Fusion vs fission: clean, green nuclear energy technologies explained
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 China said 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 isotopes to 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.
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.