Doctoral Dissertations

Date of Award

5-2000

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Joseph R. Peterson

Committee Members

Spiro D. Alexandratos, Samuel D. Gilman, Laurence F. Miller, Richard G. Haire

Abstract

Management and disposal of radioactive materials is a problem which will require serious considerations for nations utilizing nuclear energy. As a result, the handling of such wastes has taken on many different aspects in an effort to minimize its interaction with our planet's ecosystem. It is expected that engineered barriers and protective containers in geologic waste repositories will. Thus, likely fail during the first ten thousand years, information concerning the characterization and understanding of actinide speciation and migration processes in selected solid-matrix materials will be needed to provide important information for the storage, transport, and ultimate disposal considerations of transuranic and high-level wastes.

It was expected that although selected solid-matrix materials may have the ability to hinder radionuclide migration through diffusive and sorptive mechanisms, organics and colloidal materials may also have a significant impact on radionuclide transport. While the study of pseudo-colloid formation between actinide metals and geologic materials has been addressed by the scientific community, the behavior of real colloids has not received the same attention. With regard to real colloids of actinides, only those of plutonium have been given extensive attention in the literature. Given the significance of neptunium in actinide wastes, one goal of this work was to explore the formation of neptunium colloids. To this end, a colloid of neptunium, believed to be the first real neptunium colloid, was prepared and characterized. The environmental consequences of real, colloidal neptunium transport have not been addressed by the scientific community.

A secondary objective of this work was to explore how the speciation of ionic americium(III), neptunium(V), plutonium(III), plutonium(VI), and real colloids of plutonium(IV) affects their potential migration in the environment. The objective was to augment data and improve insight for a field in which the standards for experimental conditions can only be called "loose," at best. The solid matrix materials utilized in this effort were chosen for their interest as possible geologic barriers to actinide migration. They included sand, silicic acid, bioglass, activated charcoal, kaolin, and bentonite.

Colloid instability arises largely due to changes in solution pH or the loss and/or exchange of counter ions. Both can lead to precipitation and are primary concerns during investigations of real colloids. Several trends in actinide migration behavior were identified here. As the valences of ionic actinide species increased, their ability to sorb on solid-matrix materials tends to decrease, likely due to increased stability afforded by oxygenated species (e.g•9 actinyl ions/ MOa*''/ where M is an actinide, and x is the charge based on the actinide's oxidation state). Activated charcoal, kaolin, and bentonite were found to be good agents for retarding ionic actinide migration. Electrostatic interactions dominated kaolin and bentonite sorption of ionic actinides, when solution pHs were above their respective zero point charges (ZPCs). Chemical interactions dominated sorption processes for activated charcoal and sand when solution pHs were below their respective ZPCs. Notwithstanding, these same "barrierti" materials had significantly lower sorption capacities for the real plutonium(IV) colloids tested.

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