ab initio Molecular Dynamics Simulations of Storage Pond Radionuclides and Related ions

Lynes, Olivia and Kerridge, Andrew and Austin, Jonathan (2019) ab initio Molecular Dynamics Simulations of Storage Pond Radionuclides and Related ions. PhD thesis, Lancaster University.

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Abstract

An important problem in the nuclear power industry in the UK is the reprocessing of the legacy waste storage ponds at Sellafield in Cumbria. Understanding the solvation structure of the ions present in these ponds, as well as the stability of their hydroxide complexes, is vital for effective clean-up. This work used ab initio molecular dynamics (AIMD) to characterise the solvation structure of Mg2+, Ca2+, Sr2+, Cs+, U6+ in the form of uranyl (UO22+), La3+ and Lu3+. These ions have been found in the legacy storage ponds and have previously been studied through gas phase or implicit solvation Density Functional Theory (DFT) methods. The properties of the first solvation shell have been categorised, and when compared to current experimental and computational literature the results are in excellent agreement, justifying the solvation model developed. The understanding of the solvation structure of the ions in the storage ponds has been developed further, with the introduction of hydroxide ions to replicate the storage ponds alkaline conditions. The coordination and bonding of the hydroxide complexes was characterised, as was the proton transfer behaviour, through quantifying the Proton Transfer Events (PTEs) of each system. The introduction of hydroxides generally led to reduction in coordination number and bond length of the first solvation shell. It was found that PTEs were more prevalent away from the central ion of the system and occurred more frequently in the less charge dense ionic systems, where direct hydroxide coordination to the ion is less prevalent. The final focus of the work was a DFT examination of the adsorption of Sr2+ onto a solvated CeO2(111) surface. The results showed a preference for some ion coordination to the surface, which lessened when hydroxide ions were introduced to the solvation model. The aim of this chapter was to investigate the validity of the surface-solvation model using a surface relevant to the nuclear waste disposal problem for use in future AIMD simulations of the fuel pond environment.