Forensic study of early stages of the Chernobyl accident: Story of three hot particles

Abstract Three contrasting hot particles ejected from the core of the 4th Unit of Chernobyl nuclear power plant at an early stage of Chernobyl accident were studied using complementary analytical methods: including γ-spectrometry, SEM-E(W)DX, EBSD, Raman spectroscopy and Secondary Ions Mass Spectrometry. The particles span range from dispersed UO2+x fuel to a fragment of zirconia with traces of U and to chemically and structurally complex Zr U O particle. These particles represent wide variety of processes in the reactor during the accident development and likely originate in spatially distinct domains. Whereas the fuel particle is virtually unaltered, the zirconia particle records interaction of zircaloy with fuel and structural steel, albeit rather short one. Finally, the last particle comprises remarkable mix of various phases and was most likely formed at an advanced stage of the accident when significant interaction of the fuel with surrounding materials and eventual displacement of the reaction products took place. This particle alone reproduces rather wide range of interactions encountered during various in-pile experiments. Ubiquitous presence of Fe in the Zr U O phases reveals interaction of spacer grids with Zr U O melt. Conditions leading to formation of these particles at early stages of the accident are discussed.