Ruthenium Volatilisation from Reprocessed Spent Nuclear Fuel – Studying the Baseline Thermodynamics of Ru(III)

Spent Fuel Management at Sellafield includes the reprocessing of spent nuclear fuel from stations across the UK and also the reprocessing of spent fuel from overseas customers. At Sellafield, methods have been developed for the processing of high level wastes, including highly active liquors (HAL), which is a result of reprocessing irradiated nuclear fuel.

This Highly Active (HA) raffinate / waste stream is concentrated in evaporators and storage tanks in the Highly Active Liquor Evaporation & Storage (HALES) facility before feeding to the Waste Vitrification Plant (WVP). Here, the resultant HAL feed is calcined and combined with glass before pouring into containers to produce an immobilised HA wasteform.

Ruthenium is a fission product possessed of two relatively long lived stable isotopes: Ru-103 (t_1/2 = 39.8 days) and Ru-106 (t_1/2= 1 year). Both isotopes form part of the inventory of HA waste raffinate during reprocessing of spent fuel. Volatilisation of fission products in nuclear waste generally occurs at high temperature – apart from ruthenium where volatilisation occurs at the lower temperature stages of the vitrification process.

Given its volatile nature and high specific radioactivity, ruthenium presents a strong challenge to the nuclear industry in effectively managing its abatement. Part of the challenge is to fully understand the highly complex solution chemistry under conditions relevant to HA waste streams and associated abatement systems.

Experimental work within the National Nuclear Laboratory (NNL), UK has demonstrated that the presence of oxidising metal ions in HA waste (e.g. Ce(IV)) can enhance the volatility of ruthenium through a chemical conversion of Ru(III) species to what is assumed to be RuO₄. A better understanding of these species, their electrochemical processes and reaction kinetics is required to underpin the empirical evidence gathered to date, in particular to develop gravimetric, electrochemical and spectroscopic analytical methods that will improve the understanding of ruthenium speciation in high nitric acid environments, establish the kinetics of inter-conversion between ruthenium species and establish the mechanism by which metal ions such as Ce(IV) may oxidise ruthenium.

We have studied the baseline electrochemical behaviour of ruthenium and present here for the first time the intrinsic thermodynamics of uncomplexed Ru(III) using electrochemical methods to determine as bought RuCl₃ to be a mixture of Ru(III) and Ru(IV). We have achieved this via cyclic voltammetry of RuCl₃ in HClO₄ which yielded two peaks, namely at 0.55 and 0.95 V. A similar peak has been seen by Maya [1] at 0.9 V, reported to be tetramer oxidation. Analysis of the solution using UV-vis shows as bought RuCl₃ to be a mixture of Ru(III) and Ru(IV). Subsequently a successful method to electroreduce the mixture to a pure solution of Ru(III) was developed. Cyclic voltammetry of the electroreduced Ru(III) shows an absence of the previously seen peak at 0.55 V, suggesting this peak was associated to the oxidation of Ru(IV), whilst the previously observed peak at 0.95 V had been shifted to ~ 0.8 V suggesting that this peak was not in fact an observation of tetramer oxidation, as proposed by Maya. The latter peak was further interrogated using a scan rate dependence study, which shows the peak potential increasing as scan rate increases showing we have an irreversible, kinetically slow process occurring. A linear relationship between the square root of the scan rate against peak current indicates the oxidation is occurring in solution. The n number of this transition was experimentally determined to be 0.33 and we have confirmed a diffusion coefficient of   4x10^-10 m² s^-1 for RuCl₃. This shows we have a trimer in solution which is oxidised from: 3 Ru(III) to Ru(III)-Ru(IV)-Ru(III).

Future work will focus on the ruthenium nitrosyl species found in spent fuel to understand what effect NO complexation has on the system with the aim of comparing the uncomplexed ruthenium system with the complexed ruthenium system.

[1] Maya, L., J. Inorg. Nucl. Chem., 41, 1978, 67