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Crystalline zircon (ZrSi0 4 ) have been discovered during detailed examination of the so called "Chernobyl lavas" in the destroyed 4th unit of the nuclear power plant. The zircon was formed during interaction between the melting nuclear fuel and construction materials (metallic zirconium, concrete, sand). The mineralogical characteristics and chemical compositions are being studied. It is shown that zircon has increased elementary cell parameters (A 0 = 6.617+0.002, C 0 = 5.990+0.002) and high contents of uranium (6.1 -12.9 percent). The morphologic relations between the zircon and other phases in the lavas including uranium oxides, zirconium oxides, and metallic zirconium are being studied. The principal mode of formation of the high uranium zircon is presented.

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The results of a detailed study of uranium phases in the matrix of fuel-containing masses (Chernobylite) and in "hot" particles formed during the accident at the 4th unit of the Chernobyl Nuclear Power Plant are reported here. The data are compared with those for uranium phases obtained experimentally under laboratory conditions. It is shown that an interaction between nuclear fuel and structural zirconium metal in the initial stage of the Chernobyl NPP accident took place before the explosion of the core. This process occurred during an extremely short time interval and was accompanied by formation of the phases Zr-U-Ο and UO x with a Zr impurity. Evidence is obtained that before the moment of explosion a temperature >2500-2600 °C was reached in a local section of the reactor, where the melting of the Zr-U-Ο phase and, partly, of the UO, with Zr impurity phase took place.

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