严重事故下堆芯熔融物迁移行为模拟分析

Simulation Analysis of Core Melt Migration Behavior Under Severe Accident

  • 摘要: 为深入了解严重事故机理以及影响事故进程的关键因素,本文结合我国三代核电机组不同类型堆芯支承结构的设计特点,利用严重事故分析程序开展了熔融物迁移进程模拟分析,深入研究了熔融物迁移进程不同阶段的现象以及堆芯支承结构对迁移进程的影响。结果表明:严重事故初期,由于压力容器水位下降速率有限,堆芯熔化阶段出现熔融材料滞留分层,形成稳定可冷却的氧化物硬壳,导致内部熔池不断径向扩张,发生初始侧向迁移;随后熔融物继续沿侧向和竖向同时发生二次迁移,3类堆芯支承结构对熔融物二次迁移的阻碍特性不同,A类支承结构在初始侧向迁移后,熔融物尚未接触到支承板,后续侧向迁移熔融物质量流量较小,大部分熔融物沿竖向相继熔穿下堆芯板、支承柱和支承板进入下腔室,从而延缓熔融物迁移进程;B类和C类支承结构在初始迁移后至压力容器蒸干前,熔融物即可基本接触并熔化支承板,共同形成下腔室稳态熔池。最终稳态熔池中大部分熔融物,特别是金属熔融物,均需要通过二次迁移进入下腔室。

     

    Abstract: The simulation analysis of melt migration process in severe accidents is of great significance for understanding the accident mechanism and evaluating the prevention and mitigation strategies of severe accidents. Due to the limitations of experimental research on severe accidents, computer simulation analysis is usually used to simulate the process of melt migration. At present, the calculation model of the severe accident analysis program mainly focuses on the formation and cooling process of the melt pool in the initial stage of core melting and the steady state of the lower chamber. However, there are few quantitative analysis studies on the secondary migration stage of the melt during the formation of the melt pool, and the influence of the support structure in the simulation analysis of the core melt migration process has not been effectively identified. The core support structure is an important channel for the melt to enter the lower chamber from the active zone, and the main source of metal mass in the final steady-state melt pool. Its failure mechanism has an important influence on the melt migration process and the effectiveness of IVR mitigation measures. Based on the design characteristics of the three types of core support structures of the third generation nuclear power unit in China, in this paper the severe accident analysis program was used to carry out the analysis of the melt migration process, and deeply study the phenomenon of different stages of the melt migration process and the influence of the core support structure on the migration process. The results show that in the early stage of the severe accident, due to the limited water level drop rate of the pressure vessel, the melt is retained and stratified in the core melting stage, forming a stable and cooling oxide hard shell, resulting in the continuous radial expansion of the internal melt pool and the initial lateral migration. Subsequently, the melt continues to undergo secondary expansion and migration along both the lateral and vertical directions. The three types of core support structures have different hindrance characteristics to the secondary migration of melts. After the initial lateral migration of the type A support structure, the melt has not yet reached the support plate, and the mass flow of the subsequent lateral migration melt is small. Most of the melts are successively melted through the core plate, the support column and the support plate into the lower chamber along the vertical direction, thereby prolonging the time of the melt migration process. After the initial migration of the type B and type C support structures to the pressure vessel before evaporation, the melt can basically contact and melt the support plate to form a steady-state melt pool in the lower chamber. Finally, most of the melts in the steady-state melt pool, especially the metal melts, need to enter the lower chamber through secondary vertical migration. In addition, the MAAP program is limited by the simplified model and empirical relationship, and lacks the consideration of the stress distribution of the support plate structure. As a result, there may be a certain deviation between the calculation results in the secondary vertical migration stage of the melt and the serious accident mechanism, which affects the final penetration position of the support plate.

     

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