fDENTIFYING SUITABLE
ROCK FORMATIONS
Agencies of the US government have been studying geological disposal since the
1950s. At first, salt looked the most promising and then tuff, shale, basalt and granite
came under consideration. Each has advantages and disadvantages.
Salt, basalt and tuff, found in the western US, can ab-
sorb radioactivity (sorption), seal (heal) their own frac-
tures because they are plastic, do not allow much ground-
water flow because of low permeability, and can conduct
heat away from the waste.
Whereas most salt formations have minimal ground-
water, research indicates that the presence of heat and
radiation might cause groundwater to migrate toward the
waste and dissolve the salt around a repository. Some of
these rock types might not withstand the extreme heat
produced by spent fuel.

fIacture in Wolf River Batholith

GRANITE

Granite is a stable, strong igneous (formed by heat)
rock. It has coarse cystalline grains, and may withstand
more heat than the other rocks (high thermal loading).
High thermal load is advantageous, because spent fuel
has much higher temperatures than reprocessed (mili-
tary) waste.
But granite has disadvantages as well. Little is known
about granite at deep levels. Much of Wisconsin's granite
is covered by a thick glacial overburden of soil and rock
debris, which makes investigation at deep levels rather
difficult.
Fractures may be a major problem, because they
could allow groundwater to flow into the repository, be-
come contaminated with radiation and return to the
human environment. Unfractured granite has very low
permeability to groundwater flow, but some granite is
fractured, at least at shallow levels. Little is known about
fractures and groundwater flow at greater depths.
Another area of uncertainty is the effect of heat and
radiation on granite. Scientists debate the suitability of
granite for waste disposal. More study is needed, but in-
vestigating these issues takes time.
Granite usually fractures during the drilling and
blasting of excavation. DOE believes that improved tech-
nology will minimize fracturing, but some scientists are
not convinced. Again, more research is needed, but re-
cent findings in Canada support these concerns.
Fractures in granite heal very slowly as water depos-
its minerals, which build up to seal the fractures. Deep
fractures might also heal from extreme pressure of over-
lying rock. Fractures in granite do not heal as easily as
those in salt, tuff or basalt.
Puritan Batholith granite outcropping

High-Level Nuclear Waste

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