Lesson 3 Background Notes
Construction and Tunneling
Three types of excavation equipment or tunneling techniques are used on the Yucca Mountain
Project: tunnel-boring machine, alpine miner (roadheader), and drill and blast. The type of equipment
or technique selected is based on the strength of the rock, the diameter of the tunnel, and/or the
need to minimize disturbance to the surrounding rock.
Tunnel-Boring Machine
|
The 7.6-meter-diameter tunnel boring machine is prepared for the Exploratory Studies Facility excavation. |
The tunnel-boring machine is a large piece of automated tunnel construction equipment. This machine
is used in road construction, in excavating water diversion tunnels, and for many other applications.
A 7.6-meter diameter tunnel boring machine was used to construct the initial Exploratory Studies
Facility at Yucca Mountain. In comparison, a 10-meter tunnel boring machine was used to make the
“Chunnel,” the tunnel beneath the English Channel connecting England and France.
The Exploratory Studies Facility was
excavated from October 1994 to April
1997. This tunnel is 8 kilometers long
and 7.6 meters in diameter. It was
built in a U-shape to provide access
into Yucca Mountain. Scientists
accessed the rock beneath Yucca
Mountain in order to do field tests on
the rock in place. Some of these
underground tests lasted many years.
Some testing underground will
continue even after waste is placed
into the mountain.
The Exploratory Studies Facility will
become the main tunnel to haul waste
underground in the initial phases of
waste emplacement. |
Tunnel-boring machines are generally very effective over long distances and in relatively good rock
conditions. Tunnel boring machines are usually used in combination with conveyor belts to remove
the waste rock, called muck.
|
Back view of the tunnel boring machine as it navigates a path through the mountain using a lazer guidance system. |
Once their job is done, the machines are often left buried inside the ground. The cost of removing
them is usually too high to salvage them. In general, a tunnel boring machine cannot be removed
back through the tunnel because too much construction has occurred behind it (lights, pipes, electrical
lines and supply structures, steel supports, conveyor belt, ventilation ductwork).
Tunnel boring machines make round tunnels. As soon as the machine passes a location, concrete
blocks, called inverts, that are round on one side are placed behind the machines. The inverts provide a flat structure to place rail for bringing
material into the tunnel. Tunnels, or drifts into
which waste is to be emplaced, will not have
concrete inverts placed in them. In those drifts,
steel framing and crushed rock will support
rails and waste packages.
At the end of the waste emplacement phase at
Yucca Mountain, all underground mining
equipment will be removed because metals,
rubber, and oils left underground over a long
period may alter environmental conditions.
Some equipment, such as a tunnel boring
machine, may be left in places where it cannot
impact the radioactive waste packages.
Alpine Miner
|
The alpine miner has a rotating boom that claws its way through the rock to form small rooms off of the main tunnel and lobster-like arms that gather the rock for removal. |
Another machine, called an alpine miner, was used on a limited basis at Yucca Mountain to excavate
small underground rooms for conducting scientific studies (depending on their size, these rooms are
called niches or alcoves). The alpine miner has a boom arm with tungsten carbide picks that rotate
against the rock walls and dislodge the rock.
Drill and Blast
Drill and Blast is another method of excavation used to a limited extent at Yucca Mountain. A crew of
miners first drilled a pattern of boreholes into the rock walls and then filled the holes with explosives.
After the miners leave the tunnel area, the explosive is detonated remotely to fracture and break the
rock away from the walls. Other teams return underground to scale or knock down loose rock and
remove the muck (waste rock).
|
The drill and blast method of excavation was used to start the South Portal and North Portal entrances of the Exploratory Studies Facility at Yucca Mountain. |
If DOE is granted a license to begin construction of the underground emplacement tunnels at Yucca
Mountain, tunnel-boring machines will be the primary choice of equipment for most of the excavation
work. The machines for the emplacement tunnels will be about a 5.5-meters in diameter.
Other future access and perimeter tunnels will require larger tunnel boring machines.
For additional information see the DVD titled: Yucca Mountain: The Making of an Underground
Repository (Order free of charge from the Yucca Mountain Information Center, 1-800-225-6972)
Waste Packages
Waste packages are the disposal containers for the nuclear waste that would be placed in the repository
at Yucca Mountain.
Waste packages are designed to contain the radioactive material and protect it for tens of thousands
of years. Specifically, the waste packages are designed to:
- Isolate radioactive particles during the operational period of the repository
- Contain radioactive particles during the postclosure period of the repository
- Provide criticality protection during waste package loading and emplacement
- Manage the decay heat for the potential repository
- Provide identification (i.e., each waste package will be uniquely labeled and its contents identified)
- Enhance the safety of personnel, equipment, and the environment
- Prevent adverse reactions involving the waste form
- Maintain structural integrity during loading, onsite transportation, emplacement, and retrieval
- Resist corrosion in the emplacement drift environment
- Provide physical and chemical stability for the waste form
- Promote heat transfer between the waste form and the outside environment
- Facilitate decontamination of the waste package’s outer surface
|
Alloy 22 will be used for corrosion resistance on the outside 2-cm of the waste package. A thin sample of this metal alloy that is equal to the thickness of one U.S. quarter would provide protection for 16,000 to 160,000 years, depending on the corrosiveness of the environment. The thickness of the alloy 22 to be applied to the outside of the waste package is equal to 12 quarters thick. |
The waste packages have been designed to use materials that perform well under the anticipated
conditions at Yucca Mountain. They will consist of two thick metal cylinders, one nested within the
other. The inner cylinder would be made of stainless steel to provide structural strength. The outer
cylinder would be made of a nickel alloy (called Alloy 22) that is highly resistant to corrosion. Two key
aspects of the waste package are the construction and the material used in the waste package. The
waste package provides both structural protection and corrosion protection. The cylinder shape will
be used because moisture will run-off the surface and because a cylinder can withstand greater
impact from falling rocks than a box-like shape.
The highly corrosion-resistant material of the waste package would protect the underlying structural
material from corrosion, while the extremely strong internal structural material would support the
thinner corrosion-resistant material of the exterior.
The design analyses performed on the waste package include evaluations of structural strength,
thermal performance, criticality safety, and shielding properties. Using data from these extensive
analyses, scientists do not expect corrosion or any other mechanism to breach the waste packages
for tens of thousands of years or longer. Some testing of the corrosion resistant waste package
materials have shown that they can rapidly deteriorate under extreme conditions. Such extreme
conditions cannot occur within Yucca Mountain, however, according to scientific studies designed to
define the range of potential conditions in the mountain.
Selecting Corrosion Resistant Materials
A special metal alloy called Alloy 22 was selected for the outer layer of the waste package based on
tests that show that this material will last for thousands of years in an environment like Yucca Mountain.
Experts selected Alloy 22 after completing a multi-step analysis and design process. Factoring in the
long-term effects of such conditions as heat, humidity, and water chemistry, scientists methodically
tested and selected materials that could withstand the environment in the emplacement tunnels.
Based on these studies, experts projected the corrosion rate of a variety of materials over tens of
thousands of years.
In addition, this and other engineered-barrier materials were chosen to withstand the stress and wear
of handling, emplacement, and possible retrieval of radioactive waste, and for the ability to withstand
the high heat that it generates.
Testing of these materials will continue for many years to provide a better scientific basis for longterm
safety (using Total System Performance Assessment modeling).
|