Gamma-Radiolysis Kinetics and Its Role in the Overall Dynamics of Materials Degradation

Ryan P. Morco, Department of Chemistry, The University of Western Ontario

Abstract

The γ-radiation emitted during radioactive decay of the wastes in a nuclear fuel container can affect container corrosion. When a metal/water system is exposed to γ-radiation both the solid metal and the liquid water (or humid air) absorb energy. This energy dissipates mainly as heat in the metal but induces decomposition of water molecules to yield a range of chemically reactive species. The different chemical environments induced in the metal and liquid phases can change the driving forces for surface reactions and thereby influence the rate and pathway of metal corrosion.

This thesis presents the development of radiolysis kinetic models that predict radiolytic oxidant concentrations relevant to used fuel container (UFC) corrosion in the anticipated deep geologic repository (DGR) environments. The different DGR environments were addressed by constructing three different radiolysis kinetic models: (1) water radiolysis model (WRM), (2) humid air radiolysis model (HARM), and (3) groundwater radiolysis model (GWRM).

The HARM predicts that HNO₃ will be the dominant oxidizing species formed during humid air radiolysis with its concentration increasing linearly with dose rate. The extent of corrosion damage caused by HNO₃ on a copper surface was conservatively estimated from the determined transfer rate of HNO₃ in the gas phase to droplets in contact with the container. A simplified rate formula for the overall radiolytic production of HNO₃ in humid air was proposed.

The radiolysis kinetics of deaerated and aerated water at temperatures ranging from 25 to 80 ^oC was studied using the WRM. The model predicts that the key oxidants formed by radiolysis will be H₂O₂ and O₂. In saline groundwater, GWRM predicted the formation of an intermediate, HOCl. The model calculations were verified using experimental data performed with pure water and chloride solutions.

Radiolytic corrosion of Cu in saline solution was also investigated by performing experiments with γ-irradiation and with chemically-added radiolytic oxidants to simulate the effects of radiation. The results demonstrate that the continuous radiolysis production of reactive species at low levels has different effects on corrosion kinetics from the one-time addition of these species at concentrations equivalent to their overall radiolysis yields over long times.