[en] This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The authors have successfully used neutron scattering techniques to investigate physicochemical properties of elements, compounds, and alloys of the light actinides. The focus of this work is to extend the fundamental research capability and to address questions of practical importance to stockpile integrity and long-term storage of nuclear material. Specific subject areas are developing neutron diffraction techniques for smaller actinide samples; modeling of inelastic scattering data for actinide metal hydrides; characterizing actinide oxide structures; and investigating aging effects in actinides. These studies utilize neutron scattering supported by equilibrium studies, kinetics, and x-ray diffraction. Major accomplishments include (1) development of encapsulation techniques for small actinide samples and neutron diffraction studies of AmD_2.4 and PuO_2.3; (2) refinement of lattice dynamics model to elucidate hydrogen-hydrogen and hydrogen-metal interactions in rare-earth and actinide hydrides; (3) kinetic studies with PuO₂ indicating that the recombination reaction is faster than radiolytic decomposition of adsorbed water but a chemical reaction produces H₂; (4) PVT studies of the reaction between PuO₂ and water demonstrate that PuO_2+x and H₂ form and that PuO₂ is not the thermodynamically stable form of the oxide in air; and (5) model calculations of helium in growth in aged plutonium predicting bubble formation only at grain boundaries at room temperature. The work performed in this project has application to fundamental properties of actinides, aging, and long-term storage of plutonium