23.04.2013 Views

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

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

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

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

B.4<br />

B.2 DIMENSIONS AND PROPERTIES OF HOST ROCKS AND MEDIA<br />

The host rock must have the properties and dimensions to assure geologic isolation<br />

(Office <strong>of</strong> <strong>Waste</strong> Isolation 1977). One method for defining the required dimensions <strong>of</strong> a<br />

repository medium is use <strong>of</strong> an "equilibrium release fringe concept." The concept assumes<br />

that the repository system contains the waste within a known or definable zone for the<br />

necessary time period. After a period <strong>of</strong> time, the competing factors <strong>of</strong> radioactive decay<br />

and chemical migration processes will produce an equilibrium zone or fringe that will not<br />

move or will move so slowly as to be insignificant. Using these definitions, a three-<br />

dimensional zone consisting <strong>of</strong> host rock material, repository and waste, is defined on the<br />

basis <strong>of</strong> host rock and waste package properties beyond which no waste or activity beyond a<br />

specified range is expected to migrate for the necessary time period. The specified range<br />

lies between the values for radioisotope concentration at the maximum natural concentration<br />

found in the world and the average U.S. natural background concentration. This condition<br />

is defined as an equilibrium condition, i.e., any material or activity released beyond the<br />

fringe or boundaries <strong>of</strong> the zone would be within the range <strong>of</strong> that which occurs naturally.<br />

The size <strong>of</strong> the zone <strong>of</strong> effect will probably change throughout the repository's his-<br />

tory. After sealing, the zone will be very nearly the size <strong>of</strong> the repository and the fringe<br />

will be located by radiation effects. At a later time in the repository's history, when the<br />

canisters and overpack material may have lost their integrity as barriers, the waste will<br />

be partially in contact with the host rock. The waste may then move slowly into the host<br />

rock by diffusion, concentration gradients or whatever forces are present to move it. The<br />

fringe bounding the zone <strong>of</strong> effects will expand as the zone slowly moves out from the<br />

repository. The size <strong>of</strong> the zone <strong>of</strong> effect and the location <strong>of</strong> the equilibrium fringe will<br />

depend on the host rock properties, the form <strong>of</strong> the waste, the activity and thermal state<br />

<strong>of</strong> the waste at the time the canisters became ineffective as containment, and other factors<br />

such as presence <strong>of</strong> water.<br />

The location <strong>of</strong> the equilibrium release fringe is difficult to predict, particularly<br />

over time periods greater than several thousand years. Simulation by modeling may furnish<br />

some estimates if the necessary input data are available. The modeling would proceed under<br />

the assumption that no intrusions or disruptions occurred.<br />

The required dimensions <strong>of</strong> the host rock relate closely to the radius <strong>of</strong> equilibrium<br />

release and are established as a function <strong>of</strong> the medium's properties and <strong>of</strong> engineering<br />

design <strong>of</strong> the repository. Important media properties that affect the radius <strong>of</strong> equilibrium<br />

release can be classified as thermal, chemical, and hydrologic.<br />

The host rock dimensions must be large enough with respect to the repository dimensions<br />

to adequately disperse or contain all <strong>of</strong> the perturbations and loads induced by the reposi-<br />

tory. These dimensions will depend directly on site-specific geologic properties <strong>of</strong> the<br />

host rock. The host rock must also be <strong>of</strong> sufficient thickness to ensure that excavation and<br />

construction can proceed on several depth levels and over many acres <strong>of</strong> lateral extent.<br />

Adequate thickness <strong>of</strong> the zone adds assurance that the specific medium is <strong>of</strong> sufficient mass<br />

and extent to contain the waste and buffer the repository from materials with different

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!