Assessment of Debris Bed Formation Characteristics Following Core Melt Down Scenario With Simulant System

In a fast reactor safety analysis determination of the molten core conditions when it reaches the core catcher plate is one of the main factors after a postulated MFCI event. If large fragmentation and quenching is accomplished in the coolant column no major problems for main vessel attack would occur. If instead, a significant amount of melt would remain as a solid molten cake, potential for lower head penetration would exist. In the present study towards development of a model for core melting and debris settling on to a core catcher plate, early phase of liquid stream fragmentation progression due to hydrodynamic consideration was investigated with woods metal melt water system. The system selected simulates the hydrodynamic physical properties more closely that of liquid UO2-sodium system. Assessment of debris-bed forming characteristics was carried out with different coolant column and different melt temperatures with melt inventories up to 20 kg released from a nozzle of 8 mm diameter. Bed height, debris spread area, jet break up length and repose angle obtained are presented for a melt release rate of ∼ 600 g/s. Only solidified debris constituted the bed for a melt temperature of 100°C and water temperature of 29 °C, with 720 mm water height. The estimated average bed height, bed porosity and heap repose angle were 15 cm, ∼0.6 and 43° respectively. Solidified central columnar lump of height 30 cm was seen for a water column of 360 mm. Relative bed forming characteristics for melt temperatures of 120 °C and 220 °C are also presented. High speed video imaging was taken to assess the stream break up distance and heap formation dynamics. Bulk coolant temperatures close to the melt stream were also monitored. Dependence of particulate debris and bed characteristics on melt temperature, interaction height and melt inventory have been brought out.