Phase composition of lava-like fuel-containing materials of Unit 4 of the Chornobyl NPP. Black ceramics

More than three decades after the accident, the uranium-containing crystalline phase Rameauite K2Ca[(UO2)3O3(OH)2]2·6(H2O) and crystalline phases that do not contain uranium U: magnesia MgO, Katoite Ca2.916Al2(SiO4)1.104(O4H4)1.89, calcium magnesium silicate CaMgSiO4, magnesium aluminate MgAl2O4, and magnesium metasilicates MgSiO3 and Mg2SiO4 were found in the black ceramics of lava-like fuel-containing materials (LFCM) of the Unit 4 of the Chornobyl nuclear power plant. The presence of urania UO2.338(U4O9), chornobylite Zr1−xUxSiO4 and, possibly, iron α−Fe (ferrite) have been confirmed. Most of the uranium in the crystalline phases is in the form of Uranyl-oxide hydroxy-hydrate, Rameauite K2Ca[(UO2)3O3(OH)2]2·6(H2O), rather than urania UO2.338(U4O9). For the first time, the phases of black ceramics are divided into three groups according to their ”origin”: before, during and after the accident. Part of urania UO2.338(U4O9) and iron α−Fe (ferrite) was formed before the accident. During the accident, a new part of urania UO2.338(U4O9), magnesia MgO were formed, and also chornobylite Zr1−xUxSiO4 was synthesized. Calcium magnesium silicate CaMgSiO4, magnesm aluminate MgAl2O4, and magnesium metasilicates MgSiO3 and Mg2SiO4 apparently began to form as a result of lava crystallization during its cooling and continued this process after the accident. Rameauite K2Ca[(UO2)3O3(OH)2]2·6(H2O) and Katoite Ca2.916Al2(SiO4)1.104(O4H4)1.89 were formed after the accident. It is shown that for 36 years after the accident, a number of physical and chemical processes took place and are currently taking place in LFCM black ceramics: urania oxidation, formation of Rameauite K2Ca[(UO2)3O3(OH)2]2·6(H2O) and Katoite Ca2.916Al2(SiO4)1.104(O4H4)1.89 and glass phase crystallization. The studies used the methods of electron microscopy and X-ray phase analysis in combination with a new method of processing X-ray diffraction data for multiphase materials with a low content of phases. The results obtained will be used to develop methods and technologies for management and predicting the behavior of fuel-containing materials resulting from accidents at nuclear power plants in Chornobyl and Fukushima.