The COMET-L3 experiment on long-term melt–concrete interaction and cooling by surface flooding Original Research Article

The COMET-L3 experiment considers the long-term situation of corium/concrete interaction in an anticipated core melt accident of a light water reactor after the metal melt is layered beneath the oxide melt. The experimental focus is on the cavity formation in the basemat and the risk of a long-term basemat penetration by the metallic part of the melt. The experiment investigates the two-dimensional concrete erosion in a cylindrical crucible of 60 cm in diameter fabricated from siliceous concrete in the first phase of the test, and the influence of surface flooding in the second phase. The initial mass of the melt was 425 kg steel and 211 kg oxide. Decay heating in the two-component metal and oxide melt is simulated by sustained induction heating of the metal phase that is overlaid by the oxide melt. In the initial phase of the test, the overheated, highly agitated metal melt causes intense interaction with the concrete, which leads to fast decrease of the initial melt overheat and reduction of the initially high concrete erosion rate. Thereafter, the erosion by the metal melt slows down to about 0.07 mm/s into the axial direction. Lateral erosion is by a factor of 3 smaller. Surface flooding of the melt is initiated at 800 s. Flooding does not lead to strong melt/water interactions and to penetration of water into the melt. Concrete erosion continues with about 0.040 mm/s until the melt reaches the maximum erosion limit of the crucible. Post-test analysis of the solidified melt was performed after the crucible was sectioned. The solidified melt shows no indication of water ingression from the upper surface. Tight surface crusts explain poor heat removal to the flooding water and the ongoing concrete erosion also after the top flooding. Details of the experiment are reported. The experiment shall be used for validation of models and computer codes for safety assessment.