Various types of Chernobyl fuel containing masses named black “lava”, brown“lava”, porous “ceramic” and “hot” particles that formed during first daysof the accident at the Chernobyl Nuclear Power Plant 4th Unit were studiedby methods of optical and electron microscopy, microprobe and x-raydiffraction. Data about their chemical, phase and radionuclide compositionare summarized. The products of interaction between fuel, zircaloy andconcrete, produced under experiments in laboratory were examined forcomparison with samples of Chernobyl “lava” and “hot” particles. Thebehavior of nuclear fuel in first days of the Chernobyl accident was athree-stage process. The first stage occurred before the moment of theChernobyl explosion and was exceptionally short-lasting, perhaps, less thana 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 localpart of the core, which is estimated to be not more than 30% of whole corevolume. The second stage lasted for about 6 days since the explosion, duringwhich there was interaction between uranium products of the destroyedreactor: UOx, UOx with Zr, Zr-U-O, with the environment and silicatestructural materials of the 4th Unit. The third stage, after 6 days involvedthe process of final formation of the radioactive silicate melt or Chernobyl“lava” at one of the sections of the destroyed 4th Unit. During this stagethe 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 mineralsof uranium identified are: UO4·4H2O studtite; UO3·2H2O epiianthinite; UO2·CO3 rutherfordine; also, Na4(UO2)(CO3)3 was identifiedtogether with the sodium carbonate phases Na3H(CO3)2·2H2O and Na2CO3·H2O. These minerals formed due tothe interaction between fuel-containing masses or “lava”, water and air. Thematrices of the “lava” do not contain significant amounts of sodium. Thesource of sodium may be water that has penetrated into the “Sarcophagus”.All identified secondary minerals of uranium are highly unstable, and theircontinued formation can seriously endanger the radiological situation of the 4th Unit.

A summary of the results collected during the studies of the products of achemical interaction between uranium oxide fuel and Zircaloy cladding in theChernobyl accident is presented in this paper. The reaction products aremainly Zr-U-containing phases with different U/Zr ratio and are described onthe basis of electron microprobe and X-ray diffraction (XRD) analyses. TheZr-U-bearing phases were discovered among the inclusions in different typesof Chernobyl fuel-containing masses (”lava”) inside the destroyed 4th Unitand in hot particles collected up to 12 km from the 4th Unit along the WestPlume. A correlation of data on the chemical composition and phaseinterrelations obtained in investigated samples with a phase diagram ofZr(O) - UO2 shows, that a temperature >1900 °C was reached ina part of the core before the explosion. The detection of hot particle withsegregated morphology points out that liquid immiscibility existed betweenU-rich and Zr-rich melts. This and other observations indicate that the coretemperature 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 ofrelatively high specific activity (hot particles) with a wide range ofchemical compositions. The composition of a hot particle bears informationabout its genesis. Particle sizes ranged from a few to 100s of micrometers.Data on a hot particle, found in Berlin, Germany, is presented and discussedin context with earlier measurements on other particles to understand theirgenesis. The chemical composition was determined by electron probe microanalysis. Our particles are either reactor fuel (one) or fission productalloys (nine). The alloys were formed during normal reactor operation.Strongly varying concentrations of Fe and Ni suggest that at least some ofour particles reacted with molten structural material of the reactor. Theparticles were mobilized by fuel oxidation or fuel dust generation duringthe accident. The fission product composition can only be explained if weassume that the alloys remained in the solid state in the course of theaccident. Some particles may have been ejected during the explosion, otherslater while the reactor was burning. Activities (103Ru and 106Ru, originally up to 160,000 Bq) of our ten year oldparticles were re-measured but were no longer detectable. No long-livedγ-emitters were found. The 99Tc activity was calculated and foundto only lBq. The γ -spectrum of the fuel particle still shows137Cs (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 compositionaldata, thermodynamic models of solid-aqueous interactions were developed toclarify the leaching behaviour of various materials within the Shelter. The“Selektor-A” code, based on a convex programming approach to Gibbs freeenergy minimization, was used for the calculations. A built-in flexiblehybrid thermodynamic database for the systemNa-K-Ca-Mg-Cl-S-N-H-O-Si-P-Fe-Al-Sr-Cs was extended with the criticallyselected and matched parameters for aqueous species and solid phases in theU-Zr-Si-O-H subsystem, secondary U-minerals, mineral phases of fullyhydrated Portland cements and U-bearing zircons. Modeling results show thatthe “Shelter waters” can selectively leach a significant quantity of U andSi from the fuel-containing masses, while Zr, Fe, Ca, Mg and some othercomponents are rather insoluble. Serpentinite, assemblages of fully-hydratedphases of Portland cements, and oxidation products of steel structuralelements are estimated to be sufficiently stable in the aqueous environmentof the Shelter. Our calculations also define some feasible pathways forsecondary mineral formation from evaporation of Shelter water solutions andinteractions between these waters with the mineral matter inside theShelter.

The Shelter constructed above the destroyed Unit-4 of the Chernobyl NPPcontains 20 MCi of nuclear fuel. More than 1000 m3 ofintermediate-level radioactive water is disposed in its basement. Thepurpose of this work was to simulate migration of the radionuclides 90Sr and 137Cs from the Shelter into the subsurfaceenvironment to evaluate their migration rate and migration paths. Amathematical model accounting for the coupled transport of water andradionuclides in variably saturated media was used. The lack of suitableexperimental and field studies excluded the possibility of completevalidating the model. Results of the simulations will be useful for futurefield studies.

The morphology and composition both chemical and radionuclide of the maintypes of the solid-phase “hot” particles formed following the accident onthe Chernobyl NPP have been studied by SEM, electron microprobe andgamma-spectrometry methods. Differences in many isotopes including: 106Ru, 134Cs, 137Cs dependent upon thehot particle matrix chemical composition was observed. The classification ofhot particles based upon the chemical composition of their matrices has beendone. It includes three main types: 1) fuel particles with UOxmatrix; 2) fuel-constructional particles with Zr-U-0 matrix, 3) hotparticles with metallic inclusions of Fe-Cr-Ni. Moreover, there are morerare types of hot particles with silicate or metal matrices. It was shownthat only metallic inclusions of Fe-Cr-Ni are concentrators of 106Ru, which caused this nuclides assimilation in the moltenstainless steel during the initial stages of the accident. Soilscontamination of non-radioactive lead oxide particles in the Chernobyl NPPregion were noticed. It was supposed that part of metallic lead, droppedfrom helicopters into burning reactor during first days of accident, wasevaporated and oxidized accompanying solid oxide particles formation.