DISPOSAL OF 233 U IN THE WASTE ISOLATION PILOT PLANT

The United States is considering the disposal of excess uranium-233 ( 233 U), which is a weapons-usable, alpha-emitter with many properties similar to those of plutonium (Pu). Seven processing options were identified to convert the 233 U into a form acceptable for disposal into the Waste Isolation Pilot Plant (WIPP). Processing is required to meet two WIPP waste acceptance criteria (WAC): (a) criticality control and (b) safeguards. There are multiple approaches to meet these criteria. Uranium-233 can be converted to non-weapons-usable 233 U by mass dilution with transuranic waste (TRUW) or by isotopically diluting the 233 U with 238 U (<12 wt % 233 U in 238 U). Uranium-233 can be made critically safe by the limiting fissile mass per drum, adding gadolinium, or isotopically diluting 233 U with 238 U (<0.66 wt % in 238 U). The isotopic dilution options would require changes in the current WAC. Isotopic dilution techniques were not considered when the WIPP WAC was originally developed because the expected concentrations of 233 U in waste streams at that time were very low. There are also legal uncertainties. WIPP accepts only defense-related TRUW. Some-but not necessarily all-of the 233 U contains sufficient plutonium such as to be considered TRUW if it were declared waste.