Experiment QUENCH-20 with BWR geometry simulation bundle was conducted at KIT on 9th October 2019 in the framework of the international SAFEST project. The test bundle mock-up represented one quarter of a BWR fuel assembly with 24 electrically heated fuel rod simulators and two B4C control blades. The rod simulators were filled with Kr to inner pressure of 5.5 bar at peak cladding temperature of 900 K. The pre-oxidation stage in the flowing gas mixture of steam and argon (each 3 g/s) and system pressure of 2 bar lasted 4 hours at the peak cladding temperature of 1250 K. During the following transient stage, the bundle was heated to a maximal temperature of 2000 K. The cladding radial extensions and failures due to inner overpressure (about 4 bar) were observed at temperature about 1700 K and lasted about 200 s. During the period of rod failures also the first absorber melt relocation accompanied by shroud failure were registered. The interaction of B4C with steel blade and ZIRLO channel box was observed at elevations 650…950 mm with formation of eutectic melt. The typical components of this melt are (Fe, Cr) borides and ZrB2 precipitated in steel or in Zr-steel eutectic melt. Massive absorber melt relocation was observed 50 s before the end of transition stage. Small fragments of the absorber melt moved down to the elevation of 50 mm. The test was terminated with the quench water injected with a flow rate of 50 g/s from the bundle bottom. Fast temperature escalation from 2000 to 2300 K during 20 s was observed. As result, the metal part (prior β-Zr) of claddings between 550 and 950 mm was melted, partially released into space between rods and partially relocated in the gap between pellet and outer oxide layer to 450 mm. The bundle elevations 850 and 750 mm are mostly oxidized with average cladding ECR 33%
