Ruthenium: Nuclear's Volatile Problem

Ruthenium is a fission product possessed of two relatively long lived stable isotopes: Ru-103 (half life = 39.8 days) and Ru-106 (half life = 1-year). Both isotopes are present in UK spent fuel and so have had to be accounted for during the reprocessing or disposal of that fuel. At a number of stages during the processing of spent fuel, ruthenium can be exposed to high nitric acid, high temperature conditions that may lead to its transfer into the gas phase as ruthenium tetroxide.

Two such stages are the dissolution of spent fuel into concentrated nitric acid at the start of reprocessing, and the vitrification of ruthenium into a glass waste form after reprocessing has occurred.

Volatilisation is to be avoided as the resultant gas phase ruthenium may then redeposit within metal pipework elsewhere in the plant which will then have to be decontaminated. However, ruthenium volatilisation occurs at unexpectedly low temperatures. Whilst RuO2 is not seen to volatilise below 900oC, gaseous ruthenium oxides have been seen to evolve from solutions of Ru in nitric acid at temperatures as low as 150oC - making the management of ruthenium difficult during reprocessing and vitrification.

Thus, given its volatile nature and high specific radioactivity ruthenium presents a strong challenge to the nuclear industry in effectively managing its abatement. Key challenges are to fully understand the highly complex solution/solid state chemistries that obtain not only under conditions relevant to dissolvers, evaporators and vitrification plants, but also in the decontamination methods used in its clean up.

Using a combination of chemical, analytical and engineering approaches, we shall seek to address these challenges in this PhD. The specific objectives of the PhD will be to:

1) Develop gravimetric, mass spectroscopic, electrochemical and spectroscopic analytical methods that will improve the understanding of ruthenium speciation in high nitric acid environments and oxidation state interconversion during oxidative / thermal treatment of same.

2) Using these methods, to establish the kinetics of interconversion between ruthenium species, most especially Ru(III) to Ru(IV) and Ru(VIII) and Ru(IV) to Ru(VIII), and the resultant product distributions of these processes.

3) To establish the influence that Ru(III) complexation may have on these interconversions and the role that RuO2 may have in supporting or inhibiting volatilisation.

4) Establish the mechanism by which other fission product metal ions such as Ce(IV) may oxidise and thus potentially volatilise ruthenium.

5) Investigate the role that key NOx species such as NO and HNO2 may have on oxidising Ru(III) directly or inhibiting the putative Ce(IV)-driven oxidation of Ru(III).