Radioactivity defined as the spontaneous emission of particles (alpha, beta, neutron) or radiation (gamma, K capture), or both at the same time, from the decay of certain nuclides that these particles are, due to an adjustment of their internal structure.
Radioactivity can be natural or artificial. In natural radioactivity, the substance already has radioactivity in the natural state. In artificial radioactivity, the radioactivity has been induced by irradiation.
A radionuclide is all the radioactive cores of the same kind. All radioactive cores forming a radionuclide have a well-defined radioactivity, which is common to all of them and that identifies them; the same way that a type of chemical reaction identifies the elements involved.
Quantitatively, radioactivity is a statistical phenomenon. Therefore, it should be observed to rate the behaviour of a set of nuclei of the same species. By the law of large numbers, we define a constant λ as the probability of radioactive decay of a nucleus per unit time. With this definition, the number N of radioactive nuclei of the same species found in a substance in a time t is given by N = No · e-λt, where No is the number of radioactive nuclei that existed before time t. In fact, hardly a radioactive substance is formed by a single radionuclide, although each of its components disintegrate becomes a different kernel, which as well, can also be radioactive.
The initial radionuclide is called parent and the derivative, child. This situation may continue throughout multiple affiliations and the set of all of them is called radioactive series or family. In this case, the relationship that gives the number of current radioactive nuclei is more complex because, in addition to considering the number of each one in the initial moment, we should consider that, by disintegration of some ones, other are formed.
The problem is simplified when you want to achieve radioactive equilibrium (also called secular equilibrium in the natural radioactive series), when a sufficiently long time has passed since the process of affiliation has started. Then, the rate of the decays is imposed by the radionuclide, which has the smallest radioactive constant.
Natural radioactive nuclides
In nature there are about 300 different nuclides, from which 25 are radioactive with a sufficiently long period so that they exist even today; other 35 have a much shorter period. Continuously they are created and they decay into radioactive series.
Artificial radioactive nuclides
Over 1000 artificial radionuclides have been created and identified. The radioactive series are called with the name of the parent nuclide of longer periods. There are four. Three of these are natural radioactive series: thorium´s series, uranium´s series and actinium´s series, ending in their own stable isotopes of lead. These isotopes have respectively the mass numbers of 208, 206 and 207. Regarding the neptunium series, as radionuclides that compose it have a short period compared to the length of the geological eras, this series is not from the nature and has been obtained artificially. The last nuclide of this series is the isotope 209 of bismuth.
Origin of radioactivity
In 1896, Antoine-Henri Becquerel discovered radioactivity. He observed that, in studies on the phosphorescence of substances, a mineral of uranium was able to gloss photographic plates, which were kept at his side.
The nuclear fuel is the material that has been adapted for use in nuclear power generation.
When it comes to nuclear fuel we can be talking about the same item or set that is made ready it can be used for containing the material itself, but also other elements.
The most widely used process involving the nuclear fission fuel.
There are different types of nuclear fuel, but the most common fuel is consisting of fissile elements such as uranium, generating chain reactions in nuclear reactors. The most common isotope in the fission is uranium-235.
Production processes nuclear fuel forms the set called the nuclear fuel cycle.
Another nuclear process is much less used nuclear fusion. In nuclear fusion nuclear fuel used are lightisotopes such as tritium or deuterium.
There are other elements such as plutonium-238, among others, that are used to produce small amounts of nuclear energy by radioactive processes to radioisotope thermoelectric generators atomic piles or other disintegration.
A nuclear reactor is an installation capable of initiating, controlling and maintaining nuclear reactions (fission usually) chain occurring in the core of the facility.
The composition of the nuclear reactor is formed by the fuel, coolant, control elements, structural materials and, in the event that it is a thermal nuclear reactor, the moderator.
Nuclear reactors can be classified as rapid thermal reactors and reactors.
Thermal reactors are those which function by delaying (moderating) the faster neutrons or increasing the proportion of fissile atoms. To slow the neutrons, called slow neutrons, a moderator is required which can be light water, heavy water or graphite.
Fast reactors are not required to moderate the speed of electrons and using fast neutrons.
To build a nuclear reactor is necessary to have enough fuel, we call critical mass. Having enough critical mass means having enough fissile material in good condition to maintain a chain reaction.
The provision of neutron absorbers and control rods to control the chain reaction and stopping and starting of the nuclear reactor.
In the reactor core occurs and manteiene the nuclear chain reaction in order to heat the water to be used for driving the turbine of the plant.
Components nuclear reactor core
A nuclear reactor consists of the following components:
Nuclear fuel is a material capable of fission enough to reach critical mass, that is, to maintain a nuclear chain reaction. Is positioned so that it can quickly remove the heat produced by this nuclear reaction chains.
In nuclear power plants using solid fuel. The nuclear fuels vary depending on the type of reactor used but generally uranium derivatives.
In general, a fuel element is constituted by a quadrangular arrangement of fuel rods, as seen in the image. While Russian nuclear reactor VVER pressurized water is constituted by a hexagonal arrangement.
The guide tubes are attached to the fuel support grids in this way is able to maintain the centers of the fuel rods and tubes guíaa the same distance.
The mechanical design of the different fuel elements is identical. Some contain bundles and control rods containing burnable poisons or other neutron sources.
To ensure the quality of the fuel elements, there are numerous inspections and testing of both raw materials and the final product.
The control rods beams provide a rapid means for controlling the nuclear reaction. Allow rapid changes reactor power and eventually stop in case of emergency. They are made of neutron absorbing material (boron carbide or alloys of silver, indium and cadmium, etc.) and typically have the same dimensions as the fuel elements. The reactivity of the core increases or decreases by raising or lowering control rod, that is, modifying the presence of neutron absorbing material contained in them in the nucleus.
For a reactor operated for a period of time must have an excess of reactivity which is maximal with fresh fuel and decreases over the life of the same until it is canceled, when the refill is made of fuel.
In normal operation, a nuclear reactor is the control rods fully or partially extracted from the nucleus, but the nuclear plant design is such that any fault in a security system or reactor control, always acts in the sense of security of introducing reactor completely all the control rods in the reactor core and carrying a safe stop in a few seconds.
The resulting neutron fission reaction have high kinetic energy (high speed gain). The higher your speed is less likely to fisionen other atoms so that this speed should be reduced to encourage new chain reactions. This is achieved by elastic collisions of the neutrons with nuclei makes moderator element.
Among the most commonly used moderators are light water, heavy water and graphite.
In order to use the heat energy given off by nuclear fission reactions using a refrigerant. The function of this heat refrigerant and transport aboserver. Coolant must be corrosion, with a large heat capacity and should not absorb neutrons.
The most common refrigerants are gases, such as carbon dioxide and helium, and liquid as the light water and heavy water. There are even some liquid organic compounds and metals such as sodium, also using for this function.
In a nuclear chain reaction, a certain number of neutron tends to escape from the region in which it occurs. This neutron leakage can be minimized with the existence of a reflecting means to redirect them into the reaction region. In this manner serves to increase the efficiency of the reactor. The medium reflector surrounding the core must have a low capture cross section for not reducing the number of neutrons and to reflect as many of them.
The choice of material depends on the type of reactor. If we have a thermal reactor, the reflector can be the moderator, but if we have a fast reactor reflector material must have a large atomic mass to reflect neutrons in the nucleus with its original speed (inelastic scattering).
When the reactor is in operation, it generates large amounts of radiation. Protection is needed to isolate the installation workers caused by radiation from fission products.
Therefore, biological shielding is placed around the reactor to intercept these emissions.
The materials used to build this shield are concrete, water and lead.
Uranium is the most widely used nuclear fuel in nuclear fission reactions.
For the particular makes the uranium so different from the other substances we must first consider some basic nuclear physics.
Atomelectronscomprises surrounding a core; in turn, a core consists of protons and neutrons. A protonhas a positive charge; aneutronhas no electric charge and is neutral. The positive charges of protons outward violently push attempt. But within the compact volume of a new class of core strength makes an appearance: an attractive force short range, immensely powerful, equally acts between protons and neutrons (which from this point of view, are all nucleons). The short-range nuclear force holds them together, opposing the repulsive effect of the positive charges of the protons. Thus, the neutrons act as "nuclear cement".
However, in a core which contains 92 protons (which is a uranium core) repulsive force between protons is expiring nuclear force. While there are 146 neutrons, the nucleus can hardly remain intact. This form of uranium containing 238 nucleons in total, called uranium-238. The next most likely arrangement is a uranium nucleus containing three fewer neutrons, uranium-235. Theseatomswith lighter nuclei comprise about 0.7% of uranium that occurs naturally (if the cores have the same number of protons, these nuclei thereof chemical element. well, all 92protoncore is the nucleus of anatomof uraniumatomswhose nuclei have the same number of protons but different numbers of neutrons are called isotopes of the element eg. uranium-238 and uranium-235 areisotopesof Uranium nucleus of uranium-235 has a unique property among more than two hundred types. nuclei found in nature in significant quantities The core of uranium-235 and is under a voltage close to the internal break,. a strayneutronapproaching you can break it completely.