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

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space disposal in an intermediate position below mined repositories because of the low rank-
ing of space disposal on the basis of radiological effects during the operational period.
6.2.5.2 Subseabed
No clear preference emerges between the subseabed disposal concept and the island mined
repository concept. However, because of significant uncertainties regarding the long-term
radiological integrity provided by island geologic and hydrologic systems, subseabed appears
to be superior to the island mined repository concept for continued development as an alter-
native to mined repository waste disposal. An additional advantage may be provided by
subseabed's unique characteristics as a genuine conceptual alternative to mined repositories
in comparison with island disposal, which is basically a variant (with additional uncer-
tainties) of the mined repository concept. Uncertainties remain to be resolved concerning
the long-term integrity of the emplacement media; development of transportation, emplacement
and monitoring technology; resolution of potential international conflicts; and development
of corrective action capabilities. Research will still be required, especially with the
objective of resolving the waste isolation potential of the subseabed sediment. Should this
capability be demonstrated conclusively, engineering development of the system could
proceed.
6.2.5.3 Very Deep Hole
Although not possessing any clearly defined advantages over the mined repository con-
cept on the basis of currently available evidence, the very deep hole concept ranks gener-
ally high on most of the assessment properties. Very deep hole offers potential for a high
degree of geologic barrier performance in the post-operational period and some possibility
of superior working conditions compared to mined repositories. A key issue is the value of
manned in-situ examination of the actual placement location to understand the condition and
environment into which the waste package is to be placed. Significant problems remain how-
ever, including the need for substantial development of drilling technology, improved under-
standing of the geologic environment at very deep hole depths, and analytical verification
of the postoperational integrity of very deep hole repositories and performance of packages
at the requisite temperature and pressure. Since deep hole technology is being developed
for other reasons (e.g., for geopressured methane and for geothermal purposes) it is likely
that increased information will be available regarding these uncertainties. An additional
problem is the difficulty of providing adequate corrective action capability, Thus, the
very deep hole concept, though having potentially superior characteristics to other alterna-
tives, is also characterized by greater uncertainties. For these reasons, although con-
tinued development of the very deep hole cohcept as a long-term alternative to mined
repositories is recommended, the priority of development is considered to be secondary to
the subseabed concept. The considerations of potential problems with corrective action and
the relatively unadvanced status of technology weigh heavily in this decision.