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THE long-anticipated nuclear renaissance has arrived. In his State of the Union address last month, President Barack Obama announced plans for the US to build a new generation of nuclear power plants, and his budget for 2011 proposes large funding increases for the industry.
Several European countries are also likely to restart their nuclear power programmes soon. The UK plans to increase to 20 per cent the proportion of its electricity generated from nuclear.
A return to nuclear power is attractive right now for many reasons. It promises to help cut carbon emissions and reduce imports of fossil fuel. What's more, unlike renewables, it can ensure a stable baseload electricity supply whatever the weather.
However, nuclear energy also creates problems of its own, not least the risk of Chernobyl-style accidents and the production of radioactive waste that takes tens of thousands of years to decay. One thing Obama did not spell out is how the US will deal with a new generation of waste now that it has abandoned plans for a storage facility at Yucca mountain.
There is a way of returning to nuclear while overcoming all these concerns: hybrid nuclear fusion. The concept has been around for decades, and has been discussed in the technical literature and at the International Atomic Energy Agency. But it has not yet been explained to governments, industry, researchers and the public.
Hybrid nuclear fusion combines the two forms of nuclear power, fission and fusion, in a single reactor. This has several advantages over fission alone: it minimises the environmental impact, reduces risks, enlarges reserves of nuclear fuel and is more flexible to operate.
Fission, the process behind conventional nuclear power, harnesses energy from the radioactive decay of uranium and other fissile materials. Fusion, meanwhile, is an experimental technology that extracts energy from processes similar to those occurring inside the sun, where hydrogen atoms are fused together to form helium.
"Pure" fusion is often touted as the solution to all our energy problems, and it has undeniable advantages over fission. It produces no long-lived nuclear waste and needs no fuel other than water. But it could take another 50 years to make fusion technically and economically viable - if it can ever be made to work at all.
One problem with fusion is the size of the reactor core. To make a fusion reaction self-sustaining requires a plasma volume of about 3300 cubic metres, more than three times the proposed volume of ITER, the world's most advanced fusion project now under construction in France.
Another unsolved issue is how to construct a reactor wall, or "blanket", capable of withstanding intense bombardment from high-energy neutrons generated by the plasma. Materials that can do this do not yet exist.
Hybrid nuclear power potentially solves both these problems. First, the blanket is itself a fission reactor that burns nuclear fuels and generates neutrons. In the process it absorbs high-energy neutrons from the plasma, reducing the energy flux reaching the outer wall by a factor of 50, meaning that existing materials could be used.
Second, a hybrid reactor's plasma ball can be much smaller than in a pure fusion reactor - about the same size as ITER's, in fact - because energy generated by fission can be fed back into the plasma to keep it burning.
Hybrid reactors have other advantages too. One is that the fission reaction can burn a range of fuels, including the long-lived high-level nuclear waste produced in conventional fission reactors. It "transmutates" these waste products into isotopes that decay over a hundred years rather than tens of thousands. Not only does this eliminate some of the nuclear industry's waste problems, it also potentially helps to rid the world of plutonium and other weapons-grade materials.
Hybrid reactors can help rid the world of plutonium and other weapons-grade materials
Hybrid reactors also sidestep looming shortages of the high-grade uranium required to fuel conventional reactors, as they can run on non-enriched uranium and thorium. Low-grade uranium and thorium are plentiful in most parts the world. And because the fissile material produced in the blanket remains at well below critical mass, hybrid reactors have a much lower risk of suffering an accident than conventional reactors, as runaway reactions and consequent meltdown are impossible.
Finally, the power output of a hybrid reactor can be easily varied. That would allow nuclear power to be combined with renewables, which are inherently unpredictable, to provide baseload power.
There is growing interest in hybrid reactors. The Institute of Plasma Physics in Hefei, China, a world-class fusion research centre, is planning to build a prototype by 2020 in collaboration with China's growing nuclear industry. Other countries, including those participating in ITER, are also looking at R&D programmes on hybrid reactors. Last month the UK science minister, Paul Drayson, suggested that nuclear research in the UK and elsewhere should consider hybrid systems. The US energy secretary, Steven Chu, has also mentioned hybrid reactors. Existing fusion research programmes, such as the Culham Centre for Fusion Energy in Oxfordshire, UK, can contribute a great deal to these efforts.
Workable hybrid technology is still some way off, but given the inherent problems with fission and the uncertainty over fusion it has to be worth pursuing. Even modest-sized reactors could provide affordable and almost limitless energy for all. Hybrid fusion deserves wider understanding and support from governments, scientists and environmentalists.
Julian Hunt is professor of climate modelling at University College London and a former fusion researcher.
Graham O'Connor is a former senior scientist at the ITER project
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