The mass of the nucleus is about 1 percent smaller than the mass of its individual protons and neutrons. This difference is called the mass defect. The mass defect arises from the energy released when the nucleons (protons and neutrons) bind together to form the nucleus. This energy is called the binding energy. The binding energy determines which nuclei are stable and how much energy is released in a nuclear reaction. Very heavy nuclei and very light nuclei have low binding energies. This implies that a heavy nucleus will release energy when it splits apart (fission), and two light nuclei will release energy when they join (fusion).
The hydrogen 2 nucleus, for example, composed of one proton and one neutron, can be separated completely by supplying 2.23 million electron volts (MeV) of energy. Conversely, when a slowly moving neutron and proton combine to form a hydrogen 2 nucleus, 2.23 MeV are liberated.
The mass defect and binding energy are related by Albert Einstein's formula, E = mc2. In 1905, Einstein developed the special theory of relativity. One of the implications of this theory was that matter and energy are interchangeable with one another. This equation states, a mass (m) can be converted into an amount of energy (E), where c is the speed of light. Because the speed of light is a large number and thus c squared is huge, a small amount of matter can be converted into a tremendous amount of energy. This equation is the key to the power of nuclear weapons and nuclear reactors.
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The combination of two lighter nuclei to produce a heavier nucleus is called:
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Editor's note: This article was originally published on January 30, 2013. It has been revised, updated and republished.
Inside the sun, fusion reactions take place at very high temperatures and enormous gravitational pressures
Look up during the day to see one of the most powerful examples of a nuclear reactor: the sun. Inside the sun, fusion reactions take place at very high temperatures and enormous gravitational pressures.
The foundation of nuclear energy is harnessing the power of atoms by splitting apart, a process called fission, or combining them, called fusion. Both fission and fusion alter atoms to create energy, but what is the difference between the two?
Fission, a term coined by scientists LIse Meitner and Otto Frisch, is named after the term “binary fission” in biology to describe cell division. Just as cell’s divide, in fission an atom splits into smaller particles. Fission takes place when a large, somewhatunstable isotope (atoms with the same number of protons but different number of neutrons) is bombarded by high-speed particles, usually neutrons. These neutrons are accelerated and then slammed into the unstable isotope, causing it to fission, or break into smaller particles. During the process, a neutron is accelerated and strikes the target nucleus, which in the majority of nuclear power reactors today is Uranium-235. This splits the target nucleus and breaks it down into two smaller isotopes (the fission products), three high-speed neutrons, and a large amount of energy. This resulting energy is then used to heat water in nuclear reactors and ultimately produces electricity. The high-speed neutrons that are ejected become projectiles that initiate other fission reactions, or chain reactions.
Conversely, fusion takes place when two low-mass isotopes, typically isotopes of hydrogen, unite under conditions of extreme pressure and temperature. Atoms of Tritium and Deuterium (isotopes of hydrogen, Hydrogen-3 and Hydrogen-2, respectively) unite under extreme pressure and temperature to produce a neutron and a helium isotope. Along with this, an enormous amount of energy is released, which is several times the amount produced from fission.
While fission is used in nuclear power reactors since it can be controlled, fusion is not yet utilized to produce power. Some scientists believe there are opportunities to do so. Fusion offers an appealing opportunity, since fusion creates less radioactive material than fission and has a nearly unlimited fuel supply. These benefits are countered by the difficulty in harnessing fusion. Fusion reactions are not easily controlled, and it is expensive to create the needed conditions for a fusion reaction. However, research continues into ways to better harness the power of fusion, but research is in experimental stages, as scientists continue to work on controlling nuclear fusion in an effort to make a fusion reactor to produce electricity.
Both fission and fusion are nuclear reactions that produce energy, but the processes are very different. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy. While different, the two processes have an important role in the past, present and future of energy creation.
All of the energy we produce comes from basic chemical and physical processes. That’s mostly been accomplished throughout history by burning carbon-based material like wood, coal and gas—or by harnessing power from the sun, wind, and water. Fission and fusion are two physical processes that produce massive amounts of energy from atoms. They yield millions of times more energy than other sources through nuclear reactions. You can check out the difference between the two in this video below.
Video courtesy of the Department of Energy
Fission occurs when a neutron slams into a larger atom, forcing it to excite and split into two smaller atoms—also known as fission products. Additional neutrons are also released that can initiate a chain reaction.
When each atom splits, a tremendous amount of energy is released.
Uranium and plutonium are most commonly used for fission reactions in nuclear power reactors because they are easy to initiate and control.
The energy released by fission in these reactors heats water into steam. The steam is used to spin a turbine to produce carbon-free electricity.
Fusion occurs when two atoms slam together to form a heavier atom, like when two hydrogen atoms fuse to form one helium atom.
This is the same process that powers the sun and creates huge amounts of energy—several times greater than fission. It also doesn’t produce highly radioactive fission products.
Fusion reactions are being studied by scientists, but are difficult to sustain for long periods of time because of the tremendous amount of pressure and temperature needed to join the nuclei together.