**6. Conclusion**

394 Advances in Crystallization Processes

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.

Inlet

**5.2 Concept of crystal purification apparatus** 

Fig. 10. Schematic diagram of KCP

Perforated tray Wash nozzle Shaft seal

Melter

Outlet (Purified crystals) Wash nozzle Shaft seal

Intermediate bearing Outlet

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

(Molten dirty crystals)

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.

Separation of Uranyl Nitrate Hexahydrate Crystal

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