Actinides in alkaline media: dissolution, mineral associations, and speciation in hanford waste tank sludge simulants

We have investigated the leaching behavior of actinides from simulants of tank waste sludges derived from the BiPO₄, Redox, and PUREX processes, the principal chemical separations processes that operated at the Hanford site during 40+years of plutonium production. Fundamental investigations of the speciation of uranium and neptunium in solutions representative of proposed alkaline sludge washing liquors have also been completed. Correlation of the results from sludge leaching indicate that, while Am and Pu are generally not appreciably dissolved from the sludges into alkaline solutions in the absence of oxidants, Np and U can be mobilized during alkaline sludge washing. Leaching of sludges with acidic solutions and strong complexing agents indicate considerable association of all actinide ions with Cr, Fe, and Mn oxides in the sludge simulants. Electrochemical experiments conducted in strongly alkaline solutions have defined the formal potentials of Np in strong base and reveal that mononuclear hydrolysis products dominate the speciation of neptunium (and by analogy U and Pu) in concentrated alkali. The results of these observations are discussed in the context of alkaline sludge washing procedures associated with waste tank remediation. This work supported by the Assistant Secretary for Environmental Management under U.S. Department of Energy Contract Numbers W-31-109ENG-38 at Argonne National Laboratory and DE-AC03-76SF0098 at Lawrence Berkeley National Laboratory. (author)