Batch Tests with unirradiated uranium metal fuel program report

Although the general environment of the proposed repository at Yucca Mountain is expected to be oxidizing in nature, the local chemistry within fuel canisters may be otherwise. The combination of low dissolved oxygen and corrosion of metallic fuels, such as Hanford's N-Reactor inventory, may produce reducing conditions. This condition may persist for periods sufficient to affect the corrosion and paragenesis of fuels and their reaction products. Starting in September 2001, unirradiated metallic uranium fuel was examined during batch tests under anoxic conditions. A series of tests carried out under inert atmosphere highlighted the rapid corrosion of the metallic uranium in EJ-13 water at 90 C. During the oxidation of the uranium, uranium dioxide fines spilled from the fuel surface generating copious amounts of colloids. The proportion of uranium-associated colloids accounted for nearly 50% to >99% of the uranium in solution after a brief period where no colloids were detected. The colloids were identified as individual (<10nm) and agglomerated uranium dioxide spheres as large as a few hundred nanometers in size. Silicate and alumino-silicate clays of diverse size and shape were also identified. The bulk size distribution as measured by dynamic light scattering was consistent with the microscopy observations in that the polydispersity indices were large, indicating a wide distribution of colloid particle sizes. The colloids were found to persist for periods beyond the scope of these tests and are at least partly stable. The anoxic experiments suggest that at least two mechanisms are responsible for uranium corrosion. The initial corrosion period is variably long but may last more than one month during which there is no net release of gas. Calculations of oxygen concentration in the vessel at the time of vessel closure show that this period is not consistent with the presence of dissolved oxygen, which would suppress H2 production in undersaturated conditions. After this induction period, the fuel begins to produce H2 gas until the coupon completely disaggregates into fine UO2+x powder