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

6.36
The heat effects in the peripheral edges of the zone would be similar to effects found in a
mined repository.
Transport of Radionuclides in Rock Melting. Under normal operating conditions, the cas-
ing in the emplacement well should prevent contact of radioactive waste with any aquifers
that would overlie the disposal cavity. However, during waste charging, it is conceivable
that some radioactivity could migrate out of the cavity into the surrounding rock. But, if
the cavity were maintained approximately at atmospheric pressure, the tendency of water under
hydrostatic pressure to flow into the cavity should minimize the importance of this transport
mechanism.
During the rock melting phase, transport of radionuclides out of the waste-rock mixture
would presumably be inhibited, because no water would be present in the melt and a portion of
the surrounding zone of heated rock (Taylor 1977). (This is the "heat barrier" effect refer-
red to earlier.) However, the radionuclide leaching capabilities of the high-pressure and
high-temperature water vapor existing in this region would have to be characterized.
Finally, after the waste-rock matrix had cooled and solidified, it must be assumed that
water would reenter the matrix and leach at least some of the radionuclides out of the matrix
volume. Leaching potential at elevated pressure and temperature would have to be determined.
As the radionuclides were transported to the relatively cool rock away from the repository,
existing data on radionuclide transport in rock should be applicable (Klett 1974, Burkholder
et al. 1977, de Marsily et al. 1977, Pines 1978, EPA 1978). It is possible that leaching
data on other waste forms could also be useful (Brownell et al. 1974, Ralkova and Saidl 1967,
Schneider 1971b, Mendel and McElroy 1972, Lynch 1975, and Bell 1971).
Effect of Superheated Water on Glasses in Rock Melting. Data from recent investigations
of the devitrification of glass by water at high pressure and temperature (McCarthy et al.
1978 and McCarthy 1977) could be useful in determining the availability of radionuclides to
water from vitrified rock present in the resolidified waste-rock matrix. However, the appli-
cability of the conditions under which these data were obtained to the rock melt concept
would have to be established.
Safety Studies: Disposal of HLW with Rock Melting. During the cavity charging portion
of the presealing phase, HLW in such forms as solutions or slurries would be directly intro-
duced into the repository cavity. The various operations that would be involved in carrying
out this phase of the process are not as unique as the postsealing phase. Consequently, the
probabilities for the release of radioactivity to the environment can be estimated for each
step of this phase. This can be done both for normal operation and for assorted accident
scenarios. In general, sufficient data exist to prepare a risk analysis for this phase of
the rock melt concept.
After cooling of the waste-rock matrix to the point where water could contact the waste,
it may be assumed for purposes of modeling that the waste dissolves, and transport through
the surrounding rock is initiated. Calculations for risk analysis of this postsealing phase