The formation of uranium minerals is still continuing in Chernobyl Unit No. 4. Yellow products of alteration that stain the surface of Chernobyl “lava” have been examined by SEM and X-ray diffraction methods. Secondary minerals of uranium identified are: UO4·4H2O studtite; UO3·2H2O epiianthinite; UO2·CO3 rutherfordine; also, Na4(UO2)(CO3)3 was identified together with the sodium carbonate phases Na3H(CO3)2·2H2O and Na2CO3·H2O. These minerals formed due to the interaction between fuel-containing masses or “lava”, water and air. The matrices of the “lava” do not contain significant amounts of sodium. The source of sodium may be water that has penetrated into the “Sarcophagus”. All identified secondary minerals of uranium are highly unstable, and their continued formation can seriously endanger the radiological situation of the 4th Unit.

Various types of Chernobyl fuel containing masses named black “lava”, brown “lava”, porous “ceramic” and “hot” particles that formed during first days of the accident at the Chernobyl Nuclear Power Plant 4th Unit were studied by methods of optical and electron microscopy, microprobe and x-ray diffraction. Data about their chemical, phase and radionuclide composition are summarized. The products of interaction between fuel, zircaloy and concrete, produced under experiments in laboratory were examined for comparison with samples of Chernobyl “lava” and “hot” particles. The behavior of nuclear fuel in first days of the Chernobyl accident was a three-stage process. The first stage occurred before the moment of the Chernobyl explosion and was exceptionally short-lasting, perhaps, less than a few seconds. It was characterized by reaching a high temperature, ≥2600 °C, in the epicenter of accident and formation of a Zr-U-O melt in a local part of the core, which is estimated to be not more than 30% of whole core volume. The second stage lasted for about 6 days since the explosion, during which there was interaction between uranium products of the destroyed reactor: UOx, UOx with Zr, Zr-U-O, with the environment and silicate structural materials of the 4th Unit. The third stage, after 6 days involved the process of final formation of the radioactive silicate melt or Chernobyl “lava” at one of the sections of the destroyed 4th Unit. During this stage the melt's lamination occurred, followed by a break-through of the “lava” reservoir on the 11 th day of the accident and penetration of the “lava” into space under the reactor.

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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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