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
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
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
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
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
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
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
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
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
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
01:24, two explosions occurred.
One was a steam explosion; the other resulted from the expansion of fuel
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
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
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
project is intended to help the people living in the area contaminated
by the 1986 disaster reduce their exposure to radioactivity.
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
Three Mile Island