Read more: “How to dismantle a nuclear reactor“
FROM the safety of a computer screen in the control room, I can see a robot scoop up a chunk of asbestos from the reactor floor. I am at Sellafield, the nuclear complex on the coast of Cumbria in north-west England, watching remotely controlled machinery crawl through the defunct Windscale Advanced Gas-Cooled Reactor, gradually stripping out the last of its guts.
The mammoth task of dismantling the reactor started in the early 1990s but is only now finally nearing completion. With its totemic golf-ball shaped exterior, this was the prototype of a brave new generation of British-designed nuclear power plants built between the 1960s and the 1980s. It joined 26 more defunct stations of an earlier design, now dotted around the UK coastline like beached ships. These older plants are so full of radioactive debris that nobody will try to disassemble them until the end of the century at least.
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This problem is hardly unique to the UK. All the world’s civilian nuclear power plants have to be shut down within a few decades of being built (see “How to dismantle a nuclear reactor”). Years of sustained nuclear chain reactions bombard the metal vessel of the reactor with neutrons. The metal and anything the coolant touches become highly radioactive, brittle and prone to failure so that the whole plant eventually becomes a liability. Because the process of making a power plant permanently safe can take longer than the reactor’s operating life, only 17 have so far been fully decommissioned worldwide, and the backlog of closed reactors is now well past 100. And still we keep building more.
“The backlog of closed reactors is now well past 100, and still we keep building more”
But a new generation of power plants built from better materials raises the possibility of an intriguing new solution. The future, says Mike Burke, director of research at the (NAMRC) at the University of Sheffield, UK, lies in nuclear reactors that get around the decommissioning problem by prolonging a reactor’s life – from a few tens of years to a hundred or more.
About 10 years ago, the first plans emerged to change the future of nuclear energy with a new breed of super-efficient reactor that operates at higher temperatures and pressures. Using materials like molten salts or helium gas to cool the reactor and transfer its heat to turbines, these “Generation IV” reactors will burn fuel with greater efficiency and could generate far less highly radioactive spent fuel than their predecessors. India plans to kick-start the Generation IV programme with the commission of its Prototype Fast Breeder Reactor early in 2013.
Fast-breeders burn plutonium and other long-lived radioactive isotopes in spent fuel. More than that, they turn it into new fuel, turning waste into energy.
This is by no means a new concept; fast breeders were among the first research reactors. But they have never been used for commercial power generation. Before they can digest the fuel, fast breeders rely on a method called reprocessing, which can create weapons-grade material. Rather than being discarded, the spent fuel elements are chopped up and dumped in nitric acid. The acid dissolves the highly radioactive products of the fission reactions, including uranium and plutonium, which are then retrieved. This both makes the remaining waste safer for disposal and provides uranium and plutonium for new fuel.
“Nuclear becomes a truly sustainable energy source if you can develop a fuel cycle that incinerates its own waste, and that is what we can do,” says Burke.
There’s just one problem. Burke says the new reactors aren’t being designed with greater longevity in mind, and the intense reactions in a fast-breeder, for example, could actually reduce its lifetime to just a couple of decades.
To work, the technology must be combined with more sustainable materials. These are what Burke’s group is researching at the , which was founded in 2005 as part of a British effort to revive stagnating nuclear research. A critical issue is finding materials that can better withstand the stresses created by the chain reactions inside a nuclear reactor.Uranium atoms are bombarded with neutrons that they absorb. The splitting uranium atoms create energy and more neutrons to split yet more atoms, a process that eventually erodes the steel reactor vessel and plumbing.
See graphic: “Where are the world’s nuclear reactors?“
Bad reactions
The breakdown that leads to a reactor’s decline happens on the microscopic level when the steel alloys of the reactor vessels undergo small changes in their crystalline structures. These metals are made up of grains, single crystals in which atoms are lined up, tightly packed, in a precise order. The boundaries between the grains, where the atoms are slightly less densely packed, are the weak links in this structure. Years of neutron bombardment jar the atoms in the crystals until some lose their place, creating gaps in the structure, mostly at the grain boundaries. The steel alloys – which contain nickel, chromium and other metals – then undergo something called segregation, in which these other metals and impurities migrate to fill the gaps.
These migrations accumulate until, eventually, they cause the metal to lose shape, swell, harden and become brittle. Gases can accumulate in the cracks, causing corrosion.
A key part of extending reactor lifespans will be to build them and their plumbing out of better materials that will not weaken under constant bombardment, says Tim Abram, head of nuclear fuel technology at Manchester.
One solution is to change the composition of the steel by creating new super-alloys with better properties. Xin-Ming Bai and colleagues at the US government’s Los Alamos National Laboratory in New Mexico are using nanotechnology to create smaller-grained alloys. In the nanocrystalline materials they created, fewer atoms were dislodged, creating less damage to the structure of the alloy. Moreover, the atoms dislodged into the gaps at the grain boundaries returned to their original places, a process dubbed “self-healing” (). This research could be the key to radiation-tolerant reactor vessels with lifetimes potentially several times greater than today’s models.
Metals that can withstand all the assaults of a nuclear reaction will be crucial both for lengthening the life of reactor vessels in existing designs and building successful Generation IV reactors. Enough of these tweaks could allow them to operate continuously for hundreds of years.
A reactor that does not need to be shut down after a few decades will do a lot to limit the world’s stockpile of nuclear waste. But eventually, even these will need to be decommissioned, a process that generates vast volumes of what the industry calls “intermediate-level” waste.
Despite its innocuous name, intermediate-level waste is highly radioactive and will one day have to be packaged and buried in rocks hundreds of metres underground, while its radioactivity decays over thousands of years. It is irradiated by the same mechanism that erodes the machinery in a nuclear power plant, namely neutron bombardment. Find a way to make the reactor less radioactive when it has to be dismantled, and the decommissioning problem suddenly looks much less daunting.
Here, too, the Manchester work can help. One reason the reactor vessel becomes so radioactive has to do with the large amounts of cobalt present in reactor steel.
Under neutron bombardment, cobalt turns into radioactive cobalt-60. Low-cobalt alloys would leave less of this troublesome isotope in the waste. Another option is to create a material that will absorb the cobalt and that can also be used to decontaminate decommissioned reactor vessels. German and Indian researchers recently created a polymer that does just that ().
But there’s more to intermediate-level waste than the reactor. Take, for example, the graphite used in the UK’s gas-cooled reactors to slow the neutrons in the chain reaction. Some future designs, such as the , will also be graphite-moderated. When it is shelled with neutrons, graphite accumulates problematic radioactive gases that need to be regularly vented – gases like chlorine-36, with a half-life of 300,000 years.
The UK’s reactors contain about 90,000 tonnes of that radioactive graphite, which adds yet more waste that upon decommissioning requires ultra-safe disposal. But the Dalton Institute scientists are working both to find ways to decontaminate graphite and to manufacture it in ways that prevent the build-up of radioactive gases. They are working on novel ways to vent these gases from the graphite (). Then the non-radioactive graphite can be disposed of by conventional means.
Toxic legacy
The Dalton Institute work will make the future crop of nuclear power plants much safer to decommission and could fortify future reactors for a much longer life. This will include fast-breeders, which could also help dispose of an even more toxic byproduct of the nuclear past: the spent fuel from the world’s reactors.
That highly radioactive waste remains lethal for thousands of years and is without doubt nuclear energy’s biggest nightmare. Efforts to “green” nuclear energy have focused almost exclusively on finding ways to get rid of it. The most practical option is disposal in repositories deep underground. Yet, seven decades into the nuclear age, not one country has built a final resting place for its most toxic nuclear junk. So along with the legacy waste of cold-war-era bomb making, it will accumulate in storage above ground – unless the new reactors can turn some of that waste back into fuel.
Without a comprehensive clean-up plan, the wider world is unlikely to embrace any dreams of a nuclear renaissance. But if a new generation of reactors can be built to limit new waste and even reduce the legacy of decades of nuclear power generation, they might augur a truly green era of nuclear energy.
Read more: “How to dismantle a nuclear reactor“
When this article was first published it incorrectly attributed work done by the University of Manchester’s Dalton Nuclear Institute to the Nuclear Advanced Manufacturing Centre. This has now been rectified.
