Testing and modelling the behaviour of platinoids during vitrification of high level radioactive waste: Part 4. Effect of spinel

摘要

We reported previously on the behaviour of ruthenium, rhodium, and palladium (platinoids) during vitrification of high level radioactive defense waste. The experiments were conducted in an industrial scale melting furnace 'DuraMelter~(TM)1200' using simulated waste. Mathematical modelling was used to describe settling of platinoids and to calculate furnace lifetimes for various operating conditions. This article deals with the effects of spinel on the behaviour of the platinoids in the cold cap and in the melt. Experiments were conducted in a gradient furnace and in crucibles, and the results compared with respective findings in the DuraMelter~(TM)1200. Under oxidising melting conditions palladium formed metallic particles. Palladium data measured in a gradient furnace can be used to model its behaviour in a large scale furnace. Ruthenium and rhodium participated in spinel formation. Ru-Rh-spinel from the cold cap dissolved upon entering the melt in the DuraMelter~(TM)1200. Ruthenium recrystallised as RuO_2 in the melt. Rhodium partitioned between RuO_2, the melt, and a new spinel, whose liquidus temperature was high enough for the spinel to coexist with the melt at 1150°C. The behaviour of ruthenium and rhodium in the DuraMelter~(TM)1200 could be simulated only partially by gradient furnace and crucible experiments. These results suggest that to model settling of ruthenium and rhodium, data must be gathered in a large furnace. Under mildly reducing conditions (no sulphate reduction) formation of platinoid alloys competed with that of spinel. We discuss uncertainties associated with data acquisition and mathematical modelling of platinoid settling.