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What do you get when you cross an accelerator with a nuclear reactor?

Source: www.guardian.co.uk

By: Corrinne Burns  

Professor Bob Cywinski is every inch the academic: a wavy-haired, bearded man with a voice like hot coffee poured on a Sunday morning. But he is also a man with a dream: to change the nuclear landscape of the UK.

 Conventional nuclear power (fission) is controversial and carries inherent risks, but no other energy source has a chance of securing our energy needs for the future. Nuclear fusion – for many scientists the ultimate goal of energy production – is still a long way off.

 Cywinski is part of a team of scientists who are working towards an entirely new type of nuclear reactor: one that could be operated safely and without generating long-lived radioactive waste. This new reactor could even consume the toxic waste generated by conventional nuclear reactors, removing it from the ecosphere.

 It’s called the Accelerator-Driven Subcritical Reactor (ADSR), or Energy Amplifier, and in a recent lecture hosted by the Leicester Literary and Philosophical Society, Cywinski outlined his vision of an ADSR-powered future.

 The concept was first proposed in 1993 by Nobel prizewinning physicist Carlo Rubbia. The basic idea – and what distinguishes it from all other nuclear reactors – is the coupling of a particle accelerator, like the ones at Cern, with the reactor core.

 That may sound bizarre upon first reading, but there’s good science here.

 Conventional reactors are fuelled by uranium – specifically, the uranium isotope U-235. That’s a lively old isotope that likes to split: it is “fissile”. When U-235 splits, it releases neutrons, and these go on to initiate an energy-generating nuclear chain reaction by splitting still more U-235 atoms.

 But there are downsides to the use of uranium-235 as fuel: first, it produces plutonium as waste. Second, the uranium-235 fuel cycle is what engineers call “critical”: once it gets going it’s self-sustaining, so there is a risk – albeit a tiny risk – of loss of control.

 In the ADSR proposed by Rubbia, we wouldn’t use uranium-235 as nuclear fuel at all. Instead, we would shift two spaces to the left in the periodic table, to uranium’s unsung cousin: thorium. Despite being named for the god of thunder, thorium sits quietly in the Earth as a safe, unreactive mineral – and it sits there in great abundance, especially in Welsh earth.

 Unlike uranium-235, the thorium atom does not easily split, making it safe to store and handle. But we need a fissile atom to initiate the energy-generating nuclear reaction. Since thorium is not fissile, it must be converted to something that is.

 That’s where the particle accelerator comes in.

 In an ADSR, the thorium-containing reactor core would be coupled to a particle accelerator. This would fire up a beam of protons before slamming them into a block of lead inside the reactor core. The bombardment induces the lead to release neutrons, in a process called spallation. Those neutrons are then smashed into the thorium atoms, turning them into atoms of uranium-233, which is fissile – and so the reaction begins.

 It’s still nuclear fission, but a crucial safety difference between a conventional nuclear reactor and an ADSR is that in the latter the reaction operates at subcritical levels: it is not self-sustaining. So in the event of a problem, all the operator has to do is switch off the proton beam. Almost immediately, the reaction will cease.

 Furthermore, the small amount of toxic waste generated by the thorium/uranium-233 fuel cycle ceases to be radioactive after a few hundred years, rather than the thousands of years during which uranium waste remains toxic. Better yet, an ADSR could actually utilise, as fuel, the plutonium waste created by current reactors, eliminating toxic waste while generating further energy.

 But surely that particle accelerator needs a lot of energy to operate? Yes, it does. However, you get far more power out at the other end. That’s where the ADSR’s unofficial name – Energy Amplifier – comes from. The Thorium Energy Amplifier Association, ThorEA, calculates that an ADSR would generate 600MW of electrical power – pretty much the same as a conventional power station.

 Yes, the accelerator will require power input – around 20MW – but that power can be taken from the ADSR’s own output, leaving an excess 580MW of electric power.

 So what we have, in principle, is a reactor running off stable, abundant fuel, producing an excess of energy, with no danger of meltdown. If ADSRs are really this perfect, how come we don’t already have one?

 The problem is that, for the moment, our available options for the accelerator are limited. Commercial accelerators are pretty big, not to mention expensive to build and run. We can’t have a Cern in every city. If we’re going to have ADSRs as standard power stations, we have to get around this.

 That’s where Emma comes in. The Electron Model of Many Applications, Emma for short, is a new type of accelerator designed to be the perfect partner for an ADSR. A prototype Emma lives under the grounds of the Daresbury Laboratory in Cheshire. Emma’s unique selling point is that she is a new hybrid of a cyclotron and a synchrotron, combining the advantages of both into a compact, economical form.

 Last month, Nature Physics published the first results of Emma’s operation, showing that she is indeed, despite her petite proportions, capable of stably accelerating electrons to the kind of velocities needed.

 Emma is a proof-of-principle for the new hybrid. She accelerates electrons, not protons, so will never be connected to an energy amplifier. But what we learn from Emma will be used to construct proton versions in the near future.

 Given sufficient investment, ADSRs could be operational in the UK by 2025. But do we really need them, given that conventional reactors, however unpopular, do still work? Yes, says Cywinski, because it’s time to move on. As he puts it: “The Stone Age didn’t end because we ran out of stone.”


Corrinne Burns  




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