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

6.186
tion of facilities. The failure of a launch vehicle presents a potential single mode fail-
ure for space disposal and rapid rescue from incorrect earth orbit would likely be required
to prevent public exposure.
Although, based on present evidence, any of the concepts could probably be conducted
with radiation doses no greater than those currently permitted in fuel cycle facilities,
substantial differences in cumulative radiation exposure might exist among the concepts.
The above analysis suggests the following order of decreasing preference among concepts
based on relative radiological effects during the operational period: mined repository;
very deep hole; island mined repository; subseabed; space.
Post-Operational Period
Based on present evidence, any of the five concepts compared here has the potential to
perform satisfactorily in the post-operational period (Section 6.2.3). However, proba-
bilities of satisfactory performance differ and will be used as the basis of this compari-
son. Factors to be considered in evaluating the post-operational radiological integrity
include failure of engineered containment to perform as expected, failure of natural bar-
riers to perform as expected, compromise of repository integrity by catastrophic natural
events exceeding design standards, and compromise of repository integrity by inadvertent
human activity. From the standpoint of all four considerations, space disposal probably
would provide the greatest certainty of satisfactory waste isolation in the post-emplacement
period. In addition, the probability of satisfactory containment for several hundred years
is seen as equally likely for the remaining concepts (see Section 6.2.3) although the per-
formance of the package in the very deep hole is somewhat uncertain. Thus this discussion
will focus on the prospects for longer-term isolation.
The effectiveness of natural barriers is seen to be potentially the greatest for the
very deep hole concept because of the extreme depths involved. This assumes that depth
alone will provide the single most effective barrier; however, uncertainties regarding the
long-term integrity of the hole seal remain to be resolved. The mined repository concept
relies on shaft seals as a barrier also but appears to offer greater probability of satis-
factory long-term integrity due to the ability for human access during sealing operations.
The possibility of disturbing the stability of the host sediment by emplaced waste might
render the performance of the subseabed option less than that of mined geologic. The lack
of understanding regarding behavior of island hydrologic systems under natural or waste-
perturbed conditions raises significant questions as to the performance of the island mined
repository in the long-term. For this reason the island mined repository concept is consid-
ered to be the least acceptable of the concepts on the basis of potential performance of
natural barriers.
Of the four non-space concepts, very deep hole appears on the basis of its remote depth
to offer superior protection from catastrophic natural events. Little distinction on this
basis can be made between the subseabed, and mined repository concepts. Mined repositories
on islands appear susceptible to catastrophic natural events associated with changes in
future ocean levels.