Physical and chemical properties of fuel-containing materials in the "Ukryttia" facility

Our investigations in this field were initiated in 1993 by Prof. I.R.Yukhnovskii. Two major problems to be explored were formulated: the first one was dedicated to the processes of radionuclide migration in soils and subsoil water of Chornobyl’s zone, and the second one to the interactions of water and aqueous solutions with fuel-containing materials (FCM) in the Chornobyl’s Shelter ('Ukryttia' facility). Initially, the investigations were performed within a joint department of ICMP and 'Ukryttia' ('Shelter') International R&D Center of NASU (Head of Department: M.V.Tokarchuk). Later we were joined by other research groups, including those of Profs. I.V.Stasyuk, M.F.Holovko, I.M.Mryglod. The studies were carried out in a close collaboration with scientists of 'Ukryttia' ('Shelter') International R&D Center of NASU (now Institute for Safety Problems of Nuclear Power Plants, O.V.Zhidkov’s group), Institute of Physics of NASU (A.G.Naumovets’ and I.M. Yakovkin’s group), Institute for Nuclear Research of NASU (V.I.Sugakov’s and V.M. Pavlovich’s groups), Institute of Theoretical Physics of NASU (A.G. Zagorodnii’s group), Institute of Surface Chemistry of NASU (V.M.Mishchenko’s group), Center of Mathematical Modelling within Ya. S. Pidstryhach Institute of Applied Problems of Mechanics and Mathematics (Ye.Ya.Chaplia’s group), Ivan Franko Lviv National University (A.S.Voloshinovskii’s group), Lviv National Polytechnic University (I.M.Krip’s group), Odessa National University (G.S.Drahan’s group), SSE “TechnoCenter" (S.B. Kumshaev’s group). A series of most important problems was formulated; those included the studies of radiation stability of FCM; mechanical, dielectric, and magnetic properties of lava-like fuel-containing materials (LFCM); phenomena of dust formation and dynamics of submicronic radioactive dust; oxidation processes and interaction of FCM with aqueous solutions, etc. These studies stimulated a series of important experiments, which allowed to establish the nuclear physical and chemical characteristics of different modifications of residual nuclear fuel remaining in the destroyed block of Chornobyl’ Nuclear Power Plant and formulate recommendations concerning their handling at different stages of transforming the 'Ukryttia' facility into an environmentally safe system.

Amongst our major achievements in this field are the following.

• Several models of LFCM have been developed and tested, which allow one to explain the experimental behavior of their major physical (dielectric, mechanical, magnetic) properties and take into account the factor of radiation defect formation as a major reason of their changes in time. They form a basis, on which a complex model for the fuel-containing material behavior, suitable for the long-term forecast, can be constructed.
• Importance of water in the 'Ukryttia' facility has been indicated. The obtained results allowed us to establish a series of factors (radionuclide leaching, complex formation in aqueous solutions and precipitations, destructive role of water at interactions with the LFCM surface, adsorption phenomena, etc) that should be taken into account in the forecast model to be constructed and at monitoring in the 'Ukryttia' facility. The possible role of water even increases, if the molecular-sieve properties of the LFCM are taken into account.
• Preliminary studies of LFCM stability were performed, with taking into account the factor of their internal self-irradiation. Major mechanisms and factors of LFCM degradation were analyzed. A time to start of spontaneous bulk self-destruction of these materials has been estimated; it has been shown that this quantity had been previously overestimated by a few orders of magnitude: the intensity of LFCM self-destruction will essentially increase in the nearest years.
• A method for calculations of submicronic dust particle dynamics in air, the corresponding algorithms and programs, suitable for making long-term forecasts, have been developed. It has been shown that submicronic dust particles before their precipitation can stay in the atmosphere for years and travel by thousands kilometers. The other factor, stimulating more intensive studies of submicronic particles containing radioactive elements behavior in water, is related to their established influence on the local chemical properties via the hydrolysis effects that changes the conformational properties of biologically active molecules and explains the toxicity mechanism of such particles.
• Mechanisms of spontaneous generation of submicronic particles from the LFCM surface were explored. In the framework of the cellular automata model, the processes of self-destruction of solid materials, containing the radioactive fuel, were studied. A series of characteristics was obtained, applicable for description of material surface roughness and dynamics of surface degradation. In the framework of this approach we reproduced the experimentally obtained histograms of the size distribution of dust particles and explained their major peculiarities. The risk factors associated with the possibility of LFCM bulk self-destruction and related to it sharp increase of dust generation intensity were indicated.

Several new perspective approaches to the problem of FCM monitoring and other technology problems have been proposed. Those are, in particular: monitoring of concentration and distribution of submicronic dust particles in the 'Ukryttia' facility using optical diffraction methods; controlled leaching of radioactive elements by alkaline aqueous solutions; methods for improvement of dust collection schemes; utilization of efficient sorption materials, synthesized using the modified natural carbonate-containing clays of Yaniv’ (Lviv region) sulfur deposits for radionuclide elimination from the aqueous solutions, etc.