The future of nuclear waste storage

Materials World magazine
,
3 Oct 2016

The world’s high-level nuclear waste, produced over decades by hundreds of nuclear power stations scattered across the globe, largely sits above ground in temporary storage, awaiting a more permanent home. That is about to change, as James Perkins reports.

Convincing communities to host a ‘nuclear waste dump’ for 100,000 years is an understandably difficult sell for governments. That’s why, after 60 years of nuclear power there is no operational deep geological storage (DGS) facility for nuclear waste.

The science is settled that this is the best way to store the waste, hundreds of metres underground and surrounded by favourable geology but, after decades of work, the two most advanced projects, Yucca Mountain, USA, and Cumbria, UK, have recently been cancelled. However, with typical Nordic pragmatism, Finland became the first country in the world to grant a construction license for a DGS mine in December 2015. The €3bln Onkalo spent nuclear fuel repository, located on the island of Olkiluoto, near the construction site of the troubled European Pressurised Reactor, is expected to come online in 2020.

Olkiluoto has hosted a working nuclear power plant since 1978 and was first identified by nuclear power company Teollisuuden Voima (TVO) as a potential site for DGS in 1987. It was officially selected in 2000, the local council issued a building permit in 2003 and excavation began the following year. Posiva, a joint venture of Finland’s two nuclear power providers, received the construction licence.

The approval of construction at Onkalo is a significant moment for the nuclear industry. As Ben Heard, Director of Think Climate Consulting and PhD Candidate at the University of Adelaide, Australia, says, ‘The absence of a service for used fuel has proven a serious bottleneck in global nuclear development. It brings an element of uncertainty to the decision to develop a nuclear power sector and provides a powerful rhetorical weapon for the opponents of nuclear technology.’

Others are set to follow in Finland’s footsteps. Sweden, which developed the KBS-3 storage system to be used at Onkalo, approved plans for a repository in June. The UK, 40 years after the Flowers Report, is developing its approach to a national geological screening process and released results of a public consultation in April 2016. Meanwhile, the South Australian Nuclear Fuel Cycle Royal Commission, in May 2016, recommended the South Australian Government investigate building DGS to host the growing amount of high-level nuclear waste (HLW) emanating from Asia and the rest of the world. 

This DGS renaissance has arrived for a few reasons – confidence has grown in the KBS-3 technology and in geologists’ ability to select sites, and governments have become more attuned to the needs of local communities. Finally, the amount of nuclear waste is growing and there is a lot of money invested in finding a home for it. A conservative estimate states a 138,000t South Australian DGS facility would reap AUS$257bln in revenue, with costs of AUS$145bln, over its 120 year lifetime. To put it into perspective, South Australia’s GDP in 2015 was AUS$98.6bln.

How best to store it

The global inventory of used fuel is estimated to be in the order of 390,000tHM in 2015, according to the International Atomic Energy Association (IAEA). By 2090, this is anticipated to be more than one million tHM. Fission products and transuranics are part of the used fuel pellets, which are ceramics cladded with metal. They are radioactive and generate large amounts of heat. The most radioactive components have decayed within 500 years, but the fuel requires isolation and containment from the environment for 100,000 years to return to the same level of radioactivity as the uranium had when it was mined.

DGS uses a combination of engineered and natural barriers to isolate the material. With the KBS-3 method, the waste is first stored for 30 years, then casted in iron canisters, then cased in copper alloy (CuOFP) capsules, before being deposited in a layer of bentonite clay hundreds of metres below the surface of the earth.

Bruce Yardley, Emeritus Professor of Metamorphic Geochemistry at University of Leeds and Chief Geologist to the Radioactive Waste Management Directorate of the Nuclear Decommissioning Authority, UK, acknowledged questions raised by Stockholm’s Royal Institute of Technology in 2012 about the long-term resilience of the copper canisters, but told Materials World, ‘others argue that the criticisms are not applicable to a repository setting’. He said, ‘The granting of the licence in Finland, after a very thorough review, gives me confidence in the KBS-3 method.’

Finding the right geology

A DGS facility has two requirements – it must allow waste to be stored deep, in a stable environment and, through a combination of geology and engineering, must ensure radionuclides are not transported to the surface in a way that causes harm. Radionuclides can make their way to the surface dissolved in groundwater, as a gas, or through human intrusion by drilling or mining.

The three preferred geologies for DGS are salt, strong rocks with low water permeability, such as granite, and mudstones or clays. Salt is currently used as a host for intermediate level waste in New Mexico, USA. Finland is now licensed to host its spent repository in granite, while France and Switzerland have plans for facilities with mud as the host. 

Yardley says the three long-term issues are tectonic and volcanic activity, alongside the possibility of future glaciations. ‘[These are] are sufficiently predictable on a million-year timescale that [...] they can be avoided'. A new ice age is expected in the northern latitudes within the timeframe, but Yardley says, ‘With good understanding of the past glacial history of a region, it should be possible to make good predictions of likely future impacts and design a site accordingly.’

Also, even minor ‘uplift’ underground can have a serious impact. ‘In the timescales that need to be considered for waste disposal, even quite modest tectonic uplift rates could result in hundreds of metres of uplift and erosion over the relevant timescale.’ 

Convincing the local community

Deep geological disposal is agreed amongst experts as the preferred way of storing HLW, but finding a suitable site and convincing the local people to accept their home becoming a 'nuclear waste dump', has not been easy. Finland has shown a way forward, through allowing communities to ‘volunteer’, then embarking on long-term consultation, scientific study and compensation for nearby communities, but it has been a slow process.

Yardley says, ‘The science of geological disposal has been well-established for some time, but what is changing is largely to do with social aspects. Developing co-operation and trust with local communities is something that takes a very long time and the progress we are seeing now is the result of concerted efforts over the last two decades.

‘Other countries, such as the UK, are committed to working with local communities in a similar fashion, but are further back on the curve. I think we will see a number of countries begin work on constructing facilities in the next decade.’

Dr Jonathan Cobb, Senior Communications Manager for the World Nuclear Association, agrees, ‘A characteristic of the project in Finland is that it has strong local public and political support. The same is true for the proposed repository in Sweden and this support is one of the reasons why these two projects have made good progress.’

Heard believes the movement in this area could become a seminal moment in the fight against climate change. ‘The availability of the first approved service could bring with it a profound tilt in investment away from fossil fuel development and toward nuclear development,’ he said. ‘While that’s great for the nuclear sector, the more important issue for me is that we are talking about a serious departure from high-greenhouse gas emission pathways for fast-growing nations.’

Are we wasting the fuel by burying it?

Heard says the material currently being stored above ground has not been used to its full potential. ‘The technology to extract around 150 times more energy from used fuel and depleted uranium is demonstrated and available right now in a reactor design that is commercially advertised but not yet licensed. Several other advanced nuclear reactors are in the development pipeline with great recycling potential.’

According to the World Nuclear Association, recycling the U-238 isotope that remains in most nuclear fuels would decrease waste to one-fifth of its current level, also ensuring the radioactivity of the material decreases more rapidly. Prototypes for recycling plants, fast breeder reactors and fast neutron reactors are under development right now, but are encountering problems. For example, Japan has spent almost 30 years and US$25bln on its nuclear fuelrecycling programme, which is yet to become operational, and most of the experimental fast neutron reactor programmes have been shut down.

Heard remains optimistic, 'Given the timeframes in question, I think there is more than enough evidence that the smart move is to store above-ground in secure and retrievable containers and participate actively in bringing these technologies into commercial operation. So, while a repository is central to the plan today, I would not be surprised to see a course-correction toward recycling in the next five-to-fifteen years.’

Dr Bruce Yardley, Chief Geologist to the Radioactive Waste Management Directorate of the Nuclear Decommissioning Authority, explains three particularly effective types of geology for a disposal facility.

Salt deposits

They are widespread and comprise of either thick salt beds or cross-cutting salt domes that have risen through younger rocks. As well as being dry, salt is weak, so caverns constructed in salt will creep and collapse over the operational life of the facility, sealing the waste in the salt – this makes it very unlikely that water will be able to penetrate.

Strong rocks with very low permeability and water content, such as granite

Their strength means that large caverns can be constructed, and the cost of construction is relatively low. Normally, granite bodies contain some fractures and local zones of higher permeability. Water will be present in these and is able to move through the rock along connected cracks. However, if these are very sparse, then the amounts of water able to flow through a repository will be extremely small, even over long time periods. There are many examples of this type of rock containing old saline groundwater that has moved little over many thousands of years.

Mudstones or clays

A stiff mudstone still contains water, but only in small pores between clay particles, where it is effectively immobile – the pore water is normally an ancient brine. Mudstone is too weak to excavate in large caverns, but tunnels can be bored that are several metres in diameter, and where the rock has been cracked by construction these cracks will reseal with time if water gains access because of the swelling properties of the clay minerals when wetted. As with a repository in salt, canisters embedded in a mudstone repository are kept effectively dry and there is no water flow to transport radionuclides from corroded canisters towards the surface.