Understanding Ruthenium Volitility in Nuclear Fuel Recycling

Until the closure of reprocessing two years ago, spent nuclear fuel management at the UK’s Sellafield plant involved spent fuel reprocessing by the PUREX (Plutonium Uranium Redox Extraction) process. This produced a Highly Active (HA) aqueous waste predominantly comprised of the fission products and activation products formed in spent fuel, a significant fraction of which are platinum group metals (PGMs) such as palladium, rhodium, and ruthenium. Using evaporators, this HA raffinate is concentrated into storage tanks in the Highly Active Liquor Evaporation and Storage (HALES) facility. From there, it is fed to the Waste Vitrification Plant (WVP) where the resultant Highly Active Liquor (HAL) feed is calcined and combined with glass to produce an immobilised HA waste form.<br/><br/>Amongst the PGMs in the HA raffinate, Ru has two relatively long-lived isotopes: Ru-103 and Ru-106, with half-lives of 39.8 days and one year, respectively. While the volatilisation of fission products in nuclear waste typically occurs at high temperatures, Ru is an exception, as its volatilisation can occur at lower temperatures, including during the early stages of the three-stage vitrification process. These stages are:<br/><br/>The HAL is evaporated to dryness - completed at 150°C.<br/>The solid residues are calcined (inc. denitration to oxides) at 500-600°C.<br/>The calcined oxides are fused into a glass melt, at 1000-1100°C.<br/><br/>Due to its volatility and high specific radioactivity, Ru presents significant challenges in waste management. Indeed, vitrification at Sellafield has been scrutinised following the accidental release of 3.1 GBq of volatile Ru-106 (specific activity = 1.239 x 10¹⁴ Bq/g) from the Waste Vitrification Plant (WVP) in 1997. To prevent future incidents, understanding the complex solution and thermal chemistry of Ru is crucial. It is widely accepted that the volatilisation of Ru occurs due to a redox state change from Ru(III) to Ru(VIII), forming RuO₄. However, the effects of different atmospheric conditions and intermediate chemical steps in this process are not well understood.<br/><br/>One possible route for this volatilisation involves the conversion of soluble Ru(III) in the nitric acid based HA raffinate to particulate RuO₂ during stage one of the drying/calcination/vitrification process described above. This is followed by atmospheric oxygen-driven oxidation of the solid RuO₂ to volatile RuO₄ at the same temperature or slightly higher. The work described here focuses on this latter process of the volatilisation of RuO₂.<br/><br/>The work described here is focused on the oxidation of solid RuO₂ to RuO₄. In this, RuO₂volatilisation has been studied as a function of temperature using ThermoGravimetric Analysis (TGA) employing a range of temperature vs time profiles under both nitrogen and oxygen atmospheres. Using commercially available RuO₂ powders, mass losses due to dehydration are observed during TGA runs up to 1000°C. This has resulted in the development of a 3-cycle pretreatment of the samples during TGA runs so as to dry the material before detailed analysis of volatilisation at a range of hold temperatures. When held at 1000°C in nitrogen atmospheres, the amorphous as-received RuO₂ simply recrystalises into larger RuO₂ crystals with a well-defined rutile structure. However, when held at 1000°C for 1650 min in an oxygen atmosphere, total mass loss is observed, with all the RuO₂ sample being volatilised. At 950°C for 1650 min in an oxygen atmosphere, slow mass loss is observed, with half of the RuO₂ sample being volatilised. These mass loss studies have been carried out at range of temperatures, including, 400°C, 600°C, 760°C, 860°C and 950°C. During these experiments, mass loss, and thus volatilisation, is observed at all temperatures including as low as 400°C. Experimental data shows that rate of mass loss increases with temperature, which allows Arrhenius parameters to be calculated and these results to be reported at the conference.