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

5.9
thereby providing all, or a major part, of the resistance to the environment required by the
package longevity criterion; it can provide a degree of uniformity to a variety of canister
types, applied at the repository to accommodate acceptance criteria. The canister and over-
pack together can be referred to as the "container."
Overpacks for use in the repository are designed especially for chemical durability,
with less emphasis on properties such as impact resistance that are mainly important during
handling and transportation. Thus, a wide range of materials, in addition to metals, are
being studied. These include various ceramic materials, graphite and carbon materials, a
wide variety of glasses and specially selected cements.
Emplacement Hole Backfill
Backfill materials are designed to fulfill one or more of several functions:
* Sorbing the limited amount of water that may be present in a repository rock,
e.g., from brine inclusion migration in salt.
* Impeding the movement of intruding ground water to and from the waste package.
* Selectively sorbing radioiostopes from ground water in the event of the canister
breach.
* Modifying ground-water chemistry and composition (e.g., pH, Eh, etc.) to reduce
corrosion rates or minimize waste form leaching.
* Providing mechanical relief by accommodating stresses on the waste package induced
by rock movement.
* Serving as a heat transfer medium.
Several layers of filler or backfill material can be utilized, if desired, as shown in
Figure 5.1.2; thus, different materials specially designed for specific purposes can be
included for optimum functioning of the overall waste package system. Most of the filler
or backfill materials being considered are naturally occurring clays, sand or crushed rock
that are readily available in large quantities.
In addition to backfill in the emplacement holes, backfill material is also placed in
rooms and corridors when the repository is closed. The room and corridor backfill, depend-
ing upon the material and method of emplacement, can perform the same functions described
for the hole backfill. The degree of structural support provided may be important in pre-
serving repository integrity by limiting the subsidence of room and corridor ceilings. The
permeability and porosity of the backfill material may affect the amount of water entering
the repository and the time it takes for the repository to become saturated.
Mechanically emplaced crushed rock is used for backfilling the conceptual repository
described in this Statement. The use of an engineered sorption barrier as backfill is dis-
cussed in Appendix K. Other backfill materials and methods of emplacement are discussed in
NUREG/CR-0496 (NRC 1979).