Bin it, sink it, bury it – we still don’t know what to do with our radioactive waste. Is Finland offering an answer with the world’s first deep repository?
BENEATH a patchwork of green fields and beech forests not far from the city of Brunswick, Germany, lies an environmental time bomb. Known as Asse II, it is an abandoned salt mine used as a makeshift store for hundreds of thousands of drums of radioactive waste, dumped there during the 1960s and 70s. In 1988, groundwater began seeping through the walls of the mine as many had feared, threatening disaster.
Since then, a fractious debate has raged over how best to deal with the mine’s contents. Each week, hundreds of litres of brine entering the chambers are collected and stored with the drums of waste, and the mine’s structure is becoming unstable. So a decision had to be made: should engineers backfill the chambers, abandon the mine and leave the waste there in perpetuity, or should they remove it all? Both options are risky. Removing the waste will be complex, take decades and expose workers to radioactivity. If the waste stays and the mine eventually floods, groundwater may become contaminated, potentially exposing those living nearby to deadly radioactive particles.
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Asse II is both a cautionary tale and a microcosm of the global debate on how to safely dispose of spent nuclear fuel. Most experts say that the best way is to bury radioactive waste deep underground in rock vaults, where it will be shielded from the ravages of earthquakes, war, hurricanes and ice ages. “Not much happens if you’re 1000 feet underground,” says Charles Forsberg, a nuclear engineer at the Massachusetts Institute of Technology.
Yet in all but a handful of countries, attempts to create these storage sites have led to impasse. The US was once a leader in such efforts, but has abandoned plans for what would have been the pioneering Yucca Mountain waste facility in Nevada. Only Finland has a deep store that is approaching completion. The Finnish government is expected to give final approval in the next 12 months, allowing engineers to begin carving out the deep storage chambers. But significant hurdles still face such projects in Finland and in other countries that are on track to build repositories, including Sweden and France. Do we really understand enough to bury nuclear waste safely?
One thing is certain: major decisions are urgently needed. At reactor sites across the US, more than 65,000 tonnes of spent nuclear fuel and other waste – mostly uranium, but with nasties like plutonium in the mix – are temporarily stored in steel-lined concrete containers, a dirty legacy that is growing by 2000 tonnes each year. Globally, there is roughly 350,000 tonnes of nuclear waste (see diagram) and stocks of the most dangerous – “high-level” – material are increasing by about 12,000 tonnes annually. China, India and Russia look set to add most to this build-up (see diagram): China alone has 29 reactors under construction and aims to increase its nuclear generating capacity 25-fold by 2050.
At present, much nuclear waste is simply stored above ground. This makes sense in the short term, as the waste’s radioactivity will fall to one-thousandth of its initial level within half a century. At that point, probably the safest option is to move the stuff into deep underground storage.
Finding suitable sites is not proving easy, however. Repositories need stable geology and should be located in sparsely populated areas. Groundwater is bad news as it can bring radioactive particles back to the surface, so the rock needs a low permeability, and its chemistry should limit the ability of radionuclides to dissolve in the event that water does reach the waste.
According to Forsberg, an estimated 10 per cent of the Earth’s surface could be suitable for geological repositories, whether beneath dry land or under the seabed, “but the fraction that is politically acceptable is much smaller”, he says. In the US, UK, Germany and Canada, for instance, successive governments have failed to settle on acceptable disposal sites. In the US, plans for the Yucca Mountain repository, which ate up $11 billion in total, were finally abandoned in 2010. The scale of public alarm over waste stores appears to have taken politicians largely by surprise.
It’s a different story in Scandinavia. On the edge of the Baltic in western Finland, engineers have been drilling through ancient granite bedrock on the pine-cloaked island of Olkiluoto for nearly a decade. Beneath the site, a tunnel spirals downwards 420 metres to Onkalo, as the world’s first permanent repository for spent fuel is named. If the site’s construction licence, now under review, is approved by 2014 as scheduled, then Posiva, the Finnish company that owns Onkalo, expects the final phase of construction to begin in 2015, with the first waste canisters being placed underground in 2022. Eventually some 9000 tonnes of spent fuel will be stored here. Burial will continue for about a century, after which the tunnel will be sealed up.
So what is the secret of Finland’s success? Ever since its quest for a waste repository began in the 1970s, a key part of its strategy was to give citizens a voice in the process, including the power of veto up to the point at which approval for final disposal was needed. “There’s been a commitment and political will to take care of this problem,” says Risto Paltemaa of the country’s Radiation and Nuclear Safety Authority. Sweden, which also has a repository licence under review, has followed a similar approach. In the US, by contrast, the Yucca plan was imposed on Nevada, a state that does not have a single nuclear power plant, provoking a political backlash and resentment among residents.
In addition, the utility companies that own the nuclear power plants in Finland and Sweden are also responsible for figuring out how to dispose of their waste. “It’s much easier in some respects when you own the problem, and you know you own the problem,” says Forsberg. But in countries such as Japan and the US and UK, government bodies are closely involved in managing the disposal of radioactive waste, so the process is subject to complex layers of decision-making and party politics. “The problem is that the time it takes to site, build and license a repository is longer than any political alignment of power,” says Forsberg.
This could be changing. In 2012 a committee of experts appointed by the US government to examine the issue recommended that responsibility for nuclear waste management be transferred to an organisation independent of the Department of Energy. It also called for a new strategy based on building consensus at the community level. The Canadian government adopted a similar approach in 2007. “Most countries now seem to agree that consensual siting is the way to do it,” says Charles McCombie, a nuclear waste disposal specialist based in Baden, Switzerland. “You’ve got to go back to square one.”
This begs a question: what if nations like these still can’t find an acceptable way to dispose of their waste domestically. Could an international facility solve the problem?
Global candidates
In 1990 a UK-based company called Pangea Resources launched a project to identify regions of the globe with the best credentials for deep repositories, and to explore the potential for multinational cooperation. The project identified Argentina, southern Africa, western China and Australia as being geologically suitable, and concluded that Australia was the most favourable in terms of political and economic stability. The company was about to open discussions with the Australian government in 1999 when details were leaked to the press and the project hit a brick wall. “Politicians had a knee-jerk reaction and cut off any debate before it could even get started,” says McCombie, who was CEO of Pangea at the time. The Australian government promptly prohibited disposal of foreign spent fuel on Australian soil.
Pangea’s successor, the (ARIUS), is still exploring options for multinational disposal, primarily in Europe, the Arabian Gulf and Asia. McCombie, who is also president of ARIUS, says he is trying to get like-minded countries together to agree on a solution, and then they can examine the pros and cons of hosting a site. “We are not seeking to prematurely home in on any one country.”
Critics claim that such a scheme would allow some nations to shirk responsibility for their own high-level waste. Yet an international facility could offer advantages of improved security, help small nuclear nations which lack suitable geology within their borders, and allow the costs to be shared. That last point is key given Onkalo’s mammoth price tag, estimated at $4.7 billion. This is being funded through a tax paid by the Finnish energy utilities, but in other countries the government may have to foot the bill.
Then there is the issue of just how safe our waste-disposal systems are. The designs for both Finland’s and Sweden’s repositories are based on a model called KBS-3, developed by SKB, a nuclear waste specialist firm set up by Sweden’s nuclear utility firms. KBS-3 encloses spent fuel rods in 5-metre-long copper capsules. Once sealed, the capsules are put into holes in the repository and the gaps are backfilled with clay. The idea is that the copper will resist corrosion for thousands of years, and any water reaching the clay will cause it to swell up, creating an impermeable barrier.
Although SKB contends that the copper capsules will protect the contents from water for at least 100,000 years, and probably a million years, Peter Szakalos, a corrosion expert at the Royal Institute of Technology in Stockholm, Sweden, sees things differently. As certain radionuclides decay, they give off gamma rays, producing heat. As a result, capsule temperatures will peak at around 100 °C and then cool, reaching 50 °C after 1000 years, Szakalos says. His concern is that hot copper is , and he says SKB’s analysis did not factor in the presence of hydrogen and hydrogen sulphide gas, and that corrosive salts are likely to accumulate on the hot copper. These will eat through the metal far faster than SKB calculates, he argues. “The first 1000 years is the critical part. It is likely that the copper canisters will collapse in this period,” Szakalos warns. We need more research, he adds.
Others are developing chemical tricks that could help lock up the most hazardous radionuclides even if storage capsules corrode away. A team at the University of Leeds, UK, is examining a form of iron hydroxide called green rust that can immobilise dangerous elements and which could be used as a lining for waste capsules. Tests show it can chemically reduce elements like uranium and technetium, making them less soluble and in some cases binding them to the rust’s molecular structure. In a 2011 study, Bo Christiansen, then a geochemist at the University of Copenhagen, Denmark, showed that green rust can also , a particularly troublesome by-product of nuclear fission with a 2.1-million-year half-life. But since green rust is extremely reactive, it can only be used in underground storage if chemists find a way to control conditions, to stabilise it for thousands of years.
An even more ambitious plan is to use radioactive decay itself to transform the waste, turning it into a more stable material. Chris Stanek, a materials scientist at Los Alamos National Laboratory in New Mexico, is trying to understand the changes that take place inside nuclear waste as it decays. What interests him most are the so-called daughter products of radioactivity – the elements and crystal phases formed by decay – and .
Toughen up
Products such as barium-137, formed from caesium-137, and zirconium-90, formed from strontium-90, have significantly different atomic sizes and charges from their parents, so they can form crystalline phases with different volumes, even creating forces which could break up the waste. Current storage concepts don’t really consider this, warns Stanek.
Their work also points to an intriguing possibility: that this transmutation – or radioparagenesis, as Stanek calls it – could be used to our advantage, to reverse-engineer nuclear waste. Zirconium dioxide, for example, is highly stable and therefore a desirable end product. Working backwards from this state, Stanek and his colleagues argue that it could be beneficial to reprocess waste so that the strontium-90 in it is chemically converted into strontium dioxide. The strontium subsequently decays to an isotope of yttrium, which in turn decays into zirconium. Making zirconium dioxide is a simple example, he says, but it shows the potential to “design” .
“It shows the potential to ‘design’ nuclear waste that becomes increasingly stable over time”
Such technology may prove unnecessary, however, if waste can be transmutated in particle accelerators (New Scientist, 30 May 2012) or stored far below ground. Researchers are studying the idea of disposing of waste in boreholes some 5 kilometres deep. Here water exchanges very slowly with aquifers, so any radionuclides that do leak out would take millions of years to return to the surface.
But why stop there, argues environmental scientist Fergus Gibb from the University of Sheffield, UK. Gibb calculates that the hottest waste, sealed into tungsten capsules half a metre across, would generate enough heat to melt granite. Placed at the bottom of a deep borehole, these capsules would . As they move downwards, the granite above would cool and solidify, sealing them underneath.
Gibb’s scheme won’t help dispose of cooler, intermediate-level waste like the material stored at Asse II. The German government has now decided to remove the stuff from the mine, so it will have to find a conventional storage solution when the first drums emerge – sometime around 2033. But for the hottest, dirtiest spent fuel, a slow, one-way journey to the centre of the Earth, entombed inside what Gibb calls a “granite coffin”, may prove to be the cleanest end of all.
This article appeared in print under the headline “Eternal challenge”
