The primary waste form resulting from nuclear energy production is spent nuclear fuel (SNF). There are a number of different types of fuel, but they are predominantly uranium based, mainly UO 2 or, in some cases, metallic U. The UO 2 in SNF is a redox-sensitive semiconductor consisting of a fine-grained (5-10 μm), polycrystalline aggregate containing fission-product and transuranium elements in concentrations of 4 to 6 atomic percent. The challenge is to predict the long-term behavior of UO 2 under a range of redox conditions. Experimental results and observations from natural systems, such as the Oklo natural reactors, have been used to assess the long-term performance of SNF.