Deep thought on disposal3 November 2022
Successfully creating a deep geological disposal site for high-level waste has long been a key challenge for the nuclear industry. Now, a series of breakthroughs seems to show light at the end of the tunnel
While nuclear generation is now regarded as a key player in the clean energy transition, long-term disposal of nuclear waste remains a key challenge for the industry. Deep geologic disposal is widely seen as the most viable solution when it comes to the permanent disposal of high-level waste, notably spent fuel assemblies, but there has been notably little progress on developing physical repositories. At least that was the case until recently as this year has witnessed a number of significant developments. Among mounting evidence that progress on final repositories is now accelerating, a site for Switzerland’s deep geological repository has now been selected.
Over a three-year drilling campaign which began in 2019, the National Cooperative for the Disposal of Radioactive Waste (Nagra) – the body responsible for the Swiss disposal programme – drilled nine boreholes across three potential siting regions, each of which features a layer of Opalinus Clay over 100 metres thick that is very tight and solidly bedded. This Opalinus Clay layer will host the Swiss repository and is the most important long-term safety barrier for containing the radioactive materials.
While the regions differ, for example the depth of the Opalinus Clay layer can vary as can the adjacent rock strata, after an initial programme of 18 months, Nagra was able to confirm that it would be possible to construct a safe deep geological repository within all of the three regions being considered. Further exploration and subsequent drilling operations across the regions of Jura Ost, No¨rdlich La¨gern and Zu¨rich Nordost ultimately saw a total of 10,000 metres of bores drilled and more than 6000 metres of drill cores recovered as part of the CHF170m (US$170m) campaign. Having produced a full subterranean map and despite the coronavirus and minor technical difficulties while drilling, Tim Vietor, Head of Geology and Safety and member of the Nagra Executive Board, confirmed that not only is it possible to construct a repository, but there is also sufficient space for a combined repository that will hold low, intermediate and high-level waste materials. The Swiss Nuclear Energy Act stipulates that Switzerland’s radioactive waste must be disposed of in a deep geological repository. Nagra has proposed a combined repository that is suitable for all types of radioactive waste.
Nagra announced that it was pleased with the results in No¨rdlich La¨gern, where the Opalinus Clay is very tight and a coral reef lies above the clay layer. In September Nagra announced that it had selected the region located in the Zu¨rcher Unterland (lowlands) north of Zurich in Canton Zu¨rich as the preferred repository location. Although all the sites have similar host rock, differences in the confining geological units mean that No¨rdlich La¨gern is better suited for repository construction than expected and has the largest safety reserves. In addition, the site also has the largest underground area suitable for construction, giving the greatest flexibility for the final layout of the repository. In 2015, Nagra had been concerned that constructing the repository in No¨rdlich La¨gern would be more challenging from an engineering perspective. However, further investigation showed that Nagra’s initial assessment had been too cautious.
Nagra CEO Matthias Braun noted that the development of a siting proposal for a deep geological disposal site can thus begin and Nagra is now transitioning from the research phase to the licensing and implementation phase. Nagra will now prepare the general licence applications, which it expects to submit to the Federal Council in 2024. However, around 30 years or so will pass before Nagra can actually start waste emplacement operations, it says.
The surface entrance to the repository is to be constructed in the Haberstal district area of Stadel within the Canton Zu¨rich. Nagra also plans to construct encapsulation plants at the Zwilag interim storage facility which already exists and has been in operation for many years. It is located in Wu¨renlingen in Canton Aargau.
Overcoming technical storage challenges
Along with progress on a physical site for long-term geological waste disposal Nagra is among many research group that is also working on addressing some of the technical challenges this solution presents. For example, scientists from the US Lawrence Berkeley, Sandia and Los Alamos national labs, are collaborating with Nagra on the HotBENT project which is looking how well bentonite retains its safety functions when exposed to long-term heating. Bentonite is a natural, clay-based material that is placed around canisters of buried, high-level waste. Because it swells when water reaches it, this characteristic helps prevent any waste transport.
“The concern is that heat emitted by underground nuclear waste will change the geophysical and geochemical properties of the bentonite buffer and the host rock,” explained LianGe Zheng, Berkeley Lab’s lead scientist on HotBENT. “For this long-term series of experiments, we will evaluate the thermal, hydrological, chemical, and mechanical changes in the bentonite and how that affects the material’s safety function over time,” Zheng added.
The HotBENT project is being led by Nagra but also includes the US DOE and partners in Canada, Japan, and the UK among others. By demonstrating bentonite tolerance of higher temperatures, the ambition is that the project could allow for more radioactive waste to be safely stored in subsurface repositories by allowing tighter spacing between waste canisters and reducing the overall repository footprint.
Field testing began in September 2021 at Switzerland’s underground Grimsel Test Site. With four heaters installed, the temperature is being slowly increased over several months to eventually reach 200°C. This is double the current maximum allowable temperature under consideration for repositories. However, previous research has suggested that even at twice the maximum temperature, bentonite does not lose much of its beneficial properties with the ability of the bentonite to swell only decreasing by about 4% at most in the simulations. Over the initial 18-month testing programme, the scientists will be looking at changes in the materials which may affect the ability to swell.
In the longer term, the construction of the Grimsel site will allow a partial dismantling of one sector after five years of heating in around 2026. Full dismantling of this sector is anticipated after a heating and hydration phase after about 20 years in around 2041.
“If the Swiss test site can largely verify the modelling results, then the bentonite buffer may be able to retain much of its protective function at much higher temperatures than previously considered,” said Zheng.
Canada’s Nuclear Waste Management Organization (NWMO) is also working with Nagra and recently announced progress on its waste disposal programme with a full-scale demonstration of its engineered barrier design at its test facility in Oakville, Ontario.
“All elements of the demonstration performed as expected and according to plan,” said Chris Boyle, Vice-President and Chief Engineer at the NWMO.
The Canadian high-level waste repository will be built more than 500 metres underground and the design uses a series of five engineered and natural barriers. Two areas are currently being considered for the repository location, the Wabigoon Lake Ojibway Nation-Ignace area in northwestern Ontario and the Saugeen Ojibway Nation-South Bruce area in southern Ontario.
During the test, containers designed for used nuclear fuel were moved into a simulated repository space and the remaining space was then filled with a loose granular bentonite clay material. Once assembled, each used fuel container together with its bentonite-packed buffer box weighs some 8000kg. In-depth analysis is now underway to assess the results.
While the US has not identified a new site for long-term disposal of high-level waste since Congress stopped funding the Yucca Mountain site in 2010, work there does continue. In August, US-based Deep Isolation and Amentum signed a Memorandum of Agreement to commercialise Deep Isolation’s nuclear waste disposal technology which features directional drilling. The company argues this is an alternative to the conventional deep excavation approach. Initial targets for joint work include Europe and Pacific nations and the companies estimate there is a market worth more than $30bn.
“The world is changing fast, and it’s imperative for the success of nuclear energy that we solve the nuclear waste challenge,” noted Deep Isolation CEO Elizabeth Muller.
Approvals take a step forward
Along with technical and physical developments there have been important political and regulatory moves this year too. For example, in France Cige´o, the deep geological disposal project for High and Intermediate Level waste is the subject of a declaration of public utility (DUP). The move, which saw the government decision of decree published in the official journal in July, marks a key point in the licensing process of the Cige´o project for the French National Radioactive Waste Management Agency (Andra). The repository will stand on the border between the Meuse and Haute-Marne departments and is designed to hold high level waste at a depth of 500 metres in a stable geological layer. The design includes the potential recovery of waste packages already disposed of and over a duration of not less than 100 years. The aim is to ensure reversibility for the disposal facility over future generations. This reversibility will be mainly achieved by progressively building up the facility, with adaptability of the design and the flexible disposal system, integrating technological progress and adapting to any potential changes to energy policy.
Although a significant development, the declaration of public utility for Cige´o is not a construction licence but the construction licence application is expected to be submitted to the French Nuclear Safety Authority (ASN) towards the end of this year.
Sweden has also announced a significant development this year as the government decided to allow SKB to build a final repository in Forsmark in O¨sthammar Municipality along with an encapsulation plant in Oskarshamn. The next step in the licensing process is for the Land and Environment Court to establish relevant conditions while the Swedish Radiation Safety Authority will also decide on permit conditions under the Nuclear Activities Act. SKB has been researching and developing technology for the final repository for more than 40 years and the search for a site for a geological repository began in 1992. Nonetheless, only when all licences are in place can actual construction start. It will take about 10 years to build the spent fuel repository after final approvals are in place.
“We are now looking forward to implementing Sweden’s largest environmental protection project,” said SKB’s CEO Johan Dasht in a statement.
The case will now return to the Land and Environment Court and the Swedish Radiation Safety Authority.
While progress has been recorded across numerous nations, Finland is by far the leader in developing a physical storage location. Construction of a geological disposal facility 400-450 metres beneath the ground in Olkiluoto is nearly complete. The first waste is expected to be placed in 2024 and in July excavation of the first five actual final disposal tunnels was completed. Holes for approximately 180 disposal capsules will be drilled in the first five disposal tunnels, the total length of which is about 1700 meters.
Known as ONKALO, the site was selected following preliminary screening of the entire country, before detailed site at four shortlisted sites. While all of the sites would have been suitable, Olkiluoto had the largest available area and a large portion of the spent fuel was already stored there. The government took the final decision in 2001 and tunnel excavation started at the beginning of May 2021. Posiva is the responsible body and is owned by the country’s nuclear generation companies.
While deep geological disposal is the clear winner as the long-term solution to high-level waste like spent fuel, it is also evident that even in those nations like Finland, Sweden and France that are leading the development race, actual storage is some way off. Given this means the continued use of wet and dry storage of spent fuel assemblies on-site considerable research is also underway here too. The IAEA, for example, has recently launched a new Coordinated Research Project (CRP) to explore the longer-term implications of spent nuclear fuel storage systems. The Agency notes that most on-site storage systems were originally designed to last 20–50 years. However, the duration of spent fuel storage prior to long-term disposition has been steadily increasing over decades. Some six decades of wet storage, and four of dry storage have been seen and now durations of 100 years or more are typical. In some countries periods of up to 300 years and being envisaged. Nonetheless, while good performance has been reported and valuable operational experience gained, work is now underway to develop the scientific basis for extending the duration of currently licensed storage approaches and additional monitoring and inspection techniques as these systems reach the end of their original design lifespans.
The new four-year study “Performance Assessment of Storage Systems for Extended Durations” aims to gather operational experience and research results on the performance, monitoring and inspection of spent fuel storage systems.
The main objective of the programme is to strengthen technical knowledge on the long-term behaviour of wet and dry spent fuel storage systems as well as inspection and monitoring technologies. Specific research objectives include identifying degradation mechanisms of materials used in wet and dry spent fuel storage systems, developing new methods for maintenance and inspection on storage systems and transport casks and developing techniques to monitor the containment of dry storage systems, among other areas of research.
Worldwide the nuclear industry will inevitably create deep geological disposal locations that will become operations over the coming decades and 2022 will stand out as a big year in terms of key development breakthroughs. As an industry this is vital for the future, but for many nations with active nuclear programmes it is still some way off becoming reality.