<P> The remains of the No. 4 reactor building were enclosed in a large cover which was named the "Object Shelter". It is often known as the sarcophagus, its purpose is to reduce the spread of radioactivity from the wreckage and to protect the wreckage from the elements . It was not intended as a (radiation shield), it finished by December 1986, at a time when what was left of the reactor was entering the cold shut - down phase; the enclosure was built quickly as occupational safety for the crews of the other undamaged reactors at the power station, with No. 3 continuing to produce electricity into 2000 . </P> <P> The accident motivated safety upgrades on all remaining Soviet - designed reactors in the RBMK (Chernobyl No. 4) family, of which eleven continued to power electric grids as of 2013 . </P> <P> The disaster began during a systems test on 26 April 1986 at reactor 4 of the Chernobyl plant near Pripyat and in proximity to the administrative border with Belarus and the Dnieper River . There was a sudden and unexpected power surge . When an emergency shutdown was attempted, a much larger spike in power output occurred . The second spike led to a reactor vessel rupture and a series of steam explosions . These events exposed the graphite moderator of the reactor to air, causing it to ignite . The resulting fire sent week long plumes of highly radioactive fallout into the atmosphere over an extensive geographical area, including Pripyat . The plumes drifted over large parts of the western Soviet Union and Europe . According to official post-Soviet data, about 60% of the fallout landed in Belarus . </P> <P> 36 hours after the accident Soviet officials enacted a 10 - kilometre exclusion zone which resulted in the rapid evacuation of 49,000 people and their animals primarily from the town of Pripyat, the nearest large population centre . Although not communicated at the time, an immediate evacuation of the town following the accident was not advisable as the road leading out of the town had heavy nuclear fallout hotspots deposited on it . Up to around that point, the town itself was comparatively safe due to the more favourable wind direction . Shelter in place was to be the best safety measure to take for the town, before the winds began to change direction . </P>

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