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The nuclear reactors used at present in the nuclear power industry produce energy obtained from a neutron-induced fission reaction.
This is a reaction in which an incident neutron enters a heavy target nucleus (fissionable nucleus) to form an unstable compound nucleus.
A nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus is called neutron capture
The obtained compound nucleus is excited to such a high energy level that the nucleus splits into two large fission fragments.
In the course of this fission, a large amount of energy is released in the form of radiation and fragment kinetic energy.
What is most important, this fission process may produce more than one free neutrons that are capable of inducing further fissions and so on.
This sequence of fission events is known as a nuclear fission chain reaction.
The nuclear fission chain reaction occurs when one single nuclear reaction causes an average of one or more nuclear reactions
The average number of neutrons from one fission that cause another fissions is called the effective neutron multiplication factor
- the reactor state with the effective neutron multiplication factor greater than unity is called a supercritical state;
- the reactor state with the effective neutron multiplication factor equal to unity is called a critical state;
- the reactor state with the effective neutron multiplication factor less than unity is called a subcritical state
The chain reaction can take place only in the proper multiplication environment and only under proper conditions
Obviously, if one neutron causes two further fissions, the number of neutrons in the multiplication system will increase in time, and the reactor power will also increase in time
Moreover, this multiplication environment (the nuclear reactor) behaves like an exponential system. This means that the power increase in time is not linear, but exponential
In order to stabilize such multiplication environment, it is necessary to increase the non-fission neutron absorption in the system (by inserting control rods and, if necessary, increasing the concentration of a liquid absorber)
On the other hand, if one neutron causes less than one further fission, the number of neutrons in the multiplication system will decrease in time, and the reactor power will also decrease in time.
In order to sustain the chain reaction, it is necessary to decrease the non-fission neutron absorption in the system (by withdrawing control rods and, if necessary, decreasing the concentration of a liquid absorber)
The liquid medium used to remove the heat produced by the nuclear fission reaction is called a coolant
There are water cooled reactors and liquid metal cooled reactors
For making the capture of neutrons by heavy nuclei in water cooled reactors more efficient, the neutrons produced by the fission reaction should be slowed down to thermal velocities.
Such neutrons are called slow or thermal neutrons; the process of slowing down their velocity is called moderation, and the medium used to do so is called a moderator.
The water used in water cooled reactors serves both as a coolant and moderator
Liquid metal cooled reactors operate with fast neutrons, and there is no need to moderate them in such reactors
