Interaction of UO₂ and Zircaloy During the Chernobyl Accident

Abstract

A summary of the results collected during the studies of the products of a chemical interaction between uranium oxide fuel and Zircaloy cladding in the Chernobyl accident is presented in this paper. The reaction products are mainly Zr-U-containing phases with different U/Zr ratio and are described on the basis of electron microprobe and X-ray diffraction (XRD) analyses. The Zr-U-bearing phases were discovered among the inclusions in different types of Chernobyl fuel-containing masses (”lava”) inside the destroyed 4th Unit and in hot particles collected up to 12 km from the 4th Unit along the West Plume. A correlation of data on the chemical composition and phase interrelations obtained in investigated samples with a phase diagram of Zr(O) - UO2 shows, that a temperature >1900 °C was reached in a part of the core before the explosion. The detection of hot particle with segregated morphology points out that liquid immiscibility existed between U-rich and Zr-rich melts. This and other observations indicate that the core temperature locally was above 2400–2600°C.

Experimental Approaches to the Thermodynamics of Ceramics Above 1500°C

“…Interaction of uranium oxide with zirconium metal is of interest for modeling of severe nuclear reactor accidents. The Zr–U–O phase diagram was studied extensively and was used to reconstruct conditions during accidents at Three Mile Island and Chernobyl reactors.…”

“…Pseudo‐binary Zr(O) – UO 2 phase diagram. Top: from Skokan with U/Zr ratio in particles from Chernobyl accident . Bottom: from experiments of UO 2 dissolution in Zircaloy‐4.…”

“…4) display unmistakable evidence of liquid immiscibility in UO 2 -Zr melt. 84 The particles were formed by containerless quenching in air of the melt formed by irradiated fuel with irradiated cladding material-conditions not yet approached by laboratory experiments.…”

Synthesis, characterisation and corrosion behaviour of simulant Chernobyl nuclear meltdown materials

“…During the meltdown of the reactor core, temperatures reached in excess of 1600°C, which caused the uranium nuclear fuel to melt with the zirconium cladding. 1,2 This molten material interacted with structural reactor building materials, including steel, concrete, serpentine and sand, forming~100 tons of a glasslike lava that was transported under its own weight to sub-reactor rooms, solidifying in large masses, such as the widely reported "elephants foot". 1,[3][4][5] Known as lava-like fuel-containing materials (LFCM), these highly crystalline, radioactive, glass-like slags, were found in two main forms, known as Brown and Black lavas.…”

“…While analysis of samples taken from Chernobyl LFCM was performed over 20 years ago, [1][2][3][4][5][6][7][10][11][12][13][14][15][16] the highly radioactive nature of the materials limited the types of characterisation performed. Samples of the material are limited due to the difficulties and dangers associated with their collection (unstable building, high radiation levels), and only a few specimens of the lava that were sampled from within the reactor building have been studied.…”