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

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6.35<br />

cause <strong>of</strong> the limited time, the high temperatures involved, and the radioactivity levels in<br />

the system. Considerable technology in this area has yet to be developed, as discussed in<br />

the following section.<br />

Retrievability/Recoverability. <strong>Waste</strong>s disposed <strong>of</strong> by this concept would possibly be re-<br />

trievable for a short period. Prior to melting, most <strong>of</strong> the liquid or slurry could be re-<br />

moved. After the melt has begun, well techniques for the molten rock-waste mixture might be<br />

possible. However this is unproven and would likely be an expensive and difficult process.<br />

Postclosure recovery <strong>of</strong> the solidified waste form would require extensive mining and excava-<br />

tion <strong>of</strong> large quantities <strong>of</strong> hot and molten rock containing waste.<br />

6.1.2.3 Status <strong>of</strong> Technical Development and R&D Needs<br />

Present State <strong>of</strong> Development<br />

Substantial fundamental and applied research would be required for continued development<br />

<strong>of</strong> the rock melting disposal concept. This method is in the conceptual stage and no experi-<br />

mental work has been undertaken to support its feasibility.<br />

Rock Melting Process. Generally, rocks are multiphase mixtures <strong>of</strong> a number <strong>of</strong> minerals<br />

characterized by a melting interval, as noted earlier. Because any two samples <strong>of</strong> a partic-<br />

ular type <strong>of</strong> rock will have slightly different mineral compositions, they will also have<br />

slightly different melting intervals. As we have seen, the boundaries <strong>of</strong> these intervals<br />

(liquidus and solidus temperatures) depend on several parameters.<br />

If the composition <strong>of</strong> the rock in which a waste repository were to be located has been<br />

well characterized, the melting properties <strong>of</strong> that rock could be predicted with some preci-<br />

sion, and if the thermal conductivity, thermal diffusivity, and the heat <strong>of</strong> fusion <strong>of</strong> the<br />

rock were also known, the melting "history" <strong>of</strong> the HLW/rock melting phase could be predicted.<br />

Clearly, it would be prudent to experimentally verify such predictions by means <strong>of</strong> prototype<br />

experiments; however, it should not be necessary to carry out an extensive series <strong>of</strong><br />

such experiments to verify the current predictive capability for estimating the rate <strong>of</strong> rock<br />

melting and the total amount <strong>of</strong> rock melted for a particular set <strong>of</strong> waste repository con-<br />

ditions.<br />

Effects <strong>of</strong> Heat on Rock Properties. The properties <strong>of</strong> rock subjected to high thermal<br />

gradients would be important inputs to determining the condition <strong>of</strong> the rock enclosing the<br />

molten waste-rock matrix. While the radius <strong>of</strong> this molten zone should be small compared with<br />

the extent <strong>of</strong> the geologic formation in which the repository would be sited, the zone's<br />

properties would have to be known so that an appropriate structural and safety analyses could<br />

be carried out.<br />

The inner edge <strong>of</strong> this zone would be defined by the maximum radius <strong>of</strong> rock that had been<br />

heated to liquid formation. The outer radius <strong>of</strong> the zone could be roughly characterized as<br />

that location beyond which the rock had not been measurably affected by heat from the HLW.

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