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

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B.7<br />

Rocks are named and described according to their texture and mineralogy. However,<br />

their overall behavior may depend on details <strong>of</strong> petrography such as mineral composition,<br />

rock fabric, fluid inclusions, exotic mineral accumulations in joints and fractures, and<br />

trace element chemistry.<br />

Petrography is important in determining the suitability <strong>of</strong> the host rock. These data<br />

will be collected and evaluated for specific sites during the site selection process. The<br />

basic properties <strong>of</strong> rock fabric and composition are discussed in Section 5.1.<br />

Many chemical interactions are possible among mineral and fluid phases <strong>of</strong> the host<br />

rock, ground water, metal canisters, backfill material, and waste. The range <strong>of</strong> possible<br />

chemical interactions is described in Section 5.1. However, additional study <strong>of</strong> geochemi-<br />

cal aspects is warranted.<br />

Thermal properties <strong>of</strong> high diffusivity and conductivity and low thermal expansion are<br />

normally considered to be desirable. These properties result in maintaining lower waste<br />

temperatures and minimizing mechanical deformation (expansion). Design <strong>of</strong> a repository<br />

should restrict thermal loading so that excessive thermal expansion does not fracture the<br />

host rock and thereby increase permeability.<br />

Chemical properties <strong>of</strong> different host rocks vary greatly, and the range <strong>of</strong> possible<br />

chemical reactions both before and after the containers may be breached may be significant.<br />

Favorable reactions between waste and the host rock include formation <strong>of</strong> insoluble radioac-<br />

tive compounds, formation <strong>of</strong> compounds containing water (thus reducing the quantity <strong>of</strong> free<br />

water), and sorption <strong>of</strong> radionuclides by the host rock. However, if corrosive fluids are<br />

produced by heating <strong>of</strong> the host rock, they may attack the canisters and result in early loss<br />

<strong>of</strong> this barrier. Chemical reactions between waste and host rock that form highly soluble<br />

or low melting-point compounds would be unfavorable. Chemical reactions can affect chemical<br />

transport by changing the composition and quantity <strong>of</strong> fluids and by changing the ionic<br />

strength <strong>of</strong> these fluids. Chemical reactions can produce liquids and gases under high pres-<br />

sure, and can change pH, Eh, viscosity, or density. Such factors can affect rock strength<br />

and rate <strong>of</strong> physical or chemical decomposition <strong>of</strong> the host rock.<br />

Permeability is an important hydrologic property <strong>of</strong> the host rock and must be known to<br />

determine the rate <strong>of</strong> migration <strong>of</strong> radionuclides toward the biosphere. Very low permeability<br />

implies a relatively small radius <strong>of</strong> equilibrium release if other considerations are<br />

also favorable.<br />

Properties <strong>of</strong> the medium also affect repository capacity and waste placement geometry.<br />

For example, low thermal conductivity <strong>of</strong> the host rock would require lower waste loading.or<br />

greater spacing between canisters to maintain acceptable repository temperatures.<br />

The total system <strong>of</strong> waste form, repository, surrounding geologic environment, and<br />

effects <strong>of</strong> waste disposal must be considered to identify any possible site-specific determinants<br />

<strong>of</strong> the radius <strong>of</strong> equilibrium release. If possible, the host rock dimensions should<br />

encompass the radius <strong>of</strong> equilibrium release. However, the radius may extend beyond the host<br />

rock and isolation <strong>of</strong> waste will be achieved by additional barriers in accordance with the<br />

multibarrier concept.

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