Nuclear Fission and Nuclear Fusion

Nuclear fission is the process by which a nuclide splits into two or more smaller nuclides. Nuclear fusion is the reverse. In nuclear fusion, two or more nuclides combine to form a larger nuclide. Both nuclear fusion and fission have the potential to release very high amounts of energy. Nuclear binding energy is the amount of energy needed to split a nucleus to its nucleons. Nuclei with a higher nuclear binding energy per nucleon are more stable and require more energy to break apart. The nucleus with the highest binding energy per nucleon is iron that has an atomic mass of 56, the most common isotope of iron.

Nuclei lighter than iron-56 become more stable by undergoing fusion to become heavier (toward iron). For nuclei lighter than iron, fusion releases energy, and fission requires energy. Nuclei heavier than iron become more stable by undergoing fission, to become lighter (toward iron). For nuclei heavier than iron, fission releases energy, and fusion requires energy.

Nuclear fusion occurs naturally in the sun, where hydrogen nuclei fuse into helium nuclei. Nuclear fusion can potentially be used for energy production as well. However, nuclear fusion starts at very high temperatures, typically millions of degrees Celsius. There is no practical way to control the fusion mass at such high temperatures; no substance can be a solid at such temperatures. Magnetic containment of fusion mass is theoretically possible, but no practical systems have been developed so far.

A chain reaction is a nuclear reaction in which the products of the reaction can trigger more nuclear reactions. The most studied chain reaction is fission of uranium-235. The reaction begins with a neutron striking uranium-235. This makes the uranium unstable. The uranium splits into two smaller, but still large, nuclei, releasing two or three neutrons (${}_0^1{\rm{n}}$). This reaction releases considerable amounts of energy by either of the two following methods: The neutrons released during this reaction can strike other uranium-235 atoms, causing them to undergo fission reactions as well. If uncontrolled, and conditions are right, the entire mass of uranium will undergo nuclear reaction very quickly, resulting in an explosion. A nuclide that can undergo a nuclear chain reaction is called fissile. Note that the term fissile applies to nuclides, not elements. Uranium-238, for example, is not fissile. Uranium-235 is.

Not all neutrons that are released during the fission of uranium-235 will trigger new reactions. To trigger a reaction, the neutron must strike at another nucleus. Some neutrons will not strike any nuclei. Some neutrons will strike non-fissile nuclei. Neutrons also need to be at a range of relatively low speeds to trigger fission. Most of the neutrons that are produced will be too fast. For these reasons, even with a fissile substance, a chain reaction will not always occur. The minimum mass of fissile substance that can sustain a chain reaction is called the critical mass. If there is subcritical mass, the amount of fissile material is insufficient to sustain a chain reaction. The term critical mass depends on many factors, including temperature, purity, and shape of the fissile material.

Heavy elements, like uranium-235, undergo spontaneous fission. Spontaneous fission is a form of radioactive decay. In subcritical masses, the neutrons produced by the spontaneous fission will not trigger a chain reaction. In a supercritical mass, the amount of fissile material is sufficient to start a chain reaction resulting from spontaneous fission. A supercritical mass will undergo a chain reaction on its own.

In a nuclear reactor, a nuclear reaction occurs at a controlled rate. The nuclear reaction produces heat, which is eventually converted to electricity. A nuclear power plant has four basic elements: fuel, control rods, moderator, and coolant.

Nuclear reactors use material that is fissile, which means it contains nuclides that can undergo a nuclear chain reaction, like uranium-235, as fuel. Uranium-235 is an uncommon isotope of uranium. The most commonly occurring isotope (nuclei with the same number of protons but different numbers of neutrons) of uranium is uranium-238, which is not fissile. Uranium-235 is less than 1% of a sample of uranium found in nature. Uranium fuel must be first enhanced, typically to 3–5% uranium-235 content, before it is used as a fuel.

A control rod is a rod that can be inserted into a nuclear reactor to capture neutrons and slow down the rate of a chain reaction. Control rods are made from an isotope with a high chance of neutron capture. Isotopes of cadmium or boron are often used. The rods are physically immersed into the nuclear reaction where they capture free neutrons, slowing down the reaction. It is possible to stop the nuclear reaction entirely by inserting more control rods.

A neutron moderator is a substance in a nuclear reactor that slows down fast neutrons, enabling them to initiate nuclear reactions. In most nuclear reactors in the United States, the neutron moderator is water. Neutrons collide with water nuclei, which causes the neutrons to slow down. The moderator becomes hot during the nuclear reaction, and this heat is transferred out of the system to produce electricity. Some reactor designs use other substances, such as molten sodium, as the moderator.

The coolant is responsible for cooling the moderator by transferring the heat out of the system. The coolant is often water. The water is heated to produce superheated steam that is used to rotate a turbine. The steam is then condensed and cooled further by a secondary cooling system before being cycled to cool the moderator again. The main reactor and the moderator are radioactive. They are kept in a structure, which is commonly a building, called a containment system, that shields against radiation and contains the main reactor and the moderator of a nuclear reactor.