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

6.1.2.1 Concept Summary
6.28
6.1.2 Rock Melt
The rock melt concept for radioactive waste disposal calls for the direct emplacement of
reprocessed liquid or slurry HLW and remote-handled (RH) TRU into underground cavities.
After the water has evaporated, the heat from radioactive decay would melt the surrounding
rock, eventually dissolving the waste. In time, the waste-rock solution would refreeze,
trapping the radioactive material in a relatively insoluble matrix deep underground. The
waste and rock should achieve reasonable homogeneity before cooling, with resolidification
completed after about 1,000 years. Rock melting should provide high-integrity containment
for the radionucles with half lives longer than this period. Spent fuel and secondary wastes
(hulls, end fittings, and contact-handled (CH) TRU are not suitable for rock melt disposal
unless they could be safely and economically put into a slurry for injection. Otherwise,
they would be disposed of using some other form of terrestrial disposal, such as a mined
geologic repository.
The waste-rock solidified conglomerate that would ultimately result is expected to be ex-
tremely leach resistant, to the extent that it might provide greater long-term containment
for the waste isotopes than a mined geologic repository. Because less mining activity would
be involved, the cost advantages could be substantial (Bechtel 1979a).
After emplacement, the waste would be considered to be irretrievable, although it could
probably be recovered at great expense during the charging or waste addition period while
cooling water was still being added. However, the recovery operation would become much more
complex and expensive with time as the size of the charge increased (Bechtel 1979a).
There are several technological issues to be resolved and considerable R&D work would be
needed before this concept could be implemented. Primary needs would be for better under-
standing of heat-transfer and phase-change phenomena in rock to establish the stability of
the molten matrix and for development of engineering methods for emplacement.
6.1.2.2 System and Facility Description
System Options
The reference concept for rock melt disposal of nuclear waste has been developed from a
number of options available at each step from the removal of spent fuel from the reactor to
disposal in the rock melting repository.
Various options to be considered are summarized in Figure 6.1.3. The bases for selec-
tion of options for the reference concept (those blocked off) are discussed in detail in var-
ious documents listed in Appendix M. In addition, a number of options for variations within
the concept were considered. These options could improve the concept by changing the cavity
construction method or the waste form, or by eliminating cavity cooling (Bechtel 1979a and
DOE 1979).