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Nuclear fission, not to be confused with fusion.
Fission does not respond to changes in temperature and pressure like chemical reactions do. In a nuclear reactor, the fission chain reaction can be sped up by removing rods of cadmium, which absorb neutrons. These are in place to prevent the reaction from occurring too quickly. Remove them, and the chain reaction may proceed out of control.
A chain reaction has products or byproducts that cause the reaction to continue. One example is a state of nuclear critical mass, in which an atom of u-235 decays to produce fast neutrons (along with other fission fragments), which crash into other u-235 atoms, which release more neutrons. The number of neutrons in the environment increases, and if this is not controlled, then there is a nuclear explosion. That is how an atomic bomb works. Another example is a state of instability in snow on a mountain side. If snow begins to move at the top of the mountain, it pushes the snow below it to give way, this pushes the snow below it to give way in turn, going down the mountainside until the snow runs out or the mountain levels out. This is an avalanche. Another example is a situation where the electric grid is overloaded to the point of instability. A failure in a transformer can cause a power surge that causes another failure, this causes other power surges in other places, resulting in other failures. This produces widespread power outage. Chain reactions continue until some sort of equilibrium is attained, or until the unstable features of the situation have lost their energy. In human terms, the results are often destructive or at least dangerous.
When an atom has a nuclear reaction that converts it to a different element, that is a form of radioactive decay. Even the chain reaction that takes place when an atomic bomb explodes is radioactive decay, taking place more rapidly than usual.
chemical reaction takes place
Nuclear fission, not to be confused with fusion.
This answer can not be accurately answered without first knowing what type of nuclear reactor it is. A pressurized-water nuclear reactor is probably the most common so I'll briefly talk about some factors that affect the number of chain reactions in that type of nuclear reactor. First of all, the temperature of the water entering the reactor core will affect the number of chain reactions. The colder the water, the greater the number of reactions. This is primarily because the colder water is more dense and thus releases more neutrons which speeds up the chain reaction rate. Of course, reactor operators, wishing to control the number of chain reactions in order to make the reactor stable may raise or lower "rods" which will be made of a material that tends to absorb neutrons and thus lowering the chain reaction rate. These are the two most common factors affecting chain reaction rate. Would delve further into this question but it gets rather complicated at this point.
Since the continued chain reaction of a nuclear fission reactor depends upon at least one neutron from each fission being absorbed by another fissionable nucleus, the reaction can be controlled by using control rods of material which absorbs neutrons. Cadmium and boron are strong neutron absorbers and are the most common materials used in control rods. A typical neutron absorption reaction in boron is In the operation of a nuclear reactor, fuel assemblies are put into place and then the control rods are slowly lifted until a chain reaction can just be sustained. As the reaction proceeds, the number of uranium-235 nuclei decreases and fission by- products which absorb neutrons build up. To keep the chain reaction going, the control rods must be withdrawn further. At some point, the chain reaction cannot be maintained and the fuel must be replenished
Fission does not respond to changes in temperature and pressure like chemical reactions do. In a nuclear reactor, the fission chain reaction can be sped up by removing rods of cadmium, which absorb neutrons. These are in place to prevent the reaction from occurring too quickly. Remove them, and the chain reaction may proceed out of control.
The uranium 235 atoms in the nuclear fuel are what actually fission, or split into two other atoms. The uranium is in ceramic fuel pellets that are inserted into fuel rods, that make up fuel elements, that are in the reactor core that is located in the reactor vessel of the nuclear power plant. After the fuel has been in the reactor it begins to produce plutonium 239 atoms within the fuel which will also undergo a fission reaction.
In the reactor core, which is the volume filled with the fuel assemblies
Nuclear power plants are self starting because there is always a small number of fissions taking place in the uranium fuel, so as soon as the reactor is made critical by withdrawing the control rods the chain reaction starts.
In the core of a nuclear reactor
In a fission reactor, control is implemented by inserting control rods into the reactor. These are made of a material that absorbs neutrons, and prevents a reaction from taking place.
Nuclear fission takes place in the nuclear fuel rods that are placed in the reactor core that is situated in the reactor pressure vessel. The reactor pressure vessel is usually situated inside the reactor containment.
The sun's nuclear reactions are fusion reactions at extremely high temperatures and pressures, while the nuclear reactor's nuclear reactions are fission reactions at typical temperatures and pressures for earth.
Yes, all natural radiation (in the rocks) is a result of fission (but this fission is not part of a chain reaction like in a fission bomb). However, it is theoretically possible for natural processes to concentrate radioactive elements (uranium) to the extent where a natural nuclear fission reactor (a chain reaction like in a nuclear power plant) will form. Oklo in in Gabon is the only known location for this to have happened and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago.