Migration behavior of plutonium in compacted bentonite under reducing conditions controlled with potentiostat

The electro-migration method was applied to study the migration behavior of plutonium in compacted bentonite under reducing condition. The Reducing environment was controlled with iron ions supplied by anode corrosion of iron coupon using potentiostat. Ten micro liter of tracer solution containing 1 kBq of ²³⁸Pu was spiked on the interface between an iron coupon and bentonite before assembling. The iron coupon was connected as the working electrode to the potentiostat and was held at a constant supplied potential between - 500 to 0 mV vs. Ag/AgCl reference electrode for up to 7 days. Plutonium migrated from anode toward cathode as far as lmm from the interface, and the penetration length of plutonium grew deeper as the higher potential supplied. This result indicated plutonium migrated as a chemical form with positive charge through bentonite. Concentration profiles were fitted by convection-dispersion equation to obtain two migration parameters, Da, apparent dispersion coefficient, and Va, apparent migration velocity. Apparent migration velocity of plutonium was one-tenth of that of ferrous ion at the potential condition of -500 to +0mV vs. Ag/AgCl. Apparent diffusion coefficient of plutonium was estimated as ~10^-13 m²/s by comparing the values of migration velocity of Pu and Fe. These results indicate that the reducing condition changes the chemical form of plutonium, and accelerate the migration of plutonium in compacted bentonite.