Characterization of black and brown Chernobyl “lava” matrices: The formation process reviewed

Abstract

Detailed understanding about the chemical and physical properties of the Chernobyl “lava” and other radioactive meltdown products is of paramount importance for the support of decommissioning operations and nuclear accident modelling. In this study, we provide new results about the chemical composition and structural properties of the Chernobyl “lava” matrix obtained by electron microprobe analysis and confocal Raman spectroscopy. Based on the compositional data, a principal component analysis (PCA) was conducted to visualize compositional features of the black and brown “lava”, considering data from previous studies. In addition, an inverse modelling approach was performed to assess fractional contributions of construction materials that potentially contributed to the “lava” formation process. The results of the PCA show three varieties of “lava”. Different fractional contributions of UO2-fuel and Zr-cladding indicate the formation of at least two distinct sources of coium melt, respectively, for the black and brown “lava”. For the brown “lava”, the high concentration of Mg is explained by the melting and assimilation of 23% serpentine stemming from the lower reactor shield. The black “lava” shows a high contribution of concrete (43%). Significant differences in the Fe concentration of the black “lava”, as well as macroscopic flow patterns are indicative for a progressive melt formation. However, the cooling of the “lava” occurred relatively fast, forming a metaluminous glass with local variations in the degree of polymerization. Sub-microscopic inclusions of (U1-xZrx)O2 and (Zr1-xUx)O2 solid solutions point to a fractionation of U from an highly oversaturated melt. Based on the new results, the current hypothesis about the “lava” formation process is discussed and reviewed, questioning the existence of one homogenous source of melt and its stratification into layers of black and brown “lava”, before spreading and solidification of the melt.