钠冷快堆堆芯熔融物射流碎裂特性理论研究

Theoretical Research on Fragmentation Characteristics of Molten Corium Jet during Severe Accident in Sodium-cooled Fast Reactor

在钠冷快堆严重事故下,堆芯熔融物可能掉入冷却剂中并与液态金属钠相互作用,导致熔融物的碎裂及凝固,并在堆芯捕集器或下封头内重定位形成堆芯碎片床。熔融物的射流碎裂特性直接关乎堆芯碎片床的冷却及再临界行为。本文基于线性稳定性理论、运动学方程和交界面修正拉普拉斯定律,推导出考虑沸腾和凝固效应的熔融物射流表面不稳定性增长方程,建立了液态金属钠中熔融物射流碎裂模型,并提出了典型环境中熔融物射流碎裂准则。随后使用熔融物射流碎裂模型对COSA实验结果进行了对比分析。本研究结果将为钠冷快堆严重事故的评估论证提供可靠工具,对严重事故缓解措施的设计也具有重要的指导意义和参考价值。

Under severe accident in sodium-cooled fast reactor(SFR), molten corium may fall into the coolant and cause molten fuel-coolant interaction(MFCI). The molten corium is fragmented and solidified to produce debris during MFCI. The debris is relocated on the core catcher or lower head to form core debris bed. The coolability and recriticality of core debris bed are significantly affected by the fragmentation characteristics of molten corium jet. Based on the linear stability theory, kinematic equations and modified Laplace’s law at the contact interface, the growth of surface disturbance of molten corium jet was derived to consider the effect of boiling and solidification behavior. The fragmentation model of molten corium jet was established. Then the criteria for fragmentation of molten corium jet under typical conditions were acquired. The model developed considers the effects of three factors: the relative velocity between melt and sodium, sodium boiling and melt solidification. Among them, the relative velocity is reflected by the classical K-H instability, and sodium boiling and melt solidification are reflected by the impact pressure coefficient Cpband bending stiffness Ds. Furthermore, the proposed model was compared with COSA experiment results. For the three molten aluminum experiments under non-boiling conditions, when Dsis around 2.0×10-3N·m, the predicted mass median diameter(DMM) are in good agreement with the experimental values of test Al-1 and test Al-4. While the experimental results of Al-2 are in good agreement with the results of model with Dsat 1.0×10-2N·m because the initial temperature of aluminum is lower and the solidified layer is thicker. For the two stainless steel experiments under boiling conditions, when Cpbis in the range of 10~6-10~8 Pa·m-1and Dsis in the range of 10-5-10-6N·m, the predicted DMMs are in good agreement with the experimental values. Since sodium is an opaque medium, it is difficult to obtain the visualization results of jet fragmentation, such as the thickness of solidified layer(which can be used to calculate Ds) and the interface disturbance amplitude(which can be used to calculate Cpb) during the fragmentation process. In the follow-up, the key parameters of the fragmentation process can be obtained by numerical simulation method(such as MPS). Then the model can be revised and improved in detail. The research in this work not only provides a reliable tool for the evaluation of severe accident but also offers important guiding significance and reference value for design of mitigation measurements of SFR.