Electrodeposition in Ionic Liquids for Fission Product Recovery

There is more than 57,000 tons of spent fuel being stored at U.S. nuclear power plants today(1).  The spent fuel rods contain a mixture of unreacted fuel and, radioactive and non-radioactive fission products.  This is both an environmental and security concern long-term.  Additionally, the unreacted fuel could be recycled back to the reactor, making it more sustainable.  A goal for long-term handling of the power plant waste is to separate the fission products by half-life and then only sequester the much smaller volume, long-half life products.

To address this challenge, we are examining controlled electrodeposition of the fission products in ionic liquids.  Ionic liquids, salts with melting points below 100°C, can be used as non-flammable, minimally volatile, selective extractants for the separation of fission products from the aqueous system resulting from the acid digestion of spent fuel rods(2).  Ionic liquids are also favorable electrolytes.  They have wide electrochemical potential windows, moderate-to-high conductivities, and can be tuned to control the solubility of the reactants by changing the anion-cation pair(3).  They are also considered recyclable.  The wide electrochemical window is especially appealing for the electrodeposition of metals with redox potentials frequently outside of the water electrochemical window.

Noble metals are present in the fission product waste and have relatively low degrees of radioactivity(4), making them interesting for both separation for the product stream but also for further use.  Additionally, they can be used as model systems for other radioactive fission product streams. 

We have investigated the electrodeposition of noble metals in ionic liquids. Ionic liquid selection is very important for both the electrochemical properties and solubility properties.  Figure 1 shows the solubility of PdCl₂ in four ionic liquids.  It can be seen that the ionic liquids with stronger ligands ([DCA]^-) solvate the palladium salt, while those with weaker ligands ([NTf₂]^-) do not.  Table 1 shows structures of the ionic liquid used.  Palladium (II) chloride electroreduction is easily obtained using ionic liquids.  Figure 2 shows a two-wave reduction of PdCl₂in [Bmim][DCA].

Impact of ionic liquid selection and electrochemical conditions on the electrodeposition efficiency, deposition composition and morphology, as well as the phenomena associated with the electrodeposition will be presented.

References

1.             Fact sheet on licensing Yucca Mountain, U.S. NRC, 2012, http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/fs-yucca-license-review.html. Date accessed 11/26/12.

2.             J. F. Wishart, Energy & Environmental Science, 2, 956 (2009).

3.             H. Ohno, Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Inc., Hoboken, New Jersey (2005).

4.             M. Benedict and T. H. Pigford, Nuclear Chemical Engineering, McGraw-Hill Book Company, Inc., New York (1957).