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

6.1.6.1 Concept Summary
6.100
6.1.6 Well Injection
Well injection technology was initially developed by the oil industry for the disposal of
oil field brines. These brines were usually pumped back into the original reservoir and, in
some cases, used to "drive" the oil toward a producing well. The well injection concept has
subsequently been used for the disposal of various natural and industrial wastes. The tech-
niques developed in the oil industry handle liquid wastes only - particulate matter can cause
blocking of the pores in rock.
A well injection process using grout was developed by Oak Ridge National Laboratory
(ORNL) for the injection of remotely handled TRU liquid radioactive wastes into shale strata
(ERDA 1977). This technique is also suitable for grout slurry wastes, and a new facility is
now under construction at ORNL for liquid and slurry waste injection (ERDA 1977). Well
injection could be a low cost alternative to deploy and operate because of the widespread use
of the required techniques and the "off-the-shelf" availability of the main components. Two
reference methods of well injection are considered in this section: deep well liquid injec-
tion and shale grout injection.
Deep well injection would involve pumping acidic liquid waste to depths of 1,000 to 5,000
m (3,300 to 16,000 ft) into porous or fractured strata suitably isolated from the biosphere
by overlying strata that are relatively impermeable. The waste may remain in liquid form and
might progressively disperse and diffuse throughout the host rock. This mobility within the
porous host media formation might be of concern regarding release to the biosphere. Ques-
tions have also arisen regarding the possibility of subsequent reconcentration of certain
radioisotopes because of their mobility. This could lead to the remote possibility of criti-
cality if, for instance, the plutonium is reconcentrated sufficiently. Isolation from the
biosphere would be achieved by negligible ground-water movement in the disposal formation,
particularly towards the surface, retention of nuclides due to sorption onto the host rock
mineral skeleton, and low probability of breeching by natural or man-made events. The con-
cept is not amenable to a multiplicity of engineered barriers.
For shale grout injection, the shale would first be fractured by high-pressure water
injection and then the waste, mixed with cement and clays, would be injected into suitable
shale formations at depths of 300 to 500 m (1,000 to 1,600 ft) and allowed to solidify in
place in layers of thin solid disks. The shale has very low permeability and probably good
sorption properties. The injection formations selected would be those in which it could be
shown that fractures would be created parallel to the bedding planes and would therefore re-
main within the host shale bed. This requirement is expected to limit the injection depths
to the range stated above. Direct operating experience is available at ORNL for disposal of
TRU wastes by shale grout injection. The grout mixes have been designed to be leach resis-
tant and hence the concept minimizes the mobility of the incorporated radioactive wastes.