**3.2 Crystal ratio of uranyl nitrate hexahydrate**

The cooling curve in the U crystallization is shown in Figure 5. The feed solution was placed in the crystallizer and cooled 45.0 to 3.3°C over 150 min. When the temperature of the feed

Separation of Uranyl Nitrate Hexahydrate Crystal

from Dissolver Solution of Irradiated Fast Neutron Reactor Fuel 389

feed solution and UNH crystal ratio in the batch cooling crystallization process. These experimental results show high HNO3 concentration in the feed solution increased with increasing the UNH crystal ratio in the batch cooling crystallization process. These results

01234567

concentration (mol/dm<sup>3</sup>

)

Decontamination factor (−) Before washing After washing

Decontamination factor (−) Before washing After washing

HNO3

Fig. 7. Relationship between HNO3 concentration in the feed solution and U crystal ratio

U 4.18 × 102 − − Pu 4.00 × 101 8.5 24 Ba 3.70 × 10−2 3.9 4.7

95Zr 1.84 × 107 7.5 46 95Nb 3.32 × 107 8.2 55 106Ru 3.18 × 108 13 79 125Sb 6.56 × 107 12 129 137Cs 1.04 × 109 2.2 3.4 144Ce 2.42 × 109 15 164 144Pr 2.42 × 109 15 164 155Eu 7.72 × 107 14 118 237Np 2.38 × 104 3.3 6.7 241Am 9.02 × 108 9.8 109 242Cm 2.20 × 107 8.2 115

Table 1. Composition of the feed solution and DFs of metals for UNH crystal

were in agreement with the reported experimental data (Hart & Morris, 1958).

30

(g/dm3)

(Bq/cm3)

40

50

U crystal ratio (%)

Element Feed solution

Nuclide Feed solution

60

70

80

solution reached 23.8°C at 74 min, a small increase in temperature was observed in the feed solution. This indicates the start of crystallization, where heat is released by nucleation.

Figure 6 shows the appearance of UNH crystal recovered the dissolver solution of irradiated fast reactor "JOYO" Mk-III core fuel. After crystal washing, lemon yellow crystals were obtained on a filter.

Fig. 5. Cooling curve of feed solution during U crystallization

Fig. 6. Appearance of UNH crystal after crystal washing

The crystal ratio of UNH in the dissolver solution of irradiated fast neutron reactor fuel was examined by changing in HNO3 solution in the feed solution. The crystal ratio, R*c,j*, is calculated by the following equation.

$$R\_{c,j} = 1 - \frac{\mathbf{C}\_{F, \mathbf{H}^+} \mathbf{C}\_{M,j}}{\mathbf{C}\_{M, \mathbf{H}^+} \mathbf{C}\_{F,j}} \tag{2}$$

where *CF*,H+ and *CM*,H+ are H+ concentration in the feed solution and mother liquor, respectively, and *CF,j* and *CM,j* are metal *j* concentration in the feed solution and mother liquor, respectively. Figure 7 shows the relationship between HNO3 concentration in the

solution reached 23.8°C at 74 min, a small increase in temperature was observed in the feed solution. This indicates the start of crystallization, where heat is released by nucleation.

Figure 6 shows the appearance of UNH crystal recovered the dissolver solution of irradiated fast reactor "JOYO" Mk-III core fuel. After crystal washing, lemon yellow crystals were

> Feed solution Coolant

0 50 100 150 200

Time (min)

The crystal ratio of UNH in the dissolver solution of irradiated fast neutron reactor fuel was examined by changing in HNO3 solution in the feed solution. The crystal ratio, R*c,j*, is

, ,H ,

where *CF*,H+ and *CM*,H+ are H+ concentration in the feed solution and mother liquor, respectively, and *CF,j* and *CM,j* are metal *j* concentration in the feed solution and mother liquor, respectively. Figure 7 shows the relationship between HNO3 concentration in the

*C C*

*C C* + +

*c j*

*R*

, ,H <sup>1</sup> *<sup>F</sup> <sup>M</sup> <sup>j</sup>*

= − (2)

*M F j*

obtained on a filter.


Fig. 5. Cooling curve of feed solution during U crystallization

Fig. 6. Appearance of UNH crystal after crystal washing

calculated by the following equation.

0

10

20

Temperature (

℃)

30

40

50

60

feed solution and UNH crystal ratio in the batch cooling crystallization process. These experimental results show high HNO3 concentration in the feed solution increased with increasing the UNH crystal ratio in the batch cooling crystallization process. These results were in agreement with the reported experimental data (Hart & Morris, 1958).

Fig. 7. Relationship between HNO3 concentration in the feed solution and U crystal ratio


Table 1. Composition of the feed solution and DFs of metals for UNH crystal

Separation of Uranyl Nitrate Hexahydrate Crystal

increased by a factor of 109 and 115, respectively.

U crystallization process will required experimentally.

0

Fig. 8. Abundance ratio of Pu(NO3)62− in HNO3 solution

20

40

60

Abundance ratio of Pu(NO3

)

2- (%)

6

80

100

120

from Dissolver Solution of Irradiated Fast Neutron Reactor Fuel 391

The experimental results indicated the DFs of Am and Cm for the UNH crystal were 9.8 and 8.2 before washing, respectively. These elements remained in the mother liquor and attached on the surface of the UNH crystal. The adhesion of liquid impurities was washed away with HNO3 solution. After washing, the DFs of Am and Cm for the UNH crystal

In the experiments, the behavior of Cs was evaluated in the U crystallization process. The DF of Cs showed 2.2 and 3.4 before and after washing. It is reported that alkali metals react with tetravalent actinide elements and form a double salt in a HNO3 solution (Staritzky &

This reaction indicates that an abundance of Pu(NO3)62− is advantageous for forming of Cs2Pu(NO3)6. Figure 8 show the abundance ratio of Pu(NO3)62− in HNO3 solution (Ryan, 1960). The abundance ratio of Pu(NO3)62− increases with an increase in HNO3 concentration. In the crystal growth of UNH, a certain amount of H2O molecules is needed in the feed solution. The mother liquor of HNO3 concentration is higher than that of feed solution after the U crystallization. Therefore, the formation of Cs2Pu(NO3)6 is easy in the course of U crystallization. Anderson reported that double salt of Pu nitrate, (C9H7NH)2Pu(NO3)6, Rb2Pu(NO3)6, Tl2Pu(NO3)6, K2Pu(NO3)6, (C5H5NH)2Pu(NO3)6 in addition to Cs2Pu(NO3)6 (Anderson, 1949). These materials are less in a dissolver solution of irradiated fast neutron reactor fuel. However, further investigation concerning the double salt of Pu nitrate for the

() () <sup>2</sup>

0 2 4 6 8 10 12 14

Among alkali earth metals, Ba behavior is examined in the cooling batch crystallization. The DFs of Ba was 4.7 after the crystal washing. In the precipitates formation experiments, Ba0.5Sr0.5(NO3)2 was observed using simulated high level liquid waste solution (Izumida &

concentration (mol/dm<sup>3</sup>

)

HNO3

3 23 6 6 2Cs Pu NO Cs Pu NO <sup>+</sup> <sup>−</sup> + ↔ (5)

Truitt, 1949). The reaction of Cs and Pu(IV) is expressed by the following equation.

#### **3.3 Behavior of transuranium elements and fission products in uranium crystallization process**

Table 1 summarizes the composition of feed solution and the DFs of metals in the U crystallization process. The DFs of metals, β*c,j* , are calculated by the following equation.

$$\mathcal{B}\_{c,j} = \frac{\frac{\mathbf{C}\_{F,j}}{\mathbf{C}\_{F,\mathbf{U}}}}{\frac{\mathbf{C}\_{P,j}}{\mathbf{C}\_{P,\mathbf{U}}}} \tag{3}$$

where *CF,*U is U concentration in the feed solution, and *CP,*U and *CP,j* are U and metal *j* concentrations in the UNH crystal, respectively.

Plutonium behavior in the U crystallization process depends on the Pu valence in the feed solution. In this study, the Pu valence in the feed solution was changed to Pu(IV) by NO*<sup>x</sup>* gas bubbling. After the crystallization, almost all the Pu remained in the mother liquor and attached to the surface of the UNH crystal. The mother liquor on the surface of crystal was efficiently removed after the UNH crystal was washed.

The DF of Np was 3.3 and 6.7 before and after washing, respectively. These experimental results implied Np was present in the form of solid impurities in the mother liquor because its DF was not improved by the crystal washing. Generally, Np can exist simultaneously in three stable oxidation state; Np(IV), Np(V) and Np(VI), in a HNO3 solution. The oxidation states of Np are interconvertible in HNO3 medium and exhibit different behavior in the reprocessing. Its valence is strongly affected by oxidation and reduction reactions with agents used in the reprocessing and other co-existing ions. All the kinetics of the oxidation and reduction reactions is not elucidated. One of these reactions is the oxidation of Np(V) by HNO3, which is the principal influential reaction as follows.

$$2\text{NpO}\_2^+ + 3\text{H}^+ + \text{NO}\_3^- \leftrightarrow 2\text{NpO}\_2^{2+} + \text{HNO}\_2 + \text{H}\_2\text{O} \tag{4}$$

In this reaction, HNO2 plays the important role of oxidation and reduction between Np(V) and Np(VI). This reaction shows that higher HNO3 and lower HNO2 concentrations bring about more oxidation of Np(V) to Np(VI). When the U ions crystallize as UNH in HNO3 solution, it requires a certain amount of H2O. As a result, the HNO3 concentration of the mother liquor is higher than that of the feed solution. It brings about more oxidation of Np(V) to Np(VI) in the mother liquor. When the Pu valence is changed to Pu(VI), Pu(VI) is co-crystallize with U(VI). The chemical behavior of Np(VI) is similar to that of Pu(VI), and it is likely to co-crystallize with U(VI) in the course of U crystallization. Since the Np was incorporated into the UNH crystal, it is difficult to remove from the UNH by the crystal washing operation. If the Np valence is adjusted to Np(IV) or Np(V), the behavior of Np would be different from that of Np(VI). The addition of reductant agent, e.g., U(IV), is effective for preventing from the Np oxidation to Np(VI). Thereby, the Np might remain in the mother liquor after cooling the feed solution.

Americium and Cm are supposed to recover by an extraction chromatography process in the NEXT, and are desired to remain in the mother liquor in the U crystallization process.

**3.3 Behavior of transuranium elements and fission products in uranium crystallization** 

Table 1 summarizes the composition of feed solution and the DFs of metals in the U

, ,U , ,U

*F j F c,j <sup>P</sup> <sup>j</sup>*

*C C C C*

*P*

where *CF,*U is U concentration in the feed solution, and *CP,*U and *CP,j* are U and metal *j*

Plutonium behavior in the U crystallization process depends on the Pu valence in the feed solution. In this study, the Pu valence in the feed solution was changed to Pu(IV) by NO*<sup>x</sup>* gas bubbling. After the crystallization, almost all the Pu remained in the mother liquor and attached to the surface of the UNH crystal. The mother liquor on the surface of crystal was

The DF of Np was 3.3 and 6.7 before and after washing, respectively. These experimental results implied Np was present in the form of solid impurities in the mother liquor because its DF was not improved by the crystal washing. Generally, Np can exist simultaneously in three stable oxidation state; Np(IV), Np(V) and Np(VI), in a HNO3 solution. The oxidation states of Np are interconvertible in HNO3 medium and exhibit different behavior in the reprocessing. Its valence is strongly affected by oxidation and reduction reactions with agents used in the reprocessing and other co-existing ions. All the kinetics of the oxidation and reduction reactions is not elucidated. One of these reactions is the oxidation of Np(V) by

<sup>2</sup> 2NpO 3H NO 2NpO HNO H O 2 3 2 22

In this reaction, HNO2 plays the important role of oxidation and reduction between Np(V) and Np(VI). This reaction shows that higher HNO3 and lower HNO2 concentrations bring about more oxidation of Np(V) to Np(VI). When the U ions crystallize as UNH in HNO3 solution, it requires a certain amount of H2O. As a result, the HNO3 concentration of the mother liquor is higher than that of the feed solution. It brings about more oxidation of Np(V) to Np(VI) in the mother liquor. When the Pu valence is changed to Pu(VI), Pu(VI) is co-crystallize with U(VI). The chemical behavior of Np(VI) is similar to that of Pu(VI), and it is likely to co-crystallize with U(VI) in the course of U crystallization. Since the Np was incorporated into the UNH crystal, it is difficult to remove from the UNH by the crystal washing operation. If the Np valence is adjusted to Np(IV) or Np(V), the behavior of Np would be different from that of Np(VI). The addition of reductant agent, e.g., U(IV), is effective for preventing from the Np oxidation to Np(VI). Thereby, the Np might remain in

Americium and Cm are supposed to recover by an extraction chromatography process in the NEXT, and are desired to remain in the mother liquor in the U crystallization process.

++ − + ++ ↔ + + (4)

*c,j* , are calculated by the following equation.

= (3)

β

β

**process** 

crystallization process. The DFs of metals,

concentrations in the UNH crystal, respectively.

efficiently removed after the UNH crystal was washed.

HNO3, which is the principal influential reaction as follows.

the mother liquor after cooling the feed solution.

The experimental results indicated the DFs of Am and Cm for the UNH crystal were 9.8 and 8.2 before washing, respectively. These elements remained in the mother liquor and attached on the surface of the UNH crystal. The adhesion of liquid impurities was washed away with HNO3 solution. After washing, the DFs of Am and Cm for the UNH crystal increased by a factor of 109 and 115, respectively.

In the experiments, the behavior of Cs was evaluated in the U crystallization process. The DF of Cs showed 2.2 and 3.4 before and after washing. It is reported that alkali metals react with tetravalent actinide elements and form a double salt in a HNO3 solution (Staritzky & Truitt, 1949). The reaction of Cs and Pu(IV) is expressed by the following equation.

$$\text{2Cs}^+ + \text{Pu(NO}\_3\text{)}\_6^{2-} \leftrightarrow \text{Cs}\_2\text{Pu(NO}\_3\text{)}\_6 \tag{5}$$

This reaction indicates that an abundance of Pu(NO3)62− is advantageous for forming of Cs2Pu(NO3)6. Figure 8 show the abundance ratio of Pu(NO3)6 <sup>2</sup>− in HNO3 solution (Ryan, 1960). The abundance ratio of Pu(NO3)62− increases with an increase in HNO3 concentration. In the crystal growth of UNH, a certain amount of H2O molecules is needed in the feed solution. The mother liquor of HNO3 concentration is higher than that of feed solution after the U crystallization. Therefore, the formation of Cs2Pu(NO3)6 is easy in the course of U crystallization. Anderson reported that double salt of Pu nitrate, (C9H7NH)2Pu(NO3)6, Rb2Pu(NO3)6, Tl2Pu(NO3)6, K2Pu(NO3)6, (C5H5NH)2Pu(NO3)6 in addition to Cs2Pu(NO3)6 (Anderson, 1949). These materials are less in a dissolver solution of irradiated fast neutron reactor fuel. However, further investigation concerning the double salt of Pu nitrate for the U crystallization process will required experimentally.

Fig. 8. Abundance ratio of Pu(NO3)62− in HNO3 solution

Among alkali earth metals, Ba behavior is examined in the cooling batch crystallization. The DFs of Ba was 4.7 after the crystal washing. In the precipitates formation experiments, Ba0.5Sr0.5(NO3)2 was observed using simulated high level liquid waste solution (Izumida &

Separation of Uranyl Nitrate Hexahydrate Crystal

crystal ratio of U was about 84%.

Feed solution inlet

**5.1 Principal of crystal purification** 

from Dissolver Solution of Irradiated Fast Neutron Reactor Fuel 393

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

> Rotary-driven cylinder with bladed

Crystal outlet

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

Cooling jacket

Mother liquor outlet

Fig. 9. Schematic diagram of annular type continuous crystallizer

engineering-scale crystallizer, crystal separator and related systems.

**5. Purification of uranyl nitrate hexahydrate crystal product** 

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

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

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 UNH crystal were washed away with the HNO3 solution.
