Experimental Study on the Coolability of Molten Core Materials Discharged Into a Depth- and Volume-Limited Sodium Plenum

Abstract During a severe accident in sodium-cooled fast reactors, molten core materials could be discharged from the core region toward the lower sodium region of the reactor vessel through coolant channels, such as control rod guide tubes. Typical SFRs have a sodium plenum with limited depth and volume, such as the core inlet plenum located under the core region. Therefore, it is important to evaluate the coolability of molten core materials discharged into a depth- and volume-limited sodium plenum. In the present study, to deepen the understanding on the coolability of molten core materials discharged into such a sodium plenum, conditions under which molten core materials form solidified fragments were discussed based on an experiment discharging a molten fuel simulant (molten Al2O3) into a test vessel filled with liquid sodium. The data obtained in this experiment suggest that the molten Al2O3 formed solidified fragments under a condition beyond the cooling limit (the volume ratio of sodium to molten core materials: more than 10 times) estimated from existing studies, such as M-series experiments performed at the Argonne National Laboratory in the 1970s. Sodium convection flow induced by sodium vaporization (vapor bubble growth) and condensation during the Al2O3 discharge might have contributed to such efficient cooling.