Nuclear reactions can generate a lot of energy. However, not all nuclear reactions are the same. There are two very distinct types of nuclear reactions: fusion reactions and fission reactions. What are the differences and similarities between nuclear fission and nuclear fusion?

As you have learned, all atoms are made of sub particles called protons, neutrons, and electrons. In the simplest terms, nuclear fission occurs when large, unstable atoms split into smaller atoms to achieve a more stable state. Nuclear fusion is the opposite of fission. Fusion occurs when smaller atoms bind together to form a larger, more stable atom. In both cases, the reaction occurs to bring the atom to a more stable state by a reduction of potential energy.

Nuclear fission reactions occur when the nucleus of an atom is split into fragments. When this occurs, smaller fragments are created. Most often, the result is two fragments with almost the same number of particles as the starting atom. How then is energy released in this reaction? You can compare the mass of the starting atom and the mass of the final products. You will find that the mass of the final products is slightly less than the mass of the starting atom. Yes, in this type of nuclear reaction, mass is lost! This means that matter is lost. This loss of matter is known as the mass defect. Remember, the law of conservation of mass is for nonnuclear changes. Nuclear reactions are described by conservation of mass-energy. The small amount of lost mass is converted directly into large amounts of energy. The energy created from this reaction can be used to generate electricity in power plants or for the huge explosions of atomic bombs.

One type of atom used for fission reactions is an unstable isotope of uranium, known as uranium235. In order for this reaction to occur, a neutron must collide with the uranium 235 nucleus, creating uranium 236. This collision creates enough energy to split the newly formed uranium 236 nucleus into smaller nuclei. A huge amount of energy is released in the process. As the nucleus splits and releases energy, more neutrons are produced. These may collide with another uranium 235 nuclei, causing it to undergo fission. This will create more energy and produce even more neutrons. The reaction will become something called a chain reaction if enough neutrons are produced during collisions to keep the reaction going.

The minimum amount of material that is needed for the reaction to keep going is called the critical mass. The trouble with fission reactions is that radiation and nuclear waste products are created in the process. This is a problem, as many nuclear power plants use fission to produce energy, producing a lot of radioactive by-products as a result.

Conversely, in fusion reactions, the nuclei from atoms with low atomic weights combine to create heavier nuclei. This reaction does not require neutrons in order to occur. Two light nuclei must collide with a great deal of energy. Fusion reactions require extremely high heat. In fact, the heat required can exceed several million degrees Celsius. (Yes, several million!) To put these types of temperatures in perspective, the temperature at the surface of the Sun is around 5,600ºC. Meanwhile, the temperature near the core of the Sun is calculated to be around 15 million ºC. The temperature of the Sun stated here should give you a hint of a nuclear reaction that you can observe. The Sun is a product of an ongoing fusion reaction that has been occurring for billions of years.

Why is such high heat required for these reactions? Remember, the protons of atoms are all positively charged. In order to combine atoms, the repulsion of the positive protons of the combining atoms must first be overcome. Extremely high heat gives the nuclei the kinetic energy needed to overcome the repulsion. And as shown with the Sun, fusion reactions are also very exothermic. Therefore, fusion reactions have the capability of producing more energy than is required to initiate the reaction. This creates a self-sustaining reaction that holds great promise for energy production. A fusion reaction will sometimes begin with a fission reaction to get it started. In the right conditions, it can sustain itself, once started. One of the benefits of fusion reactions is that there are no radioactive by-products. On Earth, fusion reactions result in the element helium. Heavier elements, such as beryllium or carbon, are produced by fusion in stars. Hydrogen bombs are produced by fusion reactions in which several isotopes of the element hydrogen are combined to produce helium.

How can you remember the difference between these two types of nuclear reactions? Think in terms of vocabulary that you may already know. A fissure is a crack or opening that can appear on the surface of Earth. It is a separation. A fission reaction will, therefore, "separate" an unstable atom into parts. A fusion reaction will "fuse" two atoms together.