Materials for High-Level Waste Canister/Overpacks in Salt Formations

Abstract

Studies on the corrosion and mechanical behavior of TiCode-12 and other titanium alloys, for use as candidate canister or overpack barriers in a high-level waste repository or test facility in salt, are reported. The corrosion behavior of TiCode-12 was evaluated as a function of: brine composition, temperature, time, pH, oxygen concentration, and gamma radiolysis. Uniform corrosion rates are in the range of 0.1 to 10 ..mu..m/yr; pitting or crevice corrosion has not yet been observed. The highly adherent, passivating titanium oxide film that provides the corrosion protection is being evaluated via electrochemical polarization and surface analytical techniques to enable modeling of the corrosion mechanism(s). An increase in the corrosion rate by a factor of about 2 was observed for sensitized TiCode-12; changes in the alloy microstructure are being analyzed in order to model this phenomenon. Alterations in the chemistry and processing procedure of TiCode-12 are being evaluated to optimize corrosion, mecahnical, and mill-producibility properties for high-level waste package applications. Slow strain rate testing of TiCode-12 revealed no apparent susceptibility to stress corrosion cracking; no significant changes in tensile properties were observed, but alterations in fracture mode were determined to be caused by internal hydrogen content. Hydrogen effects on titanium alloymore » mechanical properties and crack susceptibility are being studied. Some hydrogen embrittlement occurs at hydrogen concentrations in the range of 200 to 300 ppM by weight, but the strength of TiCode-12 is not affected at concentrations up to 1100 wppM. A TiCode-12 HLW canister-package is proposed that could provide long-term containment integrity and significantly minimize total HLW isolation system costs when compared to other waste package design concepts.« less