As Fukushima and Chernobyl have demonstrated, the consequences of an accident in a nuclear power plant are international. Extreme conditions, far from thermodynamic control are encountered and changes in the uranium fuel will occur. The storage of spent fuels for a long period of time will also show phase alterations of uranium. These topics need close attention to ensure the highest standard of safety. In this proposal we look at one uranium(VI) compound that is known to control solubility of uranium in the environment and has a structure that will give important information on what happens under extreme pressures encountered in a serious accident. A key objective is to determine the effects of extreme pressure on the solid state structure via neutron diffraction. The key elements of structural control are the water molecules so neutron techniques are the ideal probe for this effect.