Nuclear fusion and fission are both natural physical processes that release energy as a result of interaction between atoms. These energies are a magnitude greater than those of chemical reactions. While fusion and fission are natural phenomena without which life on earth could not exist, it is the man-made application of these forces that most often attracts attention. The use or misuse of nuclear energy has come to define much of our modern world, creating promise and threat in equal measure.
Put simply, nuclear fission is the splitting of an atom into two or more atoms of a lower atomic weight. When the total mass of the smaller atoms is less than that of the original atom, the difference in mass is converted into energy. As Einstein taught us with his famous equation E=mc2, a small amount of mass will convert to a large amount of energy. This is because of the huge energy potential that is bound up in an atomic nucleus.
Nuclear fission occurs naturally all the time. Heavy elements such as Uranium and Thorium continuously undergo slow, spontaneous fission that generates radioactivity and heat. This heat warms the planet’s crust and molten core. The rotating core generates the magnetic field that protects all life from deadly cosmic and solar radiation. Heat from radioactive decay is also thought to drive plate tectonics.
In 1913, Danish scientist Niels Bohr conceptualized the atom as a kind of “miniature solar system,” with electrons orbiting a nucleus in set locations he described as shells. When an electron moved between shells, radiation was either emitted or absorbed. Many experiments were conducted in the 1920s and 1930s to explore and refine the atomic model further.
With the realization that bombarding the nucleus of a heavy atom with energetic particles could start a chain reaction, the possibility of a bomb became real. The United States initiated the Manhattan Project, culminating in the dropping of the atomic bomb on the Japanese cities Hiroshima and Nagasaki.
While the destructive potential of a fission reaction was clear, there were more promising applications for the future. As a power source, nuclear energy was millions of times denser than conventional fuels. Attention turned to the design of commercial-scale fission reactors. The first to go online was at Shippingport, Pennsylvania in 1957 and could generate 60MWe.
Enthusiasm for nuclear power saw the commissioning of dozens of reactors over the following decades, peaking at 107 reactors in the United States by 1990. While it had many advantages, practical experience in the operation of these facilities also highlighted serious issues. The byproducts of fission, particularly high-level radioactive waste, could remain hazardous for many years. Nuclear accidents like those at Three Mile Island in 1979 and Chernobyl in 1986, demonstrated that even advanced engineering could not mitigate all the risks involved in generating nuclear power from fission sources.
One possible answer to this problem was nuclear fusion. In theory, fusion could generate even larger amounts of energy than fission without creating hazardous waste.
Nuclear fusion is the opposite of fission, in that it involves the fusing of two or more atoms together to form a new, heavier element. The newly formed atom will contain slightly less mass than the sum of the atoms that were used to create it. The missing mass is converted into energy. The energy output of fusion is several times greater than that achieved in the fission process. While fusion does produce some radioactive by-products, they are extremely short-lived in comparison to fission.
The most obvious natural example of nuclear fusion is our Sun. The tremendous heat and gravity at the center of the Sun cause hydrogen elements to fuse together in a series of complex interactions to form helium, producing tremendous amounts of energy in the process. The sun has been undergoing this hydrogen-helium fusion for about 4.5 billion years and is expected to continue for at least another 5 billion before it runs out of hydrogen fuel.
Achieving a sustainable fusion reaction has been a much rockier path than the fission efforts of the 1940s. This is due to a fundamental barrier engineers face, which is how to overcome the electrostatic repulsion between atoms and force them to fuse without expending more power than is gained. In nature, this is achieved in a regime of extremely high temperatures, on the order of millions of degrees. Many decades and billions of dollars have been spent around the world, and it is still unclear when, if ever, a functioning nuclear fusion power plant will become operational.
In an era of carbon-neutral power sources, nuclear energy could have a role to play. New fission reactor designs can efficiently re-process radioactive waste and use it to generate more power. Nuclear fusion however remains the holy grail of power generation. If it can be achieved our energy worries will be over.