Mechanism of fuel release to coolant from breached oxide fuel in liquid metal fast reactor

Many results of run-beyond-cladding-breach (RBCB) testings in fast reactor have shown negligible fuel release to coolant. However, there are several RBCB testing results showing significant magnitudes (0.2-20g) of fuel loss. The mechanism of it has not yet been clear. In this study, detail data review have been conducted for the many results of RBCB testing in EBR-II and BR-2. Besides, various cases of calculation have been performed for two dimensional analyses of temperature distribution around fuel pin breached sites, using the finite element method code FINAS. Then, synthetic considerations for the data and analyses have been made in order to clarify the mechanism including dominant parameter of the significant fuel release. It is concluded in the present study that there are six kinds of microstructural morphology for fuel release, and that the rate-determining mechanism is attributed to sodium-boiling-cycle-erosion (SBCE). Based on this mechanism, a new unitary thermo-hydraulic index is introduced as a dominant parameter to express the SBCE initiation condition. (author)