The explosion at Unit 4 of the Chernobyl nuclear power station was the worst nuclear accident in history. There are still some uncertainties regarding the exact causes and events leading to the accident, though the key factors are now known. The accident occurred during an experiment to test the behaviour of an electrical system which powered the station in the event of a failure of the main electricity supply. In order to conduct the experiment, the reactor power was reduced which (possibly due to a problem in the operation of the automatic control rods) led to the reactor being in an unstable state39 and operating outside its design parameters.

At 01:23 on the morning of the 26th of April 1986, the experiment began, despite the fact that:

(i) The reactor power output was well below that required by the experi­mental procedure;

(ii) Certain reactor safety systems had been deliberately disabled in order to carry out the experiment; and

(iii) The number of control rods in the reactor was only half the minimum required for its safe operation.

Thirty seconds after the experiment began, the reactor power began to increase rapidly and ten seconds later the operators attempted a full emergency shut down by re-inserting the control rods. The reactor power was now increasing exponentially, leading to a failure in the pressurised cooling water system. Eight seconds later, the reactor exploded (an explosion of steam, not a nuclear explosion) scattering burning core debris over the surrounding area.

Over 100 firemen were called to the scene and they worked with plant personnel to put out many small fires in the reactor building and on the roofs of Unit 4 and the adjacent Unit 3 building. This work exposed the emergency workers to extremely high doses of radiation. During the days after the explosion, heli­copters were used to dump thousands of tonnes of various materials onto the exposed reactor core. These materials included boron, lead, sand and clay to smother the fire, absorb radiation and reduce nuclear reactions in the molten core material. In total, 1800 helicopter flights were made39 at great risk to the pilots. Despite the heroic efforts of firemen, helicopter pilots and many other emergency workers to put out the fire, the reactor continued to burn for ten days.