The Star Builders

by Arthur Turrell

  Moments after the explosion, the fireball had consumed everything within miles. Coral from the atoll that was blown into the air was heavily contaminated by neutrons, making it radioactive. Surrounding islands were showered with radioactive debris, and their residents were, eventually, evacuated. A one-hundred-mile-long plume of fallout spread eastward from Bikini Atoll to Matashichi Oishi and the other fishermen.

  There’d been an exclusion zone around Bikini Atoll, but, tragically, it hadn’t been big enough. Kenneth Ford, one of the original designers of the hydrogen bomb, was on the team that put together Castle Bravo. It had been his job to do the calculations that would predict how much yield, in energy, the bomb would release. Working with primitive computing and the knowledge of nuclear reactions then available, he came up with a figure of seven megatons; that’s an explosive force equivalent to 7 million tons of TNT. The actual yield of Castle Bravo was fifteen megatons. That’s a dangerous miscalculation, and it was the difference between life and death for some of the fishermen. Fifteen megatons is equivalent to one thousand of the atomic, or fission, bomb that was dropped on Hiroshima going off at once, in the same place.

  The incorrect reckoning happened because some of the lithium-6 (lithium with six particles in its nucleus) in the assembly was actually lithium-7, which Ford expected to be inert, but which can capture neutrons and turn into tritium in the right conditions. Tritium can undergo fusion and release energy. As it does so, it produces neutrons that can cause more fission reactions, releasing more energy, and so on. The result was a much bigger explosion than anticipated, and the terrible events that befell the crew of the Lucky Dragon No. 5. The US was eventually forced to pay compensation.2

  “It does not seem possible, at least in the near future,” the great nuclear physicist Enrico Fermi wrote in 1923, “to find a way to release these dreadful amounts of energy—which is all to the good because the first effect of an explosion of such a dreadful amount of energy would be to smash into smithereens the physicist who had the misfortune to find a way to do it.”3

  Fermi was wrong (unusually) and we’ve had the ability to smash ourselves into smithereens using nuclear reactions for decades. Clearly, nuclear technologies are a double-edged sword that can be used to help or harm humanity. We’ve heard a lot in this book about how they might be able to help. Now we’re going to ask whether nuclear fusion can be harmful. The sharp side of nuclear technologies is so dangerous that the word “nuclear” on anything can put people off. (That’s why even life-saving nuclear magnetic resonance imaging machines lost the word “nuclear” as they began to appear in hospitals.) Star builders want us to embrace their favored nuclear technology—fusion for energy—but we need to know the risks. And there’s no getting away from the fact that fusion is a key part of the most devastating nuclear technology in existence.

  Nuclear weapons have changed war more profoundly, and more suddenly, than anything in history. Unlike the awful loss of life and property in the 1945 firebombings of Tokyo (100,000 dead) and Dresden (25,000 dead), total annihilation can now be achieved with just one bomb. The atomic, or fission, bombs used on Hiroshima and Nagasaki created a Hell on Earth. At Hiroshima, all living creatures and plants within half a mile of the blast center were immediately vaporized, leaving only black charred residue. Fourth-degree burns that penetrated the skin occurred even at a distance of 2.5 miles from ground zero. Eyewitnesses reported that victims’ skin hung off their bodies. More than 90 percent of the buildings in Hiroshima were gone. Those who didn’t die from the blast were engulfed by the subsequent fires, and many who survived both were killed by radiation sickness. The total death toll was probably 200,000. The bomb dropped on Nagasaki probably killed another 150,000 people. The destruction wrought by nuclear weapons is so effective at making entire cities vanish that, paraphrasing Arthur C. Clarke, it’s as if the technology were magic, albeit of the darkest and most terrible kind.

  Nuclear weapons are among the most dangerous threats to humans. A big enough nuclear war would likely plunge the world into a nuclear winter in which the Sun’s rays would be blocked by soot and debris, temperatures would dip to levels not seen for twenty thousand years, and those not killed immediately would likely starve to death. There are more than ten thousand nuclear weapons in existence today. We must all do what we can to ensure that they’re never used.4

  There are two types of nuclear weapons, those based on fission (atomic bombs) and those based on fusion and fission (hydrogen bombs). Hydrogen bombs, like the one that obliterated Bikini Atoll in 1953, are much more deadly than atomic bombs. The arsenals of the world’s nuclear powers are now stocked with them.

  But it would be incorrect to assume that peaceful nuclear fusion designed to provide energy poses the same kind of danger.

  Happily, a fusion reactor just can’t be rigged up to explode like a hydrogen bomb. Controlled fusion reactors designed to produce power are fundamentally different from hydrogen bombs. The proliferation of nuclear weapons always requires fissile material: you can’t build either kind of nuclear weapon without the isotopes that undergo nuclear fission, usually special types of uranium or plutonium. In an atomic bomb, fission of these isotopes provides the explosive energy. In a hydrogen bomb, fission provides the initial trigger that kicks off the fusion reactions. Fusion reactors, however, involve no fission at all—so they don’t work like a bomb of any kind, and can’t be made to.

  A more legitimate concern is that a fusion reactor could be used to create the material to assemble a nuclear weapon. It’s scientifically possible for someone to take uranium-238 or thorium-232 into a fusion power plant and expose it to fusion-produced neutrons to breed enough fissionable plutonium-239 and uranium-233, respectively, to get the basic materials for a fission bomb. One study calculates that the fastest this could be done with no attempt to hide what was going on and with full access to the uranium and thorium precursors would be a period of months. I asked the star builders whether this meant fusion energy was a risk for nuclear proliferation.5

  “It’s always a risk,” Dr. Mark Herrmann, NIF’s director, conceded, “but the advantage with fusion is there’s no need for any fissile material to be around.” His strong belief is that fusion is safer from a proliferation point of view than fission, and it’s a view shared by everyone I spoke to at Lawrence Livermore. Don’t forget that one of the laboratory’s primary missions is preventing the proliferation of nuclear weapons. The other star builders felt the same: “Fusion is far, far lower risk [than fission],” CEO of Tokamak Energy Jonathan Carling said, “because it doesn’t involve any fissile material like uranium or plutonium.”

  There’s no need to have any materials at a fusion reactor site that could be used either to build an atomic bomb or as the trigger for a hydrogen bomb. “You can’t break into a fusion power plant and steal some stuff and build a bomb with it,” Dr. Nick Hawker, CEO of First Light Fusion, told me. Besides, fissionable isotopes and their precursors are detectable in small quantities, so it’s relatively easy to check for them. Inspection equipment at a reactor would need only seconds or minutes to find out what was going on. And all of the materials needed to breed the isotopes for a bomb are—as Nick Hawker pointed out to me—very tightly controlled.

  So while the star builders agree that there’s a risk of proliferation, they believe that it’s far lower for fusion than for fission. In the case of a fission plant, it could take weeks to openly and aggressively create the critical mass for a bomb, and more than one nation has been accused of clandestinely creating the ingredients for nuclear weapons using fission power as a cover. Although everyone agrees it is far harder to use fusion power as cover for a weapons program, star builders will need to bear the risk of proliferation in mind if they’re serious about making fusion one of the planet’s main energy sources.6

  When it comes to all things nuclear, though, the concerns extend beyond proliferation.

  “If you think about what worries people most,” Jonathan Car
ling told me, “it’s a meltdown, because when we’ve seen other events, like Fukushima or Chernobyl, it hasn’t been good. Fusion plants can’t have a meltdown.” All star builders agree. “There’s no risk,” said Nick Hawker, “that a fusion reactor can melt down: if you interrupt it, it just stops.”

  Ian Chapman puts it like this: “In fusion, if you want to stop a reaction, it takes milliseconds; the hard thing is to keep it going. It’s easy to stop, it’s really easy to stop, so there is just no risk of chain reaction in the same way.”

  Meltdowns occur because, once they begin, fission reactions can be hard to stop. Fission reactions happen in a chain, with each reaction giving rise to the next. Enough uranium fuel is kept within the reactor chamber to keep this chain going. A fission power plant is balanced to keep the chain in check so that the number of reactions is sustained without growing explosively, or spluttering out and stopping altogether. Using the analogy of a pandemic, it’s like keeping the effective reproduction number, R, at 1. In a fission power plant meltdown, the chain gets out of control, the heat rises substantially, and parts of the plant can literally melt.

  You might worry that a fusion reactor could get out of control too, burning through more of its fuel than star builders intend. Yet because controlled fusion relies on temperature, confinement, and density, rather than a chain, fusion can’t race out of control in the same way. In contrast to fission, fusion often needs energy to be put in to keep the plasma confined. In the case of magnetic confinement fusion, what activates the process is externally applied heating or magnetic fields. Stop them suddenly and you’d get a reactor disruption, but the plant wouldn’t explode or melt down: fusion reactions are hard to start and easy to stop. In inertial confinement fusion, stop the driver—in NIF’s case, that’s the laser—and the whole plant shuts down. There’s no runaway effect in play.

  And, unlike with the fission process, there’s only a tiny amount of fusion fuel in a fusion reactor at any one time, so the maximum amount of energy that can be released is severely limited. The biggest planned tokamak will have, at most, two grams of hydrogen in it at any time; far less than is present in a nuclear weapon. Putting any more hydrogen into either a magnetic or inertial fusion machine would stop the conditions for fusion from being reached: the reactors are naturally self-limiting. Tokamaks can only run with a certain hydrogen density—any more and they can’t get hot. And, in inertial confinement fusion, there’s just not the driver energy to implode capsules much bigger than the ones that are currently being fielded. The only type of fusion in which the whole fuel supply is kept within the reactor is gravitational confinement fusion in stars.7

  The star builders are keen to stress that fusion’s safety is part of why they’re so passionate about it. Fusion reactors can’t be turned into bombs, undergo uncontrollable chain reactions, or melt down, because they don’t involve nuclear fission. What fission and fusion do have in common, though, is radioactivity, which poses a genuine danger.

  Radioactive!

  Radioactivity is perhaps the most misunderstood aspect of nuclear technology, and it’s a big reason why many people are suspicious of nuclear power—especially nuclear fission.

  Radioactivity seems mysterious and dangerous partly because it’s invisible to the naked eye. Although fire or flooding are much more common risks to human life, they at least can be seen, understood, and avoided. To understand the risks of radioactivity from fusion, let’s start with what radiation actually is.

  There’s particle radiation, formed of high-energy electrons, protons, neutrons, and other fundamental particles. And there’s light radiation, such as X-rays or gamma rays. Activated, or radioactive, substances have atoms that are unstable and will occasionally decay into other atoms. Physicists often think about radioactive materials in terms of the time it takes for half of the atoms to decay, the half-life. Fissile uranium-235, for example, has a half-life of around 700 million years.

  When radioactive atoms do decay, they often ping out one of these types of radiation. The weak nuclear force is behind these decays. The radiation that is described in textbooks is usually said to be one of three types: fast helium nuclei (alpha radiation), electrons (beta radiation), or very energetic light (gamma radiation). Alpha, beta, and gamma. But, guess what, put a load of energy into any particle and it can still cause lots of damage—a bullet is only harmful because of how fast it’s going.

  So I don’t think the alpha, beta, gamma names given to radiation are particularly useful, and they mostly exist for historical reasons because those types of radiation were discovered first, and they’re the most common.8

  A better way to think about radiation is that it’s composed of particles or sometimes packets of light that have enough energy to break up atoms and molecules as they go, potentially creating more radiation in the process. For instance, the carbon-14 found in tooth enamel decays when a neutron in its nucleus becomes a proton, pinging out an electron (beta radiation) as it does. The fast neutrons created by nuclear fusion reactions can also change atoms and induce radioactivity.

  It may come as a surprise to many people, but radiation is completely natural. Low doses pose almost no danger and are unavoidable. You’re being strafed by radiation as you read this; approximately every single second of every day, every square meter of the planet is hit by hundreds of high-energy “cosmic rays,” bits of radiation that originated outside our solar system. Scientists are still not 100 percent sure how cosmic rays get all the way to Earth at such high energies, but they’re a normal part of our universe. When they hit the upper atmosphere, they can create cascades of radioactive isotopes, including carbon-14.9

  Radioactive substances can also be found in the earth, and the early radioactivity pioneers, such as Marie Curie and Ernest Rutherford, did their first experiments on these. These ores originally come from outer space and were created in supernovae. You may wonder how, if these atoms are unstable, and formed in supernovae, they haven’t all decayed yet. The instability of some of these “unstable” atoms is not instability as we would understand it in our everyday lives; the atoms decay over a period so long that it is hard to imagine—as long as 20 billion billion years.10

  The extent to which radiation released by radioactive material is dangerous for biological creatures like ourselves is measured in “dose equivalent,” the amount of energy dumped into your body as radiation goes through it. It’s counted in sieverts, or Sv. What is or isn’t radioactive, and how radioactive a given thing is, is quite surprising. A banana, for example, doses you with one-ten-millionth of an Sv because of the potassium it contains. A single chest X-ray is roughly twenty-millionths of an Sv. An eight-hour flight from London to New York is forty-millionths of an Sv.

  Most people receive around 4 milli Sv (mSv) a year—one hundred times more than a single flight from London to New York—just from natural background radioactivity and medical scans.11 (“Milli” means thousandth, as in millimeter.) Cosmic rays are responsible for 10 percent of annual exposure. At that dose, radiation is not dangerous. To pose a risk to health, the doses must be either much larger or be received over a short period. It might be a surprise, but in normal times and with safe operation, even the radioactivity from being near a nuclear fission plant is negligible. For example, in the UK, nuclear fission workers’ annual occupation-related exposure is just 0.18 mSv.12

  A dose of 100 mSv per year or more is associated with an increased risk of cancer, and symptoms of radiation sickness would usually only appear after doses of a few hundred mSv over a short period of time. It’s useful to put these numbers into some perspective. Two of the most famous nuclear fission disasters are the Three Mile Island incident and the reactor meltdown at Fukushima. In the former, some people were exposed to 1 mSv, and those in the Fukushima exclusion zone in the two weeks after that meltdown would have had a 2 mSv dose—far lower than the level associated with cancer. However, six workers at the Fukushima plant received higher doses than that level, and tr
agically, there has subsequently been one death that can be directly linked to radiation. The doses pale in comparison to those received at Chernobyl, the world’s very worst nuclear meltdown, however. Thirty people received fatal doses of more than 8 Sv—that’s eighty times the cancer limit and eight thousand times the dose at Three Mile Island. But it’s not just nuclear power that is associated with radioactivity. Coal power plants release uranium and thorium into the atmosphere as they burn fuel. Some analysis estimates that coal ash carries one hundred times more radiation into the environment than a safely operated nuclear power plant producing the same amount of energy.13 So radiation is everywhere, and the doses can vary widely both in their magnitude and in the extent of harm they cause.

  This all sounds very depressing. But radiation isn’t all bad, far from it. It can be really useful. It’s commonly used to sterilize food. There’s a good chance that it’s keeping you safe right now. Within most smoke detectors is a small amount of radioactive matter that emits helium nuclei. Composed of two protons and two neutrons, helium nuclei are positively charged, and cause other particles in the air to become charged when they crash into them. The stream of helium nuclei in your smoke alarm sets up a small current that can be detected. When smoke is present in the air, the helium nuclei get absorbed by it, less current flows, and the alarm goes off.

  Radioactive carbon-14 is fantastic for accurately dating organic objects that are thousands of years old. When animals or plants die, the amount of radioactive carbon-14 in their bodies is fixed at a known fraction. Just as with teeth, you can work backward to find out how long ago it must have been that the plant or animal died. This technique works for up to around fifty thousand years, after which there’s too little carbon-based radioactivity to measure. Evidence from radiometric dating has revolutionized our understanding of human prehistory in all kinds of ways. Thanks to very long-lived radioactive sources, Rutherford was able to argue in a 1904 lecture that the Earth was billions of years old (geologists already realized this, but the physicists hadn’t been so sure). The latest radiometric dating uses the decay of uranium to lead within zircon crystals to show that the Earth is at least 4.38 billion years old.14