Since the Yucca Mountain
project in the U.S. was defunded in 2010,
the notion of disposing of spent nuclear fuel (SNF) in deep boreholes
has been reinvigorated, most recently by private companies proposing
to utilize lateral drilling technology to excavate boreholes for SNF
disposal in sedimentary rock. It is claimed that this approach will
alleviate site characterization efforts and expand the availability
of potential disposal sites. However, long-term safety will hinge
upon the prevalence of geochemically reducing, highly saline, and
slow-flowing fluids around the waste emplacement zone, and to quantify
these parameters in fluids sampled from depths >1 km will present
a challenge. Regional data indicate only a narrow geographical extent
of such conditions in the conterminous United States. Furthermore,
models of radionuclide transport from disposal boreholes must take
into account processes that may accelerate degradation of the canisters,
plug, and SNF itself, such as radiolysis and attack by constituents
of hydrothermal brines, coupled with hydrogeologic features that promote
advective groundwater flow. This review summarizes some geologic considerations,
most notably those related to geochemistry, that challenge the long-term
safety case for deep borehole disposal of SNF.