Management of Commercially Generated Radioactive Waste - U.S. ...

5.3
3. Tectonic stability of the repository area and region. Proper consideration of
this important factor will reduce the likelihood of deformation or disruption of
the host rock and thus increase the probability of repository integrity.
4. Hydrologic regime (i.e., surface-water and ground-water considerations). This is
important because the existence of connected water channels could provide potential
pathways for waste transport away from the repository.
5. Resource potential of the repository site and area. A low resource potential is
desirable to avoid loss of any economic resource by the repository existence and
to reduce the likelihood of future exploration activities for resource recovery.
6. The multibarrier safety feature. This combines the redundant isolation features
provided by the rock properties, the geologic setting, and engineered barriers to
give overall added confidence that the waste will remain isolated.
These six factors are discussed in the following sections.
5.1.1.1 Disposal Media Properties
Four geologic media are examined in this Statement to illustrate a range of rock prop-
erties for a radioactive waste repository: salt deposits (bedded and dome), granite, shale
and basalt. All four rock types possess properties that are favorable for waste isolation.
These, as well as.some unfavorable characteristics, are discussed in the following pages.
For the purpose of this Statement, the physical properties of a disposal medium
describe the characteristics of both the host rock and surrounding rock mass. The disposal
rock material is characterized in terms of its texture, i.e., the size, shape, and arrange-
ment of the component crystal grains. Texture is a consideration in the assessment of a
medium's behavior under stress and heat, and its hydrologic flow potential.
Rock mass structures include the discontinuities of bedding and joints. Bedding
refers to variations in texture because of changes in the sedimentation process by which
the rock was formed. It may be present in both sedimentary and metamorph.c rocks. Joints
are fractures along which little or no displacement of the rocks has occurred. They are
generally formed by extensional release of confining earth pressures. Descriptive features
of these discontinuities include orientation, width, spacing, filling material, waviness,
and extent (length). The potential for the transport of waste material correlates with the
number and extent of host rock discontinuities.
The rock properties of principal interest for waste disposal are those related to
strength, stress-strain, thermal, and hydrologic characteristics. These properties and
characteristics are discussed and presented in tabular form in Appendix B. For comparative
purposes index properties defined as unit weight and natural moisture content are included
in the tabulation.
Substantial strength is desirable for engineering design of subsurface repository
facilities, especially in maintaining tunnel integrity. Strength properties provide the
durability or resistance of a material to processes such as erosion and weathering and