Salt Disposal of Heat-Generating Nuclear Waste

pressure > 1 atm). A range from 100°C to 110°C is an important transition that is
compositionally dependent for reasons alluded to above.
Geochemistry of the near field during the thermal period, because of its strong
dependence on the thermal-mechanical performance of the repository and the
availability of brine, has bearing on decisions that are made during repository
design and on repository performance. For example, given the information
presented in Section 2.4, it is anticipated that salt will deform rapidly to enclose
the waste, the heat will dry out the salt by boiling or general heating (i.e.,
accessible brine will be driven away from the thermal source, leaving the salt
around the waste drier), and contact between brine and the waste packages will
thereby be diminished during the thermal period. If this is, indeed, the case,
geochemistry during the thermal period would have little impact on repository
performance except in the case that a postulated disruptive event breaches the
repository. Further, it is acknowledged that uncertainty in the estimates of the
geochemistry of the near-field environment during the thermal period would
contribute to the uncertainty of the overall performance of a HLW repository in
salt.
The scientific basis for, and likelihood of, thermally enhanced encapsulation
could play a role not only for performance assessment, but also for selection of
the ultimate disposal concept. For example, if the salt rapidly encapsulates the
waste and keeps the waste dry during the thermal period, it may be possible that
certain waste forms could be disposed without a waste package (i.e., borosilicate
glass logs or calcined waste logs). The focus of scientific research in this case
would demonstrate that in the absence of brine, these waste forms would not
degrade at high temperatures, and even if the waste form did degrade, the
resulting degradation products would not be mobile.
Further, if the salt rapidly encapsulates the waste and keeps the waste dry during
the thermal period, reducing chemical conditions in the near field of a salt
repository may not prevail (however, if waste environment is dry there is no
liquid to solubilize the waste). Generally, reducing chemical conditions that are
expected in the near field reduce the solubility of actinide elements and possibly
other elements, a benefit for repository performance. This is one of the features
that makes the WIPP a robust repository for TRU waste. However, bedded and
domal salt formations by themselves do not generally provide reducing
environments, but they do provide a highly impermeable system that allows the
reaction of materials to deplete any oxidants introduced into the system during
operations.
A HLW repository in salt would be expected to have reducing near-field
conditions during the thermal period only if reducing materials such as steel are
sufficiently reacted with brine. Reducing near-field conditions would not be
expected if corrosion-resistant alloys such as Alloy 22 or TiCode 12 were used for
the waste packages (or if no waste packages were used) because reaction would
be much less with such materials. An additional mechanism for producing
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