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

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would be stored as a liquid, it is more probable that it would be solidified (calcined or
vitrified) if an extended storage period were envisaged.
6.1.2.5 Potential Impacts Over the Long Term (Postemplacement)
Although repository-related human activity would be minimal once emplacement and
repository decommission activities were complete, impacts could occur because of the pos-
sible mobility of the molten waste material in the geologic environment. Potential events
and impacts are described below.
Potential Events
For risk analysis purposes, the postemplacement phase of the concept is treated in a
manner similar to other geologic disposal alternatives (see Section 5.6). As noted earlier,
after the waste-rock matrix cooled to the point where liquid water could contact the waste,
it is assumed that the waste would dissolve, and transport through the surrounding rock
would be initiated. Clearly, the degree of risk calculated on this basis would be strongly
site specific, and would depend on factors such as the depth of the repository, presence and
location of aquifers, water quality, and sorptive properties of the rock.
Possible pretreatment of the wastes to minimize potential adverse postemplacement
effects would depend on the waste form as well as the geologic media characteristics.
Potential Impacts
Basically, the environmental considerations involved in evaluating the long-term impact
of rock melting are how much of the radioactivity in the repository would reach the
biosphere, when it would get there, and what its effects would be.
The heat barrier effect is discussed in Section 6.1.2.3. Following total resolidifi-
cation (1000 years), when the heat barrier no longer existed, most fission products would
have decayed to innocuous levels. The toxicity of the residual radionuclides in the resoli-
dified waste-rock matrix at that time should be significantly less than that of a typical
uranium ore body from which the nuclear fuel was originally extracted.
Mixing of the HLW with the molten rock, as well as the physical and chemical properties
of the cooled and resolidified waste-rock matrix, would determine the rate at which radio-
active species could be leached and transported by ground water. It might be possible to
design some mitigating measures to significantly retard leaching rates of all or some of the
radioactive species present.
It is possible that the heat barrier effect would retard the start of effective leaching
of radioactivity until radioactive decay had essentially eliminated the fission products as
significant health hazards; thus, it might be necessary to consider only the TRU products.
Transportation of radioactivity by ground water would have to be evaluated on a site-
specific basis, although different scenarios could be postulated to obtain order-of-
magnitude estimates of the time required for radiation to appear in the biosphere and of the
concentrations of radioactive species that would be present in the water. In modeling the