**1. Introduction**

262 Mass Transfer - Advanced Aspects

Span, R., Wagner, W. (1996). A new equation of state for carbon dioxide covering the fluid

Stumm, W., Morgan, J.J. (1996). *Aquatic chemistry: chemical equilibria and rates in natural* 

Suekane, T., Mizumoto, A., Nobuso, T., Yamazaki, M., Tsushima, S., Hirai, S. (2006).

Tang, Y.P., Himmelblau, D.M. (1965). Effect of solute concentration on the diffusivity of

Total, [online], available: http://www.total.com/en/special-reports/capture-and geological-storage-of-co2/capture-and-geological-storage-of-co2-the-lacq-

Unver, A.A, Himmelblau, D.M. (1964). Diffusion coefficients of CO2, C2H4, C3H6, and C4H8 in water from 6°C to 65°C, *Journal of Chemical and Engineering Data*, 9, 3, 428-431. Weatherill, D., Simmons, C. T., Voss, C. I., and Robinson, N. I. (2004). Testing density-

Wilke, C.R., Chang, P. (1955). Correlation of diffusion coefficients in dilute solutions. *Am.* 

Xu, T.F., Apps, J.A., Pruess, K. (2003). Reactive geochemical transport simulation to study

Yasunishi, A., Yoshida, F. (1979). Solubility of carbon dioxide in aqueous electrolyte

*Journal of Physical and Chemical Reference Data*, 25, 6, 1509-1596.

carbon dioxide in water, *Chemcial Engineering Sciences*, 20, 7-14.

demonstration-200969.html, [accessed: 22.3.2011].

 *waters*, New York: John Wiley and Sons.

ROM).

 *Resources*, 27, 547-562.

*Inst. Chem. Eng. J.*, 1, 264-270.

 *Geophysical Research*, 108, B2, 2071.

solutions, *J. Chem. Eng. Data,* 24, 1, 11-14.

region from the triple-point temperature to 1100 K at pressures up to 800 MPa.

Solubility and residual gas trapping of CO2 in geological storage, *Proceedings of the 8th International Conference on Greenhouse Gas Control Technologies GHGT-8* (CD-

 dependent groundwater models: two-dimensional steady state unstable convection in infinite, finite and inclined porous layers, *Advances in Water* 

mineral trapping for CO2 disposal in deep arenaceous formations", *Journal of* 

Pyrochemical processes appeared today gives an interesting option for future nuclear fuel cycles in several aspects. These latter will have to provide high recovery yields for actinides elements, (taking into account the sustainability requirement) to be safe, resistant versus proliferation risks, and cost-effective. This lead to a rather prolific research today, with many innovative concepts for future reactors, future fuels, and obviously future processes. Pyrochemical processes seems in this context to offer significant-established or presumed-advantages: (i) low radiolytical effects versus solvent processes (which increases the ability to process high burn-up, short-time cooled hot fuels); (ii) ability to dissolve new ceramic or dense fuel compounds; (iii) presumed compactness of technology (low number of transformation steps, small size of unit operations) [Uozumi, 2004; Willit, 2005].

Partitioning and transmutation (P&T) concept is nowadays considered as one of the strategies to reduce the long-term radiotoxicity of the nuclear wastes [Kinoshita et al., 2000]. To achieve this, the efficient recovery and multi-recycling of actinides (An)*,* especially TRU elements*,* in advanced dedicated reactors is essential. Fuels proposed to transmute the actinides into short-lived or even stable radionuclides will contain significant amounts of Pu and minor actinides (Np, Am, Cm), possibly dissolved in inert matrices (U free), and will reach high burn-ups. Pyrochemical separation techniques offer some potential advantages compared to the hydrometallurgical processes to separate actinides from fission products (FP) contained in the irradiated fuel. The high radiation stability of the salt or metallic solvents used, resulting in shorter fuel cooling times stands out.

The aim of the separation techniques which are currently being investigated, both hydrometallurgical and pyrometallurgical ones, is to optimize the recovery efficiency of minor actinides minimizing at the same time the fission products (FP) content in the final product. Special attention is devoted to rare earth elements (REE) mainly due to its neutronic poison effect and the high content into the spend fuel. In addition, REE have similar chemical properties [Bermejo et al., 2006, 2007, 2008a, 2008b; Castrillejo et al., 2005a, 2005b, 2005c, 2009; De Cordoba et al., 2004, 2008; Kuznetsov et al., 2006; Novoselova &

Electrochemistry of Tm(III) and Yb(III) in Molten Salts 265

The lanthanide concentrations were determined by taking samples from the melt which

The potentiometric study was carried out using an Autolab PGSTAT30 potentiostat-galvanostat (Eco-Chimie) with specific GPES electrochemical software (version 4.9). The electrochemical

The electrochemical set-up for potentiometric investigations is shown in Fig. 1. The inert working electrode was prepared using a 5 mm vitreous carbon rod (SU - 2000) which was located in BeO crucible with the investigated melt. It was immersed into the molten bath between 3 - 5 mm. During the experiments Ln3+ ions were electrochemically reduced to Ln2+ ions up to ratio Ln3+/Ln2+ equals one. The counter electrode consisted of a 3 mm vitreous carbon rod (SU - 2000) which was placed in quartz tube with porous membrane in the bottom with solvent melt and located in vitreous carbon crucible (SU - 2000) with pure solvent without lanthanide chlorides*.* The Cl–/Cl2 electrode was used as reference electrode.

**12** 

**11 10**

**13** 

**14**

**15**

/Cl2 reference electrode/counter electrode; 8- Getter of zirconium;

1 – Pt/Pt-Rh thermocouple; 2- Cover of thermocouple; 3- Section; 4- Capsule of chlorine electrode;

9- Current contact; 10- Vitreous carbon working electrode; 11- Quartz test-tube with cover; 12- Beryllium oxide crucible; 13- Vitreous carbon crucible; 14- Investigated salt system; 15- Asbestos diaphragm.

The total lanthanide concentrations were determined by taking samples from the melt which were dissolved in nitric acid solutions and then analysed by ICP-MS. The concentration of the reduced form of lanthanides was determined by volumetric method.

**9**

**8 7**

**5**

**6**

**vacuum Ar**

**1 2**

**3**

**4**

techniques were used such as potentiometry (zero current) and coulometry methods.

were dissolved in nitric acid solutions and then analyzed by ICP-MS.

**Ar Cl2**

**2.3 Direct potentiometric method** 

5- Nickel screen; 6- Alumina tube; 7- Cl-

Fig. 1. Experimental set-up for potentiometric study

Smolenski, 2010; Smolenski et al., 2008a, 2008b, 2009] to those of actinides [Fusselman et al., 1999; Morss, 2008; Osipenko et al., 2010, 2011; Roy et al., 1996; Sakamura et al., 1998; Serp et al., 2004, 2005a, 2005b, 2006; Serrano & Taxil, 1999; Shirai et al., 2000] hence separation between these groups of elements is very difficult. For this reason, a good knowledge of the basic properties of REE in the proposed separation media is very important.

The goal of these investigations is to determine the electrochemical and thermodynamic properties of some fission products (Tm and Yb), their mass transfer, and behavior in different fused solvents using transient electrochemical techniques, and potentiometric method (*emf*).
