Chapter 35: Nuclear Energy: Safety – Accidents – Chernobyl – Edited by Dr. Mir F. Ali

Exactly seven years and twenty eight days after the Three Mile Island accident in the United States, the Chernobyl accident, the most serious accident in the history of the nuclear industry, occurred on April 26, 1986 at Unit 4 (RBMK-1000 Reactor) of the Chernobyl nuclear power plant in the former Ukrainian Republic of the Soviet Union.  The explosions completely shattered the Chernobyl reactor vessel and triggered fire that continued for 10 days.  As a result, large amounts of radioactive materials were released into the environment and it changed the global opinion about the safety of nuclear energy.

Here is a brief description of RBMK reactor:  The RBMK is an early Generation II reactor and the oldest commercial reactor design still in wide operation; it features a number of design and safety flaws (such as graphite-tipped control rods, a dangerous positive void coefficient and instability at low power levels) that have since been rectified in newer designs.  The reactor pit is made of reinforced concrete and has dimensions 21.6×21.6×25.5 meters. It houses the vessel of the reactor, made of a cylindrical wall and top and bottom metal plates. The vessel contains the graphite stack and is filled with a helium-nitrogen mixture for providing an inert atmosphere for the graphite and for mediation of heat transfer from the graphite to the coolant channels.

The Chernobyl nuclear power plant was built in the wooded marshlands of northern Ukraine, approximately 130 km north of Kiev. Its first reactor went online in 1977, the second in 1978, third in 1981, and fourth in 1983; two more were planned for construction.

This area of Ukraine is described as Belarusian-type woodland with a low density.  A small town, Pripyat, was also built near the Chernobyl nuclear power plant to house the workers and their families.  There were 49,000 inhabitants living in this town.  The old town Chernobyl, which had a population of 12,500, is about 15 km to the southeast of the complex.  Within a 30 km radius of the plant, the total population was between 115,000 and 135,000.

The plan was to shut down the plant for some routine maintenance on April 25, 1986. During the shutdown, technicians were also going to run a test. The test was to determine whether, in case of a power outage, the turbines could produce enough energy to keep the cooling system running until the backup generators came online.  The shutdown and test began as planned at 1 a.m. on April 25th. To get accurate results from the test, the operators turned off several of the safety systems, which turned out to be a disastrous decision. In the middle of the test, the shutdown had to be delayed nine hours because of a high demand for power in Kiev. The shutdown and test continued again at 11:10 p.m. on the night of April 25th.

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