Ab initio modelling of materials relevant to the nuclear fuel cycle

Large amounts of highly active nuclear waste in the UK is currently under temporary storage in Highly Active Storage Tanks (HASTs) at Sellafield. Before it can be safely transported into long-term storage, there are some serious considerations that require thorough investi- gation. Barium-strontium nitrate crystals (BaxSr1-x(NO3)2) can form in the waste sludge and settle at the bottom of the HASTs. These crystals may lead to corrosion hotspots and subsequent nuclear waste leakages. As such, a thorough investigation into the fundamental properties of this material is pertinent. Whilst there have been some experimental studies into this material, theoretical methods may offer a more in-depth, as well as practical, way to study such properties at an atomic level. This thesis therefore aimed to use ab initio com- putational methods to investigate the fundamental properties of the barium-strontium nitrate complex, including defects and thermody- namic properties. Furthermore, it aimed to develop a new force field of interatomic potentials. Ab initio DFT calculations were performed to calculate the structural and elastic properties for (Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2), us- ing different functionals and methodologies. Benchmark calculations were also performed, which demonstrated the accuracy and precision of these methods in comparison to existing experimental data. The mechanical and thermodynamic properties of BaxSr1-x(NO3)2 were also calculated along with point defects and lattice parameters, with good agreement with the literature. Finally, using the previous cal- culations, a force field interatomic potential was created. These studies demonstrate the ability of DFT to produce reliable re- sults that are consistent with experimental data. This body of work therefore provides strong and rigorous foundations for which future work can be conducted to provide vital information relating to the safety of transporting nuclear waste into long-term storage in the UK.