Advanced Nuclear Power

Cheaper, Safer, and (Still) No Vats of Glowing Green Ooze

Nuclear fission is when a large atomic nucleus gets blown apart, releasing energy we can harvest to make breakfast pastries. Depending on whom you talk to, nuclear fission is either the power of the future or the power of the past. Most people today see it as an environmentalist versus nonenvironmentalist issue, but in fact the history (and likely future) of nuclear power are substantially more nuanced. As recently as the 1970s, the Sierra Club was in favor of nuclear power and opposed to hydroelectric power.

Why? The short version is that nuclear power has a relatively small environmental footprint, while damming up a river necessarily destroys the associated ecosystems. The Sierra Club has since changed its position and is now “unequivocally opposed” to nuclear power for issues related to safety, nuclear proliferation, and storage of nuclear waste. We discuss these concerns later in the chapter, but the takeaway here is this: All major forms of power available today come with trade-offs.

We’re excited about renewables and we believe they are a major part of the future energy mix, but at least for the moment, when countries remove nuclear, they tend to replace it with fossil fuels. In Mark Lynas’s book Nuclear 2.0, which argues in favor of advancing nuclear to save the environment, he claims “more than 1,000 nuclear plants were originally proposed; had they all been built, the US would now be running an entirely carbon-free electricity system.”

Even without big investments in nuclear, the United States is doing well, thanks to greater efficiency and some power from renewables. Carbon emission rates in the United States have been falling for about ten years, recently reaching levels not seen since the early 1990s. But America is almost beside the point.

China and India contain ten times the population of America, and in the coming decades those people will expect to have something like a Western lifestyle—giant TV screens, colossally inefficient cars, and ever greater portions of emulsified cheese. That means more energy, and right now more energy means more carbon in the atmosphere.

More than any other technology, nuclear power embodies the double-edged sword of innovation. There are many ways to build a reactor. All of them at some point involve elements that can be used to make atomic weapons. But they also produce no airborne particulate matter, they emit no carbon, and their fuel is relatively cheap and plentiful.

Today’s engineers are putting forward new designs, informed by the last seventy years of research, that should dramatically reduce or even eliminate the dangers of catastrophe, waste, and environmental damage, while generating relatively inexpensive carbon-free energy. And, unlike with fusion power, we already know that fission power can be cost competitive with traditional fossil fuels. Before we get into how nuclear power might be perfected, let’s pause to talk about what nuclear fission is.

There are different ways for nuclear fission to happen, but we’re interested in only one way: when you fire a neutron into a large atomic nucleus.

We discussed atomic nuclei in the chapter on fusion, but basically you’re talking about a bundle of positive particles (protons) and neutral particles (neutrons). Generally, we visualize them as little red balls with plus signs and little gray balls. We asked a few researchers if this is an accurate picture of atomic nuclei, but apparently it’s a well-kept secret because they just stopped talking to us.

Fission is a lot easier than fusion because there’s no Coulomb barrier (which you’ll remember is the huge amount of energy required to get two protons really close to each other). You’re still shooting at a positively charged nucleus, but your ammunition this time is an uncharged neutron. The nucleus doesn’t see it coming. If you hit the right kind of nucleus, it will absorb the neutron and become an unstable isotope. It’s sort of like if you had a really, really tight rubber-band ball, then you stuck a marble right in the middle. For a teeny tiny fraction of a second, it keeps it together, then BOOM!

It turns out that for a couple of large elements (about 240 times larger than regular hydrogen), when the atom splits, it’ll kick out two or three neutrons at high speed. This sets up something new—a nuclear chain-reaction: You fire in one neutron, but two or three come out. It’s like a two-for-one sale that you can use to destroy the world.

When performed in a slow, controlled manner, this reaction can be used to create power. When performed in a carefully constructed high-speed manner, this reaction can be used to create weapons that destroy cities.

The basic idea of a nuclear reactor is really simple. In fact, you could make a toy to explain it to kids.

What gets complicated is that nuclear reactors work under extreme conditions and can have particularly bad risks in the case of failure. The result is a lot of engineering and materials science that goes into keeping the simple mechanism safe and steady. But we’re just doing the basics, so we can safely ignore all the lifetime efforts of the unsung heroes of science who made nuclear reactors possible. If they wanted fame and fortune, they should’ve invented Tinder.

Here’s how a nuclear reactor works: You get water really hot. The water turns into steam and passes through a turbine. The turbine spins. You get power.

Okay, so that’s how pretty much every power plant that burns some kind of fuel works. What’s special about a nuclear reactor is the nuclear fuel itself. For a given amount of nuclear fuel, you can get two or three million times more energy than for the same amount of coal. To put that in perspective—it’s the difference between one cheese and two or three million cheeses.

By far the most common nuclear fuel is uranium, an apparently unassuming gray metal that is surprisingly heavy and emits neutrons in every direction.

We talked about hydrogen isotopes. Well, uranium has isotopes too. Like hydrogen, it’s almost entirely in two fairly stable forms: uranium-235 and uranium-238. We’ll just call these U-235 and U-238 from here on out. U-238 makes up about 99.3% of all uranium, and most of the remaining 0.7% is U-235.

So if you take some uranium-heavy ore out of a mine and refine it to pure uranium, the result will mostly be U-238. But, like with hydrogen, the rare version is the good version. U-235 does a much better job of getting the nuclear two-for-one deal when it absorbs neutrons.

For most reactors, you have to take your hunk of uranium down to the corner centrifuge man and get it “enriched” so the concentration of U-235 is around 3%–5%. Basically, you get uranium in gas form and spin it really, really, really hard. U-238 weighs a little more than U-235, so with enough centrifuges, you can separate them out.

Between 3% and 5% is actually a bit of a sweet spot—less, and you can’t run most reactors. As you get toward 20%, you’re approaching the right sort of material to make an atomic bomb. This is why the UN gets freaked out when a secretive country might be enriching uranium beyond a certain point. Over time, centrifuge technology has gotten much more energy efficient, which makes it easier to hide from UN inspectors. On the plus side, clandestine uranium enrichment has never been more green.

Now, say you’ve got your enriched uranium chunks and you want them to start emitting neutrons into each other. There’s a snag: The neutrons don’t come out of the uranium chunks in the best form for splitting atoms. Specifically, they come out too fast.

This may seem counterintuitive—shouldn’t fast neutrons do a better job of splitting atoms? Well, think of the nucleus as a happy family and the neutron as your uncomfortably racist elderly relative. The relative doesn’t just run through your family, causing it to scatter. He sits down at the table, offering unsolicited opinions on politics. The family structure soon grows unstable, between factions that will and won’t tolerate Uncle Neutron. Then BOOM. It all comes apart. Just for the sake of finishing the analogy, imagine the split causes three uncomfortably racist cousins to zoom away at high speed, lodging themselves in other Thanksgivings. The process repeats, causing (if you will) a breakdown of all nuclear families.

Just so, in an atom, the neutron doesn’t just zip through, splitting as it goes. It has to lodge in the nucleus, perhaps so it can offer unsolicited opinions on the Higgs boson. As we touched on, the nucleus can use its strong nuclear force to snag things, but this force only acts over a short distance. Slowing down the neutron gives the nucleus a better chance of capturing it.

To slow things down, you need some substance for neutrons to pinball around on for a while. Because it moderates the speed of neutrons, this substance is called a moderator. By far the most common moderator used is water, which is cheap and easy to work with. It’s also dual purpose: It can both moderate neutrons and transport energy from the reactor to the turbine.

So when you’re building a reactor at home, you take your chunks of enriched uranium and put them at separate locations inside a container. Then you pour in water. Each uranium chunk emits fast neutrons. The fast neutrons get slowed down by the water, then enter other uranium chunks, causing more neutrons to be emitted. All the fission happening causes the water to heat and turn to steam, which makes the turbine spin, and somewhere out there, a happy family enjoys electrically heated pizza rolls.

But if you aren’t careful, this process can get out of control. Since this is a two-for-one deal, that means exponential growth—one creates two creates four creates eight. If the process runs out of control, the steam can get too high pressure and explosively rupture its container. You could even have your fuel “melt down” into a gloppy pool at the reactor bottom.

If things go very badly, it’s possible that you get a nuclear reaction similar to what happens in a nuclear bomb.

We say “similar” with some care. You can’t get a nuclear-bomb-style explosion in a nuclear reactor. Not “probably won’t.” Can’t. Why? It’s reallllllly hard to make a nuclear bomb. In order to get a big boom, you need to split as many atoms as possible before it all blows apart. This is hard because every time an atom splits, it pushes other atoms away. Making a bomb requires extreme precision just to get 1% or 2% of the nuclear fuel to fission. It can’t happen by accident any more than shaking a bunch of metal parts can accidentally produce a functional watch.

What can happen is that a bomb-style reaction starts, but it blows the core apart before it gets close to anything like a nuclear bomb.

But it’s still not awesome if you get an out-of-control reaction. To keep your reactor going hot without getting out of control, you insert some substance that’s good at eating neutrons, thus slowing down the reaction. For example, boric acid absorbs neutrons, which means you can tune the speed of your nuclear chain-reaction just by injecting boric acid into the water.

All reactor designs also have neutron-absorbing “control rods” held above the system by electromagnets, so that they drop in automatically to stop the reaction if the power goes out. As we’ll see, this is an imperfect safety system, since nuclear reactions don’t stop instantly. They continue generating heat as they slowly snuff out, and once the reaction has stopped, the reactor still takes a long time to cool off.

Once you have fuel, a control mechanism, and a way to convert the energy of nuclear fission into useful power, you’ve got a nuclear reactor. It generates as much power as a big coal or natural gas reactor, but it releases no carbon into the atmosphere. None. The relatively small amount of waste it does create comes in solid and liquid form. That may not sound delightful, but it’s probably preferable to the uncontrollable gaseous emissions that come from fossil-fuel plants.

And yet, in the United States, we have about the same number of nuclear power plants as we did in the late 1980s, and a 2016 Gallup poll found that the majority of U.S. citizens are opposed to nuclear energy.

They have good and bad reasons for their views. We are going to do our best not to get too political, and to simply report what our books and interviews gave us. That said, to put our biases on the table, we think the problems with nuclear power are real, but often exaggerated and misunderstood in popular discourse. France, for instance, generates about three fourths of its electricity from nuclear power, and as far as we know they are not drowning in toxic green slime. Baguettes remain tasty, and the accordion sounds as good as it ever did.

That said, full disclosure: Zach once had a Twitter argument about nuclear power with the comedian Joe Rogan, which culminated in Zach being called a “silly pro-nuclear bitch.”

Opponents to nuclear power think it can never be made safe enough. Proponents of nuclear power think governments and activists have saddled it with an overly burdensome set of legal regulations. But the more we read and the more we talked to scientists and engineers, the more we came to think the biggest issues facing a “nuclear renaissance” were much more mundane: Licensing a nuclear plant is really time consuming, and once you have the license it costs a whole lot to build the thing.

The environmental and safety concerns are real and important, but for reasons we discuss later, even under nonideal circumstances nuclear may still be a good choice for a long time to come. But many scientists believe that innovative new nuclear designs can move us closer to ideal circumstances, with more energy for less fuel, less waste generated, more safety, and lower cost.

If they’re right, the atmosphere would be a lot cleaner, even as power consumption grows exponentially.

Where Are We Now?

Today almost all reactors are some variation on the water-based type we described above. In the early days, there was a lot more variety. The major deviations involved switching out water for something else—gas, heavy water, or liquid metal. It’s worth looking at them for a moment, since a lot of the most current ideas involve revisiting the early experiments.

Gas ended up having issues, which include the 1957 Windscale disaster, in which a reactor caught fire. In short, the problem with a gas reactor is that you have to use a solid moderator, usually graphite. Graphite is a great moderator, but it has this property that when neutrons smack into it, it can store their energy internally only to suddenly release it later. At Windscale, an actual fire was mistaken for one of these graphite energy releases. In a pretty serious whoopsie, they activated cooling fans to dissipate the heat. If you’ve ever blown air at a campfire, you can imagine the result. The fire got out of control and threw radioactive dust over the surrounding cow pastures. Gas reactors have been built since, but they have not become popular.

Heavy-water reactors are pretty much a Canadian thing. As you might expect from the Noble Canadian, their reactors use fuel very efficiently. The trick is using heavy water instead of regular, boring water. Heavy water moderates neutrons really well, meaning more neutrons actually end up fissioning uranium. This means you can use uranium in its “natural” state rather than enriching it. So why has the heavy-water reactor, like maple-flavored coffee, remained largely a Canadian phenomenon? The two have something in common—they solve a problem that doesn’t exist. Heavy-water reactors were designed for a world in which uranium is really expensive. For the moment, uranium is not a significant cost of doing business at a nuclear reactor.

Liquid-metal reactors replace water with molten metal. Beyond being awesome, there are good reasons to do this: Metal absorbs and transfers heat really well. Most metal can’t turn into gas at typical reactor temperatures. This means you can’t get a pressure explosion. If something goes wrong, control rods drop in, the reaction stops, and the metal cools down. The downside is that liquid metal can be hard to work with. For instance, early attempts at this reactor style used mercury, which releases toxic fumes.

But the choice of water reactors wasn’t entirely for technical reasons. Early on in the research process, the U.S. Navy got involved because they wanted nuclear submarines. Water is a pretty good reactor coolant when you’re at sea, so a lot of money got plowed into a particular design that was submarine friendly, called a pressurized water reactor. In part because this design was the best researched, it became the most popular. About two thirds of all reactors today are in this style. Many researchers believe that this was a fine choice for submarines, but not great for terrestrial reactor systems.

Modern Designs

Reactors built in the last decade have made evolutionary improvements over old designs, in three ways in particular: (1) They have modern computer systems, (2) they are designed to be simpler and cheaper, and (3) they incorporate passive safety features.

Modern computer systems may not sound like much, but from what we could tell, stepping into a reactor is often like taking a step back to the 1970s. The computers are enormous machines with little dials, blinky lights, and readouts via ink on paper. Also, the people have weird clothes and haircuts, but that may speak more to the fashion sense of your typical nuclear engineer than anything. Digital systems improve all of this (except the haircuts), and allow for finer adjustments to control the reaction.

“Cheap” may not be a word you want to hear in association with a nuclear reactor. Rest assured, these nuclear power plants still cost billions. But there is a lot of room for economic efficiency among old designs. A typical reactor from the old days was designed specific to its location. These newer designs are at least partially modular. Regularized parts are built off-site, then toted in and hooked up, like big nuclear Legos. The designs also tend to be simplified, requiring fewer valves, less steel, less concrete, and in general, less stuff. The result is a nuclear plant that should be far less expensive and take less time to build, while having fewer parts to break.

From the public standpoint, and the standpoint of people who will live near these things, the most exciting feature is the passive safety. If passive safety systems had been in place, the Fukushima disaster never would have happened.

The short version of the Fukushima disaster is that after an earthquake and tsunami, their control rods kicked in properly to stop the nuclear reaction. This means the nuclear two-for-one deal began to slow down, but it would still take some time to stop generating heat. So far, this is a perfectly performing fail-safe. In normal situations, once the reaction dies, diesel-powered electric pumps move water around a loop, dumping heat (but not water) into a nearby lake or ocean. At Fukushima, these systems broke down, causing the water to get really, really hot. The reactor there was clad in zirconium, probably because the engineers were too cheap to buy real diamond. Zirconium is good reactor armor because it doesn’t eat too many neutrons out of the system. But when extremely hot steam contacts it, it reacts to produce hydrogen gas. At Fukushima, the result was a massive buildup of hydrogen gas, resulting in an explosion.

Here’s how a new passive safety system works without using any electricity:

Imagine your nuclear reactor, shaped like a big cigar. Now, imagine you have a container around the cigar completely enclosing it. Inside this outer vessel are massive water tanks. After the control rods stop the reaction, the water in the core becomes high-pressure steam. That pressure causes the tanks to open, whereupon they rain water down on the hot reactor within.

This rain cools the reactor immediately, but the reactor is way too hot to get totally cooled by the bucket trick. So here’s the cute part: The water gets hot, then rises. At the top of the tank, it transfers its heat out to the surrounding air, condenses, and rains back down. Then the process starts over again. Basically, you’ve set up a little rain cycle. It’d be downright charming if it weren’t the thin line between you and a cloud of radioactive death.

The Future

There are dozens of ideas for new types of reactors, but what they share beyond the current systems is that they all are trying to push reactors to greater extremes in an attempt to make them cleaner and to get more energy per dollar. We’re going to consider some of the major proposals to achieve these goals.

Extreme Temperatures

In a typical pressurized water reactor, the core gets to about 600 degrees Fahrenheit. In the future, the hope is to get in the 1500–2000-degree range. Regardless of what substance we’re heating, this is going to be a lot tougher. So why do it?

High temperatures have two virtues. First, as a physical rule, the higher your temperature, the more efficient your engine. So more of the heat generated goes into spinning the turbine, instead of escaping out of the system. This increase in efficiency might mean as much as a 50% improvement in output. To put that in perspective, if every nuclear reactor in the United States were 50% more efficient, about one third of all coal plants could be taken off-line. Second, you can use that ultrahot gas to heat water and get what’s called process steam. Process steam is basically just really, really, really hot steam, which has all sorts of industrial applications, such as oil refining and separating hydrogen from water to make fuel cells.

Pebble Beds

One idea for how to simplify fuel delivery is called the pebble bed. Instead of a complex reactor core, you have little premade balls that do the whole job for you.

Think of nuclear pebbles as tennis balls that emit slow neutrons. Each pebble contains the nuclear fuel, the moderator, a seal, and a layer to help prevent fracture. You put them in your reactor, surround them with some gas or liquid to transfer heat out, and off they go. This could be very convenient, because it means maintenance and refueling is just a matter of putting in or taking out pebbles. And if it becomes the standard way to do things, you can industrialize pebble manufacturing.

Pebble bed technology was tried in Germany in a reactor called AVR, which is of course short for Arbeitsgemeinschaft Versuchsreaktor. They used helium to capture the heat and turn the turbine because helium doesn’t become radioactive in a nuclear core and it doesn’t react with the walls of the container. Simple, hot, efficient. Also, if you inhale reactor core gas, your voice gets funny. They got it to about 1000 degrees Fahrenheit. If a future design can safely get closer to 2000 degrees, it might be very good indeed.

The biggest knock against this system is that AVR didn’t go that well. There was no single catastrophic incident, but after the experiment ended, the site itself was left seriously irradiated. A major problem was radioactive dust, apparently created by slow, long-term friction between the pebbles. The dust contained strontium-90 and cesium-137, which are particularly nasty substances. Both are very radioactive, but in and of itself, that’s not the danger. As you remember from the previous chapter, strontium-90 is absorbed into the body like calcium. Fortunately, when not being dispersed by a giant megabomb, strontium-90 is likely to stay pretty close to the contamination site and not be a big problem for the general public. But cesium-137 is very soluble in water, and thus poses serious contamination risks. You, being mostly water, may wish to avoid it.

At a cost of several billion dollars, the German government is having to fix the radioactive dust in place using poured concrete. Also, apparently there was a crack in their system, and about a hundred pebbles got, well, stuck. Stuck is not an ideal situation for nuclear reactor fuel.

Breeding and Burning

In traditional reactors a lot of perfectly good neutrons go to waste. Some scientists think we can reuse them. See, uranium is the most common nuclear fuel, but several other substances transmute into nuclear fuel when bombarded with neutrons. For instance, when you smack an element called thorium-232 with a neutron, it goes through a series of changes before turning into uranium-233. U-233 is relatively unstable, with a half-life of about 160,000 years. So, on this four-and-a-half-billion-year-old planet, the naturally occurring U-233 is long gone. But it works just fine as nuclear fuel when humans create it in a reactor. Similarly, you can turn uranium-238 into plutonium-239, which is an excellent reactor fuel.

In order to make all this mildly uncomfortable, scientists have named these not-quite-reactor-fuel elements “fertile,” as opposed to actual nuclear fission fuel, which is “fissile.” The goal of a breeder is to turn merely fertile substances into fissile substances.

This might be a great way to reheat the nuclear leftovers, so to speak. The United States currently has around 700,000 tons of “depleted uranium” stockpiled at enrichment facilities in Ohio, Kentucky, and Tennessee. The one use of depleted uranium right now is for the military. A given chunk of depleted uranium weighs about 70% more than the same amount of lead, which makes it a great thing to hurl at enemies or use as tank armor.

There are a lot of ways to implement a breeder reactor, such as the delightfully named “breeding blanket,” which is where you wrap the core in fertile material so reactor neutrons can slam into it. So you’re recycling war material into fuel. What’s the downside? Breeders and heavy-water reactors have the same flaw—they solve the wrong problem. Why take all the effort to breed fuel when fuel is cheap?

Oh, and there’s this: Breeders are a good way to make bomb fuel without anyone noticing. All nuclear reactors make some amount of plutonium-239 when U-238 gets thwacked with neutrons. But in most reactors, this plutonium just behaves as more nuclear fuel. In a breeder, you take that plutonium out of the system to fashion it into new fuel. As it happens, this plutonium is a great material from which to make a nuclear bomb. In fact, it was the main ingredient in the fission bomb dropped on Nagasaki in 1945. Newer reactors can circumvent this possibility by directly recycling the plutonium, so that you can’t extract it without bringing along some other nasty by-products. Normally, nasty by-products are a bad thing, but if they keep rogue dictators from getting plutonium bombs, we’re all for it.

And fuel recycling has some serious virtues: Not only do you use fuel more efficiently, but at the end you create less waste. In conventional reactors, most of the radiation the fuel will ever emit will not be inside a nuclear reactor, but inside a waste storage container. This is because the current methods result in a build-up of long-lived heavy elements that current reactors can’t use as fuel. Some of these by-products will remain radiotoxic for millions of years. If we can fission them, we simultaneously destroy toxic elements and get energy. This means less waste and more energy per unit of fuel.

Some materials left over from the breeding process are still pretty undesirable, and aren’t necessarily much use for energy production. But by bombarding them with neutrons, you can “burn” them into elements that are much safer. As an extremely creepy rancher might say, “If you can’t breed it, burn it.”

Fast Reactors

It’s not as easy, but you can run a reactor with unmoderated, or “fast,” neutrons. In fact, you might even be able to use plentiful U-238 to do it, since U-238 does a reasonably good job absorbing fast neutrons. As a bonus, if you want to use the reactor to burn up nasty by-products, fast neutrons actually do a better job than slow neutrons.

So if it works, you have a reactor that uses ultracheap fuel and does a much better job burning up by-products in the process, which potentially means the waste would be dangerous for hundreds of years instead of thousands of years or more. Given the human track record for taking any interest in the future, this may be slightly cold comfort, but hey, it’s something.

The problem is that a fast reactor is sort of like driving a car that can only go at 150 miles per hour. Your neutrons are running hot and fast. So when something goes wrong, it can go wrong very quickly.

Another problem is that you have to pick a coolant that doesn’t also moderate, since you need the neutrons to keep moving fast. This basically requires you to use an unusual coolant. One idea is the sodium-cooled fast reactor. Sodium is a metal that melts at 208 degrees Fahrenheit, and it doesn’t moderate neutrons. It’s also extremely good at moving heat around. Potentially, it’s an ideal coolant for this sort of thing, but it has this one problem: Sodium reacts violently with certain substances, like . . . air, water, and nuclear engineers.

This problem means extra expenses and care have to be taken with the reactor, which (at least right now) means it’s too expensive to be commercially attractive, even if it could be made to work.

Molten Salt

Okay, maybe you’re not yet sold on molten sodium as a coolant, but how about molten salt? At least it sounds tasty.

First, no, sodium isn’t a salt. Sodium chloride (the stuff in your salt shaker) is a salt. In chemistry, a salt is a by-product of the reaction of an acid and a base, and most such combinations are not tasty on potatoes.

Second, the cool thing about molten salt is that it conducts heat really well and behaves like a fluid when very hot. If the plant shuts down, the molten salt just hardens as it cools. So no steam-pressure explosion problem, and with the right mix you avoid the explosive sodium problem. This also means that your reactor can be supercompact, which is why an early version of this design was part of the Aircraft Nuclear Propulsion Project in the United States.

YEP. Nuclear reactor on a plane. The idea was that you could have a plane with essentially limitless flight time. This sounds like one of the Spheres of Hell to your authors, but it’s pretty good if you’re looking to build a spy plane.

The airplane version didn’t quite work out, and in any case, we suspect the public wouldn’t have been delighted by the risk of nuclear plane crashes. Plus—in the age of satellites, drone planes, and intercontinental ballistic missiles, you don’t really need an aircraft that can stay aloft for weeks. But as a regular on-the-ground reactor, it has some tempting possibilities.

A lot of geeks are particularly into a breeding version of the molten-salt reactor that uses thorium. Here’s how it works: You start with your molten salt, which contains uranium and thorium (among other things that are desirable in terms of safety and efficiency). The salt flows around a loop endlessly. Here is the neat thing: In one portion of the loop you have a graphite moderator. So when the salt-and-fuel blend passes through this area, it behaves like a reactor core. When it leaves to dump its heat, it loses its moderator and behaves like the regular old molten salt blended with uranium and thorium we’re all so familiar with. The core generates heat, the molten salt dumps the heat to water, and steam turns a turbine as usual.

As the process rolls on, the thorium in the molten salt keeps getting bred to U-233, which acts as your nuclear fuel to continue the cycle. So once the process is rolling, the core breeds its own nuclear fuel as long as you keep supplying thorium. Because uranium is cheap, thorium isn’t necessarily that exciting, even though it’s cheaper than uranium. But, thorium does have this virtue—when it converts to uranium, a rare isotope called U-232 gets created.

Uranium-232 is an unstable element, and as it decays, some of it converts into thallium-208—a very dangerous isotope that emits high-energy gamma rays. Wait . . . why is this good? Well, U-233 can be used as bomb fuel. But if it contains U-232, it pretty quickly starts throwing radiation in every direction, making it dangerous to would-be bomb makers and potentially easy to detect for inspectors. If that’s so, you get the virtues of a breeder reactor with somewhat lower risk. Whether that’s enough to keep the world safe is a matter of debate. Or, as our favorite nuclear geek, Christopher Willis, tells us, “Many in the proliferation game find this argument weak sauce because it’s relatively easy to separate the thallium out.”

Dr. Per Peterson of the University of California, Berkeley is working on a different design that combines pebble beds with molten salt in a way that may improve both: “The salt-cooled pebble bed reactors that we work on, it turns out that the fluoride salts are effective lubricants, and moreover, since the pebbles are nearly neutrally buoyant in the salt, the contact forces are much, much lower, for the salt cooled. In the salt-cooled pebble bed reactors, we don’t expect to see a very significant pebble wear. In the case of the salt-cooled reactors, any radioactive material that might get out of the fuel, particularly cesium, is highly soluble in the salt coolant. Therefore, we would be able to essentially detect any leaking fuel, and the radioactive material that might leak would be held in the coolant and could be cleaned up.”

Small Modular Reactors

Okay, so all this fancy stuff is cool, but it’s not cheap. Depending on the nuclear reactor, costs typically run between $5 billion and upwards of $10 billion. And of course, the more experimental ones carry no guarantee of success.

One idea is to go simpler and get cheaper. Small modular reactors are basically miniaturized versions of the modern passive safety reactors we described earlier.

Imagine this: Your city has a large building that supplies power to the whole area. Instead of having a bunch of power plants, what happens is that a truck shows up holding a huge tube. The tube is hooked into your system and immediately adds about 50 megawatts of power output. If your city needs more power, you just order another big tube. Once the tubes have been in use for a few years, they get changed out for fresh ones. Basically, like giant batteries.



The big appeal is that by having one repeated design, you get economies of scale. And because they’re small, each unit is relatively cheap up front. This should make it easier for communities to purchase this sort of power. These smaller reactors could be especially important for countries that are relatively poor and don’t have local fuel resources.

Progress has been slow, but as of 2016 the Tennessee Valley Authority began the process of licensing several small modular reactor designs, and a company called NuScale is working on licensing a small modular reactor in Idaho.

We asked Margaret Harding about these systems. She was vice president of Engineering Quality in General Electric’s Nuclear Energy department, and now runs a consulting firm called 4 Factor Consulting, LLC. She pointed out some downsides to small modular reactors: They’re cheaper, but they aren’t cheap. In fact, we don’t even really know what the price will end up being at this point. And the feasibility of the idea relies on economies of scale. Mass production is only viable if there is mass demand, and that hasn’t developed yet.

Small modular reactors do have one other possible advantage: For a given reactor design, every setup is the same. As Dr. Peterson pointed out to us, SpaceX was able to disrupt the rocket industry, in part by building really good small modular rocket engines. Having a simple, small reactor design might make creation a lot cheaper and licensing a lot simpler. This is why all nuclear start-ups in the United States today are focusing on small reactor designs.

Concerns

Cost and Licensing

We’ll get to the more frequently considered concerns over nuclear safety in a bit, but in terms of ever getting advanced reactors to live up to their potential, the problem comes down to cost and licensing. This may seem more pedestrian than nuclear meltdown risk, but if we can’t solve these more human problems, we’re wasting an enormous amount of resources on a technology we’ll never have.

In the United States, the Nuclear Regulatory Committee (NRC) is in charge of granting permission for companies to build nuclear power plants. Although none of our interviewees complained about the stringency of the safety regulations, all mentioned how cumbersome the process of reviewing license applications was.

Part of the problem is that the NRC has an all-or-nothing licensing process, unlike, for example, the Food and Drug Administration, which has a twelve-step process.

When approval is all-or-nothing, if anything goes wrong you’re stuck starting the entire drawn-out process all over again. Having a step-by-step process is like having save points in a video game—if you get rejected, you go back to your last successful step.

Either way, licensing is expensive. Part of this is because the NRC is required to cover 90% of its budget through licensing fees. For comparison, the FDA recouped only about 45% of its budget in 2015 on fees.

The NRC is also more familiar with traditional reactor designs, which makes new developments difficult and expensive. According to Ms. Harding, this creates a big problem for new designs. They cost millions to get past the NRC, and whoever goes first gets the worst deal, because they’re paving the way for all who come after.

Separate from the bureaucracy, you need to convince the NRC that your design is safe. Given the nature of nuclear reactors, this is tough.

According to Jessica Lovering of the Breakthrough Institute, “If you’re going to use a metal in the core of your reactor it has to be able to withstand a lot of neutron bombardment, which is a really unique thing to nuclear reactors. You have to prove that your metal is not going to rupture or fracture over sixty or eighty years of heavy neutron bombardment. To do those tests you kind of have to expose metals to a lot of neutrons. There’s not a lot of facilities to do that. Particularly the United States doesn’t have the facility to do that at a higher level of neutron flux. . . . They have to go to France to do those tests on materials.”

Safety

When you take coal and you look at the particulate pollution and the effect it has on asthma, and the effect it has on the general population, the deaths are enormous.

• Margaret Harding

Ms. Harding argues that nuclear power is safer, but when it does go wrong, it creates a lasting impression. “How do people die in nuclear power plant accidents? Big accidents that everybody remembers. It’s like having an airline crash, and everybody can name it. People that die in natural gas explosions, natural gas accidents, people that die in coal-related deaths is what we call ‘chronic.’ You can’t name them. Name the last coal death. Name the last natural gas explosion. You can’t do it. That’s the nuclear problem; we’ve got these nameable accidents. You say, ‘Three Mile Island, Chernobyl, and Fukushima,’ and everybody instantly knows what they are.”

To put things in perspective, the high-end estimate for Fukushima is that the accident will cause 1,500 premature deaths. Other estimates are in the hundreds or even lower, but let’s consider the worst case. A one-time loss of 1,500 people is absolutely tragic, but is not on the same scale as deaths lost to fossil fuels. NASA’s Dr. Pushker Kharecha and Dr. James Hansen estimate that from 1971 to 2009 the use of nuclear power in place of burning fossil fuels has saved 1.84 million lives that would have otherwise been lost to air pollution-related deaths and has prevented 64 gigatons of greenhouse gas emission from being released into the atmosphere (where they could exacerbate global warning and cause additional deaths). This is absolutely not to say that nuclear reactors can’t create danger, but some concern over the possible trade-offs is in order.

Nuclear Waste

According to Dr. Peterson, “When we burn fossil fuels, the only practical method for managing the waste is to discharge [it] into the biosphere. The quantities are so enormous that we’re actually fundamentally changing the composition of our atmosphere and the chemistry of our oceans, which if you think about it is a crazy thing to do. . . . Conversely, we know that there’s a strong scientific consensus that with the really small volumes involved, the use of deep geologic disposal, deep geologic isolation, can provide safe, long-term, effective isolation of nuclear wastes, and protect public health, for essentially the indefinite future.”

So where do we put these deep geological isolation sites? No one wants a nuclear waste disposal site near their home. These sites are picked because the geological features make it particularly unlikely that nuclear waste could leak into the groundwater, even in the unlikely event that the barrels containing the waste were to break. However, at the moment, we continue to push off making a decision about where to put these waste storage sites. No one can say with 100% certainty that nuclear waste won’t get into groundwater. But given the small footprint of nuclear waste and the ability we have to put it anywhere we like, we have much better options when it comes to making choices about safety.

And that’s just for traditional reactors. If the advanced reactors work, they should eliminate much of the waste simply by reusing fuel. But Dr. Peterson notes that reprocessing adds an expense. To his way of thinking, we need nuclear now to lower fossil fuel consumption. “Is it rational for us to reduce the amount of waste that reactors make in ways that increase the cost of nuclear electricity significantly? I think that the short answer is, given that we have to develop geologic disposal anyhow, it is better for us to try to make nuclear energy as affordable as possible.”

Nuclear Proliferation

We talked earlier about how uranium can be “enriched” to increase the percent of uranium-235 in a sample. A little bit of enriching makes uranium good for use in a nuclear power plant. A lot of enriching makes it good for nuclear weapons. Pretty much the same equipment is used to enrich for power or for weapons, so the fear is that an innocent-looking power plant might be a clandestine bomb factory. This is a particular concern for rogue nations, or even apparently stable nations that want to sneak their way into the nuclear club.

All traditional nuclear reactors can create weapons-grade plutonium. The vast majority of nations have agreed not to develop nuclear weapons and to allow UN inspectors to confirm that they aren’t sneaking out at night to make atomic engines of devastation. But the temptation to cheat is great because many nations believe that having nuclear weapons deters other nations from invading.

The risk of superweapon development is unique to nuclear, and it is a risk that can be managed, but likely never be eliminated.

Another concern is that humans are sometimes evil, often stupid, and always myopic. Even when well intentioned, we seem to underestimate risk.

Terrorist attacks are a possibility that becomes more probable as more nuclear power is brought online, and nuclear power has a uniquely bad worst-case scenario. If you blow up most sources of power, the main danger is on site. If something catastrophic is done to a nuclear reactor, you can get out-of-control radioactive clouds.

Speaking of evil and stupid, let’s talk politics. One concern is not just how well the reactors work, but who is running them. At the Chernobyl disaster, a major part of the catastrophe was not just the accident, but also the failure of the Soviet government to promptly evacuate nearby cities and notify the public generally. A public “Our bad, guys” was not forthcoming until a cloud of radioactive gas had reached Sweden. Nuclear power will always contain a substantial downside risk, but the risk is seriously compounded by secretive governments that lack significant press freedom.

Ultimately, the biggest problem facing nuclear power isn’t radiation and explosions—it’s us. Waste can be stored or burned up. Explosions can be prevented. But we can’t stop stupid decisions or bad actors without eliminating humans altogether, which seems like a nonideal way of making the world safe. At least, if you’re human, anyway.

How It Would Change the World

The USA has about a hundred reactors right now. They provide 20% of all American electricity. If all fossil-fuel reactors were replaced with nuclear, and all cars were electric, we would be pretty much carbon neutral, and our main source of greenhouse gas emissions would be cow flatulence.

As Ms. Lovering points out, “Countries that have a lot of nuclear obviously have much lower carbon emissions and much lower air pollution, but another factor which a lot of people don’t think about is when you have a huge source of baseload cheap electricity, you tend to electrify a lot more things. You have a lot more electric rail or electric trolleys for public transportation. You do a lot more electric heating and so that reduces pollution from other sectors. It reduces your air pollution from cars and it reduces your air pollution from wood-burning stoves and things like that.”

This sort of calculation is even more important when you look at rapidly industrializing nations, all of which will face exponentially increasing demands in the coming decades. We tend to think of the billions of Chinese and Indian people, but there are also the 180 million people in Pakistan, the 150 million in Bangladesh, the 250 million in Indonesia, the 1.1 billion in Africa. It’s no wonder that many nations are making large investments in new nuclear power. According to the World Nuclear Association, fifteen countries are currently constructing over sixty reactors.

Although renewables are getting better and better, the current evidence suggests that when countries switch off nuclear plants, the replacement power tends to come from CO₂-intense sources, like coal. For example, Germany has one of the most prorenewable policies on the planet, yet they use about as much fossil fuel today as they did in the 1990s. Partially this is just higher overall consumption, but another big reason for this is that they’ve been slowly reducing nuclear usage.

Although renewables are useful in many places, they are not viable everywhere. Countries that are “poor” in solar and wind resources, or that have public lands they don’t want to turn into large power plants, may turn to nuclear as the most viable option.

Like fusion power, if nuclear fission reactors can be made cheaper, you’re talking about an energy source that creates no airborne pollution and has essentially unlimited fuel reserves. Cheap power means more money, more productivity, and cheaper stuff.

And then, of course, there’s space travel.

Even if renewables become superefficient and inexpensive, they probably won’t be the best way to take a trip to Saturn. Nuclear fuel has about a million times the power density of rocket fuel. You can, of course, gather solar energy for your spaceship, but solar panels are bulky and hard to maintain in space. So even if we move into a carbon-free, renewable utopia, we’ll probably still want to research nuclear fission engines, because if you want a spaceship that works like the ones in the movies, you need a propulsion system that fits onboard a reasonably sized ship, and only needs refueling every few decades.

In 1954, the chairman of the Atomic Energy Commission Lewis Strauss said, “Our children will enjoy in their homes electrical energy too cheap to meter.” The phrase “too cheap to meter” has often been used as a slur on the false promise of nuclear power. In fact, Mr. Strauss may not have been referring to fission reactors at all, but rather to a secret project to create fusion power. And, regardless of the source of his enthusiasm, scientists and engineers from the time were well aware that nuclear power might only be able to reach prices comparable to conventional fuel sources, like coal and oil. In other words, “too cheap to meter” was a phrase uttered by a nuclear booster, but never a promise from the nuclear industry.

The confusion over the usage of “too cheap to meter” encapsulates the misunderstood promise of nuclear energy. Nuclear fission is not the end of energy history. It is an alternative to fossil fuels that comes with certain benefits and trade-offs. It creates waste that worries us more, but which can be stored more easily. Fueling and operating it causes few deaths, but in a catastrophe, it can be the most dangerous power source. It can bring reliable domestic power to resource-poor countries, but it can put weapons in the hands of dangerous autocrats.

Scientists hope that with enough development, nuclear can at least become the obvious choice when compared to fossil fuels. That hasn’t come to pass yet, but in an age of rapid economic expansion, advanced nuclear power may allow everyone to enjoy a Western lifestyle without putting dangerous chemicals into our lungs or greenhouse gases into our sky.

Or, hey, if we do burn this whole thing up, nuclear propulsion might be a way to escape.

Nota Bene on Project Orion

Dr. Ted Taylor was a young physicist in the 1950s, fascinated with the idea of trying to find the smallest amount of nuclear fuel that could be used to detonate an atomic weapon. Beyond simple curiosity, there was an idea at the time that it might be possible to create a new sort of army unit: nuclear-armed infantry.

Although compact nuclear weapons were created, no such unit was ever fielded. But his work in this area positioned Dr. Taylor perfectly for something that would later be called Project Orion.

Meanwhile, scientists had recently conducted an experiment to see if you could produce tritium by bombarding certain metals with neutrons. They hung out some giant metal balls and detonated a bomb to produce neutrons. The tritium part of the experiment didn’t work well, but something surprising was discovered: Certain nonmetallic materials in the test area had withstood the blast surprisingly well.

To most people, nuclear-bomb-resistant materials and compact nuclear weapons don’t immediately suggest a new way to travel. But Dr. Taylor, along with famed physicist Dr. Freeman Dyson, were not ordinary people. They came to believe that you could bomb your way to space cheaply by detonating nuclear explosives against bomb-resistant plates on the back of spacecraft.

Here’s the basic idea:

Your space vehicle is shaped sort of like a syringe, with a “pusher plate” on the back, connected to shock absorbers, which connect to the main body of the vehicle. You drop a specially designed nuclear bomb out the back. The nuclear bomb explodes, sending a blast of plasma toward the pusher plate.

Part of the pusher plate ablates away, the shock absorbers soften the kick, and the vehicle gets a speed boost without getting too much hotter. Repeat a few thousand times and you’re in space. This may seem a teensy bit wasteful, but the project scientists were able to make serious efficiency improvements. The explosives would have been “shaped charges,” meaning that nearly 50% of the explosive debris would have been directed toward the pusher plate.

This is already fun enough, but what really got the engineers excited was a technical discovery: The bigger you make the ship, the easier the launch gets. Okay, you need a lot more bombs, but big ships turn out to be much more stable than little ones when you’re using bombs as the propulsion system.

Soon the scientists were imagining a spacecraft the size of a cruise ship—about 700 times the size of the Space Shuttle—capable of rapidly voyaging around the solar system. The largest ship they considered possible was 8 million tons, which is about 150 times the weight of the Titanic. This would’ve been a ship with perhaps thousands of crew, with onboard nuclear reactors, Star Trekking their way around space, collecting more data in a few years of the 1960s than we do now in decades.

The project went on for about two years. The researchers mostly did theoretical work, though a few test launches were performed using conventional explosives. But, in the late 1950s, funding for space projects began its shift toward NASA, which was more interested in chemical rockets.

Pretty soon after this, we lovely humans figured out how to strap a nuclear bomb to a missile and deliver it anywhere on Earth. At this point, military funding for the project dried up. Why build a giant space cruiser that—incidentally—can explore the universe, when you can blow each other up from the comfort of home?

Dr. Taylor had started on the road to Orion in 1957, after the launch of Sputnik. For eight years, he managed to secure modest funding from various agencies, barely keeping the project going from year to year. By 1965, his luck had run out, and the project died.

Project Orion was one of those odd post–World War II projects that probably could’ve worked technically, but never could’ve worked politically. Even if the radioactive by-products could be minimized, it’s hard to imagine any government tolerating a spaceship that launches itself via a Gatling gun that fires out hundreds or thousands of nuclear bombs on the way up.

And yet the dream of an enormous spaceliner on a grand tour of the solar system is so romantic, many still dream of an Orion–style grand tour. While researching for this section, we found that Mr. George Dyson’s book Project Orion was reviewed in 2002 on Amazon.com by one Mr. Jeffrey P. Bezos, also known as Amazon CEO Jeff Bezos: “For those of us who dream of visiting the outer planets, seeing Saturn’s rings up close without intermediation of telescopes or charge-coupled devices, well, we pretty much *have* to read ‘Project Orion.’”

We suspect Amazon won’t be firing off a nuclear bomb anytime soon, but Bezos does run a well-funded space launch organization called Blue Origin. Perhaps if the governments of the world continue to pull back on funding for space research, the gap will be filled by slightly insane geek billionaires. Whether this is more desirable than the insane geek middle class at NASA is, we suppose, a matter of taste.

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