**2. Principal of uranium crystallization**

384 Advances in Crystallization Processes

by the highly efficient dissolution process and the dissolver solution was adjusted to high heavy metal concentration. Then, U is recovered as UNH crystals from dissolver solution derived from fast neutron reactor fuel. Since the amount throughput will be reduced in the simplified solvent extraction process, the adoption of the crystallization process is expected to reduce the radioactive waste, equipment, and hot cell volume. In addition, U/Pu ratio in the dissolver solution is adjusted in the crystallization process to be a suitable Pu content for core fuel fabrication. In the NEXT, Np is changed to Np(VI) in the high HNO3 concentration feed solution and is co-extracted with U and Pu in the simplified solvent extraction system. The FPs in the raffinate obtained from the simplified solvent extraction process is removed using *N,N,N',N'*-tetraoctyl-3-oxapentane-1,5 diamide (TODGA) absorbent in the extraction chromatography I. The actinide elements such as Am and Cm is recovered from the solution containing actinide and lanthanide elements by chromatography with 2,6-bis-(5,6-dialkyl-1,2,4-triazine-3-yl)pyridine (R-BTP)

Clarification

Disassembling/Decladding

Dissolution

Crystallization

U

Co-extraction

Co-stripping (U/Pu/Np co-recovery)

Solvent washing

U/TRU (product) U (product)

absorbent in the extraction chromatography II.

MA recovery

FP

Ln

Am/Cm/Ln

High level liquid waste

Fig. 1. Schematic diagram of the NEXT process

crystallization process must be confirmed experimentally.

Extraction chromatography I

Extraction chromatography II

Am/Cm U/Pu/Np

Adjustment of Pu content

A dissolver solution of irradiated fast neutron reactor mixed oxide (MOX) fuel in JAEA contains a number of TRU elements and FPs than in KfK. Since U is used as blanket fuel and TRU elements are supposed to recover by other chemical process, it is need to remove TRU elements and FPs from UNH crystals in the U crystallization process. It would be also bring about reduction in the cost for the recovered U storage and the blanket fuel fabrication due to decreased radiation shielding. Therefore, the behavior of TRU elements and FPs in the U

Since U is recovered as UNH crystal for a blanket fuel fabrication in the U crystallization process, the crystal ratio of U should be evaluated with a dissolver solution of irradiated fast neutron reactor. The crystal ratio of UNH affects HNO3 concentration in the feed

Concentration

Am/Cm/Ln/FP

In a HNO3 solution, U ions are crystallized as UNH by the following reaction.

$$\text{U}\text{O}\_2^{2+} + 2\text{NO}\_3^- + 6\text{H}\_2\text{O} \leftrightarrow \text{UO}\_2\text{(NO}\_3\text{)}\_2 \cdot 6\text{H}\_2\text{O} \tag{1}$$

Figure 2 shows the solubility curves of U in HNO3 solution (Hart & Morris, 1958). The results represent the mean of two temperatures observed for the first formation and final disappearance of crystal on, respectively, slowly cooling and warming solutions with vigorous agitation. The U ions concentration decreases with decreasing temperature in the solution before reaching the eutectic point, where H2O and HNO3 start to crystallize. Thus, U crystallization process should be performed in the right region of the minimum point in this figure. A high HNO3 solution is desirable for achieving a low U concentration in the solution, therefore yielding more UNH crystals because the eutectic point shifts from right to left as the HNO3 concentration increases.

In Pu(NO3)4-HNO3-H2O system, the crystallization behavior of plutonyl nitrate hexahydrate (PuNH) was examined. Figure 3 shows the solubility curves of Pu in HNO3 solution (Yano et al., 2004). The Pu solution was prepared by dissolving PuO2 powder with 4 mol/dm3 HNO3 solution containing 0.05 mol/dm3 AgNO3 electrochemically. In the experiments, the Pu valence was adjusted as following methods. The valence of Pu was changed to Pu(IV) with a few drops of 100% H2O2. On the other hand, the Pu solution was oxidized to Pu(VI) by Ag2+ ion and Ag in the solution was separated by ion exchange. The Pu solution was cooled quickly to −20°C and then cooled at −1 °C/min to −55°C. In the Pu(IV) solution appeared to be a green quasi-liquid (crystals in liquid). In all runs, PuNH was not crystallized in the experimental conditions but crystals of H2O and HNO3·3H2O were observed. In the NEXT, PuNH would not precipitate solely in the U crystallization process.

The influence of Pu valence in the feed solution was examined in the U crystallization process (Yano et al., 2004). When Pu(IV) existed in the feed solution, the yellow crystal was observed. On the other hand, the appearance of the crystal was orange in the feed solution adjusted so that Pu valence was Pu(VI), this color likely resulting from the mixture of the yellow crystal of UNH and the red crystal of PuNH. Plutonium(VI) in the feed solution was co-crystallized with U(VI) in the course of U crystallization. The crystal yields of Pu were smaller than those of U (Ohyama et al., 2005). The fact that the crystal ratio of Pu is smaller than that of U suggests a mechanism of U-Pu co-crystallization in which U begins to crystallize when the saturation point of U is reached by cooling the feed solution, and then Pu is crystallized on the UNH crystal.

Separation of Uranyl Nitrate Hexahydrate Crystal

concentration was 4.0 g/dm3 in the feed solution.

Crystallizer

**3.2 Crystal ratio of uranyl nitrate hexahydrate** 

Fig. 4. Schematic diagram of the batch cooling crystallizer

Stirrer

Thermocouple

Cooling/heating media

Temperature recorder

> P Pump

Heater

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

Thermocouple

Temperature controller

Cooler

min.

Corporation).

from Dissolver Solution of Irradiated Fast Neutron Reactor Fuel 387

experiments. The sheared pieces of core fuel comprising 166 g of heavy metal were dissolved with 325 cm3 of 8 mol/dm3 HNO3 solution at 95°C. The valence of Pu in the dissolver solution was changed to Pu(IV) by NO*x* gas bubbling. In the crystal ratio experiments, the U and Pu concentrations in the feed solution were approximately 450 and 50 g/dm3, respectively. In co-existing element behavior experiment, the HNO3 concentration in the feed solution was 4.5 mol/dm3 in the U crystallization process. The CsNO3 solution was prepared by dissolving CsNO3 (Wako Pure Chemical Industries, Ltd.) powder in 2 mol/dm3 HNO3 solution, and added to the dissolver solution. The Cs

A schematic diagram of the batch cooling crystallizer is shown in Figure 4. The crystallizer made from Pyrex glass was used for cooling the solution volume capacity was 200 cm3, and it had a cooling jacket for cooling and heating media whose temperature was controlled by a thermostat. The feed solution was placed in the crystal vessel and was initially maintained at about 50°C. The feed solution was cooled from 50 to 4°C while being stirred. The spontaneously nucleated and grown crystalline particles were quickly centrifuged from the mother liquor at 3000 rpm for 20 min. After solid-liquid separation, the UNH crystals were washed using 8 mol/dm3 HNO3 solution at 4°C and then centrifuged at 3000 rpm for 20

The acidity of the solution was determined by acid-base titration (COM-2500, Hiranuma Sangyo Co., Ltd.) and the Pu valence in the feed solution was confirmed as Pu(IV) by optical spectrometry (V-570DS, JASCO Corporation) of the ultraviolet (UV)-visible region. The U and Pu concentrations were measured by colorimetry. The concentrations of Np, Am and Cm were measured by α-ray spectrometry (CU017-450-100: detector and NS920-8MCA: pulse height analyzer, ORTEC). The FPs concentrations were analyzed by γ-ray spectrometry (GEN10: detector and 92XMCA: pulse height analyzer, ORTEC) and inductively coupled plasma atomic emission spectrometry (ICP-AES; ICPS-7500, Shimadzu

Fig. 2. Solubility of U in HNO3 solution

Fig. 3. Solubility of Pu in HNO3 solution

#### **3. Batch crystallization with dissolver solution of irradiated fast neutron reactor fuel**

#### **3.1 Experimental procedure**

In the experiments, HNO3 from Wako Pure Chemical Industries, Ltd., was used without further purification. Irradiated core fuel of the fast neutron reactor "JOYO" Mk-III with an averaged burnup 53 GWd/t and cooling time of 2 y was used for the U crystallization

0 100 200 300 400 500 600

U concentration (g/dm3

0 50 100 150 200 250

Pu concentration (g/dm3

**3. Batch crystallization with dissolver solution of irradiated fast neutron** 

In the experiments, HNO3 from Wako Pure Chemical Industries, Ltd., was used without further purification. Irradiated core fuel of the fast neutron reactor "JOYO" Mk-III with an averaged burnup 53 GWd/t and cooling time of 2 y was used for the U crystallization

)

6 mol/dm3 HNO3

8 mol/dm3 HNO3

10 mol/dm3

)

Pu(IV) 4 mol/dm3

Pu(IV) 6 mol/dm3

Pu(VI) 6 mol/dm3

HNO3

HNO3

HNO3

HNO3

0 mol/dm3 HNO3

2 mol/dm3 HNO3

4 mol/dm3 HNO3





Temperature (

Fig. 3. Solubility of Pu in HNO3 solution

**3.1 Experimental procedure** 

**reactor fuel** 

℃)




Temperature (

Fig. 2. Solubility of U in HNO3 solution

℃)

experiments. The sheared pieces of core fuel comprising 166 g of heavy metal were dissolved with 325 cm3 of 8 mol/dm3 HNO3 solution at 95°C. The valence of Pu in the dissolver solution was changed to Pu(IV) by NO*x* gas bubbling. In the crystal ratio experiments, the U and Pu concentrations in the feed solution were approximately 450 and 50 g/dm3, respectively. In co-existing element behavior experiment, the HNO3 concentration in the feed solution was 4.5 mol/dm3 in the U crystallization process. The CsNO3 solution was prepared by dissolving CsNO3 (Wako Pure Chemical Industries, Ltd.) powder in 2 mol/dm3 HNO3 solution, and added to the dissolver solution. The Cs concentration was 4.0 g/dm3 in the feed solution.

A schematic diagram of the batch cooling crystallizer is shown in Figure 4. The crystallizer made from Pyrex glass was used for cooling the solution volume capacity was 200 cm3, and it had a cooling jacket for cooling and heating media whose temperature was controlled by a thermostat. The feed solution was placed in the crystal vessel and was initially maintained at about 50°C. The feed solution was cooled from 50 to 4°C while being stirred. The spontaneously nucleated and grown crystalline particles were quickly centrifuged from the mother liquor at 3000 rpm for 20 min. After solid-liquid separation, the UNH crystals were washed using 8 mol/dm3 HNO3 solution at 4°C and then centrifuged at 3000 rpm for 20 min.

The acidity of the solution was determined by acid-base titration (COM-2500, Hiranuma Sangyo Co., Ltd.) and the Pu valence in the feed solution was confirmed as Pu(IV) by optical spectrometry (V-570DS, JASCO Corporation) of the ultraviolet (UV)-visible region. The U and Pu concentrations were measured by colorimetry. The concentrations of Np, Am and Cm were measured by α-ray spectrometry (CU017-450-100: detector and NS920-8MCA: pulse height analyzer, ORTEC). The FPs concentrations were analyzed by γ-ray spectrometry (GEN10: detector and 92XMCA: pulse height analyzer, ORTEC) and inductively coupled plasma atomic emission spectrometry (ICP-AES; ICPS-7500, Shimadzu Corporation).

Fig. 4. Schematic diagram of the batch cooling crystallizer
