Salt Disposal of Heat-Generating Nuclear Waste

• Response of the modified system to future human-initiated borehole intrusion,
is improved because there is less moisture available, and lower permeability
around the emplaced waste.
Based on the logic presented above, an undisturbed salt repository containing hot
waste may be even more robust than that certified for non-heat-generating waste.
3.2.3 Important FEPs for a “Hot” Salt Repository
The German approach to repository performance relies on complete isolation
within the salt formation (called the isolation rock zone or IRZ). Complete
isolation is predicated on the long-term integrity of the IRZ and proof of
functional efficacy and robustness of the engineered barrier system. This concept
is entirely consistent with the expectation of salt performance assessment for
undisturbed cases, as discussed above.
As noted by Rothfuchs et al. (2010), the new approach for complete containment
is an important development for repository safety assessments. Previous safety
assessments emphasized release scenarios and compared results to regulatory
performance objectives. The engineered barrier system played an important role
in long-term safety of the repository system, particularly the seals in the shafts
and connecting drifts, which are essential features to avoid intrusion of brine from
overlying, water-bearing strata.
In the current IRZ concept, the shaft and drift seals are assumed to have finite
lifetimes. This differs with the design philosophy for WIPP in which shaft seal
system elements perform forever and are redundant. In the German concept, the
long-term sealing function is taken over by compacting crushed salt backfill. The
crushed salt backfill has essentially no barrier function in the early years.
Permeability reduction is achieved as room closure reconsolidates the granular
salt. Thus, in their concept FEPs that represent degraded function of the shaft and
drift seals, such as FEPs describing the DRZ, become less important with time.
Scenario development for an HLW salt repository is one of the first activities to
be pursued if a salt disposal option is selected for the U.S. Considerable
uncertainty in the most important FEPs still exists, and the types of human
intrusion scenarios to be considered for a HLW repository in salt might depend on
the location of the repository and treatment dictated by regulatory requirements.
Nevertheless, an understanding of the nature of scenario development for
performance assessment clarifies which FEPs are currently deemed important
based on the information summarized in this report.
Section 4 outlines a science-based approach to these data needs that begins with
the current state of the art, identifies possible weaknesses, describes investigations
that could address possible points of contention, and concurs with international
perspectives and recommendations.
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