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.

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.

A summary of the results collected during the studies of the products of a chemical interaction between uranium oxide fuel and Zircaloy cladding in the Chernobyl accident is presented in this paper. The reaction products are mainly Zr-U-containing phases with different U/Zr ratio and are described on the basis of electron microprobe and X-ray diffraction (XRD) analyses. The Zr-U-bearing phases were discovered among the inclusions in different types of Chernobyl fuel-containing masses (”lava”) inside the destroyed 4th Unit and in hot particles collected up to 12 km from the 4th Unit along the West Plume. A correlation of data on the chemical composition and phase interrelations obtained in investigated samples with a phase diagram of Zr(O) - UO2 shows, that a temperature >1900 °C was reached in a part of the core before the explosion. The detection of hot particle with segregated morphology points out that liquid immiscibility existed between U-rich and Zr-rich melts. This and other observations indicate that the core temperature locally was above 2400–2600°C.

On April 25, 1986, the nuclear reactor Unit 4 (RBMK) at Chernobyl, Ukraine, exploded. Besides molecular species, the fallout contained particles of relatively high specific activity (hot particles) with a wide range of chemical compositions. The composition of a hot particle bears information about its genesis. Particle sizes ranged from a few to 100s of micrometers. Data on a hot particle, found in Berlin, Germany, is presented and discussed in context with earlier measurements on other particles to understand their genesis. The chemical composition was determined by electron probe micro analysis. Our particles are either reactor fuel (one) or fission product alloys (nine). The alloys were formed during normal reactor operation. Strongly varying concentrations of Fe and Ni suggest that at least some of our particles reacted with molten structural material of the reactor. The particles were mobilized by fuel oxidation or fuel dust generation during the accident. The fission product composition can only be explained if we assume that the alloys remained in the solid state in the course of the accident. Some particles may have been ejected during the explosion, others later while the reactor was burning. Activities (103Ru and 106Ru, originally up to 160,000 Bq) of our ten year old particles were re-measured but were no longer detectable. No long-lived γ-emitters were found. The 99Tc activity was calculated and found to only lBq. The γ -spectrum of the fuel particle still shows 137Cs (1 Bq) and 60Co (<1 Bq).

A special geochemical environment exists within the Shelter (”Sarcophagus”) erected in 1986 over the destroyed Unit-4 of Chernobyl nuclear power plant (NPP). Based upon the available in situ and compositional data, thermodynamic models of solid-aqueous interactions were developed to clarify the leaching behaviour of various materials within the Shelter. The “Selektor-A” code, based on a convex programming approach to Gibbs free energy minimization, was used for the calculations. A built-in flexible hybrid thermodynamic database for the system Na-K-Ca-Mg-Cl-S-N-H-O-Si-P-Fe-Al-Sr-Cs was extended with the critically selected and matched parameters for aqueous species and solid phases in the U-Zr-Si-O-H subsystem, secondary U-minerals, mineral phases of fully hydrated Portland cements and U-bearing zircons. Modeling results show that the “Shelter waters” can selectively leach a significant quantity of U and Si from the fuel-containing masses, while Zr, Fe, Ca, Mg and some other components are rather insoluble. Serpentinite, assemblages of fully-hydrated phases of Portland cements, and oxidation products of steel structural elements are estimated to be sufficiently stable in the aqueous environment of the Shelter. Our calculations also define some feasible pathways for secondary mineral formation from evaporation of Shelter water solutions and interactions between these waters with the mineral matter inside the Shelter.

The Shelter constructed above the destroyed Unit-4 of the Chernobyl NPP contains 20 MCi of nuclear fuel. More than 1000 m3 of intermediate-level radioactive water is disposed in its basement. The purpose of this work was to simulate migration of the radionuclides 90Sr and 137Cs from the Shelter into the subsurface environment to evaluate their migration rate and migration paths. A mathematical model accounting for the coupled transport of water and radionuclides in variably saturated media was used. The lack of suitable experimental and field studies excluded the possibility of complete validating the model. Results of the simulations will be useful for future field studies.

The morphology and composition both chemical and radionuclide of the main types of the solid-phase “hot” particles formed following the accident on the Chernobyl NPP have been studied by SEM, electron microprobe and gamma-spectrometry methods. Differences in many isotopes including: 106Ru, 134Cs, 137Cs dependent upon the hot particle matrix chemical composition was observed. The classification of hot particles based upon the chemical composition of their matrices has been done. It includes three main types: 1) fuel particles with UOx matrix; 2) fuel-constructional particles with Zr-U-0 matrix, 3) hot particles with metallic inclusions of Fe-Cr-Ni. Moreover, there are more rare types of hot particles with silicate or metal matrices. It was shown that only metallic inclusions of Fe-Cr-Ni are concentrators of 106Ru, which caused this nuclides assimilation in the molten stainless steel during the initial stages of the accident. Soils contamination of non-radioactive lead oxide particles in the Chernobyl NPP region were noticed. It was supposed that part of metallic lead, dropped from helicopters into burning reactor during first days of accident, was evaporated and oxidized accompanying solid oxide particles formation.