The basic phase and chemical composition of Chernobyl “lava” sample from the sub-reactor premise #305/2 that is located right below reactor base plate, and which is regarded as the primary melt source, are reported for the first time. The main substance of the “lava” is the X-ray amorphous glass-like material (in wt.%): Na2O 1.99, K2O 3.81, MgO 2.97, CaO 7.51, MnO 0.29, FeO 0.14, Al2O3 8.96, SiO2 64.15, TiO2 0.29, ZrO2 5.13, UO2 4.57, total 99.81). It contains small (5 to 30 μm) inclusions of technogenic analogues of natural minerals: high-uranium zircon, (Zr0.90U0.09Si1.01O4), U-bearing baddeleyite (Zr0.90U0.10O2) and Zr-bearing uraninite (U0.83Zr0.17O2). A technogenic analogue of vorlanite, (CaU6+O4), was found to be formed on the “lava” surface during storage at Khlopin Radium Institute under laboratory conditions, which shows the low chemical resistance of “lava” to environmental impact. The obtained results are highly important for assessing the consequences of the accident at the Fukushima Daiichi Nuclear Power Plant and modeling of the long-term environmental impact to the highly radioactive materials.