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

6.180
factory performance of the island concept, while possible, is less certain because of an
incomplete understanding of island hydrologic systems.
Maintenance of isolation for the requisite period under ice sheet conditions appears
to be sufficiently questionable as to preclude this option from further consideration on the
basis of this standard of judgement. If not tethered, the packages would descend to the
ice-rock interface where the waste form packages could be pulverized by ice motion, and
waste subsequently transported to the ocean by water potentially present at the interface.
If tethered, ice sheet erosion or sublimation (possible within a 10,000-year period given
historical climatic fluctuation) could expose waste to the surface environment.
The waste-rock matrix of the rock melt concept would potentially be exposed to severe
hydrothermal alteration and leaching conditions late in the cooling phase when hot water may
be present at the periphery of the rock-waste mass. This could result in transfer of waste
to ground water. However if the surrounding geologic and hydrologic conditions were suit-
able, migration of waste to the accessible environment might be limited to acceptably low
levels. On the other hand, thermomechanical disruption of the surrounding geology by the
rock melt process might allow rapid transfer of contaminated ground water to surface aqui-
fers, especially if promoted by thermal gradients from decay heat. While there is currently
insufficient evidence to eliminate rock melt from further consideration on the basis of this
standard of judgement, satisfactory performance appears highly uncertain. Furthermore a
method for resolving this uncertainty does not appear to be available.
The host rock is the primary isolation mechanism for the shale-grout version of well
injection. Assuming a suitably stable formation of adequate sorptive potential, preliminary
calculations (Section 6.1.6) indicate that the likelihood of unacceptable quantities of
radionuclides reaching accessible ground water is small. For dilute acid injection, assum-
ing the site has suitable bounding formations, it also appears that there would be a low
probability of unacceptable quantities of radioisotopes reaching accessible aquifers. How-
ever, prediction of acceptable long-term performance of well injection will require thorough
characterization and understanding of the host formations and surrounding geology. It is
highly uncertain at this time how this could be accomplished.
The transmutation concept may not require repositories providing 10,000-year isolation
if all long-lived isotopes are eliminated. However, the 10,000-year isolation standard is
not applicable to the transmutation process per se.
The space disposal concept appears to have most merit with respect to isolation. It
has been calculated that a stable orbit would provide a minimum of 1 million years
isolation.
In conclusion, it appears that all concepts with the exception of ice sheet, rock melt,
and well injection have the potential of meeting the 10,000-year standard for acceptable
waste isolation.