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

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6.32<br />

Predisposal Treatment <strong>of</strong> the <strong>Waste</strong>. The reference concept requires a fuel reprocessing<br />

plant to recover uranium and plutonium for recycle and to generate HLLW for disposal in the<br />

rock melting cavity, as described in Appendix VII <strong>of</strong> Bechtel (1979a). This plant could be<br />

located either on or <strong>of</strong>f site, but the reference concept assumes an on-site location because<br />

<strong>of</strong> restrictions on the transportation <strong>of</strong> liquid radioactive materials. If solid pellets were<br />

produced in the packaging/encapsulation (P/E) facility, an <strong>of</strong>f-site location would be feas-<br />

ible.<br />

Site. The primary factor in selecting a site would be the suitability <strong>of</strong> the rock<br />

formations. Those rocks <strong>of</strong> greatest interest as potential media for rock melt disposal are<br />

composed <strong>of</strong> silicate minerals. Silicate mixtures are characterized by a melting interval<br />

rather than a definite melting point, the melting interval being different for each different<br />

set <strong>of</strong> minerals (DOE 1979).<br />

The melting interval is bounded by the solidus temperature (the temperature at which<br />

liquid first forms as the rock is heated) and the liquidus temperature (the temperature above<br />

which mineral crystals do not exist stably). In rock melting, these temperatures would de-<br />

pend on parameters such as pressure, chemical composition (especially the amount <strong>of</strong> water<br />

present) and the state <strong>of</strong> segregation <strong>of</strong> the rock (see Figure 6.1.5) (Piwinskii 1967, Luth et<br />

al. 1964, and Wyllie 1971a). Therefore, the ultimate.size <strong>of</strong> the rock melt cavity would de-<br />

pend on the waste decay heat level and the rock characteristics, including thermal conducti-<br />

vity and thermal diffusivity. Also , the ultimate volume <strong>of</strong> the molten rock would be influ-<br />

enced by the size <strong>of</strong> the original mined cavity. The radius <strong>of</strong> the waste-rock melt pool, as a<br />

function <strong>of</strong> time, for a typical rock melt repository is shown in Figure 6.1.6 (DOE 1979).<br />

The total site area that would be required for a rock melt repository would depend on the<br />

number <strong>of</strong> cavities, the size <strong>of</strong> the cavities, spacing between the cavities, and surface<br />

facility requirements. For this reference concept, the site area would be approximately 4<br />

km 2 (1.5 mi 2 ) (Bechtel 1979a).<br />

Granite "dry"<br />

10 ... 100<br />

8s t 80<br />

6 q q 60<br />

S U d<br />

8<br />

0U<br />

d4 d<br />

d 40<br />

d<br />

SS<br />

2 u 20<br />

SI I II<br />

600 800 1000 1200 0 200 400 600 800 1000<br />

T(oC) Time (Years from Repository Sealing)<br />

FIGURE 6.1.5. Schematic Illustration <strong>of</strong> FIGURE 6.1.6. Radius <strong>of</strong> <strong>Waste</strong>-Rock Melt<br />

Hydrous and Anhydrous Melting Pool Over Time (For Typical<br />

Intervals for an Average Cavity and <strong>Waste</strong> Loading)<br />

Granite

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