causes of Chernobyl
Answers
One of the most crucial causes of the accident is the large positive void coefficient possessed by the nuclear reactor. One characteristic of the RBMK reactor is that it can have a positive void coefficient. This means that an increase in voids or steam bubbles is associated with a rise in core reactivity. Most other reactor designs have a negative coefficient, which means that the reactor responds to the formation of steam bubbles by decreasing heat output. This is because if the coolant contains lots of steam bubbles, fewer neutrons are slowed down. The faster neutrons are, in turn, less likely to cause fission of the uranium atoms, thus resulting in a lower power output. This is an example of negative feedback that is used to prevent the reactor's heat output from reaching dangerously high levels. However, the RBMK reactor used had a positive coefficient, which means that the reactor becomes very unstable at low power levels, and vulnerable to dangerous rises in energy production levels. This was one of the reasons for the reactor explosion during the Chernobyl accident.
Another cause was a flaw in the design of control rods. Control rods are meant to control the multiplication factor k of the reactor. Since control rods absorb neutrons, a withdrawal of the rods causes an increase in k value, and vice versa. Of the control rods, 163 are inserted from the top of the reactor and are made of graphite. The rods were found to be 1.3 m shorter than stipulated, which is unacceptable. The upper portion of the rods, which acts to absorb neutrons and slow down the nuclear reaction, was filled with boron carbide. When the rods were inserted, the graphite part displaces some of the coolant, thus leading to an increase in fission rate. This is because graphite is a more powerful neutron moderator than light water, i.e., it absorbs less neutrons. This resulted in a dangerous increase in power output. Moreover, post-accident investigations determined that at the point of the accident, the number of rods in the reactor was equivalent to 8 control rods. However, according to international standards, a minimum of 15 such rods were required at all times. This flaw in the design and number of control rods was one of the important causes of the disaster as well.
Answer:
What caused the Chernobyl accident?
On April 26, 1986, the Number Four RBMK reactor at the nuclear power plant at Chernobyl, Ukraine, went out of control during a test at low-power, leading to an explosion and fire that demolished the reactor building and released large amounts of radiation into the atmosphere. Safety measures were ignored, the uranium fuel in the reactor overheated and melted through the protective barriers. RBMK reactors do not have what is known as a containment structure, a concrete and steel dome over the reactor itself designed to keep radiation inside the plant in the event of such an accident. Consequently, radioactive elements including plutonium, iodine, strontium and caesium were scattered over a wide area. In addition, the graphite blocks used as a moderating material in the RBMK caught fire at high temperature as air entered the reactor core, which contributed to emission of radioactive materials into the environment.
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