Crystal Chemistry of Early Actinides (Thorium, Uranium, and Neptunium) and Uranium Mesoporous Materials

Despite their considerable global importance, the structural chemistry of actinides remains understudied. Thorium and uranium fuel cycles are used in commercial nuclear reactors in India (thorium) and around the world including the United States of America. Neptunium, the first man-made element, is also present in used nuclear fuel because it forms in the reactor via neutron-capture reactions.

Thorium cations are usually coordinated by eight to twelve oxygen atoms, whereas uranium (VI) structures contain the uranyl ion and an additional four to six oxygen atoms. Neptunium (V) and (VI) also form actinyl ions and bond to four to six additional oxygen atoms. Phases composed of early actinide elements are important for long term storage of commercial spent nuclear fuel in a geological repository and the potential release of radionuclides into the environment. The crystallography of the early actinides (thorium, uranium, and neptunium) remains largely unexplored. The research reported herein concerns the crystal chemistry of the early actinide elements as well as uranium mesoporous materials.

Room temperature and hydrothermal synthesis techniques were used to produce single crystals of thorium, uranium, and neptunium. These crystals were then analyzed using single crystal X-ray diffraction and infrared spectroscopy. Thorium nitrate and thorium chromate compounds were also investigated. Systems explored include the uranyl peroxides, neptunyl oxyhydrates, and neptunyl phosphates.

This work reports initial experiments intended to produce uranium-based mesoporous materials. Mesoporous materials form framework structures supported by large surfactant molecules and when the organic material is removed a porous framework remains. Uranium mesoporous materials may provide a waste form or nuclear fuel material that may selectively store other radionuclides.