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

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6.39<br />

Fracturing During Cooling. During melting, the waste-rock mass would be expected to ex-<br />

pand about 13 percent. During subsequent cooling and contraction, fracturing would have to<br />

be expected in the rock zone that surrounds the molten area. Further work would be required<br />

to establish that the rock melting concept could provide containment <strong>of</strong> the waste charge<br />

under uplift and subsidence conditions.<br />

Chemical and Physical Effects on Surrounding Rock During Rock Melting. While the rock<br />

melting process can be described with some precision (Piwinskii 1967, Luth et al. 1964, Wyl-<br />

lie 1971a, and Wyllie 1971b), the effect <strong>of</strong> a large thermal gradient on various types <strong>of</strong> rock<br />

has apparently not been similarly investigated (Executive Office <strong>of</strong> the President 1978).<br />

Although in some rocks, the predicted thermal effects <strong>of</strong> a molten mass <strong>of</strong> HLW/rock extend<br />

over relatively short distances, the extreme thermal gradient would clearly produce chemical<br />

and physical effects in the rock (Jenks 1977, National Academy <strong>of</strong> Sciences 1978). These ef-<br />

fects would have to be characterized so that the rock mechanics <strong>of</strong> rock melt disposal could<br />

be adequately modeled and any possible intermediate or long-range effects identified and<br />

characterized. It would be necessary to carry out measurements over a range <strong>of</strong> pressures up<br />

to the maximum contemplated lithostatic pressure for a waste disposal cavity.<br />

Interaction <strong>of</strong> HLW with Rock. At the present time, it is not clear whether the possible<br />

chemical reactions between the HLW solution and the rock cavity walls are important to the<br />

rock melt concept. However, it is clearly desirable to know how and to what extent such re-<br />

actions take place, and to predict what the ultimate effect <strong>of</strong> 25 years <strong>of</strong> waste solution ad-<br />

dition would be. With that information, potential problems could be identified, and mitigat-<br />

ing measures could be designed and tested.<br />

After addition <strong>of</strong> HLW to the cavity were stopped and rock melting begun, it is not known<br />

how rapidly and completely the HLW would mix with the molten rock. Because relatively com-<br />

plete mixing <strong>of</strong> the HLW with the rock appears desirable (to ensure complete dissolution <strong>of</strong><br />

the HLW in the rock and subsequent immobilization upon resolidification <strong>of</strong> the matrix), it<br />

might be necessary to design the HLW rock melt disposal facility to minimize the viscosity <strong>of</strong><br />

the molten rock.<br />

Properties <strong>of</strong> Resolidified <strong>Waste</strong>-Rock Matrix. Even if it is assumed that the HLW is com-<br />

pletely mixed with the molten rock, it is not known whether some <strong>of</strong> the radioactive species<br />

in the HLW might segregate during the long cooling process to form relatively concentrated<br />

(and possibly, relatively soluble) inclusions in the resolidified waste-rock matrix (Hess<br />

1960). It is possible that the addition <strong>of</strong> certain chemicals (at the time that HLW is em-<br />

placed) could prevent such segregation, decrease the solubility <strong>of</strong> some or all <strong>of</strong> the long-<br />

lived radionuclides, or both.<br />

R&D Requirements<br />

Resolving these many uncertainties would require an extensive R&D program, such as that<br />

described below.

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