4.2 Continuous operation with uranyl nitrate solution using annular type continuous crystallizer**

The continuous operation experiments were evaluated with a uranyl nitrate solution (Washiya et al., 2010). The feed solution of 450 g/dm3 U concentration in 5 mol/dm3 HNO3 was cooled at 0°C at 20 rph. Afterwards, amount of the crystal stay was increased gradually, and it reached to steady state in 1−2 h. The moisture content in the UNH slurry obtained from the outlet of the slurry was about 40%. The UNH crystal size was

Kawamura, 1990). On the other hand, the DF of Sr was decontaminated with the uranyl nitrate solution containing Sr and Ba after the UNH crystal was washed (Kusama et al., 2005). The solubility of Ba is 0.4 g/dm3 in 400 g/dm3 of uranyl nitrate solution with 5 mol/dm3 HNO3. Therefore, Ba is assumed to precipitate as Ba(NO3)2 in the U crystallization process. The solid impurities are not removed by the crystal washing with a HNO3 solution. Insoluble residues consist of Zr and the elements of platinum group such as Ru in a nuclear fuel reprocessing. Zirconium and Mo precipitate in the form of zirconium molybdate in the dissolution process (Adachi et al., 1990; Lausch et al., 1994; Usami et al., 2010). This compound tends to form at high temperature and with low HNO3 concentration in the solution. The DF of Zr was high after the crystal washing and Zr remained in the mother liquor. The feed solution was cooled and the acidity in the feed solution increases in the course of U crystallization. Therefore, zirconium molybdate would be difficult to crystallize

The behavior of Ce, Pr and Eu in the rare earth elements was evaluated in the U crystallization experiments. The DFs of these elements achieved to approximately 102 after the crystal washing. Their solubility in the HNO3 solution was so high that there was no precipitation as solid impurities. Therefore, they remained in the mother liquor during the U crystallization and these elements in the mother liquor that was attached to the surface of

The crystallizer is designed for continuous operation adoption high throughput and equipment scale-up and is developed for the U crystallization in the NEXT (Washiya et al., 2010). Figure 9 shows a schematic diagram of annular type continuous crystallizer. A rotarydriven cylinder has a screw blade to transfer UNH crystal slurry and annular shaped space is formed as crystallization section in between the rotary cylinder and outer cylinder. A dissolver solution of irradiated fast neutron reactor fuel is fed into the annular section from the lower part of the equipment, and the coolant is supplied into the cooling jacket located on the outside cylinder. The dissolver solution is cooled down gradually and is transferred to the outlet in the upper side of the equipment. The UNH slurry is obtained in the annular section and is discharged by the guide blade attached to the rotary cylinder. The mother liquor is separated from the UNH crystal and is discharged from the nozzle located in the more upper side than solution level. The discharged UNH slurry is still accompanied with a little solution. Hence, it needs to be dried by the crystal separator as centrifugal dewatering

**4.2 Continuous operation with uranyl nitrate solution using annular type continuous** 

The continuous operation experiments were evaluated with a uranyl nitrate solution (Washiya et al., 2010). The feed solution of 450 g/dm3 U concentration in 5 mol/dm3 HNO3 was cooled at 0°C at 20 rph. Afterwards, amount of the crystal stay was increased gradually, and it reached to steady state in 1−2 h. The moisture content in the UNH slurry obtained from the outlet of the slurry was about 40%. The UNH crystal size was

at low temperature and with high HNO3 concentration in the mother liquor.

the UNH crystal were washed away with the HNO3 solution.

**4. Crystallization apparatus for continuous operation** 

**4.1 Concept of crystallization apparatus** 

process.

**crystallizer** 

approximately 600 μm and is considered to appropriate size for solid-liquid separation. The U concentration in the mother liquor was reached to steady state within 2 h, and crystal ratio of U was about 84%.

Fig. 9. Schematic diagram of annular type continuous crystallizer

To extract operational failure events comprehensively concerning to the U crystallizer and to clarify their importance, failure mode analysis was carried out by applying Failure Mode and Effects Analysis (FMEA). Significant failure events were identified with failure causes, their effects and probability of these failures were predicted by making use of operation experience. All failure events were evaluated by cause, primary and secondary effects and scored them. As results, crystal accumulation, blockage of mother liquor discharge nozzle, blockage of crystal discharge nozzle were selected as important specific failure events. To investigate how to detect non-steady condition, these three experiments were carried out with screw rotation speed decline, crystal outlet blockage and mother liquor outlet blockage. Also, the resume procedure after non-steady state was examined sequentially to consider countermeasures for each non-steady event (Shibata et al., 2009). The accumulation of UNH crystals can be detected by the torque of the cylinder screw, the liquid level in the annular section and other instruments. These experimental results show that it is possible to recover from non-steady state when the cause of the phenomena such as blockage of crystal outlet is removed by an appropriate operation. The fundamental performance of crystallizer annular type was investigated with uranyl nitrate solution. The experiment will be carried out to confirm the system performance on integrated crystallization system consisting of the engineering-scale crystallizer, crystal separator and related systems.
