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

B.5
properties. The emplacement medium should be homogeneous and uniform in properties and com-
position, and the medium should extend some distance from the repository so that the
response to the waste will be similar and more predictable. The concept of equilibrium-
release radius can be used to derive the required host rock dimensions as discussed above.
This is a consideration for modeling specific sites in the last stage of site selection.
Several engineered barriers will be built into repository design; however, they will
probably have negligible permanence compared to the lifetime of the repository. The primary
geologic barrier to waste migration will be the repository host rock itself. The effective-
ness of the barrier will depend on the responses of the host rock to long-term effects of
heating and irradiation. Rock response over the full range of expected repository condi-
tions is not adequately understood; however, uncertainties can be overcome by more conserva-
tive design'for waste emplacement.
Preliminary thermal loading analyses indicate that tensile forces will be induced near
the outer margins of the repository (Office of Waste Isolation 1978f). Thus thermal expan-
sion could create potential pathways for waste migration by fracturing or by opening pre-
existing fractures. For salt strata this is not the problem; salt is expected to deform
plastically and heal internal fractures. However, if the surrounding strata were breached
by fracturing, salt could be vulnerable to rapid solution by ground water. Therefore, ther-
mally induced permeability appears to be an important consideration for all host rock media.
Dip, inclination, or attitude of the units in the rock column or section is considered
both from a construction standpoint and as indicators of past geologic stability. Flat or
nearly horizontal units will probably be easier to tunnel through, mine and support if
needed. Steeply dipping or inclined units, in general, indicate past deformation or move-
ment and would likely be avoided if other areas can be found. Any geologic section with
units of different inclinations or dip within the rock column may indicate the presence of
erosion or weathering surfaces that might be selectively weak or permeable. Low and fairly
uniform inclination or dips are probably most desirable.
Joints, fractures and faults are generally not favorable from a geologic site-selection
point of view. They represent zones of weakness, movement, possible conduits for fluids and
regions of anomalous properties compared to the general rock mass. They also increase the
time and cost of investigations and complicate the modeling necessary for design. The pre-
sence of these features does not necessarily exclude a site; joints and fractures may be
closed or sealed by mineral deposition and would not act as conduits and may be barriers to
flow, and some faults can be shown to have had no movement for millions of years. However,
in selecting general site areas risks and benefits of areas exhibiting these features need
to be carefully considered.
A comparative survey of rock properties is included in Table B.2.1. Rock behavior and
strength properties strongly affect design and underground construction. These aspects are
discussed in following sections of this report.