|Fig. 1: View of Chernobyl taken from Pripyat. (Source: Wikimedia Commons)|
The Chernobyl Accident was a nuclear reactor accident that occurred on Apr 26, 1986 in Ukraine. At around 01:23 am on that day, reactor number 4 at the Chernobyl plant exploded. [1-4] A total of about 30 people, including operators and firemen, died as a result of direct exposure to radiation. Only 1 person was killed at the point of explosion, and a second died in hospital subsequently due to severe injuries. 28 others died as a result of Acute Radiation Syndrome (ARS) within about a few weeks to 3 months of the accident.
The fallout that resulted from the explosions was highly radioactive. It was sent up into the atmosphere and covered a wide geographical area. An estimated four hundred times more radioactive fallout was released during the Chernobyl disaster than the atomic bombing of Hiroshima during World War II. [1-3] Due to wind and other weather conditions, the plume drifted over large regions of Soviet Union, Europe (including Eastern, Western and Northern regions), Ukraine, Belarus and Russia, among many others. These areas were all badly contaminated, leading to the evacuation of hundreds of thousands of people. The majority of the radioactive fallout (an estimated 60%) landed in Belarus.
The Chernobyl reactor uses a RBMK design, a design that is unique to former Soviet Union and former Eastern Bloc countries. [1,2,5] The RBMK, or the channelized large power reactor, is a boiling water reactor. It does not have any pressure vessels; rather, the fuel assemblies are found in pressurized tubes (1000 or more). These fuel channels are separate from one another. The advantage of this is that the channels can function independently of one another and the fuel elements can be removed and replaced online. The fuel that is most commonly used is low enrichment (2%) uranium dioxide. The moderator used is graphite as opposed to light water which is commonly used in Western designs. This explains why the RBMK reactor occupies a much larger land area than most reactors found in Western countries.
One of the most crucial causes of the accident is the large positive void coefficient possessed by the nuclear reactor. [1,2] 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. [1,2] 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.
© Zhi Wei Seh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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