A Global Village
Issue 8 » Materials

Burying Our Nuclear Legacy

New Materials for Deep Geological Disposal

Georgios Katsigiannis, Imperial College London

Deep geological disposal of nuclear waste has gained growing attention from both the Government and the scientific community in the UK spearheaded by the International Atomic Energy Agency (IAEA), the Royal Society of Chemistry and the Geological Society over the past few years. Given its realistic implementation in the country, as well as its promising long-term safety potential, geological disposal appears to be the best available approach to long-term nuclear waste management in the UK.

According to the Geological Society (1999) “only deep geological disposal can provide a long-term, safe and sustainable solution for radioactive waste”, while the Royal Society (2006) argued that “the confidence that could be placed in geological disposal in the UK sites has been understated”.

Unlike above ground storage methods – referred to by some as the ‘store and wait’ approach - proposed by some environmental organisations, the design concept and sealing properties of deep geological disposal can ensure effective isolation of the nuclear waste.

Such facilities have already been adopted in a number of countries possessing significant amounts of high level nuclear waste,1 including Canada, France, Finland and Sweden. Yet repositories of this kind must still overcome public opposition.

The UK has been producing nuclear waste since the 1940s, and the development of sustainable storage solutions has been actively discussed ever since. A major programme aimed at building an underground research laboratory in Sellafield – offering a potentially sustainable nuclear waste storage option – stems from the 1980s, yet the initiative stalled in 1997.

The design concept
and sealing properties
of deep geological
disposal can ensure
effective isolation
of the nuclear waste

Over the following few years the situation was reviewed by the House of Lords and the Government, resulting in the initiation of the 2001 the Managing Radioactive Waste Safely (MRWS) programme. Some years later the Independent Committee on Radioactive Waste Management (CoRWM) suggested a short-list of options for the safe disposal of the country’s nuclear waste. In 2006 the Government agreed that deep geological disposal, combined with safe interim storage, was the best approach for its management.

Things started then to move faster. In 2008 the White Paper: ‘Managing Radioactive Waste Safely’ was published, whilst in 2009 the Joint Research Centre of the European Commission stated that “our scientific understanding of the processes relevant for geological disposal is developed well enough to proceed with step-wise implementation”.2

The Swedish Äspö Hard Rock Laboratory is a demonstration site excavated in bedrock with excellent stability properties

In the UK, the Nuclear Decommissioning Authority (NDA), the implementing body for the deep geological disposal, is developing a parametric cost model for such a facility, which amounts to £4 to £12 billion.3 By exploiting international experience in the field to carry out its own research and site specific investigation, UK scientists can reasonably aim to develop a reliable, safe and environmentally friendly permanent disposal site in the near future.

However, the development of a nuclear waste disposal site in the UK is not solely a technical or cost challenge, but social one as well. One of the most critical issues is that of site selection, which has to proceed on a volunteer and mutual agreement basis rather than a primarily geological criteria. Local residents of potential sites often exhibit ‘not in my backyard’ opposition despite favourable regional seismic conditions.

Sites in Cumbria and Romney Marsh are being examined as a possiblity. Should the discussions move ahead, the selected host area would start to store British radioactive waste from 2040 onwards.4

With the development of
any site depending on local
public opinion, it is up to
the scientific community to
communicate the design
and its advantages including
substantial job creation

With the development of any site depending on local public opinion, it is up to the scientific community to communicate the design and its advantages including substantial job creation.

Nuclear Barrier
Several concepts have been proposed for the safe deep geological disposal of high activity nuclear waste. They all have in common a combination of natural and engineered barriers that effectively isolates nuclear waste from human beings and the environment.

Digging a deep geological repository is restricted to accessible areas – mainly on-and near shore sites – within rock that is geologically stable with limited groundwater flows to depths between 250 and 1000m.5

Having been processed to a suitable form for disposal (e.g. in glass form via a procedure called vitrification), the waste is placed into a copper container of high strength and corrosion resistance. This is in-turn surrounded by a compacted layer of a key soil material called buffer, which both protects the canister against corrosive attack and rock movements, and prevents water from penetrating into the canister.

Additionally, this buffer has the potential to retard any potential leakage of radioactive substances from the canister. The last layer of the repository is composed of the rock itself, providing the ultimate effective barrier aimed at limiting the flow of groundwater, gas release or the movement of radionuclides.

Within this framework, the buffer governs the overall behaviour of the system. Buffer materials are chosen depending on both the nature of the waste to be stored and the type of rock hosting it. However they possess common physical propeties that are critical to the efficient isolation of radioactive waste, namely: very low permeability, high swelling and self-sealing potential, adequate thermal conductivity and mechanical resistance, high exchange capacity and radionuclide retention properties.

Compacted bentonite-based materials and mixtures of bentonite with sand or crushed rock satisfy these requirements, and have been proposed as potential buffer materials. Other substances have been examined in different countries with the choice depending mostly on local availability.

Deep geological nuclear waste disposl relies on a ‘buffer’ layer of a key soil material, which protects the surroundings from potential leakage of radiactive substances from the canisters

The deep geological repository to be implemented in Sellafield could be built based on the KBS-3V concept developed in Sweden. Under this model the buffer consists of one solid bottom block, 6 ring-shaped blocks surrounding the metallic canister and 3 solid top blocks. A space between the blocks and the deposition hole walls – filled with high density bentonite-briquette shaped-pellets – has to be left to facilitate the installation procedure.

Following the assembling of the system, buffer interactions with both the wet host rock and the canister take place. The dense bentonite-based materials, initially unsaturated (i.e. voids only partly filled with water), absorb water from the wet host rock and start swelling, thus filling the space left between the canister containing the waste and the rock hosting it.

Researchers over the last decades have investigated the properties and the behaviour of potential buffer material. Its mechanical behaviour has been examined through a series of standard soil mechanics tests, starting with oedometer tests which allow simple volume and load control measurements. Experiments examining swelling pressures and swelling strain, water chemistry, shearing behaviour, hydraulic and temperature gradient experienced by the buffer have also been developed.

Large scale tests – aimed at enhancing our understanding of the very complex processes taking place in the buffer in experimental conditions similar to those of an actual repository – have taken place at the Prototype Repository and the FEBEX project, both funded by the European Union. The Prototype Repository is a research programme investigating the Swedish concept of deep nuclear waste disposal KBS-3V.

Feasability studies of disposing of spent fuel in Sweden are being carried out in a prototype repository at the Äspö Hard Rock Laboratory (Äspö HRL) near the Oskarshamm Nuclear Power Plant. Swedish bedrock is both mechanically and chemically very stable, witnessing only very slow changes, and hence provides an excellent study environment.6 In this model, electric heaters are used to reproduce the heat energy flow that would emanate from the nuclear waste. Besides providing crucial information relating to the interaction of bentonite clay, copper canisters and the rock hosting it, research carried out in this prototype repository has also enabled the study of both the bedrock’s ability to filter radioactive substances, and the groundwater’s flow and chemical constituents. This project has been going on since September 2001, and is still providing valuable large-scale testing information required to start building a deep geological repository for spent nuclear fuel.

The FEBEX (Full-scale Engineered Barriers Experiment in crystalline host rock) project is based on the Spanish deep geological disposal proposal. Under this model, the copper canisters isolating the spent fuel are placed horizontally and are surrounded by bentonite blocks.7 The project included a full scale in-situ test in Switzerland, a mock up test in Madrid and several lab tests. Other large scale underground experiments are taking place in Bure (France), in Horonobe and Mizunami (Japan), in Nevada (USA) and in several other countries including Belgium, Canada, Korea and Finland.

Finding a safe, long-term disposal method for the UK’s arsenal of radioactive waste accumulated over the past decades remains a thorny problem. Despite receiving a cautious welcome by a concerned public, deep repositories offer a promising way to store spent fuel and seem to be our only hope of a viable long-term solution to our nuclear waste problem.

It’s a matter of taking responsibility, and not passing the buck. Nuclear waste is a problem that we created, it’s a problem that needs a resolution now, and it’s a problem that shouldn’t be shouldered by future generations.

Georgios Katsigiannis is a MSc student in Soil Mechanics and Environmental Geotechnics, from the Department of Civil and Environmental Engineering at Imperial College London.

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[1]   Nuclear Decommissioning Agency, 2011.
[2]   “Geological Disposal of Radioactive Waste: Moving Towards Implementation”.
[3]   NDA, February 2011.
[4]   Black R. (2012) Nuclear Waste Consultation Closes. BBC News, 23 March. [online] Available at: <http://www.bbc.co.uk/news/science-environment-17476465> [Accessed 23 July 2012].
[5]   World Nuclear Association (2010) Storage and Disposal Options. [online] Available at <http://world-nuclear.org/info/inf04ap2.html> [Accessed 23 July 2012].
[6]   Studying Possibilities for Nuclear Waste Disposal, Prototype Repository in Swedish Bedrock, Vaisala News.
[7]   ENRESA 2000.
[8]   SKB (2010) Design, Production and Initial State of the Buffer. [online] Available at <http://www.skb.se/upload/publications/pdf/TR-10 15webb.pdf> [Accessed 23 July 2012].
[9]   Gens A., Ledesma A., Pusch R., Borgesson L. (2004) THMC Processes in Engineered Barriers : The Experience from Febex and Prototype Repositories.