Piece of the Sun : The Quest for Fusion Energy (9781468310412) Read online

  This edition first published in hardcover in the United States and the United Kingdom in 2013

  by Overlook Duckworth, Peter Mayer Publishers, Inc.

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  Copyright © 2013 by Daniel Clery

  All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system now known or to be invented, without permission in writing from the publisher, except by a reviewer who wishes to quote brief passages in connection with a review written for inclusion in a magazine, newspaper, or broadcast.

  ISBN: 978-1-4683-1041-2

  To Bernadette

  who made it all possible

  And to Sam and Ellen

  for their boundless enthusiasm

  Contents

  Copyright

  Dedication

  CHAPTER 1. Why Fusion?

  CHAPTER 2. Britain: Thonemann and the Pinch

  CHAPTER 3. United States: Spitzer and the Stellarator

  CHAPTER 4. Russia: Artsimovich and the Tokamak

  CHAPTER 5. Tokamaks Take Over

  CHAPTER 6. Fusion by Laser

  CHAPTER 7. One Big Machine

  CHAPTER 8. If Not Now, When?

  Further Reading

  Acknowledgments

  Index

  About the Author

  CHAPTER 1

  Why Fusion?

  WE OWE EVERYTHING TO FUSION. OUR OWN SUN AND every star that shines in the night sky are powered by fusion. Without it, the Cosmos would be dark, cold and lifeless. Fusion fills the Universe with light and heat, and allows life to happen on Earth and probably elsewhere. The Earth itself, the air we breathe and the very stuff we are made of are the products of fusion.

  Following the Big Bang, once things had cooled down enough for neutral atoms to form, the Universe was filled with a fairly even distribution of hydrogen, the simplest atom. There was a bit of helium and some of the mysterious dark matter, but the Cosmos appeared to be just hydrogen atoms and empty space. So how did fusion transform this blank canvas into the menagerie of astronomical objects visible today and the ninety-two natural elements we find around us? First it had some help from gravity. Although gravity is a very weak force, over many millennia it acted to pull hydrogen atoms closer together. Clumps of hydrogen formed and as they got bigger they exerted more of a gravitational pull, drawing in more hydrogen.

  As these balls of hydrogen grew, the pressure on the gas in the centre of the ball increased because of the weight of all the hydrogen above it, and with this increasing pressure came higher temperature. (Think of inflating a bicycle tyre: the more you pump it up, the hotter it gets.) Higher temperature means that the atoms are moving at higher speeds and in the high-pressure core of a proto-star they collide against each other with increasing violence. At a certain temperature the collisions are so forceful that the atoms’ outer electrons – which have a negative charge – are knocked away from their nuclei which, in the case of hydrogen, are made of just a single subatomic particle, a positively charged proton. The result is a plasma; a hot maelstrom of charged particles.

  At high temperatures, plasma – an ionised gas – represents a fourth state of matter after solids, liquids and gases.

  (Courtesy of CEA France)

  Once the nascent star grows to a certain size – roughly 28,000 times the mass of the Earth – the temperature in its core reaches around 10 million °C and fusion starts. Fusion is simply the melding together of two nuclei to make a larger one but it’s not an easy thing to do because all nuclei, such as the protons knocking around in the core of a star-to-be, have a positive electric charge and similar charges repel each other. When the temperature gets into the millions of °C, however, the nuclei are slamming together with such force that they get past the electric repulsion and are hooked by another short-range force, the one that holds protons and neutrons – their uncharged companions – together in a nucleus. The two colliding protons have to get within a subatomic arm’s length before this attractive force can grab them and bind them together to make a new nucleus. But two protons don’t make a very stable nucleus by themselves, so most of these pairs split apart again almost immediately.

  Very, very occasionally one of these brief fusions is quickly followed by one of the protons decaying into a neutron. A nucleus made of a proton and a neutron – known as a deuteron – is very stable and so the new nucleus survives. Over time this process creates more and more deuterons in the heart of the proto-star and once there are enough of them other reactions start to happen. For example, one of the deuterons can fuse with another proton to produce helium-3 (two protons and one neutron) and, once there are enough of them, two helium-3s can fuse to form a helium-4 (two protons and two neutrons) with two protons left over. These reactions form the start of a chain of fusions which eventually also produces the elements lithium and beryllium.

  These fusion reactions produce heat as a by-product because a nucleus of helium-3, for example, is slightly less heavy than the pair of reacting nuclei that created it – a deuteron and a proton in this case. This mass isn’t lost; it is converted into energy during the fusion. So once this chain of reactions gets going, and untold numbers of nuclei are fusing, the heart of the proto-star becomes a raging furnace, further raising the temperature and causing more reactions. This simple process transforms the ball of gas into a fully-fledged star and it – or a very similar reaction chain – is what powers all stars, from the very first which are thought to have ignited about 150 million years after the Big Bang and throughout the 13.7 billion-year history of the Universe.

  Fusion has more tricks too. Towards the end of a star’s life, when it has burned up all its hydrogen, it starts to consume helium in a reaction chain that can produce beryllium, carbon and oxygen. When all the helium is used up, other chains begin that consume those nuclei to make even heavier ones. In this way, in the dying days of a star, all of the elements up to iron are created by fusion. Finally, when no fusion fuel remains, the remnants of the star collapse under their own gravity. If it is a large star, this collapse will release so much gravitational energy that it would blast the outer layers of the star outwards in a cataclysmic explosion, a supernova. The energy of a supernova is so intense that it causes further fusions in the heavy nuclei remaining in the star’s ash. These fusions produce all the heaviest elements from iron up to uranium and beyond.

  So, over the lifetime of a star, fusion takes the raw material hydrogen and forges it into all the other elements of the periodic table. And when the star explodes at its end, it spreads those elements out into space where they mix with fresh hydrogen and then slowly coalesce into new stars and planets. So these second-generation stars and the accompanying planets that form around them contain a mixture of elements, allowing some of the planets to form rocky surfaces, oceans, atmospheres and life. Every atom in your body, apart from the hydrogen, was created by fusion in a long-dead star.

  * * *

  Scientists spent the second half of the nineteenth century and the early part of the twentieth figuring out what made the Sun and all the stars shine. It was a mystery to them how the Su
n could pump out such prodigious amounts of energy for billions of years without running short of fuel. By the late 1930s they had worked out the rough details of the fusion reactions described above and had their answer. That answer planted the seed of an idea into a number of minds. The seed wasn’t able to grow for a while because of World War II but once that was finished it soon began to sprout. What was that seed? It was the idea that if fusion can power the sun for billions of years, could it supply similarly endless energy on Earth, if it could be mastered? The ancient Greek mythological figure Prometheus stole fire from the gods and gave it to humans, leading to progress, technology and civilisation. Could science steal the power of the Sun and rekindle it on Earth for the good of all humankind?

  Prometheus came to a sticky end – chained for eternity to a rock for his crime. Postwar scientists didn’t have a vengeful Zeus to worry about and, in fact, thought that taming fusion was going to be relatively easy. Stars make it look easy: lump together enough hydrogen, add gravity and fusion just … happens. On Earth they didn’t have some of the benefits that stars enjoyed, including the weight equivalent to many thousands of Earths pressing down on the core to heat and compress hydrogen to fusion temperatures. Scientists would have to find some other way to heat and compress hydrogen – how hard could it be?

  Although the war years had been devastating, they had produced some technological wonders. At the start, some men had still fought on horseback but the war was soon all about fast-moving armoured tanks, long-distance aerial bombardment, vast aircraft carriers and submarines. By the end of the fighting there were rockets able to hit targets hundreds of miles away, planes with jet engines and, ultimately, a bomb able to destroy a whole city. For some scientists after the war there was a sense of optimism. If they could achieve so much in six years of war, imagine what they would be able to do in peacetime.

  One of those things was to develop nuclear power. But this was not fusion, it was the other sort of nuclear reaction, fission, the process behind the atomic bombs dropped on Hiroshima and Nagasaki. Fission is, in a sense, the opposite of fusion. In fission some of the very largest nuclei known, such as uranium, are split apart into two new nuclei. The starting nucleus is slightly heavier than the fragments that result from the fission and this missing mass is converted into energy during the process. Unlike fusion, fission doesn’t need high temperature to take place: the large nucleus will split apart easily if hit by a fast-moving neutron. It was the discovery in 1938 that when some nuclei split they produce neutrons as a by-product which led to the realisation that you could start a chain reaction: one nucleus is hit by a neutron; it splits and spits out two neutrons; these hit two more nuclei which split to produce four neutrons, and so on. This chain reaction is what makes an atomic bomb – a fission bomb – possible: if you bring together a lump of one of these so-called fissile materials – such as uranium or plutonium – larger than a certain critical size, the chain reaction will start spontaneously and run out of control, exploding in a split second.

  Before the scientists involved in the Manhattan Project – the Allies’ wartime project to develop an atomic bomb – built an explosive, they tested the chain reaction in a controlled way, in the world’s first nuclear reactor. This was built, in secret, at the University of Chicago in a squash court underneath the stands of its sports stadium, Stagg Field. Known as Chicago Pile-1 because it was a pile of uranium and graphite blocks, its construction was supervised by Enrico Fermi, a celebrated Italian-American physicist. Graphite absorbs neutrons and so slows the reaction. The blocks in the pile were carefully arranged so that there was enough uranium placed close enough together to sustain a chain reaction, but not quite enough for it to run away and explode. There was no radiation screening around the reactor and no protection from possible blasts – Fermi was sufficiently confident of his calculations to decide that they were unnecessary. In the mid afternoon of 2nd December, 1942 one of Fermi’s assistants slowly pulled out a graphite control rod from the centre of the reactor. This reduction in the amount of graphite in the reactor was calculated to be just enough to allow the chain reaction to get going. Fermi watched a neutron counter and saw the number of neutrons swell as the rod was extracted. With a group of dignitaries looking on, Fermi ran the first controlled nuclear reaction for almost half an hour and then reinserted the rod to shut it down.

  After the end of the war, engineers didn’t waste any time putting this new technology to commercial use. The first nuclear reactor to produce electricity was built in 1951 in the United States. The first to supply power to the grid was in the Soviet Union in 1954. And the first truly commercial nuclear power plant began work in the United Kingdom in 1956. At the time, many expected nuclear power to produce electricity that was cheap and limitless. But there are problems with using fission as an energy source. First, uranium is a finite resource and some predictions say it may get scarce before the end of the twenty-first century. Then there is safety: because fission reactors rely on a chain reaction, it is possible for the reactions to run away too quickly and for the reactor to overheat. A reactor cannot cause a nuclear explosion like a atomic bomb because the fissile material inside is too spread out, but it can overheat and melt the core – as happened at Three Mile Island in 1979 – or catch fire – as at Chernobyl in 1986. Reactors contain many tonnes of uranium or plutonium fuel and, if they have been running for a while, also a lot of radioactive spent fuel, some of which is extremely harmful to people. When accidents happen, the danger is that this radioactive material will spread far and wide. At Three Mile Island the material was contained; at Chernobyl it wasn’t.

  Thirdly, there is the problem of waste. A typical nuclear power plant generating 1,000 megawatts (1 GW) of power produces around 300 cubic metres (m3) of low- and intermediate-level waste per year and 30 tonnes of high-level waste. These quantities are tiny compared to the waste – some fairly toxic – from a coal-fired power station of similar output. But nuclear waste, parts of which can remain radioactive for hundreds of thousands if not millions of years, is more of a problem to deal with. Low- and intermediate-level waste can be buried close to the surface and its radioactivity will decline to safe levels in a matter of decades. High-level waste needs to be disposed of in such a way that it will remain inaccessible to humans or any other species for tens of thousands of years. Just imagining how to do that is a tall order and only a few countries have got to grips with the problem by building permanent repositories deep underground that will be sealed when full. Other countries keep their waste in carefully guarded facilities on the surface. All of the world’s reactors combined produce a total of 10,000m3 of high-level waste per year.

  In the tumultuous postwar world, as a few countries raced to commercialise nuclear (fission) energy, some scientists realised that it would be a much better idea to try to generate power using nuclear fusion. The arguments in favour of fusion are compelling. First there is the fuel: fusion runs on hydrogen or, more correctly, on two isotopes of hydrogen known as deuterium and tritium. Deuterium is simply a deuteron (a proton and neutron) with an electron added; tritium has two neutrons in its nucleus. If you fuse deuterium and tritium you get helium and a neutron.

  Slam them together hard enough and a deuteron (D) and a tritium nucleus (T) will fuse, producing helium (4He), a neutron (n) and lots of energy.

  (Courtesy of EFDA JET)

  Deuterium can be easily extracted from water. One in every 6,700 atoms of hydrogen in seawater is a deuterium atom. That doesn’t seem like much but given the amount of water in the world’s oceans there is enough deuterium there to supply all the world’s energy needs for billions of years. Tritium is trickier because it is an unstable nucleus with a half-life of twelve years and so it would have to be manufactured. The easiest way to do this is with lithium, a metal used in some batteries. Lithium, when bombarded with neutrons, splits into helium and tritium. Any source of neutrons can cause this reaction and, since fusion reactors themselves are prodigious pr
oducers of neutrons, it is thought that a portion of a reactor’s neutron output could be devoted to producing tritium fuel for its own consumption. Lithium can be extracted from easily mined minerals and there is enough around to supply the world with power for several hundred years. When that runs out, there is enough lithium in seawater for several million more years.

  This seemingly vast oversupply of fuel is understandable when you consider how little fuel a fusion reactor actually needs. A 1-GW coal-fired power station requires 10,000 tonnes of coal – 100 rail wagon loads – every day. By contrast, a fusion power plant with a similar output would have a daily consumption of just 1 kilogram of deuterium-tritium fuel. The lithium from a single laptop battery and the deuterium from 45 litres of water could generate enough electricity using fusion to supply an average UK consumer’s energy needs for thirty years.

  Fusion is a nuclear process, so some might be concerned about its safety. There are safety issues associated with fusion, but they are small compared to a fission reactor. It is actually very hard to keep a fusion reaction going and if there is any malfunction in the controls of a fusion reactor the process would naturally just stop. Even if the process did want to run away it couldn’t do so for long because there is very little fusion fuel in the reactor. Unlike a fission reactor, which has years’ worth of fuel in place in its core, the fuel in a fusion reactor at any one time weighs about as much as ten postage stamps and could keep the reactor running for only a few seconds. The fuel stored outside the reactor is at no risk of reacting: it will only start to burn when heated to more than 100 million °C in the reactor.

  Tritium is a radioactive gas, so is harmful to people, but as it will be generated on site, a fusion power plant won’t keep a large supply sitting around. In the unlikely event that, for example, terrorists blew up a fusion reactor or crashed a plane into it, or even if an earthquake and tsunami hit the plant as happened at Fukushima, the amount of tritium released would not require any evacuation of nearby residents. In any event, tritium is a form of hydrogen, a buoyant gas once used in balloons and airships; its natural tendency is to drift straight up.