In both fusion and fission, nuclear processes alter atoms to generate energy. But despite having some things in common, the two can be considered polar opposites.
For the sake of simplicity, nuclear fusion is the combination of two lighter atoms into a heavier one. Nuclear fission is the exact opposite process whereby a heavier atom is split into two lighter ones.
One of the most significant differences between these two reactions is the type of fuel they require. Nuclear fusion requires a fuel that is composed of two light elements, such as hydrogen or helium, while nuclear fission requires a fuel that is composed of a heavier element, such as uranium or plutonium.
Another important difference is the amount of energy that is released during the reaction. Nuclear fusion releases much more energy than nuclear fission, making it a potentially more powerful source of energy. However, nuclear fusion is also much harder to achieve and sustain than nuclear fission, as it requires extremely high temperatures and pressures to initiate and maintain the reaction.
This deceivingly simple equation can be found everywhere, even in pop culture. It’s printed on coffee mugs and T-shirts. It’s been featured in countless novels and movies. Millions of people recognize it and can write it down by heart even though they might not understand anything about the physics involved.
Before Einstein, mass was considered a mere material property that described how much resistance the object opposes to movement. For Einstein, however, relativistic mass — which now takes into account the fact that mass increases with speed — and energy are simply two different names for one and the same physical quantity. We now had a new way to measure a system’s total energy simply by looking at mass, which is a super-concentrated form of energy.
It didn’t take scientists too long to realize there was a massive amount of energy waiting to be exploited. Through the process of fission, which splits uranium atoms a huge amount of energy, along with neutrons, is released. Interestingly, when you count all the particles before and after the process, you’ll find the total mass of the latter is slightly smaller than the former. This difference is called the ‘mass defect’ and it’s precisely this missing matter that’s been converted into energy, the exact amount computable using Einstein’s famous equation. This mass discrepancy might be tiny but once you multiply it by c2 (the speed of light squared), the equivalent energy can be huge.
Of course, this conservation of energy holds true across all domains, both in relativistic and classical physics. A common example is spontaneous oxidation or, more familiarly, combustion. The same formula applies, so if you measure the difference between the rest mass of the unburned material and the rest mass of the burned object and gaseous byproducts, you’ll also get a tiny mass difference. Multiply it by c2 and you’ll wind up with the energy set free during the chemical reaction to power a vehicle or electrical power station.
We’ve all burned a match and there was no mushroom cloud, though. It can only follow that the square of the speed of light only partly explains the huge difference in energy released between nuclear and chemical reactions. Markus Pössel, the managing scientist of the Center for Astronomy Education and Outreach at the Max Planck Institute for Astronomy in Heidelberg, Germany, provides us with a great explanation for why nuclear reactions can be violent.
“To see where the difference lies, one must take a closer look. Atomic nuclei aren’t elementary and indivisible. They have component parts, namely protons and neutrons. In order to understand nuclear fission (or fusion), it is necessary to examine the bonds between these components. First of all, there are the nuclear forces binding protons and neutrons together. Then, there are further forces, for instance the electric force with which all the protons repel each other due to the fact they all carry the same electric charge. Associated with all of these forces are what is called binding energies – the energies you need to supply to pry apart an assemblage of protons and neutrons, or to overcome the electric repulsion between two protons.”
“The main contribution is due to binding energy being converted to other forms of energy – a consequence not of Einstein’s formula, but of the fact that nuclear forces are comparatively strong, and that certain lighter nuclei are much more strongly bound than certain more massive nuclei.”
Pössel goes on to mention that the strength of the nuclear bond depends on the number of neutrons and protons involved in the reaction. What’s more, the binding energy is released both when splitting up a heavy nucleus into smaller parts (fission) and when merging lighter nuclei into heavier ones (fusion). This explains, along with chain reactions, why nuclear bombs can be so devastating.
How nuclear fission works
Nuclear fission is a process in nuclear physics in which the nucleus of an atom splits into two or more smaller nuclei as fission products, and usually some by-product particles.
Based on Albert Einstein’s eye-opening prediction that mass could be changed into energy and vice-versa, Italian physicist Enrico Fermi built the first nuclear fission reactor in 1940.
When a nucleus fissions either spontaneously (very rare) or following controlled neutron bombardment, it splits into several smaller fragments or fission products, which are about equal to half the original mass. In the process, two or three neutrons are also emitted. The resting mass difference, about 0.1 percent of the original mass, is converted into energy.
The energy released by a nuclear fission reaction can be tremendous. For instance, one kilogram of uranium can release as much energy as combusting 4 billion kilograms of coal.
To trigger nuclear fission, you have to fire a neutron at the heavy nucleus to make it unstable. Notice in the example above, fragmenting U-235, the most important fissile isotope of uranium, produces three neutrons. These three neutrons, if they encounter other U-235 atoms, can and will initiate other fission reactions, producing even more neutrons. Like falling dominos, the neutrons unleash a continuing cascade of nuclear fissions called a chain reaction.
In order to trigger the chain reaction, it’s critical to release more neutrons than were used during the nuclear reaction. It follows that only isotopes that can release an excess of neutrons in their fission support a chain reaction. The isotope U-238, for instance, can’t sustain the reaction. Most nuclear power plants in operation today use uranium-235 and plutonium-239.
Another prerequisite for the fission chain reaction is a minimum amount of fissionable matter. If there is too little material, neutrons can shoot out of the sample before having the chance to interact with a U-235 isotope, causing the reaction to fizzle. This minimum amount of fissionable matter is referred to as critical mass by nuclear scientists. Anything below this minimum threshold is called subcritical mass.
How nuclear fusion works
Fusion occurs when two smaller atoms collide at very high energies to merge, creating a larger, heavier atom. This is the nuclear process that powers the sun’s core, which in turn drives life on Earth.
Like in the case of fission, there’s a mass defect — the fused mass will be less than the sum of the masses of the individual nuclei — which is the source of energy released by the reaction. That’s the secret of the fusion reaction. Fusion reactions have an energy density many times greater than nuclear fission, making them billions of times more powerful than chemical reactions.
Nuclear fusion could one day provide humanity with inexhaustible amounts of energy. When that day may come is not clear at this point since progress is slow, but that’s understandable. Harnessing the same nuclear forces that drive the sun presents significant scientific and engineering challenges.
Normally, lighter atoms such as hydrogen or helium don’t fuse spontaneously because the charge of their nuclei causes them to repel each other. Inside hot stars such as the sun, however, extremely high temperatures and pressure rip the atoms to their constituting protons, electrons, and neutrons. Inside the core, the pressure is millions of times higher than at the surface of the Earth, and the temperature reaches more than 15 million Kelvin. These conditions remain stable because the core witnesses a never-ending tug-of-war of expansion-contraction between the self-gravity of the sun and the thermal pressure generated by fusion in the core.
Due to quantum-tunneling effects, protons crash into one another at high energy to fuse into helium nuclei after a number of intermediate steps. Fusion inside the star, a process called the proton-proton chain, follows this sequence:
Two pairs of protons fuse, forming two deuterons. Deuterium is a stable isotope of hydrogen, consisting of 1 proton, 1 neutron, and 1 electron.
Each deuteron fuses with an additional proton to form helium-3;
Two helium-3 nuclei fuse to create beryllium-6, but this is unstable and disintegrates into two protons and a helium-4;
The reaction also releases two neutrinos, two positrons, and gamma rays.
Since the helium-4 atom has less energy or resting mass than the 4 protons which initially came together, energy is radiated outside the core and across the solar system.
To shine brightly, the sun gobbles up about 600 million tons of hydrogen nuclei (protons) every second which turns into helium releasing 384.6 trillion Joules of energy per second. This is equivalent to the energy released by the explosion of 91.92 billion megatons of TNT per second. Of all of the mass that undergoes this fusion process, only about 0.7% of it is turned into energy, though.
Though scientists have been trying to harness fusion for decades, we’ve yet to fulfill the fusion dream that promises unlimited clean energy.
While it’s relatively easy to split an atom to produce energy, fusing hydrogen nuclei is a couple of orders of magnitude more challenging. To replicate the fusion process at the core of the sun, we have to reach a temperature of at least 100 million degrees Celsius. That’s a lot more than observed in nature — about six times hotter than the sun’s core — since we don’t have the intense pressure created by the gravity of the sun’s interior.
That’s not to say that we haven’t achieved fusion yet. Scientists are currently pursuing nuclear fusion using new technologies like magnetic confinement and laser-based inertial confinement. It’s just that all experiments to date put more energy into enabling the required temperature and pressure to trigger significant fusion reactions than the energy generated by these reactions.
In December 2022, nuclear fusion reached a holy grail moment when physicists at the Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility in California announced they had achieved ‘fusion ignition’, producing more energy from nuclear fusion than the energy consumed by the fusion process.
The $3.5 billion fusion energy facility at LLNL fired 192 high-power laser beams into a capsule the size of a peppercorn, in which hydrogen atoms are inserted. The laser beams effectively heated the hydrogen fuel to 100 million degrees Celsius and compressed it to more than 100 billion times that of Earth’s atmosphere, mimicking the conditions found inside stars. After a certain threshold is crossed, the intense heat and pressure caused the capsule to implode and the hydrogen atoms to fuse.
However, the path toward achieving working nuclear fusion that can power homes and industries is long and winding. The experiment expended 2.05 MJ (megajoules) of energy and produced 3.15 MJ of output, almost 50% more fusion energy than was put in. However, in nominal terms, the output energy is pitiful, maybe just enough to boil a kettle or two of water. The reaction also lasted only a billionth of a second. Furthermore, the scientists used 300 MJ of electricity to power up the lasers and kickstart the fusion reaction, so technically the overall energy balance is still very much in the red.
But the LLNL experiment is just one of over 30 different fusion energy projects currently underway across the world. One of the most important projects in the field is the International Thermonuclear Experimental Reactor (ITER) joint fusion experiment in France which uses magnets rather than lasers to confine hydrogen fuel. Its doughnut-shaped fusion machine called tokamak is expected to start fusing atoms in 2025.
Elsewhere, in Germany, the Wendelstein 7-X reactor, which uses a complex design called a stellarator, was turned on for the first time in late 2016. It worked as expected, though still inefficient like all other fusion reactors. The Wendelstein reactor, however, was built as a proof of concept for the stellarator design which adds several twists to the tokamak ring to increase stability. The UK and China have their own experimental fusion reactors as well.
Physicists at the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) are proposing a more efficient shape that employs spherical tokamaks, more akin to a cored apple. The team writes that this spherical design halves the size of the hole in the doughnut, meaning we can use much lower energy magnetic fields to keep the plasma in place.
It seems like we’re still decades away from seeing an efficient fusion reactor. When we do get our own sun in a jar, though, be ready to embrace the unexpected. Nothing will be the same again.