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

6.1.4.1 Concept Summary
6.62
6.1.4 Subseabed
In subseabed disposal, wastes would be emplaced in sedimentary deposits of the ocean bot-
tom that have been stable for millions of years. These deposits have a high sorptive capac-
ity for the waste species (except for iodine and technetium) that might leach from the waste
packages. Transport from ocean depths for any waste species escaping the sediments to the
biologically active near-surface waters is expected to be a slow process that would result in
dilution and dispersion. In addition, the great depth of the water column would consti-
tute a barrier to human intrusion.
A program has been under way since 1973 to assess the technical and environmental fea-
sibility of this concept for disposing of high-level nuclear wastes (Bishop 1974-75, Talbert
1975-78). The total seabed represents about 70 percent of the surface of the planet (of
which less than 0.0001 percent would be used) and contains a wide variety of geologic forma-
tions. Theoretically, all wastes from the once-through cycle and uranium-plutonium recycle
options could be emplaced in subseabed formations. But, because of volume considerations,
other methods of disposal may be more practicable for contact handled and remotely handled
TRU wastes.
The reference subseabed geologic disposal system for study purposes is the emplacement of
appropriately treated waste or spent reactor fuel in a specially designed container into the
red clay sediments away from the edges of a North Pacific tectonic plate, under the hub of a
surface circular water mass called a gyre (mid-plate/gyre:MPG). (However, selection of the
North Pacific as a study area in no way implies its selection as a candidate subseabed dis-
posal site.) The reference method uses a penetrometer(a) for emplacing wastes in the
sediments in a controlled manner that allows subsequent monitoring. A specially designed
surface ship would transport waste from a port facility to the disposal site and emplace the
waste containers in the sediment. A monitoring ship, which would completely survey the dis-
posal site before operations began, could determine the locations of individual disposal con-
tainers and monitor their behavior for appropriate lengths of time. The ship would also
maintain an ongoing survey of the surrounding environment.
(a) A penetrometer is a needle-shaped projectile that, when dropped from a height, penetrates
a target material. It can carry a payload of nuclear waste and instruments
designed to measure and transmit its final position and orientation relative to the
sediment surface. Penetration depth is controlled by the shape and weight of the penetrometer,
its momentum at contact with the sediment, and the mechanical properties of
the sediment.