Salt Disposal of Heat-Generating Nuclear Waste

Cross-hole and same-hole velocity measurements give a consistent picture of the
DRZ around a rectangular drift. The DRZ is well developed at mid-height on the
rib, extending into the rock approximately 2 meters. Near the back and floor, the
DRZ is shallow (1 meter or less) or not detectable. Compressional mode waves
(i.e., P waves) in same-hole measurements are most sensitive to the DRZ because
of the orientation of particle motion perpendicular to the cracking. Changes in V P
as large as 22% were observed for P waves propagating perpendicular to the drift
axis at room mid-height. The physical extent of the disturbed zone can be
determined by propagating elastic waves through successive portions of the
formation until an undisturbed zone is reached, as indicated by a constant velocity
with increasing depth from the rib.
While the laboratory data provide convincing evidence for healing, damage
regions described from geophysical velocity measurements are meters in
dimension. Damage of concern for seal systems in salt involves much larger
scales and uncertainties associated with up-scaling properties. Thus, damage and
healing information are more meaningful when acquired in situ. A series of
sealing experiments in the WIPP underground were executed over a ten-year
period from 1985 to 1995. The tests most pertinent to seal systems involved saltbased
concrete placed in 1-meter holes in the floor and rib of the repository
horizon. The seal system consisted of the seal material, the seal/host rock
interface, the zone of rock immediately surrounding the seal, and the far-field host
rock. The seals were tested for gas and brine permeability from 1985 to 1987 and
again from 1993 to 1995 (Knowles and Howard 1996). Gas permeability
determined in 1985–1987 ranged from 10 -17 to 10 -20 m 2 and improved to 10 -19 to
10 -23 m 2 when retested in 1993–1995. Additional geophysical and petrographic
studies conducted in concert with the flow testing of the concrete seals (Knowles
et al. 1998) provided evidence that the damage existed on both sides of the
concrete plug. The authors concluded that the rigid concrete inclusion provided
the requisite back stress to heal the extant DRZ.
2.4.1.7 Full-Scale Damage Healing in Salt
Salt damage healing has been demonstrated in the laboratory and in 1-meter-scale
field tests. Field evidence of damage healing on a large scale is sparse. German
researchers have published a notable example of healing around a rigid enclosure.
This case study is known as ALOHA (Untersuchungen zur Auflockerungeszone
um Hohlräume im Steinzalzgebirge). These investigations were conducted in the
Asse salt mine near Braunschweig, Germany (Wieczorek and Zimmer 1999). In
situ permeability tests were conducted on the 700-meter level of the Asse salt
mine, where a cast steel bulkhead was placed in a drift during the 1920s. Gas and
liquid injection tests were conducted in open sections of the drift and behind the
steel liner. Permeability magnitudes behind the liner are lower than 10 -18 m 2 , at
least three orders of magnitude less than the equivalent measurements made in the
adjacent open drift. These full-scale permeability measurements are evidence of
progressive DRZ healing around a rigid structure at depth in salt. Evaluation and
modeling are ongoing.
32