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 m 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.

中文翻译：

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熔融燃料-钠相互作用期间冷却剂夹带的自发热碎裂机制

MELT 设施已经对金属真皮射流与钠之间的两种相互作用进行了实验研究，这些相互作用与钠冷快堆中的严重事故有关。 X 射线成像和碎片分析表明，熔体冷却剂界面处快速形成结壳，引发热碎裂事件。射流-冷却剂界面处的传热计算，由射流速度的粒子跟踪测速表征支持，意味着在与冷却剂接触的几毫秒内形成了固体外壳。受到喷射冷却剂夹带的 X 射线成像观察的启发，提出了一种增强热碎裂的机制。地壳内壁的高压与地壳的致密化和收缩相竞争，导致地壳中产生显着的应力。热弹性应力场建模表明，断裂是由外地壳壁的切向应力决定的，与外地壳表面 SEM 成像期间观察到的 200-300 m 长断裂一致。内地壳壁处的高压似乎在短时间内占主导地位，在外地壳壁处施加拉应力，可能引发断裂。当冷却时间较长时，射流的致密化和收缩占主导地位，导致切向压缩。因此，射流内的冷却剂夹带和蒸发似乎在地壳中施加了竞争应力，这可能影响射流破裂的模式。