THE CHERNOBYL ACCIDENT 1986, Root cause of Reactor 4 failure.

“Chernobyl | Chernobyl Accident | Chernobyl Disaster – World Nuclear Association,”, 2022. (accessed Mar. 15, 2023).

Main facts

  • The Chernobyl accident in 1986 was the result of a flawed reactor design that was operated with inadequately trained personnel.
  • The resulting steam explosion and fires released at least 5% of the radioactive reactor core into the environment, with the deposition of radioactive materials in many parts of Europe.
  • Two Chernobyl plant workers died due to the explosion on the night of the accident, and a further 28 people died within a few weeks as a result of acute radiation syndrome.
  • The United Nations Scientific Committee on the Effects of Atomic Radiation has concluded that, apart from some 5000 thyroid cancers (resulting in 15 fatalities), “there is no evidence of a major public health impact attributable to radiation exposure 20 years after the accident.”
  • Some 350,000 people were evacuated as a result of the accident, but resettlement of areas from which people were relocated is ongoing.
  • On 24 February Ukraine informed the International Atomic Energy Agency that Russian forces had taken control of all facilities at Chernobyl .
  • On 9 March the Chernobyl nuclear plant was disconnected from the electricity grid. The IAEA stated that it did not see a critical impact on safety as a result.

The April 1986 disaster at the Chernobyl nuclear power plant in Ukraine was the product of a flawed Soviet reactor design coupled with serious mistakes made by the plant operators. It was a direct consequence of Cold War isolation and the resulting lack of any safety culture.

Root cause analysis

M. Lallanilla and L. Geggel, “Chernobyl: Facts about the world’s worst nuclear disaster,”, May 18, 2022. (accessed Mar. 15, 2023).

Specific issues relating to the design of the RBMK reactors were the subject
of intensive studies in the USSR after 1986, and some modifications have since been
made to these reactors and to their operating regimes. More recently, discussions
have been held at the international level on the safety of nuclear power plants with
RBMK reactors. International efforts to assist in the safety assessment of the RBMK
reactors have been intensified in recent months.

On 25 April, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested.

A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. A peculiarity of the design of the control rods caused a dramatic power surge as they were inserted into the reactor.

Aftermath of the human error accident

The interaction of very hot fuel with the cooling water led to fuel fragmentation along with rapid steam production and an increase in pressure. The design characteristics of the reactor were such that substantial damage to even three or four fuel assemblies would – and did – result in the destruction of the reactor. The overpressure caused the 1000 t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down. Intense steam generation then spread throughout the whole core (fed by water dumped into the core due to the rupture of the emergency cooling circuit) causing a steam explosion and releasing fission products to the atmosphere. About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite. There is some dispute among experts about the character of this second explosion, but it is likely to have been caused by the production of hydrogen from zirconium-steam reactions.


[1]“Chernobyl | Chernobyl Accident | Chernobyl Disaster – World Nuclear Association,”, 2022. (accessed Mar. 15, 2023).

[2]M. Lallanilla and L. Geggel, “Chernobyl: Facts about the world’s worst nuclear disaster,”, May 18, 2022. (accessed Mar. 15, 2023).