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What is nuclear fusion?
Nuclear fusion is a nuclear reaction that produces huge amounts of energy but virtually no pollution or long term nuclear waste. The process is the opposite of splitting the atom, which is how we currently get nuclear power. Instead, as the name suggests, two atoms of hydrogen are made to fuse.
It happens naturally in the core of the Sun, where temperatures of more than 10 million degrees cause hydrogen atoms to slam into each other violently enough to fuse together. When this happens, a new atom is created, and tiny particles – called neutrons – are released. These neutrons are one of the key signatures of fusion.
Why is fusion so important?
It could be the solution to all of the world's energy problems. The fossil fuels – coal, oil and gas – are running out. These took millions of years to create but in the last 300 years alone, since the start of the industrial revolution, we have almost used them all up.
With the earth's population growing and huge populations in countries such as China and India beginning to access cars, fridges and heating systems – which all require fuel – many scientists are convinced that an energy crisis is looming. The solution could be nuclear fusion.
It runs on a type of hydrogen found within heavy water, which makes up a small part of all ordinary water. Calculations have confirmed that there is enough water on earth to satisfy all the energy needs of a growing population for many millions of years. The fusion power would be harnessed in fusion reactors and – just as in a conventional power plant – would be turned into electricity and fed onto the grid. Scientists have estimated that a bathtub of heavy water could power the UK for 30 years.
Why don't we have it yet?
For many years there was a huge barrier that had to be overcome: fusion experiments always used up more energy than they created, something known as the "break even" problem.
These experiments were attempting to create conditions like those in the centre of the sun – which has a temperature of around 10 million Kelvin. Just getting the experiments hot enough to work required huge amounts of energy so, in total power terms, the experiments didn't break even.
In the UK in the 1990s, however, a series of very important experiments took place. These finally showed that more power could be generated from fusion reactions than was required to make them happen. But even though the "break even" problem had been overcome, fusion scientists still believed that it would be around 40 years before fusion power finally made it on to the electricity grids. Many sceptics were much less optimistic.
What is the claimed breakthrough in the film?
A scientist in the US has published two papers detailing how he achieved fusion from sound waves. He uses a process called sonoluminescence, which is a way of turning sound into light. This process, although known since the 1930s, was little-studied until the 1980s when scientists began to make measurements of it and realised that the light produced was incredibly hot, easily tens of thousands of degrees.
This was an incredible discovery in its own right. It meant that sonoluminescence was a very good energy focusing system. It was increasing the energy of the soundwave by a factor of a trillion, that's 1,000,000,000,000. If the energy could be increased by an additional factor of 100 then fusion was possible.
The challenge then was to make this happen, and various groups around the US set to work to achieve this goal. One, Rusi Taleyarkhan, claims to have succeeded by engineering a very high pressure system using very small bubbles which focus the energy of the soundwave even better.
What are the challenges facing "sonofusion"?
There are two main challenges. The biggest is for Rusi Taleyarkhan's results to be independently reproduced. Until now, no one but Horizon has published data on replicating Taleyarkhan's results and many scientists remain highly sceptical about this set of results, although they do not dispute the principle that sonofusion is potentially achievable.
Their scepticism focuses on Taleyarkhan's use of synthetic neutrons in his experiment. Neutrons are one of the key signatures that fusion has taken place so using synthetic neutrons in the experiment means that neutron detection has to be extraordinarily good. Horizon used the best neutron detection system available for projects specifically like this and we found no fusion neutrons.
The second main challenge facing sonofusion will come when and if the work is successfully replicated. Then it will face the same "break even" problems as other nuclear fusion systems. Scientists are hopeful that this could be overcome because in principle, it would be thermonuclear fusion, which is the right kind of fusion for energy production. One idea put forward is that sonofusion could be a route towards a more efficient, second generation of nuclear fusion.
Is this the same as "cold fusion"?
No. Cold fusion was an infamous scientific episode that started in March 1989. Two scientists claimed to have achieved nuclear fusion in a test tube at room temperature, arguing that it involved a "new kind" of nuclear reaction. It was announced at a press conference and the news went around the world. But crucially it had not yet been thoroughly peer reviewed.
Over the next few months other scientific groups tried to replicate the findings but failed to find any signs of a nuclear reaction. Rusi Taleyarkhan's breakthrough has been reviewed and published in respected journals, but it has not yet been replicated. It also is not a new kind of fusion but regular, "hot" fusion done in a new way.
What is similar is that cold fusion seemed to offer a short cut to nuclear fusion and because of its remarkable energy-focusing properties sonoluminescence could ultimately also represent a shortcut to fusion.
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