Incorporation mechanisms of actinide elements into the structures of U6+ phases formed during the oxidation of spent nuclear fuel

Uranyl oxide hydrate and uranyl silicate phases will form due to the corrosion and alteration of spent nuclear fuel under oxidizing conditions in silica-bearing solution. The actinide elements in the spent fuel may be incorporated into the structures of these secondary U6+ phases during the long-term corrosion of the UO2 in spent fuel. The incorporation of actinide elements into the crystal structures of the alteration products may decrease actinide mobility. The crystal chemistry of the various oxidation states of the actinide elements of environmental concern is examined to identify possible incorporation mechanisms. The substitutions Pu6+ <-> U6+ and (Pu5+, Np5+) <-> U6+ should readily occur in many U6+ structures, although structural modification may be required to satisfy local bond-valence requirements. Crystal-chemical characteristics of the U6+ phases indicate that An(4+) (An: actinide) <-> U6+ substitution is likely to occur in the sheets of uranyl polyhedra that occur in the structures of the minerals schoepite, [(UO2)(8)O-2(OH)(12)](H2O)(12), ianthinite, [U-2(4+) (UO2)(4)O-6(OH)(4)(H2O)(4)](H2O)(5), becquerelite, Ca[(UO2)(3)O-2(OH)(3)](2)(H2O)(8), compreignacite, K-2[(UO2)(3)O-2(OH)(3)](2)(H2O)(8), alpha-uranophane, Ca[(UO2)(SiO3OH)](2)(H2O)(5), and boltwoodite, K(H3O)[(UO2)(SiO4)], all of which are likely to form due to the oxidation and alteration of the UO2 in spent fuel. The incorporation of An(3+) into the sheets of the structures of alpha-uranophane and boltwoodite, as well as interlayer sites of various uranyl phases, may occur.