The specter of self-sustaining fission, or criticality, in the nuclear ruins has long haunted Chernobyl. When part of the Unit Four reactor’s core melted down on 26 April 1986, uranium fuel rods, their zirconium cladding, graphite control rods, and sand dumped on the core to try to extinguish the fire melted together into a lava. It flowed into the reactor hall’s basement rooms and hardened into formations called fuel-containing materials (FCMs), which are laden with about 170 tons of irradiated uranium—95% of the original fuel.
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Funny interview with Gorbachev recounts how in the hours after the disaster he couldn't get a straight answer from Chernobyl of the exact nature of the accident. He finally got his answer when Sweden called to complain about a horrendously radioactive cloud originating within northern Ukraine passing over their airspace. Hundreds of thousands of Soviet military were exposed to lethal doses of radiation in the attempt to contain the radiation. For example 600 helicopter pilots who were dumping sand and lead on the smoldering fuel pile all died quite rapidly.