Development of Reprocessing Scheme for Advanced Heavy Water Reactor Spent Fuel Using N,N-Dihexyl

DAE-BRNS SESTEC-2018; Scope Name (see announcement) BITS Pilani, Goa, INDIA, May 23-26, 2018

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Spectrophotometric Studies on Cation−Cation Interactions Between Np(V) and Th(IV) Cations in Nitric Acid Medium

P.K. Verma, P.N. Pathak*, A. Bhattacharyya, D.R. Prabhu, P.K. Mohapatra Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085 *E-mail: [email protected]

The higher-valent actinide cations exhibit fascinating coordination chemistry essentially due to the flexibility in their electronic structures. This is particularly applicable for the dioxo penta- and hexavalent lighter actinides such as U, Np, Pu, and Am. Even though the oxo moiety is assumed to be chemically inert in these cations, + there are several studies which show that pentavalent Np cation (NpO2 ) displays a tendency of bonding with charged metal ions. This class of weak bond formation has been classified as cation-cation interactions (CCI) [1].Initial studies suggested that these types of interactions of Np(V) with polyvalent metal ions are more probable in non complexing perchloric acid solutions.Majority of these studies have been performed in perchlorate media and the stability constant values have been calculated. On the other hand, these studies in nitric acid medium are rather few. Our recent spectrophotometric study has shown that Np(V) cation undergoes disproportionation reaction and the tendency for CCI becomes less favorable at elevated temperature in nitric + 4+ acid media [2]. This study has been extended for understanding possible CCI between Np(V)O 2 and Th ions under varying conditions such as nitric acid (1 - 6 M HNO 3), Th concentration (up to 0.86 M), and temperature (293-343K). An attempt has been made to calculate the stability constant values and compare with those reported in other media. + These studies show the appearance of signature peak of Np(IV) (~964 nm) in addition to NpO 2 (980 + 4+ nm) and NpO2 -Th CCI complex (~994 nm). The intensity of the Np(IV) peak increases with increased temperature while that of Np(V) peak decreases. The CCI interaction is favored at higher Th(IV) concentrations. By contrast, the intensity of 994 nm peak decreases steadily with increasing temperature suggesting the exothermic nature of the complexation process. The thermodynamic parameters for CCI formation have been calculated and compared with the reported values. The variation in the stability constant data with temperature suggests both exothermic (at low acidity) and endothermic (at high acidity) nature of CCI formation. This observation has been explained in terms of the hydration and nitrate complexation of Th(IV) ions under the conditions of these studies. Typical stability constant value for Np(V)-Th(IV) cation-cation complex for 0.86 M Th(IV) in 4 M HNO3 was 4.69±1.17 at 303 K.

0.05 0.03 293 K 0.1 M Th(IV) 303 K 0.2 M Th(IV) 0.04 0.4 M Th(IV) 313 K 0.6 M Th(IV) 323 K 0.02 0.75 M Th(IV) 333 K e e 0.03 343 K

0.86 M Th(IV) c c n n a a b

r r o o 0.02 s s b

0.01 b A A 0.01

0.00 0.00 950 975 1000 1025 1050 850 900 950 1000 1050 Wavelength, nm Wavelength, nm Fig.1: Absorption spectra of 8.5x10-5 M Np(V) in the presence of -5 varying concentrations of Th(IV) in 4 M HNO at 293 K Fig.2: Absorption spectra of 8.5x10 M Np(V) + 0.86 M Th(IV) 3 in 4 M HNO at varying temperatures 3

Key words: Neptunium, Thorium, CCI, Spectrophotometry,Thermodynamics

References 1. J.C. Sullivan, J.C. Hindman, A. J. Zielen, J. Am. Chem. Soc., 83 (1961) 3373. 2.P.K. Verma, P.N. Pathak, A. Bhattacharyya, D.R. Prabhu, P.K. Mohapatra, Dalton Trans., 42 (2013) 14058.