Lesson 3 Reading Lesson
How will a Yucca Mountain repository work?
A geologic repository is a system for permanently disposing of spent nuclear fuel and high-level radioactive waste. It includes a surfacebased operations area and an underground disposal facility for isolating the radioactive waste. In order to operate in the United States, a geologic repository must be licensed by the U.S. Nuclear Regulatory Commission.
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| Cutaway illustration of Yucca Mountain showing the position of the underground tunnels that make up the repository. |
The purpose of a geologic repository is to keep nuclear waste away from people and the environment for thousands of years by isolating it deep underground. As long as the waste stays in solid form, it will not be harmful because layers of rock and dirt will shield its radiation. However, if enough water contacted the waste, over time it could break it down into tiny radioactive particles and then carry the particles into the environment. Therefore, the strategy is to keep water from contacting the waste for as long as practicable.
Underground Facilities
In 1994, the U.S. Department of Energy began excavating an underground tunnel at Yucca Mountain called the Exploratory Studies Facility. It was completed it in 1997. The purpose of this tunnel is to provide scientists direct access for studying the underground area where the repository would be located.
The Exploratory Studies Facility is in a “U” shape, about 8 kilometers long, 7.6 meters in diameter. It started at the surface and was then gradually excavated to the west and deeper into the rock layers of Yucca Mountain. Along its north to south run, it averages about 300 meters below the crest of the mountain. It was later augmented with a side tunnel , called the cross-drift tunnel. This was done to allow access to the rock in the area of the actual tunnels in which the waste is to be emplaced (west of the Exploratory Studies Facility).
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| Inset photo - Project scientists map fractures along the walls of the main Exploratory Studies Facility tunnel. |
If the Department of Energy receives a license from the Nuclear Regulatory Commission to construct a repository, it will begin to excavate the remaining subsurface facilities, which will include more than 100 horizontal tunnels for storing the waste. These tunnels are sometimes called emplacement tunnels, or emplacement drifts. The existing Exploratory Studies Facility main tunnel will provide access to some of the emplacement drifts and another access tunnel will be built to facilitate excavation of emplacement tunnels in the northern area of the repository (shown in the figure at the bottom of this page).
The emplacement tunnels would be excavated in solid rock about 300 meters beneath the surface of the mountain and 300 meters above the water table. The tunnels will be about 5.5 meters in diameter and about 792 meters long and will be reinforced with rock bolts and perforated stainless steel sheets to prevent rock from falling on the stored waste.
The underground location of the emplacement tunnels is based on several factors:
- the thickness of the overlying rock and soil
- the characteristics of the rock (such as porosity)
- the location of the fractures in the rock
- the distance from possible earthquake faults
- the depth to the water table
- the strength of the rock itself
- the heat output of the waste packages
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| Illustration showing the depth of rock above and below the proposed repository level. |
Experts designed the underground layout of the emplacement tunnels to manage the heat that would be generated by the waste. This heat will influence moisture in the rock, humidity in the tunnels, and other conditions within the repository. In addition, the tunnels are designed so that in the very unlikely event that a significant amount of water enters a drift, it will drain, by gravity, out of the tunnels downward and away from the repository.
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| Cutaway illustration of Yucca Mountain showing the system of parallel tunnels that would eventually host the waste packages. |
A Design for Safety
With its desert climate, deep water table, and thick layers of stable rock, Yucca Mountain provides a suitable geologic setting for a repository. To provide assurance that the waste remains isolated in the underground tunnels, the Department of Energy plans to complement Yucca Mountain’s natural features with man-made, or engineered, features.
The plan for the repository is to seal the waste in extremely durable containers called waste packages, then place the containers in the deep underground tunnels. The waste packages are designed to last for tens of thousands of years. They will consist of two thick metal cylinders, one nested within the other. The inner cylinder will be made of stainless steel to provide structural strength. The outer cylinder will be made of a nickel alloy that is highly resistant to corrosion. 
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| Graphic depicting the key engineered components of the emplacement system in the underground tunnels. |
Before the repository is closed and sealed, drip shields made of another corrosion resistant (titanium) metal will be placed over the waste packages. The drip shields will protect the waste packages from dripping water and falling rock and debris.
The floor of the emplacement tunnels will contain another engineered feature called an invert. The inverts will support the waste packages in the repository tunnels. The inverts are composed of two parts: a steel frame below the waste packages and crushed volcanic rock.
The 300 meters of rock above the repository will restrict the amount of water that could reach the tunnels. However, if any water eventually drips into the tunnels, it will fall on the drip shields, and corrosionresistant waste packages that would still protect and contain the waste for tens of thousands of years. When radioactive particles manage to escape from the waste packages, the tunnel invert rock would act to slow and restrict their movement into the rock below.
With its natural and engineered features, the repository will have multiple barriers to water and radioactive particle migration. The repository is designed so that some engineered barrier components will continue to work even if others fail, assuring that the key components are unlikely to fail for the same reason at the same time.
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Top illustration - Artist’s concept of the layout for the surface waste handling facilities that would be located near the North Portal at Yucca Mountain.
Bottom photo - An operator protected by a shielded window using robotics in a “hot cell” at a radioactive handling facility. |
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Surface Facilities
When trains and trucks carrying nuclear waste arrive at Yucca Mountain, workers will have to unload the waste and then prepare it for disposal underground. This work will be done in special facilities at the surface. These facilities will be built with the safety of the workers, the public, and the environment in mind.
Spent nuclear fuel and high-level radioactive waste are highly radioactive and may also be very hot (thermally). Therefore, special facilities and controls are required to protect the workers who will be preparing the waste to go underground.
Workers will never directly handle the waste, but will use remotely operated robotic equipment to move the waste from the transportation casks to the waste packages. The workers will be in rooms that protect them from radiation, while the actual waste handling operations will occur in other shielded rooms. The workers will be able to see their operations through video displays and specially designed windows with thick glass that also shields radiation.
Engineers will use the latest design methods to construct the surface buildings to withstand the motion from potential earthquakes, shield radiation, and control contamination. Thick concrete walls plus special materials will provide radiation shielding for the workers. The buildings will also include backup systems for controlling radiation in the event that a primary control system fails.
The areas surrounding these facilities will be highly secured. A security fence, video surveillance, and security guards will protect the facilities.
Repository Operations
 If the Department of Energy receives a license to build and operate a repository, it plans to begin moving waste to the repository a few years after the license has been issued. Under the current plan, the repository will receive about 175 shipments of nuclear waste per year for 24 years. Most of these shipments will be by train, but some will be by truck.
 Nuclear waste will arrive at the repository inside special shipping containers called transportation casks. Once at the site, personnel will log and inspect each shipment. Once inspected, the truck or railcar will move to a special building where workers will remove the impact limiters and any other protective equipment used during shipment.
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| Heavy duty crane for off-loading spent nuclear fuel casks. |
Next, the truck or railcar will move to a special building where an overhead crane will transfer the heavy transportation cask to a trolley car. The trolley car will take the cask to a room that shields radiation for unloading the radioactive materials.
 Within the radiation shielding room, robotic equipment will unseal the transportation cask. A crane will lift the waste from the transportation cask and place it in a waste package for disposal. The transportation casks will be decontaminated, inspected, and shipped back to the originators for reuse.
 In a shielded closure area, robotic equipment will weld a lid to the waste package. The waste package will be filled with helium (an inert gas that will reduce corrosion); then two more lids will be installed, welded, and inspected. The waste package will then move to a loading area.
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| Robotic welding machine for closure of waste packages. |
In the loading area, a crane will place the sealed waste package onto the retractable bed of a special railcar called a transporter.
A locomotive will move the shielded transport vehicle underground through the main Exploratory Studies Facility tunnel to a pre-assigned emplacement tunnel. At the tunnel entrance, the transporter’s retractable bed will slide out. Remote-controlled equipment (emplacement gantry) will lift the waste package and move it from the transporter’s bed to its final position inside the tunnel. The waste packages will be placed end to end inside the emplacement tunnel.
Monitoring and testing for long-term performance
To assure continued repository safety, the Department of Energy is required by law to monitor the repository from the start of waste emplacement. The repository design will allow future generations to decide whether to close the repository or continue monitoring and maintaining it, for up to 300 years.
During the monitoring program, experts will continually collect and analyze data on the repository’s safety.
Retrieving the Waste
During the time the repository remains open, its design allows for the waste to be retrieved. This is an important feature if society were to find a use for the nuclear materials or a better method of disposal. The process for retrieving the waste would be done by reversing the operational steps described above.
Closing the repository
Repository closure will involve:
- Installing drip shields over the waste packages in the underground tunnels
- Removing materials and equipment that are not a part of the permanent repository
- Sealing all surface openings to the underground facilities
- Removing all surface facilities
- Restoring the environment to its original condition
Marking the Repository Site
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| Artist’s conception of warning monuments and information center on the crest of Yucca Mountain after permanent closure of the proposed geologic repository. |
When the Yucca Mountain repository is filled to capacity and sealed many decades, if not centuries, from now, the site will be marked to tell future generations of its existence. “Passive institutional controls,” or markers, will be designed to tell our descendants that the area at Yucca Mountain is not totally in a natural state — that a previous generation left something beneath the mountain that needs to remain undisturbed.
Marking the site so future societies will know that radioactive materials are located beneath the surface is a daunting and creative endeavor. The Department of Energy has reviewed some interesting plans from design engineers about how to build permanent markers at Yucca Mountain so that someone thousands of years from today would not disturb the waste. Yucca Mountain designs borrow from those of “futurists” and others, including one science fiction writer, who contributed to early designs for a marker system for the Waste Isolation Pilot Plant (a Department of Energy transuranic waste repository in Carlsbad, New Mexico that will be closed and sealed long before Yucca Mountain).
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