中文摘要：

对ATHLET-SC系统程序进行改进,实现了两流体模型下的跨临界瞬态计算。以该程序为基础,采用超临界轻水堆型（SCLWR-H）的滑压启堆方案,针对混合谱堆型的堆芯部分进行启堆工况下的热工水力动态模拟。模拟结果表明,整个启堆过程中燃料棒包壳表面温度均未超过限值（650℃）,跨临界瞬态下水的物性突变不会对堆芯燃料棒包壳传热造成不良影响。

英文摘要：

The modification to ATHLET-SC code is introduced in this paper,which realizes the simula-tion of trans-critical transients using two-phase model.With the modified code,the thermal-hydraulic dy-namic behavior of the mixed SCWR core during the startup process is simulated.The startup process is similar to the design of SCLWR-H sliding pressure startup.The results show that maximum temperature of clad-ding-surface does not exceed 650℃ in the whole startup process,and the sudden change of water properties in the trans-critical transients will not cause harmful influence to the heat transfer of the fuel cladding.