LONG-TERM PERFORMANCE ISSUES OF NUCLEAR-WASTE
PACKAGE MATERIALS
The longevity of manufactured materials in the repository environment over such
long periods of time is subject to significant uncertainty. At the same time,
the prediction of material performance is essential in the development and use
of waste packages (waste forms and waste containers). In the absence of a good
mechanistic understanding of a material’s performance and data that span a wide
range of the expected performance and physicochemical conditions, extremely conservative
assumptions need to be considered. Many of the performance predictions rely on
data collected over a relatively limited range of test conditions; thus, extrapolation
of these data requires good mechanistic understanding.16,26
Without proper data support, any benefits that the waste forms or container might
provide could be ignored; hence, it is highly desirable to improve the predictability
of the materials performance. This also requires demonstration of quality control
of the product.
Various technical issues must be addressed in the assessment of the long-term
performance of the waste package in a geologic repository.27,28
As all components of a waste package may be altered in time within the repository
environment, the environment for a waste package (both internal and external)
must be well characterized. A demonstrated understanding of factors that might
affect long-term service behavior is required for the characterization of materials
for the waste-package components. These factors include variations in characteristics
such as chemical composition, stress state, microstructure, fabrication or production
history, and thermodynamic phase equilibria. Various interactions may be expected
from gaseous or aqueous media that are in contact with the materials of the waste
package. For metallic containers, various forms of corrosion that result from
interactions with water and oxygen are important, as are the effects of hydrogen,
which may result from radiolysis of water and vapor or galvanic coupling with
borehole liner or container support structures. The environment may produce hydrostatic
or lithostatic pressure, which may alter the stress state in waste-package components.
Radiation will change the environment and create species with the potential for
accelerated degradation of the waste-package components. Microbial species, if
they are present in significant quantities, have the potential for interactions
with the waste-package materials.29
The service life of the waste package must be determined based on the consideration
of these interactions between the environment and the waste-package components,
including joints, seals, and welds. For details on the experimental programs specific
to Yucca Mountain, refer to Reference
21.
