Unit #4 of the Chornobyl nuclear complex, located at Pripyat, Ukraine, exploded in the early morning of 26 April 1986 at approximately 01:24:00 hrs Kyiv time. The explosion and the following graphite fire spewed some 185 million curies of deadly radioactive debris onto the city of Pripyat and the surrounding area. The present-day republics of Belarus, Ukraine and the Russian Federation were particularly severely contaminated by the radioactive dust spread by the air currents throughout eastern, northern and central Europe, and, indeed, around the world. Next to the atomic bombs dropped on Hiroshima and Nagasaki on the 6 and 9 August 1945, it remains (hopefully forever) the world's worst nuclear tragedy. Its consequences, which have already been enormous, shall remain with the planet earth for the next 250,000 years.
Since the very first day of the explosion, Chornobyl was and remains a political football. Despite his stated policy of glasnost, Michail Gorbachev initially attempted to suppress knowledge of the event. Some claim that Chornobyl was the last nail in the coffin of the now-defunct Soviet Union. The initial reaction of Western scientists (especially in the nuclear establishment) was to denigrate the design, construction and operation of the RBMK-1000 reactor which exploded, and to claim that such an accident could not possibly occur with "their" reactors in the West. The anti-nuclear lobby has been trumpeting Chornobyl as a perfect example of why all nuclear reactors should be shut down and banished from the face of the earth. The death toll over the past ten years attributed to Chornobyl varies from the original 32 to some 100,000. Some people claim that millions of people in Belarus, Ukraine and the Russian Federation are suffering from radiation-related diseases; others insist that these illness are psychosomatic or stress related. Costs of "liquidating the consequences of the accident" vary from several hundreds of million to several tens of billion of dollars.
Surprisingly, over the past ten years, the polarization between these extreme views has been increasing rather than decreasing. Let us hope that the next ten years leads to a convergence of views and a consensus of opinion as to the appropriate actions required to be taken.
For better or for worse, the nuclear genie was let out of the bottle some 50 years ago. The human species has been grappling with the implications of this new technological reality ever since. There is no turning back. In addition to the two extremes of nuclear weapons and electrical power, nuclear physics has spawned a myriad of technological advances which have entered our technological infrastructure. Nuclear energy is neither good nor bad. It is whatever we make of it.
Ukraine willingly gave up its nuclear weapons and signed the nuclear non-proliferation treaty. Since it has little oil, Ukraine probably has little choice but to retain the nuclear option for the production of electricity. The costs of Chornobyl have been and will continue to be enormous. Ukraine has neither the financial nor technological infrastructure to handle the problem. International help is required.
The tragedy of Chornobyl is not just a tragedy of Ukraine. It is a tragedy of the whole world and of all of humanity. Is it too much to hope that, on the tenth anniversary of the Chornobyl tragedy, we finally come to grips with the problem?
Over the past several years, an immense amount of information on Chornobyl has been archived at various websites around the world. When I recently used the AltaVista search engine at http://www.altavista.digital.com I got 200 hits for "Chornobyl" and 5000 hits for "Chernobyl". Many of these sites do not contain original information but contain links to other websites that do.
Personally, I normally start with the website of Oleh Baran at http://physics.mcgill.ca/WWW/oleh/ukr-info.html who has 19 links to other Chornobyl websites.
A Chornobyl Bibliography with 70 references can be found at
Scientists (particularly physicists interested in the accident sequence and physics of the explosion -- see sections 3 and 4 below) should check out:
http://polyn.net.kiae.su/polyn/manifest.html (Project Polyn, Kurchatov Institute, Moscow -- ambitious but incomplete; sporadic access)
http://www.nea.fr/html/rp/chernobyl/chernobyl.html (OECD Nuclear Energy Agency, Chernobyl: Ten Years On)
http://www.uilondon.org/cherntim.html (Uranium Institute, London -- accident sequence)
Soviet Archives Exhibit is to be found at http://sunsite.unc.edu/expo/soviet.exhibit/chernobyl.html
Media Watch Ukraine, Toronto has started a Chornobyl section at http://www.mwukr.ca/
A Belarus perspective can be obtained at the site of Alex Artsukovich and others:
http://ourworld.compuserve.com/homepages/castner/belarus.htm (Remco Castner; German)
http://solar.rtd.utk.edu/~ccsi/ ... (Children of Chornobyl Relief Fund)
http://www.oneworld.org/index_oc/issue196/bykau.html (Bykau personal experience)
Global radiation patterns, aerial images/photos, maps are at:
The anti-nuclear activist, Dr. Gordon Edwards of the Canadian Coalition for Nuclear Responsibility has a website at http://www.deev.com/ccnr/
Announcements of conferences, news reports, etc. are at
http://solar.rtd.utk.edu/~kasaty/chern.htm (24-26 April 1996, Mozyr, Belarus)
http://www.iaea.or.at/worldatom/thisweek/preview/chernobyl (8-12 April 1996, Vienna - summary report)
http://www.tryzub.com/CCRF/ (8 April 1996; dinner in New York)
http://www.yale.edu/rees/yui/chornobyl.html (8-9 April 1996, Yale and Columbia Universities)
http://www.cnn.com/WORLD/9604/04/cnnp_chernobyl/index.html (4 April 1996, CNN report)
http://ecolu-info.unige.ch/colloques/Chernobyl/ (3-5 May 1995, Geneva)
http://www.lasercum.com/press_releases/Chernobyl_PR_11_94 (1 Nov. 1994, Linda Dackman)
http://solar.rtd.utk.edu/~ccsi/csew/94-01/chernobyl.html (18-22 April 1994, Minsk, Belarus)
In addition to the websites listed above, I am familiar with the mailing list of the Canadian Nuclear Association at firstname.lastname@example.org
Specific questions could presumably be routed to subscribers of this list.
I am not aware of any newsgroups specifically dedicated to the Chornobyl accident, although there is often discussion on soc.culture.ukrainian, soc.culture.soviet, etc.
File from http://www.uilondon.org/cherntim.htm
The accident at Chernobyl Unit 4, on 26 April 1986, did not occur during normal operation of the reactor. It happened during a test designed to assess the reactor's safety margin in a particular set of circumstances. The test, which had to be performed at less than full reactor power, was scheduled to coincide with a routine shut-down of the reactor.
Nuclear power stations not only produce electricity, they also consume electricity, for example to power the pumps that circulate the coolant. This electricity is usually supplied from the grid. If the source of electricity should fail, most reactors are able to derive the required electricity from their own production. However, if the reactor is operating but not producing power, for example when in the process of shutting down, some other source of supply is required. Generators are generally used to supply the required power, but there is a time delay while they are started.
The test carried out at Chernobyl-4 was designed to demonstrate that a coasting turbine would provide sufficient power to pump coolant through the reactor core while waiting for electricity from the diesel generators. The circulation of coolant was expected to be sufficient to give the reactor an adequate safety margin.
A number of reports have been published that have given summaries of the events leading up to the accident. Since the reactor was destroyed, these summaries have been based on interpretation of evidence. They have not been consistent. There are three reasons for this:
(i) Different researchers have interpreted the same evidence in different ways.
(ii) With the passage of time more evidence has become available.
(iii) Some authors of reports have been biased.
The sequence of events which follows has been compiled following a review of a large number of reports and it represents what we consider to be the most likely sequence of events.
01:06 The scheduled shutdown of the reactor started. Gradual lowering of the power level began [from 3200 MW(t)].
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.
Isolation of the ECCS was a safety violation. The fact that it was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly.
14:00 The 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 enable demand to be met. Consequently, the reactor power level was maintained at 1600 MW(t) and the experiment was delayed.
Without this delay, the test would have been conducted during `day shift'.
23:10 Power reduction recommenced.
24:00 Shift change.
00:05 Power level had been decreased to 720 MW(t) and continued to be reduced.
The minimum safe operating level for this reactor was about 700 MW(t), since this design of reactor has a positive void coefficient. However, this appears not to have been communicated to station staff by the designers.
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, 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 26 control rods. There is little doubt 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 lead to overcooling of the core. Low water temperature led to reduced steam generation.
01:15 Problems noted in cooling system as a result of reduced steam generation. Key automatic trip systems were deactivated by the operator to permit continued operation of the reactor.
01:18 Operator increased feed water flow in an attempt to address the problems in the cooling system.
01:19 Some manual control rods withdrawn to increase power and raise the temperature.
Operating policy required that a minimum of 15 manual control rods be inserted in the reactor at all times. At this point it is likely that the number of rods was reduced to less than this (probably eight). However, there were a number of automatic control rods in place so that the total number of control rods was more than 15.
01:21:40 Feed water flow rate reduced by the operator because he considered that no more was required.
01:22:10 Spontaneous generation of steam in the core began.
01:22:45 Indications received by the operator, although abnormal, gave the appearance that the reactor was stable.
01:23:04 Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test.
01:23:10 Automatic control rods withdrawn from the core. An approximately 10 second withdrawal was normally required in order 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. However, at low power the expected decrease in steam quantity did not occur. Removal of the automatic control rods reduced the total number of control rods to less than the minimum allowed number of 15.
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 begins to increase uncontrollably.
01:23:40 The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core, but too slowly to be effective.
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.
01:24:00 Two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapour.
The explosions lifted the pile cap, allowing the entry of air. The air reacted with the graphite moderator blocks to form carbon monoxide. This flammable gas ignited and a reactor fire resulted.
Some 8 of the 140 tonnes of fuel, which contained plutonium and other highly radioactive materials (fission products), were ejected from the reactor along with a portion of the graphite moderator, which was also radioactive. These materials were scattered around the site. In addition, caesium and iodine vapours were released both by the explosion and during the subsequent fire.
Wally Cichowlas email@example.com states
The original statements and reports made by Soviet experts in 1986-87 tended to put most of the blame on errors by the station staff. Over the years, it has become more clear that there was enough blame to go around among
(a) operators and station staff who were complacent about safety,
(b) an unforgiving design and inadequate safety analysis of that design,
(c) an inability or unwillingness of designers and regulators to learn from past mistakes, and
(d) poor operating procedures.
As for the specific chronology and analysis, I refer you to three documents which provide the best, most comprehensive review of the accident. These should answer most of your questions in better detail than I can provide here:
(1) INSAG-1 "Summary Report on the Post-Accident Review Meeting on the Chernobyl Accident" Safety Series No.75-INSAG-1, IAEA, Vienna (1986).
(2) NUREG-1250 "Report on the Accident at the Chernobyl Nuclear Power Station" US Nuclear Regulatory Commission (January, 1987).
(3) INSAG-7 "The Chernobyl Accident: Updating of INSAG-1" Safety Series No.75-INSAG-7 (November, 1992).
(4) AECL-9426 (1987), "The Chernobyl Accident: Multidimensional simulations to identify the role of design and operational features of the RBMK-1000", by P.S.W. Chan, A.R. Dastur, S.D. Grant and J.M. Hopwood.
(5) AECL-9334 (1987), "Chernobyl - A Canadian Technical Perspective", by V.G. Snell and J.Q. Howieson.
(6) AECL-PA-10 (1986), "Chernobyl - A Canadian Perspective", by V.G. Snell and J.Q. Howieson.
CHORN_96.D26 = Chornobyl on the Internet Part 1
Copyright © 1996 Dr. W. Zuzak