The Chernobyl nuclear accident that occurred on 26 April 1986, at the Chernobyl Nuclear Power Plant in Ukraine (the Ukrainian Soviet Socialist Republic then, part of the Soviet Union). It is considered the worst nuclear power plant accident in history and is the only level 7 event on the International Nuclear Event Scale.

The Chernobyl nuclear accident began on 26 April 1986, at reactor number four at the Chernobyl plant, near the town of Pripyat in the Ukrainian Soviet Socialist Republic, during a systems test. A sudden power output surge took place, and when an attempt was made for emergency shutdown, a more extreme spike in power output occurred which led to a reactor vessel rupture and a series of explosions. This event exposed the graphite moderator components of the reactor to air and they ignited; the resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union. Large areas in Ukraine, Belarus, and Russia had to be evacuated, with over 336,000 people resettled. According to official post-Soviet data, about 60% of the fallout landed in Belarus.

Despite the accident, Ukraine continued to operate the remaining reactors at Chernobyl for many years. The last reactor at the site was closed down in 2000, 14 years after the accident.

The accident raised concerns about the safety of the Soviet nuclear power industry as well as nuclear power in general, slowing its expansion for a number of years while forcing the Soviet government to become less secretive about its procedures.

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. Fifty deaths, all among the reactor staff and emergency workers, are directly attributed to the accident. It is estimated that there may ultimately be a total of 4,000 deaths attributable to the accident, due to increased cancer risk.

On 26 April 1986, at 01:23 a.m. (UTC+3), reactor four suffered a catastrophic power increase, leading to explosions in the core. This dispersed large quantities of radioactive fuel and core materials into the atmosphere and ignited the combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke, as the reactor had not been contained by any kind of hard containment vessel (unlike all Western plants). The accident occurred during an experiment scheduled to test a potential safety emergency core cooling feature, which took place during the normal shutdown procedure.

Nuclear power reactors require cooling, typically provided by coolant flow, to remove decay heat, even when not actively generating power. Pressurized Water Reactors use water flow at high pressure to move waste heat. Once the reactor is scrammed, the core still generates a significant amount of residual heat, which is initially about seven percent of the total thermal output of the plant. If not removed by coolant systems, the heat could lead to core damage.

Following an emergency shutdown (scram), reactor cooling is still required to keep the temperature in the reactor core low enough to avoid fuel damage. The reactor consisted of about 1,600 individual fuel channels, and each operational channel required a flow of 28 metric tons (28,000 liters (7,400 USgal)) of water per hour. There had been concerns that in the event of a power grid failure, external power would not have been immediately available to run the plant's cooling water pumps. Chernobyl's reactors had three backup diesel generators. Each generator required 15 seconds to start up but took 60–75 seconds to attain full speed and reach the capacity of 5.5 MW required to run one main cooling water pump.

This one-minute power gap was considered unacceptable, and it had been suggested that the mechanical energy (rotational momentum) of the steam turbine could be used to generate electricity to run the main cooling water pumps while the turbine was still spinning down. In theory, analyses indicated that this residual momentum had the potential to provide power for 45 seconds, which would bridge the power gap between the onset of the external power failure and the full availability of electric power from the emergency diesel generators. This capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 showed that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field during the spin-down. The system was modified, and in 1984 the test was repeated, but again proved unsuccessful. In 1985 the tests were attempted a third time, but also yielded negative results. The test procedure was to be repeated again in 1986, and scheduled to take place during the maintenance shutdown of Reactor Four.

The test focused on the switching sequences of the electrical supplies for the reactor. Since the test procedure was to begin when the reactor was scrammed automatically at the very beginning of the experiment, it was not anticipated to have any detrimental effect on the safety of the reactor; so the test program was not formally coordinated with either the chief designer of the reactor (NIKIET) or the scientific manager. Instead, it was approved only by the director of the plant (and even this approval was not consistent with established procedures). According to the test parameters, at the start of the experiment the thermal output of the reactor should have been no lower than 700 MW. If test conditions had been as planned the procedure would almost certainly have been carried out safely; the eventual disaster resulted from attempts to boost the reactor output once the experiment had been started, which was inconsistent with approved procedure.

The Chernobyl power plant had been in operation for two years without the capability to ride through the first 60–75 seconds of a total loss of electric power—an important safety feature. The station managers presumably wished to correct this at the first opportunity; which may explain why they continued the test even when serious problems arose, and why the requisite approval for the test was not sought from the Soviet nuclear oversight regulator (even though there was a representative at the complex of 4 reactors).

The experimental procedure was intended to run as follows:

• the reactor was to be running at a low power, between >700 MW & 800 MW
• the steam turbine was to be run up to full speed
• when these conditions were achieved, the steam supply was to be closed off
• the turbines would be allowed to freewheel down
• generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps

Chernobyl Conditions prior to the Accident
Chernobyl Experiment and Explosion
Radiation Levels of Chernobyl Nuclear Accident
Causes of Chernobyl Nuclear Accident
Chernobyl Nuclear Accident Effects
Chernobyl Recovery Process