... Operation of nuclear reactors, thermal reactors and fast reactors ...
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Nuclear reactor

A nuclear reactor is an installation capable of initiating, controlling and maintaining nuclear reactions (usually nuclear fission) chain that occur in the core of this facility.

The composition of the nuclear reactor is formed by the nuclear fuel, coolant, control elements, structural materials and, in the case in question a thermal nuclear reactor, the moderator.

Nuclear reactors can be classified as thermal reactors and fast reactors.

Thermal reactors are those which function by delaying (moderating) the fastest neutrons or increasing the proportion of fissile atoms. To slow these neutrons, called slow neutrons, a moderator is needed which can be light water, heavy water or graphite.

Fast reactors are not required to moderate the speed of electrons and they use fast neutrons.

To build a nuclear reactor is necessary to have enough nuclear fuel, we call critical mass. Have sufficient critical mass means having enough fissile material in top condition to maintain a chain reaction.

The provision of neutron absorbers and control allows to control the chain reaction and stopping and starting of the nuclear reactor.

Nuclear reactor

In the reactor core it occurs and maintains the nuclear chain reaction in order to heat water to be used to drive turbines of the plant.

The first nuclear reactor in the history of nuclear energy was designed and implemented by the Nobel Prize in Physics Enrico Fermi under the bleachers rugby field at the University of Chicago on December 2, 1942. It was only half watt power but it served to demonstrate that a nuclear reactor is technically possible. It was used as a pilot reactor designed to produce plutonium for the atomic bomb inside the Manhattan Project instalation during the World War II.

Components of the nuclear reactor core

A nuclear reactor consists of the following components:

Nuclear fuel

Nuclear fuel. Uranium pelletsThe nuclear fuel is a material capable of fission enough to reach critical mass, that is, to maintain a nuclear chain reaction. It is positioned so that the thermal energy produced by the nuclear reaction chains can be quickly extracted.

In nuclear power plants solid nuclear fuel is used. Nuclear fuels vary depending on the type of reactor used but generally it derived from uranium.

In general, a fuel element is constituted by a quadrangular arrangement of the fuel rods. Although Russian nuclear pressurized water reactor consists VVER a hexagonal arrangement.

The guide tubes are atached to the fuel support grids, in this way it is possible to keep holding the centers of the fuel rods and guide tubes at the same distance.

The mechanical design of the different fuel elements is identical. Some contain control rod bundles and  others contain burnable poisons or other neutron sources.

Fuel rods

Nuclear fuel rods

These are the physical place where the nuclear fuel is confined. Some fuel rods contain mixed uranium and aluminum in the form of flat plates separated by a distance that allows fluid flow to dissipate the heat energy generated.

The sheets are placed in a sort of box that supports them.

Reactor core

It is composed by the fuel rods. The core has acharacteristic geometric shape, it is  cooled by a fluid, usually water.

In some reactor core it is located inside a pool of water, about 10 to 12 meters deep, or within a pressure vessel made of steel.

Control rods

The beam control rods provide a quick means to control nuclear chain reaction. Allow rapid changes in reactor power and eventual emergency stop. They are made of materials than can absorve neutrons  and usually have the same dimensions as the fuel elements. Core reactivity is increased or decreased by raising or lowering the control rods, that is, modifying the presence of neutron absorbing material contained in them in the nucleus.

For a power reactor during a period of time you must have excess reactivity which is maximal with fresh fuel and It decreases his life untill finally it is canceled. It will be the time to refuel.

Reactor core of nuclear energy

In normal operation, a nuclear reactor have te control rods fully or partially extracted from the core, but the design of a nuclear power plant is such that in case of a faulty security system or reactor control, it always acts in the sense of reactor safety introduceing all the control rods fully in the reactor core and bringing the reactor to safe stop in seconds.


The neutrons resulting from a nuclear fission reaction have a high kinetic energy. How more high is the speed is less likely to atoms fission so that this rate should be reduced to encourage new chain reactions. This is achieved by elastic collisions of the neutrons with the nucleis of the element that makes moderator.

Among the most used moderators they are light water, heavy water and graphite.


To take advantage of the heat energy given off by the nuclear fission reactions a refrigerant is used. The function of this refrigerant is absorbing heat energy and transport it. The coolant should be anticorosive with a large heat capacity and should not absorb neutrons.

The most common refrigerants are gases, such as carbon dioxide and helium, and liquids such as light water and heavy water. There are even some organic compounds and liquid metals such as sodium, that are also used for this function.


In a nuclear chain reaction, a certain number of neutrons tends to escape from the region where it occurs. This neutron leakage can be minimized with the existence of a reflecting means that redirects them within the reaction region. In this way it is possible to increase the efficiency of the nuclear reactor. The reflector surrounding the core must have low capture cross section for not reducing the number of neutrons and that reflects the greatest possible number of them.

The choice of material depends on the type of nuclear reactor. If we have a thermal reactor, the reflector can be the moderator, but if we have a fast reactor reflector material should have a large atomic mass to reflect neutrons in the nucleus with its original speed (dispersion in-elastic).


When the reactor is in operation, a large amount of radioactivity is generated. Protection is necessary to isolate the installations workers from the radiation caused by the fission products.

Therefore, a biological shield around the reactor is placed to intercept these emissions.

The materials used to build this shield are concrete, water and lead.

Nuclear reactor uses

The technology began to develop nuclear reactors for military purposes but from the fifties began to diversify for civilian purposes, in particular for the production of electricity.

In recent years, sustainability problems posed by fossil fuel used in the thermal power plants, and for independence would represent about renewable energies such as solar energy; It has been growing interest in nuclear fission reactor first and then by nuclear fusion as a means to power. The downside is that the research about nuclear fusion is very expensive, due it includes expensive facilities and do not provide immediate results, which the prochets hava international caracteristics (such as the ITER project) between several technologically very developed and rich countries. Economic resources that are available are also not the same tahn research for military purposes.

The applications of nuclear fission reactors basically fall into

  • Production of heat (thermal energy), which is used directly or for producing steam from water. The steam generated is used for mechanical work (turbine), to produce fresh water from sea (desalination), to produce hydrogen by electrolysis at high temperature, etc. Mechanical work can be used directly or to produce electricity with an alternator (NPP)
  • Naval Propulsion icebreaking vessels, nuclear submarines, military aircraft carriers, etc. It also investigates the use for rocket propulsion.
  • Production of plutonium, which can be used for military purposes, such as in atomic bombs, such as MOX fuel, oxides made of depleted uranium and plutonium that can be used and in some PWR reactors. In the latter case, in principle the concept is the reverse, in the 90s are starting to build nuclear power plants that use nuclear fuel and radioactive nuclear waste from other nuclear power plants, which turn out to be the plutonium and the "depleted" uranium resulting from the process of enriching uranium.
  • Production of radioactive isotopes used in construction (Americium smoke detectors), medicine (Cobalt-60), research, etc.
  • Production of free neutrons which are used in research and medicine.
  • Production of neutron bombs, used for military purposes.

The construction of large reactors always ends up needing more time and money than initially expected.

Nuclear fusion reactors are all still under research and development, one of the most important future applications that are expected of them is the production of electricity.

nuclear fusion reactor


Reactores nucleares

Last review: September 3, 2015

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