Nuclear Fission and Fusion
Nuclear Fission
Nuclear fission and fusion are processes that harness the power the the nucleus to releases huge amounts of energy. Both reactions act upon the same principle:
E = mc2
which tells us that energy produced is equal to the mass multiplied by the speed of light to the second power. This means that mass can be harnessed to produced large amounts of energy. Fission is the break up of nucleus. Natural, or spontaneous, nuclear fission is possible with only the heaviest nuclides, those with a mass number of 230 or greater. However, these reactions are extremely slow. 235-uranium can take 1016 years for it to break down. Induced nuclear fission speeds up this process by shooting a neutron at a large nucleus to break apart the nucleus. This releases huge amounts of energy, with more stable nuclides and neutrons produced in this process. If there is critical mass of a substance, then the neutrons are then used to start the next chain of fission reactions. For example, 235-uranium is one the naturally occurring isotopes that we can use neutrons to produce energy. Scientists will irradiate samples of 235-uranium with thermal neutrons, which will move at a fairly fast speed. Once the neutron hits the nucleus, it will become unstable and break apart into neutrons and many different daughter nuclides with atomic masses between 71 and 161, including the two below:
One gram of 235-uranium can produce up to 80 million kilojoules of energy, a much greater amount compared to the 50 kJ/ gram produced with natural fuel. If there is critical mass of uranium, then more fission will continue to occur using the extra neutrons to split additional fuel:
Fission reactors harness these kinds of reactions to produce electrical energy. Small chips of uranium are loaded into control rods that regulate the fission of uranium by limiting the amount of neutrons that can be produced. Induced fission is allowed to occur and the energy produced is used to heat some kind of cooling agent, usually water. The cooling agent is changed into steam from the heat and is used to spin turbines that produced electrical energy. The side effect of these reactors are the radioactive waste that is produced. The slag decomposes at an extremely slow rate and is harmful to the environment. Some countries have attempted to solve this program by reusing the the waste; however, environmental concerns were raised again.
If not the reactions are not controlled, then it becomes an atomic bomb. In the bomb, pure pieces of 235-uranium or plutonium are brought and held together with great force at the center. Because the naturally produced neutrons escape, it does not start a reaction. A source of neutrons is then shot into the the radioactive material, starting a chain reaction that releases enough energy to match a kilton of TNT. Intense radiation is also produced.
Nuclear Fusion
Nuclear fusion is the exact opposite of fission, it brings together two smaller nuclei to form one bigger nucleus. In the process, some mass is lost and energy is produced. An example of this is the fusion of four hydrogen atoms to form a helium atom:
The energy from the sun is produced using this reaction to fuse hydrogen together to form helium at the core. However, this kind of reaction require enormous amounts of energy in order to begin the process. Two nuclei must overcome the electrostatic forces of the positive protons in order to fuse together. Because of that and the fact that a fusion reaction is much harder to control, there has not yet been made a fusion reactor. There does exist a hydrogen bomb, which does release similar amounts of energy as the fission bomb. What seems to be promising is the fusion of two isotopes deuterium and tritium, which forms a helium atom and a neutron:
In order to do that, there must be an extreme amount of heat applied to overcome the positive forces of the nuclei. Some 108 Kelvin of heat is needed, which makes it hard to contain a fusion reaction. Hydrogen bombs provide this energy by first setting off a smaller fission reaction, then waiting to the nuclei to fuse. As the bigger nuclide is made, there are some extra particles that are given off along with energy.
