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

6.16
thermomechanical behavior would need to be studied in situ. Heater tests in a variety of
rocks at design depths would probaby be necessary to understand the complex response to local
high temperature of rock that is water saturated, stressed, and fractured.
Some aspects of thermomechanical behavior of rocks can be studied in the laboratory, how-
ever. Since fractured rock is in question, and since characterization of natural fractures
is at present impossible, these laboratory studies would involve large samples of rock con-
taining one or more joints, obtained by special sampling techniques. The samples may have to
be large (dimensions of several meters). This would require extension of present laboratory
testing techniques to test at conditions simulating the in situ environment. The areas where
study would be particularly needed include:
* Thermal cracking and other forms of degradation of rock
* Thermoelastic response of intact and jointed rock over a long time frame
* Changes in permeability caused by heating a rock mass
* Two-phase transport of fluids in fractured rock
* Hydraulic fracturing in thermally stressed rock
* Thermal conductivity of hot, saturated thermally stressed rock
* Stress corrosion due to heated ground water in thermally stressed rock.
Emplacement. Most people engaged in drilling for resource exploitation feel that, to
prevent collapse, the borehole would need to be kept full of drilling mud at all times. This
would include the period during which the canister would be lowered for the waste disposal
concept. Getting the waste canister to drop through the drilling mud could be difficult be-
cause of the close clearance between the casing and canister. The potential accidental con-
tamination of the drilling mud and lowering cable should a waste package be ruptured would
raise numerous questions regarding decontamination techniques and optimum loading methods.
Thus, in addition to a need for substantial research and development on improving the
properties of the drilling mud, techniques and equipment would have to be developed to assure
lowering and releasing the canisters at depths of 10,000 m and for decontaminating the dril-
ling mud and cable in case of canister failure during this operation.
Isolation from the Biosphere. The principal issue of radioactive waste emplacement in
very deep holes is the long-term isolation of the waste from the accessible biosphere (LBL
1979).
In addition to packaging, hole conditions, and hole sealing, a number of other condi-
tions would have to be addressed before long-term isolation from the biosphere could be as-
sured. Several of these involve geotechnical considerations, including:
* An improved understanding of the hydrologic regimes of deep crystalline and sedimentary
rock units, including porosity, permeability, and water presence.