A Mechanism for Spontaneous Thermal Fragmentation with Coolant Entrainment During the Molten Fuel–sodium Interaction

Experimental investigation of two interactions between metallic corium jets and sodium, pertaining to severe accidents in a sodium-cooled fast reactor, have been undertaken at the MELT facility. X-ray imaging and debris analysis reveal rapid formation of a crust at the melt coolant-interface, instigating thermal fragmentation events. Heat transfer calculations at the jet-coolant interface, supported by particle tracking velocimetry characterisation of the jet velocity, imply the formation of a solid crust within milliseconds of contact with the coolant. A mechanism for enhanced thermal fragmentation is proposed, inspired by observations from the X-ray imaging of coolant entrainment into the jet. Elevated pressures at the internal crust wall compete with densification and contraction of the crust, resulting in significant stress in the crust. Modelling of the thermoelastic stress field indicates that fracture is dictated by the tangential stress at the outer crust wall, consistent with 200–300 μm long fractures observed during SEM imaging of the outer crust surface. The elevated pressure at the internal crust wall appears to dominate on short timescales, imparting tensile stress at the outer crust wall which may initiate fracture. On longer cooling times, the densification and contraction of the jet dominate resulting in tangential compression. Thus, coolant entrainment and vaporisation within the jet appears to impart competing stresses in the crust which may influence the mode of jet rupture.