April 26, 1986

Chernobyl disaster

Chernobyl Unit 4 explodes during a turbine rundown test

The shutdown button was pressed at 01:23:40. Because of how the control rods were built, pressing it first made the reactor more powerful.

01:23:40, 26 April 1986

On 26 April 1986, during a planned shutdown of Unit 4, operators were testing whether a coasting turbine-generator could power coolant pumps and emergency equipment until diesel generators started after a loss of external power.

The senior reactor-control engineer pressed the emergency shutdown button. In the reactor's abnormal state, defective control-rod design initially added reactivity — the rods displaced neutron-absorbing water before their absorber sections entered the core. By about 01:24, severe shocks and explosions were destroying Unit 4.

Design, not just operators

The crew had brought the RBMK into a poorly understood low-power configuration with too little operating reactivity margin. In that unstable state, the large positive void coefficient and the defective control-rod design helped turn the shutdown into an uncontrollable power excursion.

IAEA's revised account treats the accident as a combination of design defects, deficient procedures, weak safety culture and operator actions — and says the exact initiating sequence may never be known with certainty. INSAG-7 also found that isolating the emergency core cooling system did not initiate or affect the accident's development.

The next 36 hours

Roughly 600 workers were at the site that morning. Plant personnel and firefighters suppressed numerous fires within hours, keeping them from spreading to the other operating units, while the destroyed reactor kept releasing radionuclides.

Pripyat was not evacuated that day. Its 49,360 residents were moved starting at 2 p.m. on 27 April, and the Soviet Union publicly acknowledged an accident on 28 April, after elevated radiation was detected abroad.

Sources

Researched 23 Aug 2026 7 sources 2 audit passes, 1 issue open

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

What the sources leave uncertain

IAEA's INSAG-7 says the exact initiating sequence may never be known with certainty, although the destructive role of the positive void coefficient and control-rod design is well established.
One worker was killed in the explosion and another died hours later from traumatic injuries; 28 ARS deaths occurred over the following three months. These categories should not be combined into an “instant death toll.”
Long-term cancer deaths are statistical projections with differing geographic scopes and assumptions, not individually identified casualties.
The 116,000 people evacuated in 1986 and roughly 220,000 relocated later are separate groups and time periods.