Uranyl oxide hydrates are an important and structurally diverse group of uranyl minerals. The y are common alteration products of uraninite, UO(sub 2+x), and are often found in the oxidized portions of U deposits. They occur in soils contaminated by actinides, where they impact the mobility of U. In experiments intended to simulate the alteration of spent nuclear fuel under conditions expected in the proposed geological repository at Yucca Mountain, uranyl oxide hydrates were found as early alteration products of unirradiated U0(sub 2) spent nuclear fuel. Despite the obvious importance of uranyl oxide hydrates, and uranyl minerals in general, few studies have examined the thermochemistry of these compounds. Knowledge of uranyl minerals lags behind most other major mineral groups, owing to the difficulty of studying these phases that often occur as intimate intergrowths of multiple species. Recently, considerable progress has been made concerning the crystal structures, compositions, and synthesis of uranyl oxide hydrates.
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