Thermodynamic evaluation on solidification path for U-Zr-Fe-O corium

Ohgi, Hiroshi  ; Nagae, Yuji ; Kurata, Masaki 

The analysis of small samples retrieved from the inside of Primary Containment Vessel (PCV) of Units 1, 2, and 3 at Fukushima Daiichi Nuclear Power Station detected various types of U-bearing particles. Elucidation of the formation mechanism of these particles is expected to be good practice for real debris characterization. In this study, we attempted to analyze the solidification path of U-bearing particles by the thermodynamic approach. From the thermodynamic solidification path patterns analysis, pattern II (at low oxidation condition) was identified as the available one for Units 1 & 2 particles, whereas pattern IV (at high oxidation condition) would be additionally possibly for Unit 3. From these thermodynamic analyzes, the following characteristic are speculated for the debris in PCV: 1) The debris are likely to have been solidified by gradual cooling from high temperatures to intermediate temperatures and solid-solid transition at lower temperatures may be limited. 2) Units 1 & 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.