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

6.17
* An improved understanding of in situ rock mechanical properties under the high temperature
and pressure conditions expected at the required depths and under unusual thermal
loading conditions. These properties include strength, deformation, stress state, and
permeability.
Additional R&D might be required in the areas of site selecton, site evaluation, and geo-
chemistry (LBL 1979).
Sealing. It is assumed that the sealing system for very deep holes must meet the same
time requirement for sealing penetrations used by mined repositories. The primary purpose of
the seal would be to inhibit water transport of radionuclides from the waste to shallow
ground water or the surface for the specified time period. For integrity to be maintained,
the sealing material would have to meet the following requirements.
* Chemical composition - the material must not deteriorate with time or temperature
* Strength and stress-strain properties - the seal must be canpatible with the surrounding
material, either rock or casing
* Volumetric behavior - volume changes with changes in temperature must be compatible with
the enclosing medium.
The seal system would consist not only of plugs within the casing, but also of material to
bridge the gap between the casing and surrounding rock. To minimize the possibility of a
break in containment, rigorous quality assurance would be required during the placing of
several high-quality seals at strategic locations within the borehole.
Therefore, research and development would be needed in two major areas - materials de-
velopment and emplacement methodology - to ensure permanent isolation. Materials develop-
ment would include investigating plugging materials, including special cements, as well as
compatible casing materials and drilling fluids, which might be incorporated into the sealing
system. Because the seal would include the host rock, these investigations should include
matching plug materials with the possible rock types. It is conceivable that different plug
materials would be required at different points in the same hole.
Emplacement methodology would have to be developed for the particular environment of each
hole. Considerations should include all envisioned operations in the expected environment,
casing and/or drilling, and fluid removal. Because the emplacement methodology would depend
on the type of sealing material, initial studies of sealing material development should pre-
cede emplacement methodology development. However, the two investigations would be closely
related and there should be close interaction between the two phases. In situ tests should
be performed to evaluate plugging materials. Equipment developed should include quality
control and quality assurance instrumention.
Logging/Instrumentation. Proper development and operation of a VDH emplacement system
would require the collection of reproducible, remotely sensed data on the geologic formation
from the bottom of a borehole under high temperature and pressure. Existing logging tools
are generally not designed to operate at temperatures exceeding 175 C (350 F).