Nuclear fission and dangers of radioactive substances
The radioactivity was discovered in 1896 by Henri Becquerel, a French physicist who first observed the spontaneous emission of radiation from a piece of uranium ore called pitchblende. The phenomenon observed by Becquarel was called radioactivity and uranium is a radioactive element was defined.
But because the salts of uranium emit radiation? The majority of natural elements has stable atoms, which remain unchanged with the passage of time, but there are also some atomic species unstable, that is "disintegrate", that are transformed by emitting radiation. The atoms that have the same number of protons, but a different number of neutrons of the core, are defined isotopes.
For nuclide is defined as the nucleus of an isotope that has a different mass number (protons more neutrons) and an atomic number (number protons) well-defined. In the nucleus of uranium, there are always 92 protons and neutrons can be 143, 144 or 146.
The uranium then consists of three radioactive isotopes having mass numbers as 235, 236 and 238, which are referred to as U 234, U 235 and U 238.
The uranium is part of the natural radioactive elements or that have a high atomic number and are unstable and then emit radiation already in nature. It is also possible to transform natural radioactive elements in other unstable elements, said elements artificial radioactive, which are obtained in the laboratory, by means of nuclear reactions.
According to the deviation that the radiation undergo when they are subjected to the action of magnetic fields and electric, it was possible to understand that they are made up of three basic types of alpha rays, beta and gamma.
Alpha rays consist of helium nuclei, consisting of two neutrons and two protons and therefore positively charged.
The beta rays are made of electrons and hence are negatively charged. The gamma ray radiation are ultimately electromagnetic in nature, have no mass and electric charge are very insightful and full of energy.
The nuclear fission
Some isotopes, such as uranium 235 and plutonium-239, because of their high instability are particularly suitable to undergo fission. The fission is achieved by bombarding the nucleus of these unstable elements with neutrons. To activate the fission process takes at least 7.5 kg of uranium (critical mass) before being introduced into the reactor must be pulverized, or reduced to pads. In the fission process are obtained atoms having an atomic number lower, two or three free neutrons and a huge amount of energy. The neutrons released during the fission of uranium, can in turn affect other uranium atoms triggering a process that takes the name of a chain reaction. An uncontrolled chain reaction is the principle on which rests the atomic bomb. The large amount of energy produced is calculated using the Einstein formula:
E = mc2
Since the speed of light is 300,000 km / s, the result which is obtained even with only one gram of uranium is equal to 25 million kilowatt hours of energy. The operation of a nuclear power station is similar to that of a thermoelectric plant.
The particularity of nuclear power plants is that in them the water is heated by nuclear fission. The structures in which case the controlled fission reactors are called. These can be of two types:
Are traditional and are named so because, as fuel using low percentage of uranium 235 in it the chain reaction develops slowly, producing energy with constant flow. The energy produced is used for the production of pressurized steam, which turns the blades of a turbine, in a way similar to what occurs in a thermoelectric power station. This type of reactor is used on ships and submarines and in power stations of first generation;
They represent the evolution of the earliest and are characterized by a greater power. They are also known as breeder reactors produce simultaneously both energy and new nuclear fuel. They are in fact able to transform also the uranium 238 into plutonium-239, a fissile element artificial. In this way, that 99% of uranium 238 that remains unused in reactors lens, is transformed into new nuclear fuel.
While the reserves of uranium-235 are expected depleted by about thirty years, the plutonium produced could be sufficient for another 1000 years.
The use of nuclear power plants for the production of electricity has always been the subject of much discussion.
The worst disaster was Chernobyl, in Ukraine, which occurred April 26, 1986, when it was still part of the Soviet Union. Due to some structural defects and of a series of errors by the functional of the plant, the nuclear reaction got out of control and came to the melting of the reactor core. The result was the explosion of the middle atmosphere and the shedding of a cloud of radioactive substances, which are scattered over some 2000 km to reach most of Europe.
In response to these problems nuclear power plants in Italy were closed, following a popular referendum.
Besides the risk of accidents, there is also the problem of disposing of radioactive waste, which is formed during the fission of elements. They remain radioactive for a long time and there is still no method that is able to render them harmless.
Currently the slag are closed in the bins with an inner wall of lead (which blocks the radiation) and an external stainless steel. These bins are then stored in underground storage. In some cases these materials are exported to third world countries that accept radioactive waste in exchange for money. But the most dangerous form of pollution is from radiation. These can be emitted into the environment not only by nuclear explosions, with subsequent relapse in the soil of radioactive particles (fallout), but also from waste coming from nuclear power plants, pharmaceutical plants from, from hospitals, laboratories, etc..
Radioactive elements, once released into the air, are transported from the weather over long distances and contaminate it fall to the ground and vegetation. In this way you insinuate in food chains and can therefore lead to man.
The most dangerous elements that will remain active for a long time are:
- iodine 131, which remains active for eight days and concentrates in the thyroid and can cause cancer even after some time;
- strontium-90 remains active for 28 years and can cause degenerative changes in the bones;
- Cesium 137 is active for 30 years and can cause degenerative changes in the bones and muscles;
- plutonium-239 is active for 24,000 years and is a danger to the lungs and ovaries.