(Traslated by Google. Revised on may 2013)
On April 26th, 1986, reactor No. 4 of the Chernobyl Nuclear Plant exploded, impacting the world with the greatest ecological and human tragedy of all time. Since then, the radiation have poisoned the lives of approximately 8 million people in Belarus, Ukraine and Russia, who clearly did not know the consequences the catastrophe could generate on their health.
Days after the explosion, entire communities were evacuated as radiation levels at home were extremely unhealthy. Thirteen years after the terrible accident social assistance for victims, as well as medical care and were still rare and difficult to obtain. Today, surprisingly, the city of Pripyat, in part, account for almost ghostly other side with amazing vegetation. What has not changed is the memory of a region that once had life, some cities with families, children, schools, hotels, gardens and amusement parks that were never ever to be inaugurated. Those who had to leave their homes overnight but live not forget that memory of resigned by the largest catastrophe so far, that not only took part of their lives, but physically that of many of its neighbors and has marked malformations, cancer and other ailments to as many of them.
All the contaminated area is off-limits except to some researchers and journalists (like the Fourth Millennium program, Iker Jimenez, who in his sixth season, Chapter 2, they entered it to see how it is now, on the 25th anniversary (in 2011) of the disaster. checkpoints There are strict limits on the forbidden zone and an army checkpoint and scientists and engineers within although several kilometers of the plant, which control their condition.
However, there are still some families residing in the area. After so many years have managed to survive, although, obviously, eat and drink contaminated products.
The question we ask is how they have managed to live there, and how it has grown back so vigorously vegetation with environmental radiation. Scientists believe that both plants and humans can become accustomed to low levels of radiation, although issues related to nuclear power is still little known.
Status of Chernobyl and the area most affected by radiation.
The April 26, 1986 at 1:23 am (local time) skilled in the Chernobyl power plant in Ukraine (former USSR), initiated a drill consisting of reducing the power level in the reactor number 4 as part of a controlled experiment, but ultimately resulted in a disaster due to a succession of errors.
Causing overheated reactor core fission, which resulted in two bursts, which flew can lid, allowing radiation clouds output for 10 days. People in Chernobyl were exposed to radioactivity 100 times higher than it was in Hiroshima. Northern Europe was exposed to clouds of radioactive material were carried by the wind. As later reported up to 17 European countries were touched by the cloud. It has been said that to Italy, but also came to Spain
The 70% of the radiation is estimated that fell on Belarus. This caused (and still causes) that Siguan babies born without arms, without eyes or with any of their deformed limbs.
It is estimated that over 15 million people have been victims of the disaster in some way and it will cost more than $ 60 billion medically treat all those people affected. More than 600,000 people were involved in the cleanup, many of whom are now dead or sick. Chernobyl plant was composed of four graphite core reactors. Number four exploded in the 1986 disaster and the number 2 was closed because of a fire in December 1996.
Expansion of the radioactive cloud.
As said, the accident was the result of a series of human errors (such as unplugging the protection system) and technical, but it got worse because of the design of the reactor (RBMK-1000 type) because vessel had no protection or adequate protection systems. These factors led to a catastrophic increase and almost instantaneous heat in the core also due to an error in temperature measurement. As a result there was a steam explosion in the reactor vessel by not having protection destroyed much of the building. Accumulated radioactive materials in the reactor core be started instantly released into the environment.
The explosion was so great that launched hundreds of meters contaminated reactor wreckage. Firefighters put out the main fire except the central hall of the reactor, where graphite fire continued neighboring places in the destroyed reactor.
En los días siguientes cerca de 5.000 toneladas de diversos materiales, incluyendo unas 40 toneladas de sustancias que contenían boro, 2.400 toneladas de lead, 1.800 toneladas de arena y arcilla, 600 toneladas de dolomita, trinatriufosfato y líquidos polimerizados, fueron lanzados dentro del reactor desde helicópteros del ejército para enfriar el grafito incendiado con el fin de disminuir la radiactividad. No se sabe bien aún si al tirar materiales fuera del reactor alcanzaron su meta. Según datos de observaciones posteriores sólo una pequeña parte de los materiales lanzados llegaron al reactor y ellos formaron montículos de una altura de 15m en el vestíbulo central del reactor numero 4.
After the accident he built a "sarcophagus" over the reactor to prevent the emission of radiation. It was one of the most difficult jobs in the world built by the very radioactivity and the work itself. This project was completed in November 1986 but had to be reinforced by posterioemtente fissures that contamination occurred, and have been played nowadays.
La central antes del accidente.
Unfortunately 30 people (personnel against fire) died to receive high doses of radiation. Even a helicopter was hit cool the core wires and a tower next to the reactor and fell a few feet of the damaged building. The contaminated area has been more than 130 000 km 2, only in the USSR. About 4.9 million people lived in this place before the accident. The entire population was evacuated within a radius of 30 km. directions and relocated in different areas. The question to ask is whether that radius was sufficient for the level of the disaster (July 7 on the international scale of nuclear emergency INES) when Fukushima was 6 and pollution has affected water and food even in areas more distant (eg. in Tokyo, about 240 km from the plant's water pollution levels exceeded the limits recommended for children).
The impact of the Chernobyl accident was tremendous political level. Some countries halted their nuclear energy programs, which caused that the construction of new nuclear plants in the USSR were arrested. Public opinion turned against nuclear power plants and some of them were closed. The Chernobyl accident began international activity in the area of nuclear safety and nuclear emergency planning. While security has increased dramatically since then and have established better evacuation protocols and reducing the impact of a possible accident, after Fukushima has once again put on the table if it's worth keeping an energy production system as hazardous in case something goes wrong and as happened with Chernobyl popular pressure has caused a slowdown in nuclear policy worldwide and a further review of safety systems.
In any case we must ask if this will not be temporary, and again to forget in a few months or years. We have to remember, for example, the unknown and disturbing case of existing nuclear power plant 30 km from New York on an earthquake fault, and has not become closed.
THE CHERNOBYL REACTOR.
As stated, the No. 4 reactor of Chernobyl is the type called RBMK-1000. The Soviet RBMK-1000 reactors have several unique features cooling.
Five of these features are:
1. - The reactors are reloaded fuel when in operation. Few commercial reactor designs have this distinction. Other reactors that are recharged in operation are the Canadian CANDU reactors MAGNOX united kingdom. The newest AGR reactor is also designed to handle the refueling operation. RBMK reactors were designed for this particular form because the fuel rods could be removed after a major step in the production of plutonium for weapons material supply for the nuclear arsenal of the former Soviet Union.
2 - For economic reasons used slightly enriched uranium fuel. Uranium is found in nature with only 0.7% of U-235 fissile and is enriched prior to use in most reactors. The use of low enriched uranium creates several operational problems, one of which is the inability of the reactor to operate at reduced power ranges for some time, is the phenomenon called poisoning of the reactor. This was one of the factors that contributed to the accident. Since then the RBMK fuel has been enriched with about 2.4% U-235 to help prevent poisoning.
3. - The reaction is monitored by water-cooled graphite. The combination use of graphite and water cooling is not in other reactors. This is a complex issue, but moderation graphite reaction was a contributing factor to the accident. The use of water as coolant also increases the risk of explosion, as seen in Fukushima, where the hydrogen is combined with oxygen, leading to the explosion of some of the reactors.
4.- Control rods that stop the chain reaction used graphite "heavy water" in your filters. Since the graphite-moderated reactor is, causes the reactor power is increased temporarily and makes it a scram, that is, an emergency stop of the reactor core. All commercial reactors except the RBMK, are designed for inserting the control rods and stop the chain reaction in less than 3 seconds. The RBMK took 18 seconds to fully insert the control rods and stop the reaction. This slow movement of the control rods, combined with small amounts of existing graphite momentary power increase occurred in the end the accident.
5.- - THE Soviet RBMK-1000 not used a container structure. Only this would have greatly reduced the severity of the Chernobyl accident.
What is a nuclear reactor?
It is a facility that produces, maintains and controls a nuclear chain reaction. Therefore, in a nuclear reactor using a fuel that would ensure adequate normal energy production produced by the successive fissions. Some reactors can dissipate heat from the fission, others still use the heat to produce electricity. The first reactor built in the world was operated in 1942, in the premises of the University of Chicago (USA), under the careful direction of the famous researcher Enrico Fermi. Hence the name "Fermi stack" as later called this reactor. Its structure and composition were basic when compared with current reactors existing in the world, basing his confinement safety and solid walls of graphite bricks.
Turbinas del reactor
Fissile material used in specific amounts and provisions so that allows quickly and easily extract the energy generated. Fuel in a reactor is in solid form, the most used form low uranium isotope U-235. However, there is also fissionable elements such as plutonium, which is a byproduct of the fission of Uranium. In nature there is little amount of fissile uranium (about 0.7%), so that most of the fuel used reactors "enriched", ie fuel which increases the amount of uranium-235.
They are the physical location borders the Nuclear Fuel. Some fuel rods contain mixed uranium in the form of aluminum flat sheet separated by a distance that allows the flow of fluid to dissipate the heat generated. The sheets are placed in a kind of box that supports them.
Is constituted by the fuel rods. The core has a geometric shape that is characteristic, cooled by a fluid, generally water. In some reactors the core is located inside a pool with water about 10-12 meters deep, or inside a pressure vessel made of steel.
All reactor has a system to start or stop the nuclear fission chain. This system are the control rods, able to capture neutrons that are in the surrounding medium. The neutron capture avoids new atomic nuclei fission of uranium. Usually the control rods are made of cadmium or boron.
Obtained Neutrons emerging nuclear fission at very high speeds (fast neutrons). To ensure continuity of the chain reaction, that is, to ensure that the "new neutrons" continue colliding with atomic nuclei of the fuel, it is necessary to decrease the speed of these particles (slow neutrons). Decreases the kinetic energy of fast neutrons through collisions with atoms other suitable material, called Moderator. Moderator is used as natural water (light water), heavy water, the carbon (graphite), etc.
Scheme of the reactor core.
The heat generated by the fission must be removed from the reactor core. To accomplish this process fluid is used in which the core is immersed. The fluid must not be corrosive, must possess great power of absorption heat and have few impurities. You can use light water coolant, heavy water, carbon dioxide, etc.
In a reactor there is large number of all types of radiation, which are distributed in all directions. To prevent operators from the reactor and the external environment are subjected to such radiation unduly using a suitable "biological shield" which surrounds the reactor. The materials used in the construction of shielding for a reactor are water, lead and high density concrete, whose thickness is greater than 1.5 meters.Types of Nuclear Reactors
There are two types of reactors:
- Research Reactors:
Generated neutrons used in fission to produce radioisotopes or for further studies in materials.
- Power Reactors:
These reactors use the heat generated by fission to produce electrical energy, seawater desalination, heating or propulsion systems.
There are other criteria to classify different types of reactors:
Depending on the speed of the neutrons emerging from fission reactions. He speaks fast reactor or thermal reactors. Depending on the fuel used. There Natural uranium reactors (the proportion of uranium fuel used is very close to having in nature), enriched Uranium (increasing the proportion of uranium in the fuel). According to the moderator used. You can use light water moderated, heavy water or graphite. According to the refrigerant used. Refrigerant is used as the water (light or heavy), a gas (carbon dioxide, air), water vapor, salts or other liquids. These materials may act as a kind of coolant and moderator reactors simultaneously. There are two types of power reactors most popular in the world: the Boiling Water Reactor and Pressurized Water Reactor:
Extension of the radioactive cloud.
Boiling Water Reactor (BWR) has been developed mainly in the U.S., Sweden and Germany. Use purified natural water as moderator and coolant. As fuel has enriched Uranium-238 Uranium-235, which as we know, facilitates the generation of nuclear fission. The heat generated by the chain reaction is used to boil water. The steam produced is introduced into a turbine that drives an electric generator. The steam from the turbine passes through a condenser where it is transformed back into liquid water. Then returns to the reactor to be driven by a suitable pump.
Pressurized Water Reactor (PWR) is widely used in the United States, Germany, France and Japan. The coolant is water at high pressure. The moderator can be water or graphite. Your fuel is also enriched Uranium-238 Uranium-235. The reactor is based on the principle that the water subjected to high pressure it can evaporate without reaching the boiling point, namely at temperatures higher than 100 ° C. Steam is produced at about 600 ° C, which goes to a heat exchanger where it is cooled and condensed to liquid form back into the reactor. In exchanging heat are transferred to a secondary loop of water. Water secondary circuit from heat, produce steam, which is introduced into a turbine that drives an electric generator.SAFETY IN NUCLEAR REACTORS
Basically consists of the control rods and by different monitoring instrumentation. The control rods are driven by a series of mechanical, electrical or electronic, so rapidly ensure the extinction of nuclear reactions. Monitoring instrumentation is located inside or outside the reactor core and are intended to maintain constant surveillance of these security parameters for pressure, temperature, radiation level, etc.
Consisting of a series of multiple barriers that prevent the escape of radiation and radioactive products. The first barrier, in certain types of reactors, is a ceramic material covering the uranium used as fuel element. The second barrier is the structure that contains the uranium, ie it is the fuel rods. The third barrier is the vessel containing the reactor core. In power reactors is called pressure vessel is constructed of a special steel with stainless steel liner. The fourth barrier constitutes the building housing the reactor as a whole. Is known by the name "Containment Building" and is constructed of reinforced concrete, at least 90 cm thick. Used to prevent possible leakage of radioactive products abroad, high impact resist internal or external support large pressure variations and maintain a slight negative pressure within to ensure a constant input from the outside air, so to avoid any leakage activated material.