The promise of unlimited cheap energy from nuclear fusion has been around for a long time. It’s touted as the ultimate climate saviour. The new D2N2 joint mayoral authority pins its green strategy on it. But the most the UK’s experimental reactor has run for is five seconds. And a prototype power plant is not expected until 2040. So what’s the big deal?
Physicist extraordinaire Dr Jonathan Frost explains.
KEYWORDS: Fusion, nuclear, power, energy
Bottled sunshine?
Back in 1980 as a young scientist I wanted to save the world. Nuclear fusion promised clean, unlimited, nearly free energy and it was only 40 years away. I wanted to be part of it. So I called a colleague at the Joint European Torus near Didcot in Oxfordshire to ask his advice. He was pretty blunt and said JET chooses to employ you, you don’t choose to work at JET and in any case commercial fusion power is always going to be 40 years away. I felt sorely rebuffed at the time but looking back I am grateful for his advice that steered me towards semi-conductor research instead. Fusion power is still 40 years away.
Nuclear fusion is really simple in principle. Just smash light elements together to make heavy ones. The total mass goes down and Einstein says that means we release a lot of energy. The sun does this easily by using its immense mass to scrunch all the atoms together.
It’s tempting to think we can duplicate this process closer to home. The physical solution is easy to describe but turns out to be incredibly difficult to realise in practice.
Because we can’t put the mass of 10,000 Earths in the lab, we have to use some expensive high tech equipment. First we need to put hydrogen fuel into a strong clean container. We make the chamber out of stainless steel and pump out all the air. We need to make it very strong with thick, heavy walls so that it doesn’t implode under the pressure of the atmosphere. The ultra high tech machine that I used in Cambridge to grow semi-conductor crystals was the size of a small car and cost about a million pounds. Prototype fusion reactors have a volume 1,000 times larger.
We need ultra high temperatures to make the hydrogen nuclei, the centres of the hydrogen atoms, move so fast that they have a good chance of hitting each other and fusing together. The temperature needs to be similar to that at the centre of the sun, say 10 million degrees C.
To generate this high temperature, we pass a huge electric current through the thin gas to form a rare state of matter – plasma. Now we need to make sure the super high temperature plasma doesn’t touch the edges of the container. A touch of plasma will make the stainless steel evaporate faster than a blink of an eye.
We keep the plasma away from the edges with intense magnetic fields, carefully designed to keep the plasma swirling in the centre of the vessel. The superconducting magnet coils have to be kept very cold to maintain their efficiency. We have to connect huge refrigerators to these coils.
The really challenging part comes when we try and get all that lovely heat out to boil some water to make some steam to spin the turbines to power the generators that make electricity. And there’s the problem. No viable engineering solutions to this step are on the horizon.
Which is why commercial nuclear fusion always seems to be 40 years away.