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 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.
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.
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.
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.