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

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6.15<br />

diameter <strong>of</strong> approximately 48 cm (19 in.) so that a 36-cm canister could be placed in a mud-<br />

filled, fully cased hole.<br />

No increase in the present capability to rotary drill deep wells is expected by the year<br />

2000 without some very significant effort to develop new technology. Currently, there is no<br />

industry demand to produce the technology advancement necessary. If sufficient resources<br />

were available to advance technology, a 9,000-m hole with a 48-cm (19 in.) diameter might be<br />

attainable by the year 2000. Most <strong>of</strong> the hole would be cased; however, in high strength<br />

rocks without gas pressure, the bottom part <strong>of</strong> the hole might be left uncased. Technology<br />

improvements required to reach this depth include:<br />

* New drilling muds capable <strong>of</strong> operating at temperatures <strong>of</strong> 370 to 430 C (700 to 800 F)<br />

* High-temperature drill bits, either roller cone or diamond<br />

* New drill pipe, including improved designs and use <strong>of</strong> improved (high-temperature)<br />

steels<br />

* Improved support equipment, such as high-temperature logging and surveying tools and<br />

fishing tools<br />

* Improved casing materials (high-temperature steels) and joint design<br />

* High-temperature cements and surface pumps for pumping these cements.<br />

<strong>Waste</strong> Form and Package Integrity. Criteria currently being proposed for waste forms and<br />

packages require total containment within the package for the time period dominated by fis-<br />

sion product decay (up to 1000 years). The development <strong>of</strong> materials to retain their integ-<br />

rity for this period <strong>of</strong> time at temperatures that would be reached when the ambient rock tem-<br />

perature is 200 to 300 C and under geochemical conditions that would be encountered would re-<br />

quire significant effort.<br />

Heat Transfer (Thermomechanical and Thermochemical Factors). Under a normal geothermal<br />

gradient <strong>of</strong> 20 to 30 C/km (60 to 90 F/mi) ambient temperatures in excess <strong>of</strong> 200 to 300 C (390<br />

to 570 F) are expected at a depth <strong>of</strong> 10,000 m. The heat released by radioactive decay <strong>of</strong> the<br />

emplaced waste would further increase the temperature <strong>of</strong> the surrounding rock. The magnitude<br />

<strong>of</strong> this induced temperature increase would be determined by the thermal properties <strong>of</strong> the<br />

rocks and the power output <strong>of</strong> the waste.<br />

Because <strong>of</strong> the very large height-to-diameter ratio <strong>of</strong> the column <strong>of</strong> radioactive waste,<br />

the heat flux from the waste would be mainly in the radial direction, as from an infinite<br />

cylinder. The temperature within the heat source itself would be very nearly uniform and<br />

would drop very abruptly at the ends. Therefore, from a purely thermal point <strong>of</strong> view, this<br />

geometry would be very favorable. It takes 200,000 years for heat from 5,000-m depths to<br />

diffuse to the surface (DOE 1979). The thermally induced effects on the chemical stability<br />

and mechanical integrity <strong>of</strong> the geological formation and upward driving <strong>of</strong> the ground water<br />

would be the most critical issues.<br />

The thermochemical behavior <strong>of</strong> rocks around a deep hole is not predictable at present.<br />

Since controlling factors would be the jointing, fracturing, and fluid content <strong>of</strong> the rocks,

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