The fuel pellets inside the fuel rods are almost entirely made of uranium-oxide while the encasing in which the pellets are placed is made of zirconium alloys. Part of the concrete was incorporated in the lava flow, explaining its high content of silicates, minerals composed mostly of silicon, aluminum and magnesium. When lava has low viscosity, it can flow very easily as demonstrated by stalactites hanging from valves and tubes in the destroyed reactor core. It was feared that the radioactive lava would burn through the containment structure and contaminate the groundwater. The lava-like material resulting from a nuclear meltdown is also named corium, after the core of the reactor.

Source: Forbes June 14, 2019 13:44 UTC