QUANTIFICATION OF HYPO EUTECTIC B-C-Fe-O UNDER SEVERE ACCIDENT CONDITIONS IN NUCLEAR MATERIAL BY EPMA

摘要

In the framework of a JAEA and CEA collaboration agreement, experimental and modelling studies have been focussed on the investigation of the solidification behaviour of a melt representative of the in-vessel conditions of Fukushima Dai'chi Unit 2 (1-F2). Boron was initially present in Unit 1-F2 under B_4C phase (control rod). During and after the accident, boron was able to react with the different oxide/metallic melts to form complex liquid mixtures and then solid phases after cooling (possible eutectic phase). The eventual presence of boron in the solid phases is a key point for the future corium extraction operations in Fukushima Daiechi 1-F2 NPP and for the material storage options, to accurately evaluate the potential risk of re-criticality whatever the configuration. This contribution focussed on the interaction between boron and steel. It concerned three interactive thermal tests of boron oxide and boron carbide samples with iron, in order to identify the Fe-C-B system formed at high temperature. Small-scale experiments were carried out at CEA Cadarache (PLINIUS/VITI) to characterise and model the solidification of the melt for materials containing B-C-Fe-O elements under different conditions representative of SA scenarios. Fe, B4C, Fe_2O_3, and/or B_2O_3 powders were heat-treated in an inductive furnace with the material compositions and temperature histories determined from the hypothetical scenario of the 1-F2 accident. Characterisation of the microstructure and distribution of boron in the solidified melt was then investigated using SEM/EDS with a field emission gun. The analyses of eutectic compositions in the B-C-Fe-0 system were carried out by EPMA /WDS. XRD was used to allow and confirm the potential phases. EPMA/WDS was performed taking into account the chemical shift of boron and carbon in the different states of the material detected by using LPC3 and PC2 crystal. The results indicated that the final solid is separated into metal phases (Fig. 1) based on Fe with borides or boron-carbides compounds or solid solutions, and a vitrified part with oxide phase formed from B_2O_3. The following solid phase have been identified by SEM/EPMA: Fe + Fe_2B and eutectic formation as Fe_3(B,C), Fe_(23)(B,C)_6 included in the metal matrix. The oxide part, which was a vitrified solid based on XRD spectra, was partially contaminated by the crucible material in alumina, consisting of mixed oxides of Al_2O_3-B_2O_3-FeO_x compositions and small precipitates of pure Fe. XRD confirmed the presence of a metallic phase (cubic iron with dendritic microstructure), a tetragonal Fe_2B and a cubic Fe_(23)(C,B)_6 with eutectic microstructure in agreement with SEM/EPMA. A measurement of the amorphous structure was observed at the low angle may be the result of the BO_(1.5)-rich oxide phase measured by EPMA. The identification of the formed solids is also important because it will play an important role for the decommissioning of Fukushima Daiichi reactors.