**5.1 Principal of crystal purification**

Generally, crystalline particles produced in crystallizers are often contaminated by the mother liquor which appears on the surface or inside the bodies of the crystals. The UNH crystal recovered from a dissolver solution of irradiated fast neutron reactor fuel is washed away with a HNO3 solution. Although the TRU elements and FPs on the surface of the UNH crystal are decontaminated by the crystal washing, the inclusions within the crystal and the solid impurities are not removed from the UNH crystal. Therefore, crystal purification method is studied for the purpose of further increasing decontamination performance. One

Separation of Uranyl Nitrate Hexahydrate Crystal

other by gravity and mixing.

**6. Conclusion** 

from Dissolver Solution of Irradiated Fast Neutron Reactor Fuel 395

operation (Otawara & Matsuoka, 2002). The crude crystal is supplied at the bottom of the column and then is carried to the upper side of a column by a double screw conveyor, and then part of the crystal is molten by a heating unit at the top of the column and the melt trickles downward among the crude crystal. The apparatus performs countercurrent contact between the crystal and reflux melts in the course of being conveyed upward, and the crude crystal is washed by a portion of the melt. Therefore, higher DFs of liquid impurities will be obtained by the KCP, because the liquid impurities were washed with melt in addition to

**5.3 Continuous operation with uranyl nitrate solution using kureha crystal purifier** 

The crystal purification experiments with the KCP were carried out using the UNH crystal recovered from uranyl nitrate solution containing Sr of SUS304L (Yano et al., 2009). Although the DF of liquid impurities, Eu, in the static system was approximately 2.4 at 60°C for 30 min, the DF of Sr was 50 by the KCP. The liquid impurities such as Sr was removed from the UNH crystal not only by the sweating phenomenon but also by washing with U reflux melt, which was produced by the melter at the top of the column of the KCP. On the other hand, the DF of solid impurities, SUS304L, achieved a value of 100 with the KCP. In the static system, the solid impurities remained in the UNH crystal after the sweating operation. In the KCP, the solid impurities were removed from the UNH crystals due to upward movement of the crude crystals from the double screw conveyors; the UNH crystals and solid impurities, which have different densities and particle sizes, separate from each

Experimental studies on the behavior of TRU elements and FPs in the dissolver solution of irradiated fast neutron reactor core fuel were carried out to develop a crystallization method as a part of an advanced aqueous reprocessing. The experimental results show high HNO3 concentration in the feed solution increased with increasing the UNH crystal ratio in the U crystallization process. Among coexistent elements, Zr, Nb, Ru Sb, Ce, Pr, Eu, Am and Cm remained in the mother liquor at the time of U crystallization. Therefore, portions of these elements in the mother liquor that was attached to the surface of the UNH crystal were washed away with HNO3 solution. Cesium exhibited different behavior depending on whether Pu was present. Although a high DF of Cs was obtained in the case of uranyl nitrate solution without Pu(IV), Cs was hardly separated at all from the UNH crystal formed from the dissolver solution of irradiated fast neutron reactor core fuel in the case of high Cs concentration in the feed solution. It is likely that a double salt of Pu(IV) and Cs, Cs2Pu(NO3)6 precipitated in the course of U crystallization process. Since Ba precipitated as Ba(NO3)2, its DF was low after the UNH crystal was washed. Neptunium was not removed from the UNH crystal because Np was oxidized to Np(VI) in the feed solution and thus cocrystallized with U(VI). The experimental data on the behavior of TRU elements and FPs will be actually utilized in fast neutron reactor fuel reprocessing. The continuous crystallizer and the KCP were developed, and the apparatus performance was examined with the uranyl nitrate solution containing simulated FPs. In the future, the integrated crystallization

system performance will be confirmed for part of U recovery in the NEXT process.

sweating effect. The pure crystal product exits from the top of the column.

crystal purification method, the grown crystalline particles are purified by heating up to as high as the melting point of the crystal and introducing the mother liquor to the outside of the crystal, which is exhaled along defects and grain boundaries (Zief & Wilcox, 1967; Matsuoka & Sumitani, 1988). This phenomenon is called "sweating" and is applied to organics and metals. The mother liquor and melt in the crystal are discharged by Ostwald ripening and increase in the internal pressure (Matsuoka et al., 1986). The incorporated liquid is expelled from grooves along defects and grain boundaries. It was reported that countless grooves were observed in the organic crystal after sweating (Matsuoka & Sumitani, 1988). The purification of the *p*-dichlorobenzene (*p*-DCB) and *m*chloronitrobenzene (*m*-CNB) crystalline particles by sweating was experimentally investigated (Matsuoka et al., 1986). The purity of 99.99% was obtained by a single sweating stage at temperatures about 1°C below the melting points of the pure crystals after the duration of 90 or 120 min of sweating. In the batch operation, the UNH crystal purification experiments were carried out with the dissolver solution of MOX fuel containing simulated FPs (Nakahara et al., 2011). Although the DFs of solid impurities such as Ba(NO3)2 and Cs2Pu(NO3)6 did not change in the sweating process, that of Eu increased with increases in temperature and time. In the batch experiments, the DF of Eu increased to approximately 2.4 times after 30 min at 60°C. There results indicated that liquid impurities such as Eu were effectively removed by the sweating method, but solid impurities such as Pu, Cs and Ba were minimally affected in the batch experiments.

Fig. 10. Schematic diagram of KCP

#### **5.2 Concept of crystal purification apparatus**

The crystal purification apparatus, Kureha Crystal Purifier (KCP), has been applied in industrial plants using organic matter (Otawara & Matsuoka, 2002). The schematic diagram of KCP is shown in Figure 10. The apparatus has been developed in the following fashion: feed stock is charged as solids at the bottom of the column, the heating unit is set at the top of the column, and it is possible to contact the melt with crude crystal countercurrently. The KCP features high purity, high yield, energy savings, little maintenance, and a long, stable operation (Otawara & Matsuoka, 2002). The crude crystal is supplied at the bottom of the column and then is carried to the upper side of a column by a double screw conveyor, and then part of the crystal is molten by a heating unit at the top of the column and the melt trickles downward among the crude crystal. The apparatus performs countercurrent contact between the crystal and reflux melts in the course of being conveyed upward, and the crude crystal is washed by a portion of the melt. Therefore, higher DFs of liquid impurities will be obtained by the KCP, because the liquid impurities were washed with melt in addition to sweating effect. The pure crystal product exits from the top of the column.
