The FuTure oF nuclear Fuel cycle - MIT Energy Initiative

epreSentative repoSitory deSiGnS: united StateS, SWeden, and FranCe
Four repository designs are briefly described herein and show that there are many different
ways to design a geological repository.
The Waste Isolation Pilot Plant, 18,19,20 the operating repository in the U.S for defense transuranic
waste, is in a massive bedded Permian salt deposit 658-meters underground.. Salt has
two characteristics: it is plastic so openings close over a period of decades and its existence
indicates no flowing groundwater. If there was flowing groundwater, the salt would have
been dissolved. The hydraulic conductivity of the formation is less than 10 -14 m/s and the
diffusion coefficient is less than 10 -15 m 2 /s, thus the presence of trapped 230 million-year-old
sea water throughout the formation. The primary safety case is that salt beds are massive
structures over tens or hundreds of miles. The physical size implies hundreds of thousands
to millions of years to dissolve sufficient salt to threaten such a repository. The EPA requires
at least one engineered barrier at any geological disposal site. The engineered barrier is magnesium
oxide that is embedded with the transuranic wastes to buffer the pH of any liquid in
the salt to 8.5 to 9—conditions under which plutonium is relatively insoluble and colloids do
not form. It is a chemical barrier to the movement of radionuclides in this system.
The proposed U.S. repository at Yucca Mountain for SNF and HLW is in tuff—consolidated
volcanic ash. The repository is above the water table in an oxidizing environment—the only
proposed repository in such an environment and thus a design significantly different than
other proposed repositories. SNF is to be placed into steel containers typically containing
21 fuel assemblies. The steel containers have a thick external layer of a highly-corrosion
resistant nickel alloy. The waste packages are to be placed in tunnels that will remain open
after repository closure. Above the waste packages is a titanium drip shield (semicircular
structure) to divert water as it flows from the surface through the tuff to the groundwater
below the repository horizon. The engineering barriers are designed to delay contact of the
wastes with water.
The proposed Swedish repository is in granite—a solid rock with a chemically reducing
environment. The waste package is a steel container with a thick external layer of copper.
Copper was chosen for the waste package because metallic copper has existed unchanged
for hundreds of millions of years in Swedish granite—a basis to expect long-term integrity
of the waste package. The copper waste package is placed at repository depth and surrounded
by a compacted bentonite-sand mixture between the waste package and granite. This
material has low permeability to water flow and absorbs many radionuclides. Bentonite
is typically used in earth dam construction to minimize water flow. All repository tunnels
are to be filled with a similar mixture to minimize groundwater flow in the repository. The
planned repository in Finland has a similar design.
The proposed repositories in France and Belgium are located in clay that has a very low
permeability to groundwater flow, has highly reducing chemical conditions that minimize
the solubility of most radionuclides in groundwater, and high absorption of radionuclides.
Most radionuclides (and all actinides) are highly insoluble in such environments. However,
clay has a very low thermal conductivity; thus, the waste loading per acre will be lower than
in other repositories to avoid excessive temperatures in the repository. The wastes will be
in steel waste packages. All access tunnels will be filled with low-permeability backfill to
minimize groundwater movement.
162 MIT STudy on The FuTure oF nuclear Fuel cycle