Control rods Chernobyl Unit 4 reactor

The explosion of the nuclear reactor at Chernobyl (spelling changed recently to Chornobyl) in the Ukraine on April 26, 1986 sent radioactive material as far away as Sweden.90 The current death toll is 45. There has been a huge increase in childhood thyroid cancer, with cases as far as 500 km away 91 (U. S. bomb tests have also increased the incidence of thyroid cancers in the western United States.92) There is a 30-km exclusion zone around the plant where no one is allowed to live. This was created by the evacuation of 135,000 people 93 The accident is said to have happened because of combination of the physical characteristics of the reactor, the design of the control rods, human error and management shortcomings in the design, and implementation of the safety experiment. ... 

By approximately 11 00 pm Friday evening, the local electricity demand had subsided, and the load dispatcher gave permission for Chernobyl Unit 4 to continue with its power reduction. At this point, another step in a string of unfortunate events occurred when the automatic reactor control unit did not adjust for the new lower power level. The operators had reset the reactor monitoring system to the requested level, but had failed to reset the reactor automatic controller. The reactor s response was a dramatic drop in power, down to 30 MWt, 1 % of the normal operating level (US NRC, 1987). In effect, the automatic controller inserted control rods, which drove the overall core power dramatically lower than was intended. 

The second incident occurred at Chernobyl, in Ukraine, in 1986. Graphite was used as the moderator there. When the coolant was turned off because of human error, the chain reaction went out of control. A tremendous rise in temperature followed, leading to a meltdown. During the meltdown, the graphite moderator surrounding the rods burned and radioactive smoke spewed out of the reactor. Radioactive materials were dispersed over much of Europe, Canada, and the United States. Although only a few dozen people were killed in the Chernobyl accident, many more will eventually die of cancer because of the related radiation. The type of accident observed at Chernobyl cannot happen in a light-water reactor, in which the coolant is the moderator. 

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