In April of 1986, a terrible disaster occurred in the Chernobyl power plant in Ukraine. The accident destroyed the Chernobyl 4 reactor, which was a Soviet reactor known as a RBMK reactor, a ""reactor cooled by water and moderated by graphite". In order to understand this whole situation more clearly, one must first understand how a nuclear reactor functions.
The RBMK reactor is just like all other reactors in that it uses nuclear fission to split nuclei, creating energy. In order to do this, there must be a bombarding particle, for example a neutron, and a target particle, such as uranium. The bombarding particle splits the target particle's nucleus into two, almost equal parts, and the nuclear fission releases part of the energy of the nucleus. Some of this energy comes out in the form of radiation, but most of it is heat.
Another important factor in this process is the moderator. When one tries to split the nuclei of many pieces of uranium, the neutrons move very fast, making it hard for the bombarding particle to even hit the uranium. The moderator is used in nuclear power stations to slow down the neutrons as they pass through it, making it easier to bombard many blocks of uranium at one time. Uranium is used in many nuclear power stations in the United States and Canada, but it was not used in the RBMK reactors at Chernobyl. The RBMK reactors use a pure form of solid graphite.
A reactor is made up of three main parts: the reactor vessel, the core, and the control rods. Each has its own function, making up the nuclear reactor. The reactor vessel holds all of the reactor parts. The core is a huge container that is filled with the target material, and in the case of the RBMK reactors, graphite blocks. And, the control rods are long metal rods that contain that, in RBMK reactors, contain. Their function is to help control the reaction by absorbing free neutrons. There is a device right outside the nuclear reactor that inserts and withdraws the rods to slow down or speed up the reaction.
During a reaction, the uranium or graphite fuel becomes hot. Water is then pumped through the core in the vessel and it removes the heat from the fuel. Due to this gained heat, the water then boils into steam. The steam then turns two turbines which spin electrical generators to produce electricity. The water is cooled and the process is done again.
But, during the Chernobyl accident, things were not done as they should have been. Engineers on the night shift at Chernobyl’s number 4 reactor began to experiment to see if the cooling pump system could still work using the power generated if the auxiliary power failed. The control rods were lowered in order to reduce the output to about 20% of the normal output required for the test. But, too many rods were lowered and output dropped too quickly, creating an almost complete shutdown.
The engineers worried about instability so they decided to raise the rods to increase output. But, during this time, the automatic shut down was turned off to allow the reactor to continue in such low power conditions. The engineers continued to raise the rods and the output increased to about 12%. But, then all of a sudden the power levels sky-rocketed to dangerous levels.
The reactor began to overheat and the water coolant started to turn to steam. It is believed that at this point, all but 6 rods had been removed, and the safe operating minimum is only 30. The emergency shut-off was pushed, sending the control rods back down into the reactor. But, this was a mistake because they were inserted from the top, which displaced the coolant and concentrated all activity to the core.
The power levels surged to about 100 times the normal amount, causing the fuel channels to rupture. Two explosions occurred, causing the reactor's top to be blown off and the contents of the reactor to erupt upwards. As air was sucked into the broken reactor, flammable carbon monoxide gas cause a fire in the reactor that burned for a total of nine days. Because the reactor was not in a reinforced concrete shell, which is a standard safety procedure in most countries, the building suffered severe damage and large amounts of radioactive debris escaped into the atmosphere.
The effects of the Chernobyl accident were devastating. Here is an image of all the areas affected:
The disaster released at least 100 times the amount of radiation of the bombs dropped on Hiroshima and Nagasaki. Two workers died of the explosion itself, with another 28 dying of Acute Radiation Syndrome (ARS) within a few weeks of the accident. The two main sources of radiation delivered to the people were iodine-131 and cesium-137. The total number of deaths due to the accident and radiation cannot be determined fully but there are many anticipated deaths due to the accident. The radiation has caused many cancers in its victims, especially thyroid cancer. About 4,000 cases of the cancer has been seen in victims related to the incident so far. The Greenpeace campaign group anticipates a total of 93,000 more cancer related deaths, making the death toll of the accident around 200,000.
Here is how many people were affected by the disaster:
In order to prevent more exposure, many people were evacuated from surrounding towns but complete prevention was not successful. Fire-fighters were immediately rushed to the scene to put out the fire from the explosion and all weren't put out until 9 days later. Even after the fires were extinguished, radioactive particles were still escaping the reactor. In order to eliminate this, the Soviets devised a plan to pour concrete and place steel to form a shell around the reactor so that no more radiation would escape. They named it the Sarcophagus.
The situation in Japan is similar to that of Chernobyl but not completely. Here is a video that explains why:
In conclusion, the Chernobyl accident is somewhat similar to the situation in Japan but graphite was not used in Japan. Chernobyl is a very significant disaster in world history and much was learned from the flawed structure and mistakes of the engineers.
Works Cited:
"Backgrounder on Chernobyl Nuclear Power Plant Accident." United States Nuclear Regulatory Commission .
NRC, 30 Apr. 2009. Web. 28 Mar. 2011.<http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html>.
Beegle, Bill, Peter Bleickhart, and Steven Quirk. "Chernobyl: A Nuclear Disaster." Oracle Think Quest. N.p., n.d.
Web. 28 Mar. 2011. <http://library.thinkquest.org/3426/>.
"Chernobyl Accident." World Nuclear Association. N.p., Mar. 2011. Web. 28 Mar. 2011.
<http://www.world-nuclear.org/info/chernobyl/inf07.html>.
"The Chernobyl Disaster." BBC News. N.p., n.d. Web. 28 Mar. 2011.
<http://news.bbc.co.uk/2/shared/spl/hi/guides/456900/456957/html/nn1page1.stm>.
International Chernobyl Radiological Portal of the ICRIN Project. ICRIN Project, Web. 28 Mar. 2011.
<http://chernobyl.info/Default.aspx?tabid=294>.
