安全性是钠冷快堆发展中必须考虑的问题，严重事故评价在快堆安全分析中扮演着重要角色。选择SAS4A程序为分析工具，系统考虑严重事故初始阶段主要反应性变化，并将其应用于钠冷快堆无保护失流（unprotected loss of flow, ULOF）事故分析计算。计算表明，钠冷快堆在ULOF事故早期阶段，燃料升温引入的负反应性和燃料组件轴向膨胀引入的负反应性使反应堆功率缓慢下降。随后由于冷却剂温度升高，部分区域的冷却剂钠开始出现沸腾，大量的钠空泡导致冷却剂密度迅速降低而引入大量正反应性，反应堆功率会快速升高并出现功率峰值。随后由于熔融包壳和燃料的移动而分别出现功率上升和下降现象，最终由于出现大量熔融包壳和燃料移动，钠冷快堆严重事故进入过渡阶段。

Safety is an issue that must be considered in the development of sodium cooled fast reactors, severe accident evaluation plays an important role in fast reactor safety analysis. SAS4A program was selected as the analysis tool to systematically consider the main reactivity changes in the initial stage of severe accident, and it is applied to the analysis and calculation of sodium-cooled fast reactor unprotected loss of flow (ULOF) accident. The calculations show that in the early stage of the ULOF accident of the sodium-cooled fast reactor, the negative reactivity introduced by the fuel temperature rise and the axial expansion of the fuel assembly make the reactor power slowly declines. Then, due to the increase of coolant temperature, coolant in some areas began to boil, which caused the density of the coolant to decrease rapidly. The core introduced a large amount of positive reactivity and the power increased rapidly, then a power peak appeared. Subsequently, the power increased and decreased due to the movement of the molten cladding and the fuel, respectively. Finally, due to the large amount of molten cladding and fuel movement, the severe accident of the sodium-cooled fast reactor entered the transition phase.