THERMODYNAMIC EVALUATION ON SOLIDIFICATION PATH FOR U-ZR-FE-O CORIUM

抄録

The analysis of small samples retrieved from the inside of the Primary Containment Vessel (PCV) of Units 1, 2, and 3 at the Fukushima Daiichi Nuclear Power Station (FDNPS) detected various types of U-bearing particles. Elucidation of the formation mechanism of these particles is expected to be a good practice of the real debris characterization. However, the current sample analysis could not accurately measure the oxygen concentration in the small particles, which is necessary for analyzing the debris formation mechanism. In this study, we attempted to analyze the solidification path of the particles by three-step thermodynamic approach. Step-1: The proper quasi-binary phase diagrams were drawn for the solidification path analysis. Step-2: The four prototypic solidification path patterns were identified based on the assessed phase diagrams. Step-3: The probable solidification paths of the U-bearing particles were discussed using these solidification path patterns and sample analysis results.

The following interesting tendencies of U-rearing particles were recognized: 1) Tetragonal phase, which was detected in all U-bearing particles in the sample analysis, is evaluated to stably exist only at around 1200 ℃ in the U-Zr-Fe-O system in the equilibrium condition. 2) Pattern II was reasonably identified as an only solidification path available for the Units 1 and 2 U-bearing particles, whereas pattern IV would be additionally possibly available for the Unit 3 particles.

From these thermodynamic analyzes, the following characteristic (or formation mechanism)s is speculated for these U-bearing particles:1) The debris in PCV are likely to have been solidified by gradual cooling from high temperatures (molten state) to intermediate temperatures (approximately at around 1200 °C) and solid-solid transition at lower temperatures may be limited. 2) Units 1 and 2 debris might be exposed to slightly hypo-stoichiometric conditions than Unit 3, and whereas Unit 3 debris might have a wider variation in the oxidation degree.