Study of mineral grains extracted from the Chernobyl “lava”

Results of a spectroscopic and microstructural investigation of mineral grains extracted from lava-like fuel-containing materials – Chernobyl “lava” – are presented. Raman, photoluminescence and infrared spectra reveal the presence of abundant U4+, traces of Sm3+ and Cr3+ in grains of U-containing zircon and ZrO2. No oxidation of UO2 particles is observed. The ZrO2 grains consist predominantly of the monoclinic polymorph and are often twinned. Raman spectra indicate the presence of the tetragonal polymorph, which is likely stabilized by the presence of U. It is suggested that spontaneous transformation of partially stabilized tetragonal (Zr,U)O2 to the monoclinic phase, and a corresponding volume expansion, are important factors affecting the rate of mechanical degradation of the “lava”. Transmission electron microscopy of U-rich zircon suggests the absence of precipitation of any independent U-containing phases, implying that U is fully incorporated within the zircon lattice as an impurity. Grains of rutile, corundum, quartz and natural zircon originating from construction materials of the reactor are described. Results obtained here are important for the validation of computer codes describing severe nuclear accidents.