What happened
at Chernobyl?
The world’s most devastating nuclear accident happened at Chernobyl, in
the Ukraine, in 1986. The nuclear reactor blew up releasing about 7
tonnes of radioactive material, into the atmosphere over the course of a
number of days. The contamination of the surrounding areas was
widespread and the associated problems could persist for a long time.
More than thirty died in the clean-up from burns and radiation. Many
others have become ill gradually over time.
What went wrong?
The Chernobyl reactor was a graphite-moderated water-cooled
reactor. The fuel used was uranium dioxide enriched to 2% with
uranium-235. It was decided to test certain aspects of the production
system. Similar tests had been done successfully on two previous
occasions but some electrical changes had been made since the last test.
As part of the test the emergency cooling
system was switched off breaching safety regulations.
When the test on the generator started insufficient power was delivered
to the pumps that circulate the coolant. Without the backup of the
emergency cooling system, the reactor quickly overheated.
What happened then?
The heat output soared from 6% to 50% in about 10 seconds. The control
rods could not be replaced in time. The fuel rods burst and the fuel
mixed with the water which, turned to steam and could therefore no
longer cool anything. At this stage the whole reactor building blew up
and dispersed hot radioactive material through the surrounding area
causing more than 30 fires, most of which were brought under control in
a matter of hours. However it took 10 days to get the fire in the core
of the reactor under control. This was achieved by dropping thousans of
tonnes of lead and dolomite on the reactor by military helicopter.
What was
the human and material impact?
The discharge included many short-lived radioisotopes including iodine
and caesium. Radioactive material from the plant was subsequently
detected over practically the entire Northern Hemisphere. When the fires
died down the reactor was encased in concrete. An ‘exclusion zone’,
initially some 30 kilometres in radius was established around the site
and about 116 000 people within it were evacuated to less contaminated
areas in the months following the accident. The exclusion zone was later
extended to cover an area of 4300 square kilometres. In the eight years
following the accident, more than 300 people suffered radiation
sickness, and there has been an increase in the number of thyroid
cancers in neighbouring regions, which may be related.
The future
Experts from all over the world have examined the Chernobyl incident in
detail so as to ensure that an accident like it will never happen again.
The reactor’s remains are now enclosed within a large concrete
structure.
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