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Home » 2017 » February » 23 » How to build a Nuclear reactor
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How to build a Nuclear reactor

Nuclear reactor

Nuclear reactor, any of a class of devices that can initiate and control a self-sustaining series of nuclear fissions. Nuclear reactors are used as research tools, as systems for producing radioactive isotopes, and most prominently as energy sources for nuclear power plants.

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Principles of operation: Nuclear reactors operate on the principle of nuclear fission, the process in
which a heavy atomic nucleus splits into two smaller fragments. The
nuclear fragments are in very excited states and emit neutrons, other subatomic particles, and photons. The emitted neutrons may then cause new fissions, which in turn yield
more neutrons, and so forth. Such a continuous self-sustaining series of
fissions constitutes a fission chain reaction. A large amount of energy is released in this process, and this energy is the basis of nuclear power systems.
  • Sequence of events in the fission of a uranium nucleus by a neutron.Encyclopædia Britannica, Inc.


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In an atomic bomb the chain reaction is designed to increase in intensity until much of
the material has fissioned. This increase is very rapid and produces the
extremely prompt, tremendously energetic explosions characteristic of
such bombs. In a nuclear reactor the chain reaction is maintained at a
controlled, nearly constant level. Nuclear reactors are so designed that
they cannot explode like atomic bombs.Most of the energy of fission—approximately 85 percent of it—is released
within a very short time after the process has occurred. The remainder
of the energy produced as a result of a fission event comes from the radioactive decay of fission products, which are fission fragments after they have
emitted neutrons. Radioactive decay is the process by which an atom
reaches a more stable state; the decay process continues even after
fissioning has ceased, and its energy must be dealt with in any proper
reactor design.

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Chain reaction and criticality
The course of a chain reaction is determined by the probability that a neutron released in fission will
cause a subsequent fission. If the neutron population in a reactor
decreases over a given period of time, the rate of fission will decrease
and ultimately drop to zero. In this case the reactor will be in what
is known as a subcritical state. If over the course of time the neutron
population is sustained at a constant rate, the fission rate will remain
steady, and the reactor will be in what is called a critical state. Finally, if the neutron population increases over time, the fission
rate and power will increase, and the reactor will be in a supercritical
state.


Before a reactor is started up, the neutron population is near zero. During reactor start-up, operators
remove control rods from the core in order to promote fissioning in the
reactor core, effectively putting the reactor temporarily into a
supercritical state. When the reactor approaches its nominal power level, the operators partially reinsert the control rods,
balancing out the neutron population over time. At this point the
reactor is maintained in a critical state, or what is known as
steady-state operation. When a reactor is to be shut down, operators
fully insert the control rods, inhibiting fission from occurring and forcing the reactor to go into a subcritical state.

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Reactor control:
A commonly used parameter in the nuclear industry is reactivity, which is a measure of the state of a reactor in relation to where it
would be if it were in a critical state. Reactivity is positive when a
reactor is supercritical, zero at criticality, and negative when the
reactor is subcritical. Reactivity may be controlled in various ways: by
adding or removing fuel, by altering the ratio of neutrons that leak
out of the system to those that are kept in the system, or by changing
the amount of absorber that competes with the fuel for neutrons. In the
latter method the neutron population in the reactor is controlled by
varying the absorbers, which are commonly in the form of movable control
rods (though in a less commonly used design, operators can change the
concentration of absorber in the reactor coolant). Changes of neutron
leakage, on the other hand, are often automatic. For example, an
increase of power will cause a reactor’s coolant to reduce in density
and possibly boil. This decrease in coolant density will increase
neutron leakage out of the system and thus reduce reactivity—a process
known as negative-reactivity feedback. Neutron leakage and other
mechanisms of negative-reactivity feedback are vital aspects of safe
reactor design.A typical fission interaction takes place on the order of one picosecond (10−12 second). This extremely fast rate does not allow enough time for a
reactor operator to observe the system’s state and respond
appropriately. Fortunately, reactor control is aided by the presence of
so-called delayed neutrons, which are neutrons emitted by fission
products some time after fission has occurred. The concentration of
delayed neutrons at any one time (more commonly referred to as the
effective delayed neutron fraction) is less than 1 percent of all
neutrons in the reactor. However, even this small percentage is
sufficient to facilitate the monitoring and control of changes in the system and to regulate an operating reactor safely.
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