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

6.14
Sealing. Standard oil field practices for cementing in casing have satisfactorily iso-
lated deep high-pressure gas zones from shallower formations and from the surface for time
periods measured in decades. Plugs of cement or other materials have been emplaced in aban-
doned oil and gas wells, both cased and uncased, and have maintained integrity over similar
periods of time. In these instances, it is not uncommon for the casing to corrode prior to
plug deterioration.
Logging/Instrumentation. Borehole geophysical logging techniques in existence and cur-
rently under development will permit the logging and analyses of a number of parameters
critical to the emplacement of radioactive waste in very deep holes. Caliper, acoustic,
televiewer, and other borehole geophysical devices are regularly used to verify the presence
and distribution of fractures in well bores. Electrical logs, neutron porosity loss recor-
ders, and other devices are used to verify the presence of water. Temperature logs and spin-
ner logs are used to detect water flow. While all of this equipment can be used from depths
of hundreds to thousands of feet, none of these tools can function at the temperatures
[between 200 and 300 C (390 and 570 F)] and pressures anticipated at depths around 10,000 m,
because of the electronics contained in the probe.
While rudimentary development of in situ stress measurements has been accomplished, the
down-hole techniques would require significant improvement.
Issues and R&D. Requirements
Depth of Hole. The hole depth required for adequate isolation from the biosphere would
have to be determined by the geologic medium of interest and by the history and physical con-
dition of that medium. Sedimentary rocks in some instances are considered as potential VDH
locations, but only where they are considered to be lower in elevation than circulating
groundwater, such as deep basins or hydrologically stable synclines. Crystalline rocks may
be the best geologic medium for VDH disposal. Usable hole depth in crystalline rock would be
influenced by the depth of ground-water circulation within that rock. Ground-water circula-
tion in weathered granite near the surface in a humid environment will generally be signifi-
cantly greater than in fresh granite in an arid to semiarid environment.
R&D is required to determine the depth required in various geologic media to minimize the
possiblity of significant circulation to ground-water systems. The top of the emplaced waste
would still have to be significantly below possible contact with circulating ground water,
and would have to be properly plugged and sealed against such contact.
Drilling. The discussion of the present state of development of drilling makes it clear
that emplacement of nuclear waste in very deep holes is not possible at this time given that
(1) the waste canisters will be 31 to 36 cm (12 to 14 in.) in diameter and (2) the depth re-
quired for isolation from the biosphere may be as great as 10,000 m. If it is assumed that
these two criteria are valid for the conceptual system, then a number of problems related to
drilling would have to be solved to attain emplacement in very deep holes. The key issue is
whether it will be possible to develop the technology to drill to 10,000 m with a bottom hole