What went wrong?
The fourth reactor was shut down on April 25th, the day before the explosion, to perform routine maintenance and safety checks. During testing of the systems, operators deactivated critical automatic safety mechanisms. The reactor reached dangerous and unstable levels during testing, causing steam pressure to build by heating the cooling water. Operators recognized the danger and tried to correct their mistake, but it was too late – vital components were already critically damaged, and the steam pressure continued to rise until exploding and releasing fission products into the atmosphere. A second explosion, just seconds behind the first, released fuel channel fragments and hot graphite into the surrounding area. The cause of the second explosion is still debated by experts, but the most accepted culprit was the production of hydrogen from zirconium-steam reactions.
