Products of molten corium-metal interaction in chernobyl accident: Composition and leaching of radionuclides

Abstract

During initial stages of severe nuclear accident at Chernobyl NPP on 26 April 1986 interaction between U-oxide fuel and Zr-cladding has occurred and a corium melt (Zr–U–O) formed in localized part(s) of reactor core. In this paper, we describe the properties of unique set of samples formed during interaction of the corium melt with steel in the core that was expelled from the reactor shaft into reactor premises and subsequently oxidized. These samples primarily consist of Fe–Cr oxides with inclusions of U- and Zr-based oxides. Uranium is heavily oxidized, forming U4O9 and U3O8. Nb/Zr ratio of the inclusions and presence of substantial Ni in the oxide matrix permitted us to identify precursor materials of the studied samples. The studied material was formed in two principal stages: interaction of liquefied fuel with Zr from walls of technological channels and/or guide rod, and subsequent reaction with steel parts located in a very well-defined part of the core – the bottom part of the guide rod and/or graphite/guide rod socket on the reactor base plate. The differences in texture between the samples are ascribed to a cooling regime and substrate on which the ejected melts were cooled. Submicron particles of fission products (Pd, Ru, Tc, etc.), very rare for Chernobyl samples, are scattered within the Fe–Cr oxide matrix. Their preservation was the result of highly oxidative conditions. Leaching of 137Cs and Am in hot aqueous fresh and saline solutions was studied. High leach rates of these nuclides is ascribed to highly oxidized state of U-containing phases. Results of the current study are compared with experiments on molten corium – vessel steel interaction and provide new details on progression of the Chernobyl NPP core destruction shortly before and during the explosion.