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Extraction of Using Dioxide and Dioxide for Spent Fuel Reprocessing

Kayo Sawada, Daisuke Hirabayashi and Youichi Enokida

EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan [email protected]

Abstract – For reprocessing of spent nuclear fuels, a new method to extract from spent fuel using highly compressed gases, and was proposed. Uranium extraction from broken pieces, whose average grain size was 5 mm, of pellet with nitrogen dioxide and carbon dioxide was demonstrated in the present study.

INTRODUCTION capillary tubes [1]. In this study, prior to the nitrate conversion, uranium dioxide was oxidized In cycle, reprocessing of spent in a reactor with nitrogen dioxide. It is known nuclear fuels is one of the key processes. The that is formed from uranium conventional method for the reprocessing, dioxide in nitrogen dioxide-air mixture at 523 K PUREX process, is well established one, but [2]. After the oxidation, liquefied nitrogen further development for a new process of dioxide was fed in the reactor to convert uranium reducing cost and minimizing waste is required into uranium nitrate. for the next generation reprocessing. In order to decrease the waste from the reprocessing, we are The nitrate formed in the reactor is extracted developing a new method to extract actinides with supercritical carbon dioxide. Carbon from spent fuel using a liquefied gas and a dioxide has relatively low critical point: its supercritical fluid as media. The method critical temperature and pressure are 304 K and consists of two processes, the conversion of 7.38 MPa, respectively. Supercritical carbon into their nitrates with liquefied dioxide is known to be useful for the extraction nitrogen dioxide, and the extraction of the nitrate of actinides from [3-5]. Tri- with supercritical carbon dioxide. By using n-butylphosphate (TBP) and the uranium nitrate liquefied gas or supercritical fluid as media, a complex with TBP are soluble in supercritical drastic reduction will be achieved in the waste carbon dioxide [6,7]. generation from nitric solution and solvent like n-dodecane. Liquefied nitrogen dioxide and supercritical carbon dioxide is able to be separated from the In this study, we demonstrated the uranium generated nitrates or complexes by decreasing extraction from broken pieces, whose average pressure after the conversion or the extraction. grain size was 5 mm, of uranium dioxide pellet with nitrogen dioxide and carbon dioxide. UO2

O2, NO2

Oxidation NEW METHOD TO EXTRACT Surplus O , NO ACTINIDES FROM SPENT FUEL USING A 2 2

LIQUEFIED GAS AND A Liquid NO2 SUPERCRITICAL FLUID AS MEDIA Nitrate conversion

The scheme of the proposed method is shown in Surplus NO2 Fig. 1. Nitrogen dioxide is easy to liquefy or Supercritical CO , TBP gasify by a small change of temperature at 2 atmospheric pressure since its is Extraction 294 K. Liquefied nitrogen dioxide has a higher concentration than its aqueous solution or gaseous one. In 1950’s, a team of famous UO2(NO3)2-TBP with CO2 scientists was examined the nitrate conversion of uranium trioxides with nitrogen dioxide in sealed Fig. 1. Scheme of the proposed method.

ATALANTE 2008 Montpellier (France) May 19-22, 2008 1 P3_20

EXPERIMENTAL Table 1. Experimental conditions. Experiment Parameters Value Uranium Dioxide Oxidation Temperature 523 K 3 Reactant: O2 198 cm Pieces of uranium dioxide were prepared by 3 NO2 0.5 cm * breaking a pellet of uranium dioxide. The pieces Reaction time 1 h employed in this study are shown in Fig. 2. The Nitrate Conversion Temperature 363 K average grain size was 5 mm and total weight Reactant: NO 40 cm3* was 1.2 g. 2 Reaction time 1 h

Extraction Temperature 313 K Pressure 15 MPa Extraction reagent: TBP 3 cm3 *volume at 3 MPa, 283 K

The experimental apparatus employed in the extraction was shown in Fig. 4. To extract the nitrate, 3.0 cm3 of TBP was fed to the autoclave Fig. 2. Uranium dioxide employed in this study. with supercritical carbon dioxide at 313 K at 15 MPa. The experimental condition was determined by the of TBP complex of Nitrate Conversion and Extraction Procedure uranium nitrate to carbon dioxide reported in our previous study [8]. Keeping the pressure at 15 Figure 3 shows the experimental apparatus for MPa by a back-pressure regulator, the nitrate the nitrate conversion. The uranium dioxide complexed wit TBP was rinsed off with a large pieces was stirred with and nitrogen amount of supercritical carbon dioxide, and dioxide in an autoclave (TAS-099, Taiatsu collected in collection vessels. The collected Techno Co., Japan), whose volume was 99 cm3, sample was back-extracted with to at 523 K for 1 hr. We discharged gases in the measure metal content by ICP-AES (ICPS-7000, autoclave using a vacuum pump, waited that the Shimadzu, Japan). autoclave cooled down to 313 K. Liquefied nitrogen dioxide was fed to the autoclave, mixed 5 with the uranium oxide to convert the nitrate at 4 363 K for 1 hr. The experimental conditions are summarized in Table 1. 3 9

6 P 5 6 2 8 11 10 1 7

23 1: CO2 cylinder, 2: syringe pump, 3: TBP vessel, 4: pump, 7 5: check valve , 6: autoclave, 7: stirrer, 8：heater, P 4 10 9: back-pressure regulator, 10: collection vessel 9 1 8 Fig. 4. Experimental apparatus for the extraction.

1: NO2 cylinder, 2: Ar cylinder, 3: O2 cylinder, 4: syringe pump, 5: mass flow controller, 6: check valve , 7: autoclave, RESULTS AND DISCUSSION 8: stirrer, 9：heater, 10: scrubbing bottles, 11: vacuum pump The uranium was extracted with TBP and carbon Fig. 3. Experimental apparatus for the nitrate dioxide after the nitrate conversion of uranium conversion. dioxide with nitrogen dioxide, and its amount was 33% of the loaded uranium dioxide.

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In the autoclave, there were small solid particle residues. The residue was dark blown colored. The XRD analysis shows that there was uranium dioxide in the residue. It was considered that the oxidation of uranium dioxide was not completed for 1 h. Further studies for the optimization of the experimental condition are necessary for the effective extraction.

CONCLUSION

Uranium was extracted with TBP and supercritical carbon dioxide after the nitrate conversion of uranium dioxide with liquid nitrogen dioxide.

ACKNOLIDGMENT

Present study is the result of “Development of a method to extract actinides from spent nuclear fuels using liquefied gas as a medium” entrusted to “Nagoya University” by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

REFERENCES

1. G. Gibson, J. Katz, J. Amer. Chem. Soc., 73, 5436 (1951). 2. R. McEachern et al., J. Nucl. Mater., 225, 234 (1998). 3. C. M. Wai., S.Wang, J. Chromatography A, 785, 363 (1997). 4. Y. Enokida, Proc. Super Green 2002, p. 35. Suwon, Korea, 3-6 Nov (2002). 5. Y. Enokida, I. Yamamoto, C. M. Wai, ACS Symposium Seriese, 860, 10 (2003). 6. Y. Lin, C. M. Wai, Anal. Chem., 66, 1971 (1994). 7. O. Tomioka, Y. Enokida, I. Yamamoto, J. Nucl. Sci. Technol., 35, 515 (1998). 8. K. Sawada, D. Hirabayashi, Y. Enokida, Proc. ISETS07, p. 1047, Nagoya, Japan, 23- 25 Nov (2007).

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