CONSULTING

Sequence and Origin of the Accident on 26 April 1986
The Chernobyl nuclear accident happened on 26 April 1986 at 01.23 a.m. in the course of a technical
test in unit 4. The “beyond design-basis accident” was caused by inappropriate reactor operation
at low-power level. The reactor was under extremely unstable conditions because of the
withdrawal of almost all control rods. This was a very dangerous operation in RBMK reactors as
these had positive void coefficients, meaning that runaway nuclear reactions could take place. This
duly occurred with the result of a sudden power surge, and, when an emergency shutdown was
attempted by inserting the remaining control rods, a much larger spike in power output—output
increased about 100-fold in about four seconds—which led to at least two massive steam and
hydrogen explosions and the rupture of the entire reactor vessel and a major conflagration. This
released a large volume of radioactive gases, aerosols and particulates into the atmosphere.
Radionuclides released from the explosion included very short-lived fission products, which
resulted in very high dose rates in adjacent areas.
These events exposed the reactor’s graphite moderator (1600 tons) to air, causing it to ignite. After
the initial release, larger releases of radionuclides occurred over a period of 10 days due to the
continuous graphite fire. It has been estimated that the explosions and fires released about a third
of the reactor’s radioactive inventory into the atmosphere and across much of Europe.
The accident was classified as a level 7 event (the maximum classification) of the IAEA’s
International Nuclear Event Scale (INES).
Onsite Challenges
Following two explosions, the first being the initial steam explosion, followed a few seconds later
by a second explosion, possibly from the build-up of hydrogen due to zirconium-steam reactions, a
significant part of the fuel, the graphite and structural materials were ejected. One worker, whose
body was never recovered, was killed in the explosions, and a second worker died in hospital a few
hours later as a result of injuries received in the explosions.
Fires started in what remained of the unit 4 building, giving rise to clouds of steam and dust, and
fires also broke out on the adjacent bitumen covered turbine hall roof. The chimney effect of the
ten-day-lasting graphite fire ejected smoke, radioactive fission products and debris from the core
and the building several kilometers into atmosphere. The heavier debris was mostly deposited
within 5 km of the site, but lighter components, including most fission products and noble gases,
and were blown by the prevailing winds to create the radioactive plumes, which contaminated
over 40 percent of the land area of Europe.
A first group of 14 firemen arrived on the scene of the accident on 26 April 1986 at 01:28. Over
100 fire fighters from the site and called in from Pripyat were deployed, and it was this group that
received the highest radiation exposures. Reinforcements were brought in until about 04:00, when
250 firemen were available and 69 firemen participated in fire control activities. According to
corroborating reports from various sources, 239 the fires on the roofs of units 3 and 4 were localized
239 See for example WNA, “Sequence of Events—Chernobyl Accident Appendix 1”, Updated November 2009,
see http://www.world-nuclear.org/information-library/safety-and-security/safety-ofplants/appendices/chernobyl-accident-appendix-1-sequence-of-events.aspx,
accessed 4 June 2016; and
INSAG-7, “The Chernobyl Accident: Updating of INSAG-1”, International Nuclear Safety Advisory Group,
IAEA, Safety Series No. 75-INSAG-7, 1992.
Mycle Schneider, Antony Froggatt et al. 76 World Nuclear Industry Status Report 2016