Clean, cheap nuclear energy is often touted as a means to battle climate change. But how close are we to having nuclear plants that fit the clean, green bill? What are the different technologies and what do they offer?
More than 10 per cent of the world's electricity currently comes from nuclear power plants. These existing plants all rely on nuclear fission — a chain reaction where uranium atoms are split to release extraordinary amounts of energy and, unfortunately, high levels of radioactive waste.
But a different type of nuclear reaction — nuclear fusion — has been the focus of research to develop nuclear power without the radioactive waste problem.
Nuclear fusion is the reaction that powers the Sun. It involves smashing hydrogen atoms together under extraordinary temperature and pressure, fusing them together to form helium atoms and releasing a large amount of energy and radioactive waste. But unlike fission, this radioactive waste is short-lived, quickly decaying to undetectable levels.
Nuclear fusion happens readily in stars like the Sun, because their cores reach extreme temperatures of over 15 million degrees Celsius, and pressures billions of times greater than our atmospheric pressure on Earth.
Fusion reactors would need to recreate these extreme conditions on Earth, and researchers are using two different approaches to achieve this: tokamak reactors and laser fusion.
Separate groups of scientists in Germany and China have recently announced they have made breakthroughs in nuclear fusion using tokamak reactors.
Tokamak reactors use a doughnut-shaped ring to house heavy and super-heavy isotopes of hydrogen, known as deuterium and tritium.
Normal hydrogen — which is also known as protium — consists of a single proton in its nucleus orbited by an electron. Deuterium differs in that the nucleus also contains a neutron, and tritium has a proton and two neutrons in its nucleus.
These isotopes are heated to 100 million degrees Celsius by powerful electric currents within the ring.
At these extreme temperatures electrons are ripped off their atoms, forming a charged plasma of hydrogen ions.
Magnets confine the charged plasma to an extremely small area within the ring, maximising the chance that the superheated ions will fuse together and give off energy. The heat generated can be used to turn water into steam that spins turbines, producing electricity.
Over 200 experimental tokamaks have been built worldwide, but to date they have all consumed more energy than they produce.
A massive international tokamak project — the International Thermonuclear Experimental Reactor (ITER) — aims to turn that situation around.
The ITER is designed to produce 10 times as much energy as it takes to run, becoming the first ever net energy producing fusion reactor. It is currently being built in the south of France, but with the first fusion experiments scheduled for 2027 it will be some time before we know if that goal has been reached.
In the meantime, physicists in Germany are using a variant of the tokamak, known as the Wendelstein 7-X stellarator. This uses a twisting ring design with changes in geometry and differing magnetic fields to control the plasma for longer periods of time compared to the short bursts tokamaks achieve.
Last week, physicists at the stellarator announced they had created a hydrogen plasma using two megawatts of microwave radiation to heat hydrogen gas to 80 million degrees Celsius for a quarter of a second.
At the same time, scientists in Chinasaid they had achieved temperatures of 50 million degrees Celsius (three times hotter than the core of the Sun) for 102 seconds at their experimental tokamak fusion reactor called the Experimental Advanced Superconducting Tokamak (EAST).
While tokamaks and stellarators use magnets to confine plasmas, another body of research is focusing on a different strategy to trigger fusion reactions, using high-powered lasers.
Laser fusion uses ultra-short bursts of very powerful lasers to generate the extreme temperatures and pressures needed to trigger a fusion reaction.
These laser pulses can heat and compress hydrogen isotopesto a fraction of their size, forcing them to fuse into helium and release high-energy neutrons.
The Lawrence Livermore National Laboratory's National Ignition Facility in California achieves deuterium–tritium nuclear ignition using a laser producing over two million joules of energy in a sudden pulse lasting just one nanosecond (one thousand millionth of a second).
The downside to laser fusion systems using deuterium and tritium is that they still produce high-energy neutrons (neutron radiation) which can cause other materials to become radioactive.
Nuclear fusion power could be a reality in 10 to 15 years
Emeritus Professor Heinrich Hora
An alternative laser fusion method being developed by scientists including Emeritus Professor Heinrich Hora of the Department of Theoretical Physics at the University of New South Wales, uses normal hydrogen protons and the commonly found element boron 11.
Instead of high-energy neutrons, hydrogen–boron 11 (HB11) fusion produces an avalanche of helium nuclei, resulting in extremely low levels of radioactivity — less even than produced by burning coal.
"Every HB11 reaction produces three helium particles, each of which collide with more boron to produce another three reactions and so on," said Professor Hora.
The HB11 process requires two lasers, the first to generate a powerful magnetic confinement field in a coil to trap the fusion reaction in a small area for a nanosecond, while a second more powerful laser triggers the nuclear fusion process.
"The triggering laser provides an extremely short duration pulse of just a picosecond, which is a millionth of a millionth of a second, and a thousand times shorter than the [nanosecond pulse] lasers at Lawrence Livermore," said Professor Hora.
Picosecond pulses achieve fusion through electrodynamic forces — directly converting optical laser energy into mechanical motion — smashing the target material together to trigger fusion.
Professor Hora says early HB11 fusion trials at the Prague Asterix Laser System, using high-energy iodine lasers, have generated more energy than needed to trigger the fusion process.
"For every joule of energy put into the fusion process by the lasers, the HB11 reaction generates 10,000 joules," says Professor Hora.
"Nuclear fusion power could be a reality in 10 to 15 years."
The thorium wildcard
With the goal of clean energy in mind, the focus isn't only on nuclear fusion. A cleaner form of nuclear fission is the subject of research around the globe.
Existing nuclear power stations rely on fission, using uranium 235, which is unstable and readily loses neutrons. These neutrons collide with other uranium atoms, splitting them and causing further collisions with even more uranium atoms in a chain reaction.
But all these high-energy neutrons result in large amounts of radioactivity.
Thorium fission reactors — first developed in the 1950s — could be a cleaner alternative.
Thorium is lighter than uranium, it doesn't undergo fission, and can't create runaway meltdown like uranium. Instead a seed of uranium or plutonium is injected into the thorium fuel, or a particle beam is fired at it to kick things off.
The process involves thorium 232 atoms being bombarded with neutrons to produce thorium 233 atoms, which quickly decay into protactinium 233, and then uranium 233, which undergoes fission similar to current nuclear power plants.
Unlike uranium 235, which creates self-sustaining chain reactions, thorium reactors only work as long as you keep firing neutrons, giving them an automatic failsafe to prevent meltdown.
Thorium reactors also produce just a fraction of the radioactive waste of conventional nuclear power stations, they aren't suitable for making weapons grade material, and can even be used to consume existing nuclear waste as a fuel source.
Thorium is three times as abundant as uranium, with Australia having the world's largest known reserves.
The United States, India, Israel, the United Kingdom, China, Norway, Chile and Indonesia are all examining thorium nuclear reactor projects.
An artist's impression of a cutaway view of the ITER tokamak fusion reactor in operation.
This past year has been big for nuclear fusion. First there was the announcement from Lockheed Martin claiming they could have a fusion reactor that fits in a truck. Next there is an announcement from Germany that physicists are close to finishing another fusion reactor.
I suspect that when most people read about nuclear fusion, like in this recent TIME feature on a startup called General Fusion, they just focus on the "nuclear" part. But there is a big difference between nuclear fission and nuclear fusion. Let's go over the similarities and differences.
It's All About Mass and Energy
Suppose that I had a 2 million dollars (this is clearly just a hypothetical situation). For some reason I decide to split this money two separate accounts. After doing this, I find that each account has $999,999. Yes, I am missing 2 dollars! But maybe in exchange for this missing 2 dollars, I get a whole bunch of energy. That might be ok.
This is exactly what happens with nuclear fission (fission means to break apart). If you looked at an atom, you would find it has three things: electrons, protons, and neutrons (OK, hydrogen doesn't have any neutrons). The number of protons in the nucleus tells you what element the atom is (nitrogen has 7 protons, silver has 47 protons). Then there is the atomic number atomic mass number. This tells you how many protons plus neutrons the atom has. Uranium-235 has 92 protons (because it's uranium) and 143 neutrons (because 235 - 92 = 143). Oh, one more fact for the next time you are at a party. If two atoms have the same number of protons, but different numbers of neutrons—these are isotopes (like hydrogen-1 and hydrogen-2).
But back to fission. Here is the crazy part. If you break uranium-235 into two pieces, you get krypton-92, barium-141 plus two extra neutrons. OK, that isn't crazy since all the protons and neutrons are accounted for. If you find the mass of the original uranium and the mass of all the pieces, you will find that you are missing some mass. The stuff before has a greater mass than the stuff after. That's a little crazy. It's like spitting 2 million dollars and ending up 2 dollars short. But that energy isn't really lost—it was just converted into other forms of energy. Yes, we can consider mass to be a kind of energy. This is where that famous equation comes into play.
In this expression, E is the equivalent energy, m is the mass of the particle and c is a constant that happens to be the speed of light (with a value of 2.99 x 108 m/s). Because this proportionality constant is so large (and squared), a small amount of mass can give you a HUGE amount of energy. What can you do with all of this energy you get from the change in mass? Obviously, you can heat up water and make steam. Yes, that's usually what these reactors do—they make steam to turn a turbine to generate electricity. Just like a coal burning power plant, but without the coal.
The above example looked at mass changes when you break something apart. This can also happen when you combine hydrogen and deuterium (which is just hydrogen with an extra neutron). When combining low mass elements, the product has less mass than the starting stuff and you also get energy. So, breaking large atoms gives energy (nuclear fission) and combining small atoms also gives energy (nuclear fusion).
Why Is Fission Better Than Fusion?
There are plenty of nuclear fission reactors that actually provide useful energy. As of now, there are zero useful fusion reactors. It turns out that nuclear fission isn't actually too difficult. If you take some uranium-235 and shoot a neutron at it, the uranium absorbs the neutron and becomes uranium-236. However, this uranium-236 is unstable and will break into pieces to give you nuclear fission. Even better, it also creates extra neutrons to break apart even more uranium. Oh, you can also do this with plutonium and thorium.
Fusion, on the other hand, is very difficult. Instead of shooting a neutron at an atom to start the process, you have to get two positively charged nuclei close enough together to get them to fuse. Without the electrons, atoms have a positive charge and repel. This means that you have to have super high atomic energies to get these things to have nuclear fusion. High energy particles are the problem. This is why fusion is difficult and fission is relatively simple (but still actually difficult).
Why Is Fusion Better Than Fission?
There are a couple of problems with fission reactors. First, the staring material. I think Marty McFly said it best in Back to the Future in regards to plutonium:
"Doc, you don't just walk into a store and-and buy plutonium! Did you rip that off?"
These starting materials aren't just laying around. In fact, if you went looking for some natural plutonium you wouldn't find any. The only way to get plutonium is to make it. The other problem with fission is the products. After this nuclear fission reaction, you have this left over stuff that can be both radioactive as well as chemically active. It's just nasty stuff that you have to deal with.
Nuclear fusion would solve both of these problems. It starts with simpler stuff—although deuterium isn't always so easy to find, you don't have to make it. After fusion, you get something like helium (or helium-3). Think of all the balloons you could blow up.