Transport of Weapons-Grade Plutonium and Boron Through Fractured Geologic Media

Redistribution of vitrified weapons-grade plutonium placed in the proposed Yucca Mountain repository is investigated based on the pure-colloid transport model for plutonium and the pure-solute transport model for plutonium, uranium, and boron. In the pure-colloid model, colloids carrying plutonium are assumed to settle out of groundwater in the fractures by floccu-lation. In the pure-solute model, ²³⁹Pu, ²³⁵U, and boron are transported through fractures by advection and diffuse into the rock matrix with sorption retardation. Both models show that ²³⁹Pu stays in the vicinity of the repository and decays there to ²³⁵U. All the ²³⁹Pu that originally exists in the repository reaches the bottom end of 200-m fractures as ²³⁵U. Boron spreads in the geologic medium during the glass leaching period and quickly disappears after the end of the leach time. Concentrations of ²³⁹Pu and ²³⁵U are found to be too small for autocatalytic criticality.