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

6.106
capabilities, primarily performed remotely. A heavy concrete and steel confinement building
would provide containment for the well and injection operations and shielding for the radio-
active systems.
Sealing Systems. The well hole would probably be sealed by a combination of borehole
seals and backfilling, using a procedure similar to the one discussed for the very deep hole
concept (Section 6.1.1).
Retrievability/Recovery. Liquid waste that had been injected might be partially retrie-
vable by conventional well techniques. Although much of the waste might be physically or
chemically sorbed by host geologic media, some species, in particular, 137 Cs, would be ex-
pected to remain in at least partially retrievable solution.
Shale Grout Injection
In the shale grout injection process, neutralized liquid waste or an irradiated fuel
slurry would be mixed with a solids blend of cement, clay, and other additives, and the re-
sulting grout would be injected into impermeable shale formations. The initial fracture in
the shale would be generated by hydrofracturing with a small volume of water. The injection
of waste grout into this initial fracture would generate sufficient pressure to propa-
gate a thin horizontal crack in the shale. As injection of the grout continued, the crack
would extend further to form a thin, approximately horizontal, grout sheet, several hundred
feet across. A few hours after injection, the grout would set, thereby fixing the radio-
active wastes in the shale formation. Subsequent injection would form sheets parallel to and
a few feet above the first sheet.
The principal requirement for shale grout injection is that the hydrofracture, and hence
the grout sheet, develops and propagates horizontally. Vertical or inclined hydrofractures
could result in the waste gaining access to geologic strata near the surface, and even break-
ing out of grout at the bedrock surface itself. Theoretical analyses indicate that, in a
homogeneous isotropic medium, the plane of hydrofracture develops perpendicularly to the
minor principal stress (NAS 1966). Thus, a requirement for horizontal hydrofracturing is
that the horizontal stresses exceed the vertical stresses.
On the basis of work at ORNL, approximately 40 injection wells would be required at each
of five facilities. The activity level for the shale grout injection alternative is based on
the reference concept (Schneider and Platt 1974) of 40 Ci/l activity in the initial grout.
The acceptable gross thermal loading (GTL) could be assured by controlling the number of
grout injections in the disposal formation. Depending on the fuel cycle, the maximum number
of 2-mm (0.08-in.)-thick grout layers would be five to seven per injection site.
Site. A thick sequence of essentially flat-lying shale strata would be required for
shale grout disposal, with in situ stress conditions favorable for the propagation of hori-
zontal hydrofractures. Such conditions are generally found to a maximum depth of 500 to
1,000 m (1,650 to 3,300 ft). As with deep well liquid injection, the site would have to be
located to preclude conflicts with resource development.