The recommended method for storage of spent nuclear fuels is in an underground repository for 100,000 years. Under the long timescales, containment will fail and radioactive elements will migrate through the environment. The chemistry involved must be thoroughly understood to develop robust safety cases. Whilst some progress has been achieved, there is very limited information on structural information on pressure changes; this is important as the depths of the repository suggests that this is an important factor. In this proposal we aim to examine the structural changes on a uranyl carbonate Na3K[UO2(CO3)3]. Carbonate ions are prevalent in the environment and control speciation and solubility, whilst there is an unusual interaction between the uranyl and alkali metal cations. Hydrogen bonding from the water is also important and neutron diffraction at high pressures is a unique probe.