Best of both worlds

CANADA HAS ENTRUSTED TO THE NWMO the important responsibility of managing Canada’s used nuclear fuel over the very long term. Since its inception, the NWMO has been working to ensure the plan it implements on behalf of Canadians is both technically safe and socially acceptable.

In Canada’s plan the NWMO will safely store the country’s used nuclear fuel within a deep geological repository about 500 metres underground in a suitable rock formation. The use of a deep geological repository is consistent with international best practice. This approach is the culmination of more than 30 years of research, development and demonstration of technologies and techniques.

Within the repository, a series of engineered and natural barriers will work together to contain and isolate used nuclear fuel. The five barriers are the fuel pellet, the fuel bundle, the used fuel container, the bentonite clay buffer box and the geosphere. Each of these barriers provides a different and stand-alone level of protection.

In 2010, the NWMO initiated a site selection process to seek informed and willing hosts for the project. This process is still under way.

As Canada’s plan advances towards site selection, so does the technical programme. The NWMO is now in the proof-test stage of development — testing several prototype components of the used fuel container to demonstrate manufacturability and safe implementation.

“In 2019, the rubber hit the road, as we aim to demonstrate the technology can be implemented safely and efficiently by building full-scale prototypes,” said Chris Boyle, director of engineering at the NWMO.

Used fuel container innovation

The NWMO’s used fuel container is a Canadian solution specifically optimised for used Candu fuel. It has a hemispherical head — similar in concept to the hemispherical ends of a submarine — to withstand significant pressure.

The container is made of carbon steel for strength, and coated with corrosion-resistant copper. (Copper corrodes incredibly slowly in a repository environment where there is little oxygen.) The copper is directly adhered to the steel, so the two materials act as one assembly and in the unlikely event the used fuel container becomes deformed it will retain its corrosion resistance.

The NWMO designed, developed and fabricated a first-of-a-kind system in 2019, to clad its used fuel steel containers with copper through an electroplating process. The electroplating process takes place in the NanovateTM Tank System (NTS) and the NWMO worked with industry vendors to create a purpose-built, first-of-a-kind tank. “It’s a novel tank that took previous existing technology and scaled it up, making it larger to accommodate the NWMO’s used fuel containers. We’re proving that we can do this process at the scale needed,” said Boyle.

The NWMO worked with a Canadian industry partner on the electroplating process to create copper cladding that is 4-5mm thick as plated.

Although electroplating has been around for at least 100 years — it is typically used to make copper pennies — this is one of the first adaptations of the technology to create thicker cladding. Many factors, such as temperature, chemical solution, and surface finish can affect the outcome. The NWMO and an industry partner optimised these factors to achieve high-purity copper with a smooth finish and no voids or porosity.

The NWMO has begun serial production of prototype containers and up to 20 will be produced by 2021. During the demonstration process, the organisation’s facility in Oakville, Ontario, will be used as a distribution hub for container parts, as they are shipped to various contributors to the programme for processing, such as machining, copper-coating and welding.

“Our programme has moved from concept to small-scale development trials of processing technology, to a first early container prototype, and now to proof testing and serial prototype production,” said Dave Doyle, manager of used fuel container design at the NWMO.

Non-destructive examination techniques

Following various stages of container fabrication (welding, copper coating and machining), the NWMO uses non- destructive examination to ensure the components meet the required specifications.

It uses proven technology such as ultrasonics (which works in a similar way to an ultrasound) and eddy current (an electromagnetic testing method). It has adapted these standard methods to the specific design of Canada’s used nuclear fuel container, to make them work for the container shape (ie the hemispherical heads) and the thick copper cladding.

“Super-container” approach

As part of the multiple barrier system, the used fuel container will be placed in a highly compacted bentonite clay buffer box and these will be emplaced in a deep geological repository.

The NWMO chose to use bentonite clay for the buffer box and repository backfill because it is proven to be a powerful barrier to water flow. A natural material formed from volcanic ash, bentonite clay swells when exposed to water, making it an excellent sealing material.

The method in Canada will be to pre-assemble the used fuel container and buffer box above ground and then place this ‘super-container’ in the deep geological repository. “The advantage is a lot of the assembly work can be done at surface level, potentially simplifying the emplacement concept,” said Boyle.

“Although Canada did not invent the super-container idea, we’ve made significant enhancements including ongoing testing with the objective to eliminate the need for a metal frame,” he added. The NWMO found in optimising the container for Candu fuel that it could be made smaller and lighter than for other countries. Conventional-sized equipment can be used for placement.

In 2021, the NWMO will conduct trials for full-scale emplacement of the buffer boxes into a mock emplacement room. It will test the equipment that will be used to lift the buffer boxes and place them in the mock emplacement room.

Before the trials begin, the NWMO’s engineering specialists are exploring variations in the buffer box assembly and emplacement equipment to provide the safest and most robust performance. This has already had useful outcomes. For example, bentonite is subtly affected by environmental conditions and the low moisture content in winter air causes the bentonite blocks to develop micro- cracks on the surface. To safely assemble the buffer boxes with a vacuum lift, the NWMO stores the bentonite clay in a humidity-controlled storage area to keep the surfaces smooth and free of cracks.

The deep geological repository

The NWMO will select a site with either crystalline or sedimentary rock for its deep geological repository. Both these types of rock are suitable, because their geological characteristics are stable over time and have low permeability, which means there will be little groundwater movement.

Canada’s repository will consist of a network of underground placement rooms, along with surface facilities where the NWMO will receive, inspect and repackage the used fuel into purpose-built containers.

Building on the industry standard, the NWMO continues to evolve the conceptual design of its repository. It will include a central services area, which allows for the ventilation of the underground repository through three shafts located within a single secure area. The layout has several access tunnel arms that enable the NWMO to situate placement rooms in areas with the most suitable rock.

“As we learn more about potential sites, we are continually updating our conceptual designs to improve the operations and logistics of the repository while optimising layout flexibility, ease of construction and concurrent placement and development activities,” said Chip Lee, manager of mining and repository engineering.

The NWMO has learned from other countries. For example, Finland’s Posiva is at the most advanced stage of developing a repository for used nuclear fuel of any organisation, as it received a construction licence in 2015. Posiva’s repository incorporates aspects of this flexible design.

The layout of the placement rooms in Canada will depend on the geology of the host site. In preparation, the NWMO has begun work on site-specific conceptual designs of the underground repository layout for potential siting areas in Ontario, based on information from geoscience assessments and initial borehole drilling. This will be an iterative process. As the NWMO acquires additional site specific information, it will continue to evolve the repository design.

Social engagement

Canada’s plan for used nuclear fuel will only proceed with informed and willing hosts, including municipal, First Nation and Me´tis communities. The NWMO initiated the siting process in 2010, inviting expressions of interests from communities who wanted to learn more and explore their potential for hosting the project.

Interested groups and individuals learn about the project through local engagement activities and also by visiting NWMO’s proof-test facility to see physical prototypes, speak with technical experts conducting the work and better understand how the engineered barrier system will function within the repository. To date, the NWMO has hosted more than 100 tours.

Communities involved in the siting process have also had the opportunity to contribute to it. For example, NWMO has sought out community guidance in locating socially acceptable borehole drilling sites. Indigenous Knowledge keepers have walked the land with technical specialists and cultural monitors have been present at borehole drill sites.

The NWMO is increasingly exploring partnership and community well-being with communities in the siting process.

Looking forward

The NWMO is on track to select a single preferred site for its used nuclear fuel repository by about 2023. As Canada approaches this important milestone, technical specialists at the NWMO will continue to validate the safety and effectiveness of the components of the engineered barrier system.

Combining international best practices with Canadian innovation, the NWMO has confidence that its multiple barrier system will safely manage used nuclear fuel in the long term.

Author information: Deena Waisberg, Senior writer at Canada’s Nuclear Waste Management Organization