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

5.4
breakdown into component minerals. In general, the greater the strength, the greater the
ability to resist weathering. Parameters representative of strength include cohesion or
friction angle, uniaxial compressive strength, and tensile strength.
Stress-strain properties indicate the deformation characteristics that a material will
exhibit under stress. Parameters that describe the nature of the deformation of a disposal
medium include Young's modulus, Poisson's ratio, bulk modulus, and shear modulus. These
parameters are significant in the analysis of an earth material's strength, durability, and
use properties, such as mineability, for isolation. The desirability of (or, trade-offs
between) a highly deformable medium versus a rigid disposal medium for isolation purposes
is unresolved. The ability of an earth material to deform and seal discontinuities to fluid
flow is desirable. Conversely, a rigid earth material is important to the stability of the
repository tunnel opening.
Thermal properties indicate an earth material's ability to absorb and conduct heat away
from radioactive waste. Knowledge of these properties will allow the evaluation of the
effect of the heat upon the integrity of the disposal medium. Pertinent thermal parameters
are coefficient of linear thermal expansion, heat capacity, and thermal conductivity. Heat
can physically alter an earth material by causing expansion, which, in a confined disposal
medium, can jeopardize isolation. For example, too much expansion of the rock might frac-
ture the overburden above the repository. The degree of expansion is dependent on the
ability of a host rock to dissipate heat and dependent on the amount of expansion for a
given temperature change.
Hydrologic properties are essential to assessing the potential for fluid flow. They
are evaluated by the parameters of permeability, hydraulic gradient, and porosity.
Restriction of transport of radionuclides requires as low a permeability as possible.
A host rock is an aggregate, composed of one or more naturally occurring minerals and
chemical compounds. The constituents provide the chemicals for potential reactions of the
host rock 'ith the waste mater:il. These possible reactions may increase isolation by pre-
cipitati s of insoluble materials or decrease it by converting radioactive waste into sol-
uble compounds. Possible chemical reactions among disposal media, intergranular fluids, and
waste must be defined and evaluated for their effect on isolation.
Disposal media of salt, granite, shale, and basalt are examined here and represent only
selected sample of candidate host rock types. Other host rock types may also meet the
requirements for media properties and distribution. Additional media can be grouped as hav-
ing properties similar to those of the example media. Associated disposal media are grouped
as 1) salt: anhydrite, gypsum; 2) granite: general crystalline rock, granodiorite, perio-
dotite, gneiss, syenite; 3) shale: general argillaceous rock, carbonate; and 4) basalt:
gabbro and some tuffs.