A scheduled test of the turbine system at a nuclear power station in Chernobyl, Ukraine, on April 26th, 1986, ended tragically. From a low-power condition, the reactor’s power surged uncontrollably, rapidly boiling water and igniting a steam explosion that blew the reactor’s roof off.
It started a graphite fire that released hazardous radioactive materials into the air. The catastrophe claimed the lives of 31 persons, and it is believed that an additional 4,000 cancer deaths were brought on by the radiation that was discharged.
Operators at Chernobyl retracted the control rods all the way out of the core because Xenon built up due to a rapid shut-down primarily brought on by a problem with a nearby coal plant, only to notice power instabilities due to the inherent positive coolant void coefficient of the RBMK reactor and reinsert them.
The positive void coefficient allowed the reactor to undergo a runaway power excursion, which led to a catastrophic steam explosion and graphite fire that killed 30 workers and released radiation levels that were soon detected in Finland. However, the tips of the rods were made of graphite, which displaced water without introducing neutron poison, increasing power rather than decreasing it.
PIC: history.com, Newsweek, Eurochannel, Daily Express, National Geographic