A nuclear accident
that occurred on 26 April 1986. It is considered the worst nuclear disaster in history both in cost and casualties.
It is one of only two nuclear energy accidents rated at seven—the maximum severity—on the International Nuclear Event Scale
, the other being the 2011 Fukushima Daiichi nuclear disaster
The initial emergency response, together with later decontamination
of the environment, involved more than 500,000 personnel
and cost an estimated 18 billion Soviet rubles
—roughly US$68 billion in 2019, adjusted for inflation.
The accident occurred during a safety test on the steam turbine of an RBMK-type nuclear reactor
. During a planned decrease of reactor power in preparation for the test, the power output unexpectedly dropped to near-zero.
The operators were unable to restore the power level specified by the test program, which put the reactor in an unstable condition. This risk was not made evident in the operating instructions, so the operators proceeded with the test.
Upon test completion, the operators triggered a reactor shutdown. But a combination of operator negligence and critical design flaws had made the reactor primed to explode. Instead of shutting down, an uncontrolled nuclear chain reaction
began, releasing enormous amounts of energy.
The core melted down
and two or more explosions ruptured the reactor core and destroyed the reactor building. This was immediately followed by an open-air reactor core fire. It released considerable airborne radioactive contamination
for about nine days that precipitated onto parts of the USSR and Western Europe, before finally ending on 4 May 1986.
Some 70% of fallout landed in Belarus
, . About 49,000 people were evacuated from the area, primarily from Pripyat
The exclusion zone was later increased to 30 kilometres (19 mi) when a further 68,000 people were evacuated from the wider area, and later it became the Chernobyl Exclusion Zone
covering an area of approximately 2,600 km2(1,000 sq mi).
The reactor explosion killed two engineers and severely burned two more.. During the immediate emergency response 134 station staff and firemen were hospitalized with acute radiation syndrome
due to absorbing high doses of ionizing radiation
Chernobyl's health effects to the general population are uncertain. An excess of 15 childhood thyroid cancer
deaths were documented as of 2011.
Model predictions of the eventual total death toll in the coming decades vary.
The most robust studies predict 4,000 fatalities when solely assessing the three most contaminated former Soviet states, to about 9,000 to 16,000 fatalities when assessing the whole of Europe.