Going underground

Many countries consider that the best way to dispose of nuclear waste in the long term is to bury it deep underground. Simon Morgan looks at how this could be done

It is an inescapable fact - radioactive waste exists. Some of it will remain hazardous for hundreds of thousands of years. Many countries are currently considering whether to reinvest in new nuclear power programmes, and it is a commonly held view that the waste problem must be solved before any new power stations are built. But whether or not any new waste is created, the planet’s existing radioactive waste legacy needs to be dealt with. 

Governments around the world are deciding how best to deal with the legacies of nuclear waste that have built up since the 1940s. So far, 19 countries have opted to bury their long-lived higher activity wastes deep underground. 

UK waste issues 

The UK has a particularly wide and challenging range of radioactive wastes to deal with as a result of nuclear weapons programmes and the variety of different reactor designs used in its civil nuclear power plants and the supporting R&D programmes. Many other countries don’t have the same range of radioactive wastes, with fewer types of radionuclides to worry about. 

Since the 1970s, several attempts have been made to implement solutions to the UK’s radioactive waste problem, but each has ended in failure. These failures have largely resulted from a previous lack of public engagement and support for the scientific basis behind disposal options. But in 2001 the UK government started a new approach involving much greater public participation. In July 2006 its independent Committee on Radioactive Waste Management (CoRWM) recommended deep geological disposal as the best available approach for long term management of intermediate and high-level waste in terms of safety and security - the two issues of most importance to the public (see Chemistry World, September 2006, p8). Gordon MacKerron, CoRWM’s chairman, thinks that public support is now possible: ’The results of our engagement with stakeholders and the public since April have confirmed that our recommendations are scientifically and technically robust. They also provide the basis for inspiring wider public confidence in any future process.’ 

Deep disposal 

Radioactive decay processes are well documented, but we also need to understand how the materials and components making up a geological disposal system will evolve in the very long term.  

Very slow chemical reactions such as mineralisation will occur as a waste repository slowly evolves into a natural isolation and containment system. 

A geological repository must be safe for tens to hundreds of thousands of years and the scientists and engineers involved in developing these plans need to understand the long-term chemical processes that will affect a repository and its surroundings over these immense time periods. 

Nirex, the independent organisation charged with developing and advising on safe, environmentally sound and publicly acceptable options for the long-term management of radioactive materials in the UK, has been developing repository designs around a multi-barrier, phased and reversible approach. The designs are based on emplacing waste deep underground where it is much less vulnerable to disruption by manmade or natural events. Nirex uses the designs to derive standards and specifications against which radioactive waste is packaged in the UK, and waste packaging proposals can then be assessed within Nirex’s ’letter of compliance’ process to ensure compatibility with the repository designs. Nirex’s designs build on the lessons learnt from previous failures by taking the public’s views into account. After the waste has been placed underground it will still be possible to retrieve it for a period of up to several hundred years. 

Going to waste 

Nirex’s repository concept for intermediate level waste (ILW) is based on the principles of containment, isolation and chemical conditioning. The aim is to prevent, or at worst slow down to a safe level, the release of radiotoxic substances to the environment while the natural process of radioactive decay occurs.  

Wastes are often immobilised in a cement-based grouting material within standardised stainless steel or concrete containers. Cement is the preferred material for encapsulating most ILW - it is easily produced, stable and long lived. Special formulations have been devised to tolerate the range of radiological, chemical and thermal conditions expected in a repository. 

For ILW, the waste container is important too, but on a shorter timescale. Stainless steel is used because of its excellent resistance to corrosion; waste containers are expected to resist penetration by corrosion under repository conditions for many thousands of years. After that, radionuclides can only escape from the container if they are able to diffuse through the cement block encapsulating the waste. Calculations have shown that diffusion through cement is slow. More than 90 per cent of the radioactivity will decay inside the container during the first 1000 years after repository closure and only a tiny fraction of the total radioactivity in the package will ever escape. 

To isolate this hazardous waste from the environment, waste containers would be placed in underground vaults between 300 and 1000 metres deep. These would be constructed in a stable geological environment, selected so that the rock surrounding the repository has a long and slow groundwater pathway to the surface and, ideally, favourable geochemical and mineral properties to prevent or delay the movement of radionuclides. Studies by the British Geological Survey have shown that about 30 per cent of the UK’s deep geology would potentially be suitable. 

Closing time 

Once a decision is taken to close the repository, the plan is to fill the vaults with a specially developed cement-based backfill material. This will provide a very effective physico-chemical barrier against the movement of radioactive materials into groundwater, but it has also been designed to be of low strength so that wastes can be retrieved even after the vault has been backfilled.  

The repository will eventually become saturated with groundwater after it is closed and sealed. The incoming groundwater will dissolve small quantities of the cement-based backfill, becoming more and more alkaline as it fills the vault. The high pH environment greatly slows down corrosion of the stainless steel containers. The chemistry of the cement porewater is dominated by the hydroxyl ion (OH^-), which favours the formation of low-solubility metal hydroxides, effectively reducing the amount of radioactivity that can be dissolved in the groundwater.  

The backfill also provides a high surface area for the absorption, physical adsorption and chemical adsorption processes that will partition many radioactive species from solution to the solid phase. Assessments have shown that the system can be designed and located to ensure that no more than 0.005 per cent of the radioactivity will ever reach the surface, and this will occur over a period of hundreds of thousands of years. 

Over the long lifetime of a repository, it is theoretically possible that migration of radionuclides could cause a critical mass of fissile material to accumulate. This could happen in a way similar to the process that led to the natural fission reactor which went critical and burnt itself out almost two billion years ago, deep underground at a place now called Oklo in Gabon, west Africa. However, Nirex has considered the combination of unlikely conditions and mechanisms that would have to act together - modelling studies have shown that even if a criticality did occur, it would only affect a small volume of the repository and the impact on the overall performance of the system would not be significant. 

Meeting criteria 

The UK and other countries have used well established assessment tools to show that geological disposal is viable. In the UK, the results of these assessments show that the regulatory criteria set by the environment agencies and the Nuclear Installations Inspectorate can be met. Chris Murray, Nirex’s chief executive, has a high degree of confidence in the long-term performance of geological disposal. ’There is a strong scientific and technical consensus around this option and it puts the UK broadly in line with other nations tackling the same problem. A geological solution represents the only truly long-term option and is technically viable in the UK now,’ he says. 

Many leading learned societies and professional institutions, including the RSC, also agree that deep geological disposal is the appropriate long-term management option for nuclear waste. But although there is agreement, there is also a need to promote a multi-disciplinary approach when thinking about scientific problems needing a publicly acceptable solution. A conference coming up soon in Loughborough, UK, should go some way to achieving this goal.  

It is encouraging that scientists, and chemists in particular, will play an important role in helping to solve the international problem of radioactive waste disposal. 

Simon Morgan is regulatory liaison manager at Nirex.

A two-day workshop for scientists and engineers entitled UK long-term nuclear waste management: Next steps  will take place on 6-7 November 2006 at Loughborough University, UK.  

Further Reading

• Committee on Radioactive Waste Management: Managing our radioactive waste safely; CoRWM’s recommendations to government, CoRWM Doc 700, July 2006. 
• United Kingdom Nirex Limited, The viability of a phased geological repository concept for the long-term management of the UK’s radioactive waste, Nirex Report no. N/122, November 2005. 
• Oklo’s natural fission reactor: American nuclear society.
• N A Chapman et al, 1986, Proceedings of international symposium on siting, design and construction of underground repositories for radioactive waste, IAEA, Hanover. Paper IAEA-SM 289/37.