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Chernobyl Reactor 4
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What important lessons did modern scientists learn from the Chernobyl accident?

On 26 April 26 1986, one of the four nuclear reactors at the Chernobyl nuclear plant in the north central Ukraine exploded and caused the world's worst known nuclear disaster. An improperly supervised experiment caused a steam explosion which blew off the reactor's protective covering and which released approximately 100 million curies of radionuclides into the atmosphere. More than 100,000 people were evacuated from nearby areas.  Serious nuclear fall-out affected areas of Belarus, Poland and Eastern Europe, while some of the radiation spread across northern Europe and into Great Britain.
The Soviet Union had embarked on a substantial nuclear power program in the 1960s to meet the growing need for electricity in Western Russian and the Ukraine.  By the 1980s, upwards of twenty percent of electrical demand was being met by nuclear power.  Many of the reactors were of the boiling water variety.
The failure of the Soviet authorities to immediately acknowledge the extent of the accident and to take proper containment measures led to serious criticism of Mikhail Gorbachev's policies of glasnost and perestroika.

  • 28 March 1979, Three Mile Island nuclear accident when one reactor lost coolant with a resulting partial meltdown and release of radioactivity.
  • 25 April 1986, events leading to the Chernobyl accident (when explosion and fire occurred in a graphite core reactor) began as part of a test procedure.
    • 01:06, the scheduled shutdown of the reactor started.  Gradual lowering of the power level began.
    • 03:47, lowering of reactor power halted at 1600 MW(t).
    • 14:00,  the emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later.
    • 4:00, the reactor's power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to meet demand.  Consequently, the reactor power level was maintained at 1600 MW(t), and the experiment was delayed.
    • 23:10, power reduction recommenced.
    • 24:00, shift change.
  • 26 April 1986, preparation for the test resumed.
    • 00:05, power level had been decreased to 720 MW(t) and continued to be reduced.  It is now recognised that the safe operating level for a pre-accident configuration RBMK reactor was about 700 Mwt because of the positive void coefficient.
    • 00:28, power level was now 500 MW(t).  Control was transferred from the local to the automatic regulating system.  Either the operator failed to give the "hold power at required level" signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MW(t).
    • 00:32 (approximate time), in response to the drop in power, the operator retracted a number of control rods in an attempt to restore the power level.  Station safety procedures required that approval of the chief engineer be obtained to operate the reactor with fewer than the effective equivalent of 26 control rods. It is estimated that there were less than this number remaining in the reactor at this time.
    • 01:00, the reactor power had risen to 200 MW(t).
    • 01:03, an additional pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure).
    • 01:07, an additional pump was switched into the right hand cooling circuit (part of the test procedure).  Operation of additional pumps removed heat from the core more quickly, which reduced the water level in the steam separator as more steam was generated.
    • 01:15, automatic trip systems to the steam separator were deactivated by the operator to permit continued operation of the reactor in this mode.
    • 01:18, the operator increased feed water flow to the cooling loop in an attempt to address the low water level problems in the cooling system.
    • 01:19, some manual control rods were withdrawn to increase power and raise the temperature and pressure in the steam separator.  Operating policy required that a minimum effective equivalent of 15 manual control rods be inserted in the reactor at all times. At this point it is likely that the number of manual rods was reduced to less than this (probably eight).  However, automatic control rods were in place, thereby increasing the total number.
    • 01:21:40, feed water flow rate reduced to below normal by the operator to stabilize steam separator water level, decreasing heat removal from the core.
    • 01:22:10, spontaneous generation of steam in the core began as heat was not removed from the core fast enough.
    • 01:22:45, indications received by the operator, although abnormal, gave the appearance that the reactor was still stable.
    • 01:23:04, the test began.  Turbine feed valves were closed to start turbine coasting. This was the beginning of the actual test.
    • 01:23:10, automatic control rods were withdrawn from the core.  An approximately 10 second withdrawal was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feed valves.  Usually this decrease is caused by an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. The expected decrease in steam quantity did not occur due to reduced feedwater to the core.
    • 01:23:21, steam generation increased to a point where, owing to the reactor's positive void coefficient, a further increase of steam generation would lead to a rapid increase in power.
    • 01:23:35, steam in the core began to increase uncontrollably.
    • 01:23:40, the emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core, but the insertion of the rods from the top concentrated all of the reactivity in the bottom of the core.
    • 01:23:44, reactor power rose to a peak of about 100 times the design value.
    • 01:23:45, fuel pellets started to shatter, reacting with the cooling water to produce a pulse of high pressure in the fuel channels.
    • 01:23:49, fuel channels ruptured.
    • 01:24, two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapour.

WWW sites
Start with Elena's Ghost Town. 

For some relaxation, try the on-line nuclear power plant and run the meltdown simulation to attempt to prevent a nuclear accident.  (This simulation has been know to crash a browser). A major International Conference in Vienna, 8-12 April 1996, "One Decade after Chernobyl" summed up the consequences of the accident.  Chernobyl ten years on (*.pdf)--that document is also part of a larger website www.nea.fr/rp/chernobyl/--is an excellent resource site and includes a detailed chronology of the accident, as does the Sunsite overview of Chernobyl.  For the consequences of Chernobyl, see: www.world-nuclear.org/info/chernobyl/inf07.html by the World Nuclear Association, www.usnews.com/articles/news/world/2008/04/24/chernobyl-victims-struggle-with-consequences-of-radiation-exposure.html by US News and World Report or www.nei.org/resourcesandstats/documentlibrary/safetyandsecurity/factsheet/chernobylconsequences/ by the Nuclear Energy Institute. The Chernobyl page is a set of links to further information on the disaster.  With regard to the secrecy surrounding the initial accident, see The Chernobyl Cover-Up, Time (13 November 1989), Chernobyl' or Alison Smale, Revealing Secret Spots That Evoke Dark Secrets (New York Times, 24 August 2009).  Finally, the Chernobyl project is intended to help the people living in the area contaminated by the 1986 disaster reduce their exposure to radioactivity.

Still more


Recommended Books
There are a two important U.S. Congress hearings:  The Legacy of Chernobyl, 1986 to 1996 and Beyond:  Hearing before the Commission on Security and Cooperation in Europe, One Hundred Fourth Congress, second session, 23 April 1996 (1996); Effects of the Accident at the Chernobyl Nuclear Power Plant:  Hearing before the Subcommittee on Nuclear Regulation of the Committee on Environment and Public Works, United States Senate, One Hundred Second Congress, second session, 22 July 1992 (1992).
Some interesting books include:  Arthur Hopkins, Unchained Reactions:  Chernobyl, Glasnost and Nuclear Deterrence (1993); V. Vozniak and S. Troitskii, Chernobyl':  tak eto bylo, vzgliad iznutri (1993); Piers Read, Ablaze:  The Story of the Heroes and Victims of Chernobyl (1993); Grigorii Medvedev, No Breathing Room:  The Aftermath of Chernobyl (1993); A. Iaroshinskaia, Chernobyl', sovershenno sekretno (1992); Grigorii Medvedev, The Truth about Chernobyl (with a foreword by Andrei Sakharov, 1991); Zhores Medvedev, The Legacy of Chernobyl (1990); C. Bailey, The Aftermath of Chernobyl:  History's Worst Nuclear Power Reactor Accident (1989).

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