The Star Builders

by Arthur Turrell

  And radioactivity can be used in medicine to diagnose or treat various diseases. One of the pioneers of this technique, winning a Nobel Prize for it, was Ernest Lawrence, after whom Lawrence Livermore National Laboratory is named. The technology he developed helped to slash the price of radioactive isotopes used in cancer treatment from $100,000 a gram in 1921 to just a few dollars a gram in 1935, and that was just for the salt that had to be put into the machine.15

  Another enormously useful nuclear diagnostic involves a patient purposely ingesting radioactive isotopes. Once inside the body, the absorbed isotopes reveal their location by emitting X-ray or gamma radiation. Like many technologies, radioactivity can be a tool or a terror.

  It’s the terror that more often makes the news. In the case of the Lucky Dragon No. 5, which, unluckily, found itself too close to the Bikini Atoll blast site, it wasn’t the explosion itself that did the damage, it was the radioactive rain. Fortunately, large scale radiation exposures are very rare. Meltdowns don’t happen very often.

  What many more people are rightly concerned about when it comes to radioactivity and nuclear power is what goes into and, even more so, what comes out of the plant.

  In fission, both the inputs to and outputs from the reactor are radioactive. Let’s take the “ingredients” first. The most potently fissile commonly occurring isotope is uranium-235, which is also radioactive and decays into thorium by emitting a helium nucleus. Importantly, its long half-life means not very many atoms decay per second, so it’s not the most dangerous radioactive material by a long shot.

  What comes out of nuclear fission reactors is more problematic: radioactive waste that can last for millions of years. In fission, the splitting up of large nuclei like uranium-235 creates gamma rays, an energetic neutron, and a bunch of smaller atoms that are themselves likely to be unstable. French nuclear company EDF Energy estimates that the yearly amount of nuclear waste per person in France (which generates 75 percent of its electricity from fission) is one kilogram (approximately thirty-five ounces). It’s a lot less waste by mass than fossil fuels produce, but what is created is more problematic. Most of the one kilogram is low-level waste that becomes safe relatively quickly. The really dangerous part is the 3 percent of high-level waste so radioactive that it must be actively cooled for its first forty years. It then needs to be carefully secured and stored for at least a thousand years. Only after one hundred thousand years does this waste reach the same radioactivity level as uranium ore, which can be handled with gloves.16 Countries around the world are still grappling with where to store this long-lived waste. For now, it’s mostly stored next to the plants where it was created.17

  It’s understandable that people would be concerned about the small though dangerous and long-lived waste generated by nuclear power. But, star builders contend, nuclear fusion is different from fission, even when it comes to radioactivity. Even if we assume fusion-produced waste is equally as dangerous as waste from fission (we’ll come to whether it is shortly), the most serious incidents in which people were exposed to radioactivity from fission power have all happened because of meltdowns—occurrences that can’t happen with fusion power plants.

  The inputs to fusion are arguably safer too. Deuterium isn’t radioactive. However, tritium, the other ingredient of fusion reactions, is. It has a half-life of about twelve years, and decays into helium-3. But tritium is so weakly radioactive that the electron it emits when it undergoes this decay can’t even pass through the dead layer of skin that surrounds your actual skin. Because of this, it has found some curious uses, including in a radioactivity-powered light that requires no batteries. Another use is for illuminating emergency exit signs; even when the power fails in a building, the tritium will provide a radioactive glow. Tritium is fairly safe as radioactive substances go, but it needs to be handled with care.

  I asked the star builders whether the stocks of tritium kept on-site at a fusion reactor could pose a risk to anyone. Lorne Horton, who works at the Culham Centre for Fusion Energy, says that the radiation risk from fusion fuel at JET is low: “We work at industry levels, which are smaller doses than you get from just living every day and certainly smaller than what you get from taking an airplane ride. My dose is unmeasurably small.”

  I put it to Ian Chapman that residents of Culham might be concerned that some of JET’s tritium could escape.

  “In terms of the radiological exposure in the event of a big accident and loss of containment, we’re looking at a few tens of grams of tritium,” he said. “It’s a negligible risk to the population: we have about one hundred grams of tritium here on site and we’re not under a nuclear license, we operate under the Environment Agency. It’s inside a containment vessel, inside another containment vessel, inside a concrete bio-shield—but even if all three were penetrated then it would go into an exhaust de-tritiating system that would catch it. If that were to also melt at the same time and it went out into the atmosphere, the risk is negligible because the inventory is so low.”

  Because of these low risks, fusion plants, unlike fission plants, have no exclusion zone around them. The fusion start-ups’ levels of safety procedures vary depending on whether they’re at the stage of doing deuterium-tritium fusion or not, and for now, most aren’t and so have no need for tritium (yet). Even for the biggest planned tokamak in the world, which is likely to be on the same scale as a working fusion power plant, there is no scenario that would necessitate evacuating locals—and the radiation from a plant that’s running will be a thousand times less than the natural background level of radiation.18

  “When it comes to working with ionizing radiation, the regulations are very proscriptive and we follow all of them,” Nick Hawker assures me. “The total inventory of radioactive material we’ll have for the gain experiment will be tiny in tritium; there’ll be zero of anything basically. For the power plant, there’ll be tritium, and it’s the biggest worry as it’s mildly radioactive and it’s mobile.” He went on to explain that the safety requirements for tritium are much less onerous than the ones for fission fuel.

  The fusion of deuterium and tritium directly produces fast-moving neutrons and helium nuclei, albeit in very small quantities. The helium nuclei from fusion quickly recombine with electrons as soon as the power is removed from a fusion reactor, so, in that circumstance, they pose no radioactivity problem. In fact, helium is useful, and in short supply.

  The biggest source of radioactivity from fusion reactions is the neutrons that are generated when the reactor is on and working. It is for this reason that everyone working near the reactor cores at NIF and JET has to wear dosimeters that check how much radiation they’re being exposed to.

  As Dr. Bruno Van Wonterghem, the operations manager at NIF, took me around the facility, he pointed out some of the precautions they’re taking to stop neutrons from harming anyone. NIF’s aluminum target chamber, where the laser meets the fusion fuel, is itself encased in a concrete cylinder six feet thick accessed by four-foot-thick doors. This cylinder is encased in another concrete wall, and there’s further shielding after that too. During a shot, most of the ten thousand million million neutrons created by fusion reactions escape the target chamber. As with JET, the concrete layers that surround it are packed with boron, an element that safely absorbs neutrons. Each wall of boron-rich concrete reduces the number of neutrons escaping by a factor of one thousand.

  “Neutron doses in the target bay [the space immediately outside the steel target chamber] during high-yield shots are about eighty thousand rad,” Bruno told me, calmly. “They’re beyond lethal.” The rad is a measure of how much radiation your body absorbs. Sieverts also take into account that even for the same energy, some types of radiation are more damaging. For the fast neutrons produced by fusion, eighty thousand rad is equivalent to 800 Sv. This is an extraordinary amount as just 5 Sv will kill you. There’s a reason there’s enough shielding in place in just the first twenty meters (approximately sixty-five feet) from the reactor
chamber to reduce the number of neutrons a million times over, with the dose falling even more. The multiple stages of shielding that enclose the target chamber reduce the dose to levels that pose no risk to anyone, as my radiation badge attested when I witnessed a shot from the nearby control room. And Bruno told me that the target chamber has enough shielding to cope with a fusion gain of sixty—that is, sixty times as much energy out as went in.

  We walked through one of the thick steel-frame doors, also filled with concrete, and switched from the non-radioactive zone to the radioactive zone. We were entering the target bay. In the center was the reactor chamber, inside which the laser beams meet the hohlraum. While the most lethal doses occur during a shot, radioactivity induced by the neutrons can linger. I was utterly amazed that we were able to wander around immediately outside the target chamber when the area has been exposed to beyond-lethal radiation so recently. Bruno did remind me not to touch anything. Wearing the appropriate attire, engineers were moving pieces of recently irradiated equipment around without fear. When I mentioned this, Bruno told me that it took anything from a few hours to a dozen hours after each shot for the radioactivity to fall to the background level everyone experiences every day.

  I asked the star builders how much of a risk the radiation from a typical fusion experiment poses.

  “Not much,” Mark Herrmann told me, speaking about NIF. “The chamber is very big. The doors are a couple of meters thick for high-yield experiments. There are two sets of doors. You can sit in the control room even on the highest-yield shot.”

  The reason Mark can confidently say this is because we know how to stop radiation if we need to. Even though a fusion-produced helium nucleus travels some twenty-five thousand times faster than a speeding bullet, it can be stopped by a sheet of paper. A few millimeters of aluminium will stop fast-moving electrons, while gamma rays or neutrons will only be stopped by a big block of a dense material, like lead. The biggest problem by far for fusion is the neutrons—and they’re why there are multiple layers of concrete at NIF and JET.

  However, it’s not just humans that are affected by radiation. Over a long time, the repeated bombardment of reactor chambers by neutrons has consequences. When fast-moving neutrons crash into atomic nuclei, they can cause them to become radioactive. The type of atom that is struck is what determines whether this will happen or not. Star builders have to choose the construction materials of their reactors carefully so as to minimize neutron-initiated activation. Even with the most resilient materials, over years of being strafed by neutrons, some material will become activated. This means that fusion produces radioactive waste—but it’s very different from the waste left by nuclear fission reactors.19

  “Fission plants produce waste as an inherent part of their process,” Jonathan Carling told me. “The only waste that a fusion plant produces is relatively low-activation waste of the plant itself because it gets hit by neutrons.” In fission, the spent fuel and the reactor are radioactive; in fusion, it’s only the reactor chamber that becomes radioactive, and that will be dealt with at the end of the plant’s life.

  So how easy would it be to clean up a nuclear fusion reactor site at the end of its life? JET’s been running since 1983, including with neutron-producing deuterium and tritium. It’s the closest we’ve come to a working fusion reactor and it serves as a good model of what to expect.

  “When we close JET, the plan is to be greenfield within ten years,” Ian Chapman told me. “Robots go in and remove the first walls, walls get de-tritiated… we store the tritium, throw the rest in a skip [a dumpster]. The residue is negligible. To say we’ve been operating for thirty-five to forty years and go to greenfield in ten years gives you a sense.”

  How much radioactive waste fusion reactors will create at the end of their lives depends on what the reactor chamber is made from and how long it has been in operation, but the best estimates suggest that after ten years, a nuclear fusion reactor chamber that has been creating ten times as much energy out as put in would be no more radioactive than uranium ore. After one hundred years, it wouldn’t be radioactive at all.20

  “You can put it into intermediate waste dewars [sealed flasks] until it decays and then get rid of it,” Ian Chapman says. “You don’t have the same legacy that you do with fission.”

  Radioactivity is good and bad, common and rare, dangerous and safe: it all depends on the context. Star builders have good reasons to believe that fusion reactors will pose fewer radiation risks than fission plants and leave no lasting legacy of radioactive waste.

  The Real Dangers of Fusion Reactors

  If it’s not radiation, nuclear proliferation, or meltdown, what is a big problem for star building?

  “The biggest risk,” Lorne Horton, JET’s exploitation manager, told me, “is that you fall down.” He meant this literally. Then there are the risks of being near large electricity supplies and powerful magnets, he continued. In his reckoning, radiation comes a distant third. Without any prompting, Nick Hawker also told me that working at heights was First Light Fusion’s biggest safety concern.

  “We’re not dangerous to the general public,” he explained, “and with proper process, which we have, it’s not dangerous to the team either.”

  The truth is that we won’t know just how safe a working, power-generating nuclear fusion reactor will be until the star builders get around to building one. We know that a working fusion reactor is likely to be somewhat safer than a nuclear fission plant, because fusion reactors can’t melt down and they involve fewer risks from radioactivity. But the two technologies have some similarities too—they’re both strictly regulated, use little fuel because nuclear energy is so efficient, and need a great deal of complex infrastructure in a small area.

  So as an upper bound on just how dangerous nuclear fusion will be as a power source, I thought it would be interesting to consider its cousin, nuclear fission. And that has led to a finding that may come as a big surprise to many.

  Nuclear fission power stations are the safest large-scale form of energy production on the planet. For every exajoule of energy generated by fission (and remember that the US alone consumes ninety-five exajoules of energy each year) there are around 20 deaths. To put that number into perspective, an exajoule of coal results in 6,800 deaths. By this simple metric, fission is 340 times safer than coal. It makes more sense than it might first appear; you need just two grams (less than an ounce) of uranium for every sixteen kilograms (approximately thirty-five pounds) of fossil fuels, which means a lot less mining and extraction of ores, one of the riskiest activities for fossil fuel power.

  All power sources come with risk, even energy from renewable sources. Hydroelectricity is associated with 330 deaths per exajoule, a number dominated by a single, terrible event—the failure of the Banqiao Dam in China, in 1975. It killed an estimated 200,000 people. Likewise, the 20 deaths per exajoule for fission takes Chernobyl into account.I Putting aside uranium extraction and looking at 1990 onward, fission is associated with just 3 deaths per exajoule. For wind and solar, it’s 10 and 5 deaths per exajoule, respectively, yet another reason why solar energy is a promising part of the solution to the energy crisis.21

  Nuclear disasters are big news precisely because they’re so rare. It’s akin to air disasters and car crashes: each year, car crashes kill tens of thousands of people in the US. Worldwide, plane crashes typically kill fewer than two thousand. But which do we hear more about? This asymmetry can result in poor policy choices—after the meltdown at the Fukushima nuclear plant in Japan, Germany decided to phase out its entire fleet of nuclear fission power stations. In consequence, Germany was forced to replace nuclear energy with burning fossil fuels. What happened? An estimated additional 4,600 deaths and three hundred megatons of carbon dioxide emissions between 2011 and 2017. Japan also shut down all of its own nuclear power plants in the immediate aftermath of the Fukushima accident. This caused a significant rise in energy prices, and people responded by buying less energy
during winter. As a consequence, more people died due to energy poverty—in fact, the mortality from this effect was higher than from the accident itself. We should look carefully at the evidence before overhauling policies based on instinct alone.22

  Fission can save lives if it displaces more dangerous power sources. Throughout its history of use, fission power has, it’s estimated, prevented 1.8 million air pollution–related deaths and sixty-four gigatons of carbon dioxide–equivalent greenhouse gas emissions that would have resulted from fossil fuel burning.23

  Again, all power generation involves risk; it’s inescapable. As a society, every time we use energy, we make a trade-off; we decide that the risk of mortality or morbidity is worth it for the benefits the energy offers. Our primary power source today, coal, offers a bad trade-off. Despite a small number of prominent nuclear disasters and the risks related to radioactivity, nuclear fission offers one of the best trade-offs out there.

  Nuclear fusion, star builders reason, would be substantially safer than fission because a fusion plant can’t melt down and doesn’t involve long-lived, high-level radioactive waste. Most important, fusion would generate large amounts of energy from even smaller amounts of fuel than are required for fission. If the star builders are right, then nuclear fusion, when it’s ready, will be one of the safest—if not the safest—large-scale power sources on Earth. Maybe fusion is lucky after all.