Salt Disposal of Heat-Generating Nuclear Waste

Additional analogue observations regarding bulkheads in Canada and other case
studies are reported by Eyermann, Van Sambeek, and Hansen (1995). An
experience at the Rocanville mine involved drift seal construction of brine-based
concrete, analogous to possible panel closure construction for a salt repository
room or panel. The Rocanville bulkhead was constructed in halite, following
water inflow from an overlying aquifer. A concrete bulkhead was designed to
contain the inflow and prevent additional inflow into the mine. A “temporary”
bulkhead located 30 meters from the face (i.e., the point of inflow) was
constructed and has functioned perfectly since its emplacement in 1985. Total
hydrostatic head is monitored by various accesses through the bulkhead. Design
and construction of the bulkhead took only a few weeks, and the conditions for
the construction were less than ideal. Despite these factors, the bulkhead has
resisted water pressures approaching 10 MPa since construction. This does not
unequivocally demonstrate that a DRZ developed and then healed, but it does
show that a full-scale, salt-based concrete bulkhead in salt can be constructed and
remain impermeable to a full hydrostatic head.
The most important issues summarized in this section are the descriptions and
documentation of salt damage properties, updates to technical knowledge
pertaining to the DRZ, and the possible direct application of this information to
salt repositories for HLW. Predictions of the one-way evolution of the DRZ
replicate underground observations. The size and shape of the DRZ around an
opening based on a stress-invariant criterion are similar to the size and shape
derived from sonic velocity studies and from microscopy of core damage. An
example calculation is shown in Figure 4, which is a two-dimensional finiteelement
model of a room with waste placed inside. The regions around the room
depict the extent of the DRZ. As the room closes upon the waste inside, stress
differences are reduced, and the damaged salt heals. In this particular isothermal
calculation, room closure is nearly complete in 60 years. These calculations of the
evolution (i.e., growth and healing) of the DRZ replicate observations made in the
WIPP underground. The predicted size and shape of the DRZ around an opening
based on a stress-invariant criterion (Figure 4) are similar to the size and shape
derived from sonic velocity studies and from microscopy of core damage (Hansen
2003). The model output does not provide permeability directly, but the trend and
magnitude of permeability around salt openings are understood in considerable
detail. For salt repository sealing applications, the high-permeability zone
surrounding a drift or shaft is a potentially important design issue. For
performance assessment and supporting simulations, over-estimation of the extent
of the DRZ could produce significant artifacts if modeled unrealistically.
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