A new repository waste package (WP) concept for spent nuclear fuel (SNF) is
being investigated. The WP uses depleted uranium (DU) to improve performance
and reduce the uncertainties of geological disposal of SNF. The WP would be
loaded with SNF. Void spaces would then be filled with DU (∼0.2 wt %
235U) dioxide (UO2) or DU silicate-glass beads.
Fission products and actinides can not escape the SNF UO2
crystals until the UO2 dissolves or is transformed into other
chemical species. After WP failure, the DU fill material slows dissolution
by three mechanisms: (1) saturation of WP groundwater with DU and
suppression of SNF dissolution, (2) maintenance of chemically reducing
conditions in the WP that minimize SNF solubility by sacrificial oxidation
of DU from the +4 valence state, and (3) evolution of DU to lower-density
hydrated uranium silicates. The fill expansion minimizes water flow in the
degraded WP. The DU also isotopically exchanges with SNF uranium as the SNF
degrades to reduce long-term nuclear-criticality concerns.