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The Chernobyl accident

The accident occurred on 26 April 1986 in unit 4 of the Chernobyl nuclear power plant – a reactor type of Soviet design termed RBMK. The fuel elements in this reactor type are located inside pressure tubes - surrounded by a graphite block - and cooled with water.

At the time of the accident, the reactor was in the phase of a slow shutdown according to schedule to carry out routine maintenance and test measures (revision). At the same time an experiment was planned for checking various safety features of the plant. Basic design failures of the plant in combination with failures and offences in operational management led to the reactor disaster.

Experimental programme

The experiment was to prove that in case of a loss of coolant accident and a simultaneously assumed failure of power supply it would still be possible to control the plant. In case of such an incident the reactor would immediately be shut down automatically. The mechanical energy of the coasting-down rotor in the turbine-generator set must then be sufficient for an interim supply of the power required by the reactor coolant pumps until the supply of the emergency pumps through the emergency Diesel unit is ensured.

This experiment was seen as a purely conventional experiment in the field of electrical engineering and no interactions with the nuclear part of the plant were expected.

Course of the accident

Contrary to expectations the power increased rapidly and got out of control while the experiment in Chernobyl was carried out. This resulted in a rapid increase in energy release in the fuel elements and furthermore to the destruction of the reactor core. The heat stored in the fuel was transferred to the surrounding coolant very quickly. Thus the coolant was heated up and evaporated. Due to the resulting high pressure the reactor exploded. The reactor building including its roof was destroyed and numerous fires broke out. The entire graphite block of the reactor core with a mass of 250 tons incinerated. This fire with extremely high temperatures took the reactor’s radioactive inventory to great heights of the atmosphere and caused the long-range dispersion of radioactivity throughout Europe.

Reasons for the accident

Several reasons contributed to the reactor failure:

  • Unfavourable reactor-physical properties and safety-related features of the reactor type,
  • Shortcomings in the experiment programme,
  • Unforeseen conditions while the experiment was carried out,
  • Several offences against operating rules.

Release of radioactive substances

A large number of radioactive substances were released into the atmosphere by the explosion and the reactor fire (cf. table). The massive release could be stopped only after 10 days.

Most important radionuclides released
element groupisotopes
Noble gasesKrypton and xenon isotopes
(such as krypton-85, xenon-133)
Readily volatile substancesIodine, tellurium and caesium isotopes
(such as iodine-131, tellurium-132, caesium-134, caesium-137)
Not readily volatile substancesRuthenium and strontium isotopes
(such as ruthenium-103, ruthenium-106, strontium-89, strontium-90)
Transuranic elementsPlutonium and curium isotopes
(such as plutonium-238, plutonium-239, plutonium-240, curium-242)

Distribution of radioactive substances

Due to the explosion and the thermal lift induced by the fire the released radioactive substances were dispersed into the atmosphere at very high altitudes. The prevailing airstream transported the radioactive substances over large parts of Europe.

During the passage of the radioactive air masses the occurrence and intensity of precipitation, which washed out and precipitated the radioactive substances, influenced the local distribution of the radioactive contamination. Accordingly, the intensity of radioactive contamination was very different locally. The areas most affected were areas in northern Ukraine, Belarus and western Russia.

State of 2018.05.08

© Federal Office for the Safety of Nuclear Waste Management