Core Melt Solidification Characteristics in PRV Lower Head-Experimental Results From LIVE Tests

Core melt solidification phenomena in the lower plenum of pressurized reactor vessel during external reactor vessel cooling is investigated in late in-vessel phase experiment tests under different external cooling conditions and melt pouring positions. The melt solidification behavior, which has not yet been given sufficient attention, is an important issue since it influences not only the transient but also the steady state of melt pool thermal hydraulics. A noneutectic melt (⁠$80mol%$$KNO3–20mol%$$NaNO3$⁠) was used to simulate the core melt. It has been found out that when the vessel is cooled with water during the whole test period (water cooling), the cooling is more effective than the case that the vessel lower head is first cooled with air and flooded by water (air/water cooling). Water cooling at the beginning leads to faster buildup of crust layer on the vessel inner wall and lower crust thermal conductivity compared with air/water cooling. In comparison with the air/water cooling, the water cooling also achieves shorter time period of crust growth. During the solidification period in all tests, the constitutional supercooling condition is fulfilled. Pouring position near the vessel wall results in considerable asymmetry in the heat flux distribution through the vessel wall.