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.

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
  
  
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• 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).