Abstract
Test B/C in COTELS project was performed to investigate fuel coolant interaction and molten core concrete interaction (MCCI) under water injection onto molten debris. After 60kg UO_2 corium mixture was melted in electric melting furnace, the corium was fallen into concrete trap. The molten corium in the trap was re-heated to simulate decay heat. When subcooled water was injected onto the molten debris, steam explosion was not observed. Debris was cooled after water injection and MCCI was suppressed. In 6 tests out of 10 tests, particulate debris bed was formed above continuous ingot debris. Large amount of smallest diameter particles was obtained due to the entrainment of molten corium, decomposed concrete and oxidation of metallic components in corium associated with MCCI generated gas. This favorable cooling was attributed to the existence of remaining porosity inside corium due to simulation of falling process, water ingression via eroded concrete side wall clearance and channels, and the interruption of further concrete floor erosion due to the existence of pebble bed, decomposed from concrete, below the debris.