THE elevator moves swiftly, but it still takes a minute and a half to reach the bottom. If this were a building, you would have descended 150 floors. As it is, you are now almost half a kilometre underground. Step across the cobblestones, pass through the metal door and you enter a huge hall that has been blasted into the granite. To the left a long driveway leads steeply upwards to the surface. To the right the road runs down into a circular tunnel, off which extends a labyrinth teeming with researchers, engineers, even TV crews and groups of tourists in hard hats. Welcome to Sweden’s Äspö Hard Rock Laboratory, where scientists are perfecting the techniques that will be used in the world’s first permanent underground nuclear repository.
What you notice most down here is the damp. Walking through the tunnels you hear the drips and the running water. You can smell it, too. You can even taste it – a brew of ancient brackish water from deep underground mixed with rainwater that has taken 7000 years to seep down from the surface. That makes this area near Oskarshamn a good test site, because water is the biggest obstacle to safe underground nuclear storage. Running water may carry corrosive chemicals that eat into the containment canisters, allowing radioactive particles to escape, and transport these particles back to the surface.
There are very few areas of the world where radioactive waste can be buried in dry places (see “High and dry”), so experimenting in wet conditions is essential if scientists are to develop a system that will work in most countries. When nuclear material is buried for real, the repositories will need to keep water damage at bay until the spent nuclear fuel is no more harmful than natural uranium – about 100,000 years. That means permanent waterproofing, and if nuclear power is to have a future, researchers don’t have long left to figure out how to do it.
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As concerns grow over carbon emissions from fossil fuels, and fuel prices rise with growing political instability in oil-producing regions, there is much talk of a nuclear renaissance. Governments around the world are dusting off plans to expand their nuclear capacity, and after decades on the back foot the nuclear industry senses a new dawn. Whether that dawn ever arrives depends on the public being convinced that the disposal of waste is permanent and safe.
Meanwhile, the system of temporary storage facilities is looking increasingly inadequate. Worldwide, around 260,000 tonnes of spent fuel is stored in water pools, which cool the fuel rods and shield against radiation. Many sites are already bursting at the seams. In the US, for example, most cooling pools are so densely packed that the fuel rods must be separated by neutron-absorbing boron panels to prevent the spent fuel from going critical and undergoing a chain reaction. The International Atomic Energy Agency (IAEA) estimates that by 2020 there will be 445,000 tonnes of spent fuel. Existing facilities and those under construction will not be able to hold it all: they are expected to be filled to the brim by the end of 2017. Add to this the concerns about terrorist attacks on these temporary sites, and getting the waste locked away underground becomes even more pressing.
The major nuclear nations have been aware of this for decades, yet plans to build permanent repositories have been plagued by setbacks. In the US, for example, it is 23 years since a storage site was identified at Yucca Mountain in the desert about 150 kilometres north-west of Las Vegas. Local people have always resisted, but in 2004 the US Senate finally overrode a veto from the State of Nevada. This decision, later affirmed by the Court of Appeals, made it possible for the Department of Energy (DOE) to apply for a permit to begin constructing the repository, a process that could take years. The debate is far from over, however. Last spring the DOE announced that a hydrologist from the US Geological Survey (USGS) had fabricated data on the amount of water flowing through the mountain. What this means for the safety of the proposed storage facility is as yet unknown, but it could be critical given that Yucca Mountain was chosen precisely because it is such a dry location. The affair is now being investigated by both the DOE and the USGS.
Plans to develop permanent nuclear repositories in other nuclear countries are also stalled. Japan is still looking for candidate sites and does not expect to open a repository before 2035. The UK’s nuclear waste agency, Nirex, is back at the drawing board after choosing a site beneath the Sellafield reprocessing plant in Cumbria in 1991 and then rejecting it in 1997. In 2003 the European Union urged member states with nuclear power to choose repository sites by 2008 and have them operational by 2018. The directive has since been ditched because of strong opposition. France, Belgium, Germany and Switzerland are all carrying out research into deep storage, but only Sweden and Finland are committed to taking the next step. Sweden plans to choose a site in 2011 and have its repository up and running by 2017. The Finnish authorities have already started blasting tunnels at Olkiluoto near the city of Turku. The 500-metre-deep dump is due to open in 2020.
Both Finland and Sweden will use storage techniques being developed at Äspö. The component parts of the planned repository have already been tested separately, and a full-scale trial is now up and running to examine how the complete system will perform. To that end, the research team has buried six full-size canisters – 6 meters long and weighing 25 tonnes each – 450 metres down, at the end of a 61 metre tunnel. The canisters, made of cast-iron and coated with corrosion-resistant copper, were lowered into vertical shafts lined with a clay called bentonite, and then covered with more of the clay. Bentonite swells when it is saturated with water and presses against the rock walls, encapsulating the canisters completely, protecting them from damage by running water and earthquakes. Finally the team backfilled the tunnel with 30 per cent bentonite and 70 per cent crushed granite and sealed it with concrete (see Diagram).
The burial site is fitted with sensors to measure conditions such as temperature, water pressure, water movement, humidity and tensions in the rock walls. The only difference between the test and the future real repository is that the canisters now underground contain electric heaters to mimic the heat created during radioactive decay, and not the spent-fuel elements themselves. “Here we can make mistakes, learn from them and make sure they are not repeated in a real repository,” says Britta Freudenthal from the Swedish Nuclear Fuel and Waste Management Company (SKB).
Fast-forwarding time
In another side tunnel at Äspö, engineers are testing methods to retrieve the containers. When Sweden completes construction of the world’s first permanent repository, it will have the capacity to hold 4500 canisters, but initially only 400 will be buried. During the first few years safety assessments will be carried out, and if anything goes wrong it must be possible to remove the canisters. Wrenching them from the firm grip of the bentonite could damage them, but small scale tests suggest that they can be removed safely by washing the clay away with low-pressure jets of salt water. A full-scale experiment to retrieve a canister buried in 1999 started in January and should be completed by June.
The full-scale trial is set to run for 20 years. That’s a blink of the eye compared with the lifetime of a repository, so smaller, accelerated tests are also being carried out to simulate events during the first millennium, when radioactivity will be at its peak. In the full-scale experiments the surface of the containers reaches a maximum temperature of 90 °C, as in a real repository. As a result, any water in contact with the canisters will not boil away. This is important because if it did, it would deposit corrosive salt on the copper. In the accelerated tests, the surface temperature has been increased to 130 °C, which speeds up chemical reactions in the clay so that one year is equivalent to 40 years. Additionally, the bentonite has been spiked with trace amounts of radioactive caesium-134 and cobalt-60 so the researchers can study the migration of radioactive isotopes in the clay, mimicking what would happen if containers were to corrode and leak. In 2000 three such experimental canisters were examined after one year underground, and tests showed that, as expected, there was almost no movement of radioactive particles through the bentonite. More test stands are due to be opened this year, while others will be left untouched for many years.
When Sweden first planned its repository decades ago, the underground environment was thought to be sterile. Research at Äspö has revealed that the rocks are actually teeming with microbial life, and in recent years it has become clear that microbes could pose a threat to any repository. Attention has particularly focused on bacteria that derive their energy from reducing sulphate to sulphides, which are highly corrosive. The danger is that these sulphur bacteria will form biofilms on the surface of containment canisters that will cause them to rust. The storage system being developed at Äspö should ensure that does not happen. “Experiments have shown that if the bentonite has the planned density of 2000 kilograms per cubic metre, all active bacteria are eliminated [from it] within months,” says Karsten Pedersen from Gothenburg University in Sweden. Some inactive spores do survive, but the models suggest that even these will die out over a period of 3000 years.
Other underground microbes may even help prevent corrosion by removing oxygen from the environment. One source of oxygen is groundwater that seeps down through the rocks, but tests at Äspö show that at just 25 to 70 metres below the surface, all the dissolved oxygen has been removed by bacteria that derive their energy from oxidation. Even oxygen in air trapped inside the repository tunnels after they are closed up probably won’t last long. Experiments in natural cracks at 450 metres indicate that microbes there consume the gas very quickly. “We are now working to quantify the capacity of the microbes,” says Pedersen. “For instance we would like to be sure that the microbial processes would be fast enough during a coming ice age, when large amounts of oxygenated meltwater may reach the repository.”
Microbes may also limit the movement to the surface of leaking radioactive particles. That’s because most of these take the form of metals, and many bacteria use trace amounts of metals for growth. Iron-oxidising bacteria, for example, bind iron onto rock surfaces as a rusty red deposit, and experiments at Gothenburg University have shown that these bacteria are able to oxidise radioactive metal species as well. So, radioactive particles would be fused to the rock deep underground rather than moving up to the surface. “Microbes may turn out to be an additional safety barrier in a deep repository for spent nuclear fuel,” says Pedersen.
The scientists at Äspö are confident the technology will be tested and ready by 2017, when Sweden’s permanent repository is due to open. They already seem to have won the battle for public opinion. While other countries struggle to overcome “not in my backyard” attitudes, Sweden and Finland seem to have persuaded their citizens. Last May, a nationwide opinion poll in Sweden showed that 82 per cent of the population is in favour of disposal within their own country, while 75 per cent think that the present generation should deal with the waste, provided it can be stored safely. Even people living in the two municipalities that have been chosen as likely sites are in favour of burying the spent fuel right under their feet. In Finland too there is national and local support for the planned repository at Olkiluoto.
These attitudes have much to do with the openness surrounding the research at Äspö. Anyone can download details of research reports and safety assessments from the SKB website (), and with 12,000 visitors each year, Äspö has become one of Oskarshamn’s top tourist attractions. “Most of the local people have been here to see for themselves what we are doing,” says Äspö’s director Anders Sjöland. “We are even preparing the next generation by welcoming many school classes.”
High and dry
Gone are the days when people entertained the idea of blasting nuclear waste into space. The consequences of a launch failure don’t even bear thinking about. Now burial is considered the only realistic option. Sweden, Finland and many other nuclear countries have no choice but to build their repositories in wet underground environments and devise ways to keep running water out. An alternative approach consists of identifying potential sites that are completely dry and will remain so for the next 100,000 years, even with climate change and geological processes. Promising sites include salt domes, which are almost impermeable to water, and desert.
Germany has taken the lead in investigating the salt dome option, but in 2000 its government put plans to build a repository in the Gorleben mine on hold. In the US, the Department of Energy is still pursuing the dry option at Yucca Mountain in Nevada. The plan here is to store 77,000 tonnes of high-level radioactive waste in the middle of the mountain, 300 metres below the surface and between 300 and 400 metres above the groundwater aquifers. Precipitation in the area is low – around 18 centimetres per year – and most of the rainwater evaporates quickly or runs down the mountainside. Very little water is expected to seep through cracks in the rock and reach the repository, and if it does, the heat from the radioactive waste should evaporate it before it can do any damage.
The other advantage of dry storage is that if a canister fails, movement of radioactive particles into the groundwater would be very slow as there is no water flow to transport them. The joker in the pack, however, is the possibility that a wetter climate in future might raise the water table.
