In the deep geological repository, a series of engineered and natural barriers will work together to contain and isolate used nuclear fuel from the environment.

Barrier 1: The Nuclear Fuel Pellet

The first barrier in the multiple-barrier system is the fuel pellet. Fuel pellets are made from uranium dioxide powder, baked in a furnace to produce a hard, high-density ceramic. Ceramics are extremely durable; they do not readily dissolve in water, and their resistance to wear and high temperatures make them one of the most durable engineered materials.
This picture is a close-up of a nuclear fuel pellet.

Fuel pellets are one of the most durable engineered materials.

Barrier 2: The Fuel Element and the Fuel Bundle

Each fuel bundle is composed of a number of sealed tubes called fuel elements. Fuel elements contain the fuel pellets and are made of a strong, corrosion-resistant metal called Zircaloy. The function of each element is to contain and isolate the fuel pellets.

This image shows a nuclear fuel bundle, made up of a number of rods. It also points out visually that the ceramic pellets of uranium dioxide are placed inside each rod.

Zircaloy contains and isolates the used nuclear fuel.

Barrier 3: The Used Nuclear Fuel Container

Used nuclear fuel bundles will be placed into large, very durable containers. In 2014, we refined our container design to one that is optimized for the used CANDU fuel produced by Canadian nuclear power reactors. Together with the bentonite clay buffer box (Barrier 4), the container is a key part of the engineered barrier system.
The container prevents radionuclides in the fuel from escaping into the underground environment. It is engineered to remain intact and keep the used nuclear fuel completely isolated until the fuel’s radioactivity has decreased to levels of natural uranium.
Each used nuclear fuel container holds 48 used fuel bundles in a steel basket within a carbon steel pipe. The pipe has the mechanical strength to withstand pressures of the overlying rock and loading from three-kilometre-thick glaciers during a future ice age. The pipe is protected by a corrosion-resistant copper coating.
The container has a spherical head that is welded to the core of the container. This spherical shape is designed to withstand significant pressure. The carbon steel pipe and copper coating technology for this container design are based on proven technology that is readily available in Canada. The used fuel container and supporting components will be manufactured at a container plant which could potentially be located in the host community or surrounding region, depending on interest. 
This image shows a used fuel container and points out it is 2.5 metres long and 0.6 metres in diameter, with a copper coated steel shell. It shows the fuel basket, which is placed into the steel container core.

Made from steel and copper, used nuclear fuel containers are designed to contain and isolate used nuclear fuel in a deep geological repository, essentially indefinitely.

Barrier 4: Bentonite Clay

During placement in the repository, each used nuclear fuel container will be encased in a highly compacted bentonite clay buffer box. Bentonite clay is a natural material proven to be a powerful barrier to water flow. It swells when exposed to water, making it an excellent sealing material. Bentonite is also very stable, as seen in natural formations formed millions to hundreds of millions of years ago.
The chemical properties of the bentonite clay, backfill and sealants would help to isloate any radionuclides in the unlikely event they were to escape from the container.
Each buffer box will be placed and separated from the next with bentonite clay spacer blocks. Containers will be stacked in two layers.
After the used nuclear fuel containers are placed in the repository, all open spaces in each underground chamber will also be filled with bentonite clay.
A six- to 10-metre-thick highly compacted bentonite clay seal and a 10- to 12-metre-thick concrete bulkhead will be used to seal the entrance to each placement room.
This image shows how all five barriers work together inside the host rock to encase the used nuclear fuel.

Bentonite clay is a very stable material formed naturally from volcanic ash released millions of years ago. 

Before closing the repository, tunnels and shafts will be filled with similar backfill and sealants, isolating the repository from the environment. The performance of the repository will be monitored during placement operations and during an extended postclosure period.

The chemical properties of the bentonite clay, backfill and sealants would help to isolate any radionuclides in the unlikely event they were to escape from the container.

Barrier 5: The Geosphere

The geosphere forms a natural barrier of rock, which will protect the repository from disruptive natural events, water flow and human intrusion.

The repository will be approximately 500 metres underground - the exact depth will depend on the site. It will be excavated within a sedimentary or crystalline rock formation that meets the safety and technical requirements of the project. The rock formation selected will have low permeability, which means there will be little groundwater movement. The traces of water that exist at depth, known as porewater, can take 1,000 years to move one metre through the rock and well over 100,000 years to reach the surface.

It will ensure the repository safely contains and isolates the used nuclear fuel, even under extreme scenarios.

This diagram shows the multiple-barrier system that will contain and isolate the used nuclear fuel.

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