An experimental study on layer inversion in the corium pool during a severe accident

COSMOS (COrium configuration of the molten State in the MOst Severe accidents) tests using prototypic materials have been performed to investigate the layer inversion of the heavy metallic material in the corium pool. An induction heating method using a cold crucible was implemented for melting of the UO2-ZrO2-Zr-Fe mixture. Before the main test, three preliminary tests have been performed to enhance the experimental techniques using the real core material. One main test of the COSMOS has been performed to evaluate the corium pool configuration in the lower plenum of the reactor vessel for the TLFW (Total Loss of Feed Water) sequence of the APR (Advanced Power Reactor) 1400 under the IVR-ERVC (In-Vessel corium Retention through External Reactor Vessel Cooling). A post-test examination of cutting, EPMA, and XRD for the solidified corium pool ingot in the main test has been performed to investigate the layer inversion. From the three preliminary tests, melting techniques of the real core material using a cold crucible were successfully developed. The metallurgical inspection results on the chemical information coincide with the visual observation on the centerline cut ingot in that the upper part is metal and the lower lump is an oxidic mixture with some metal clods in the main test. This means the possibility of layer inversion of the heavy metallic material in the corium pool.