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Nuclear Accidents Nuclear Waste Storage Food Irradiation Conclusion |
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Nuclear Accidents
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Chernobyl  The RBMK reactor, which happened to explode in Chernobyl, was of unique design, water and graphite cooled and moderated; nobody had seen the likes of such a thing. The characteristics flaw was that when the reactor entered into lower power it became unstable. Instability was due to the control rod design and a positive void coefficient. Prior to shutdown, the operators commenced an unauthorized test to examine how much power could be produced to operate the emergency shutdown system until the diesel engine kicked in. An increase of coolant flow and a drop in steam pressure, the operators attempted to withdraw all the control rods. The reactor became unstable and operators had to make an adjustment every few seconds. Flow of feedwater was reduced to increase steam pressure, but as the system slowly ran out of power, less coolant was pumped to the reactor. Steam pressure began to rise which caused positive void coefficient, and the nuclear reactor experienced a surge of several hundred times the full reactor power. Steam pressure caused an explosion, which blew the top of the reactor core off. A second explosion caused fragments of burning fuel and graphite to fly from the core and as air rushed in, it began to mingle with the graphite, causing the graphite moderator to erupt in flames, which blazed for 9 days. 
The problem with the control rods were that they were graphite tipped, which is neutron permeable. This means as the control rods were used to stop the flow of neutrons to decrease power, that the graphite tips actually increased the power due to their neutron permeability. A positive void coefficient, which is usually associated with the RBMK reactor design, was when neutron absorption decreased through the production of steam; therefore, more neutrons are allowed to flow freely increasing the fission reactions. This in turn creates more steam, which allows more reaction, and a problem arises. Due to this, Reactor Four, the power peaked to several hundred times of the normal nominal power. The operators were not informed of the potential hazard that their test could bring to the reactor. In addition they did not follow all operational procedures, and as a result the reactor was destroyed in a matter of a few seconds. Two deaths occurred immediately, one in the explosion and another who suffered from a heart attack. Another died the next morning after thermal burns. Twenty-eight later died within weeks later. Totaling 31 deaths after few weeks of the accident. An estimated 300 000 to 600 000 people were involved in the cleanup of the explosion. A total of about 160 000 people were evacuated in a 30 kilometer radius of the disaster. Later on a total of 210 000 people were relocated into less radioactive areas, and the initial 30 km radius zone was extended to about 4300 square kilometers. Children were exposed to enough radiation doses through contaminated milk, which in turn caused thyroid cancer. The rate increased from 5 million to 45 million after the accident. Usually a very rare disorder, cases have risen sharply, but luckily if treated early the victim can be saved. 
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