虹吸破壞現象是一種非能動的物理過程，池式快堆反應器的核能安全系統設計中利用該自然現象可有效減緩在事故狀況下冷卻劑的流失，避免爐心熔毀嚴重事故發生。虹吸破壞現象的研究具有重要的科學研究意義和工程應用價值。為深刻了解與認識虹吸破壞的相關機制，利用系統熱水流分析程式RELAP5模擬北京清華大學的虹吸破壞實驗平台的實驗結果。
研究中比對主管道中的液相流量、壓差和上水箱液位隨時間變化的模擬與實驗的結果，並對虹吸破壞現象的流動特性與兩相流相關機制進行分析。模擬結果與實驗數據和計算流體力學模擬結果進行比較，探討其中的差異。結果顯示虹吸破壞現象發生時會伴隨明顯的不穩定性，導致壓差的震盪。通過對不同下降管管徑與出口閥門不同開度的比較研究發現，系統阻力愈大其不穩定性愈強。同時也對兩相流中重要的壓降加乘因子進行深入研究。由於在實驗中管道的流體沒有完全充分發展，分析得到的摩擦加乘因子比RELAP5理論計算的值要大得多。
實際的核能裝置設計是一個完整的系統工程，需要同時對物理、熱流、材料等許多因素綜合考慮。就虹吸破壞現象而言，管道尺寸的選擇非常重要，一方面不宜太大，如本研究下降管徑不超過50 mm，否則可能對迴路壓力邊界的完整性構成威脅；一方面又不宜太小，太小難以有效發揮虹吸破壞作用而不能有效阻止水的流失，同時太小的尺寸可能也會帶來較大的流量不穩定性。

The phenomenon of siphon break is a passive physical process. It is important for the safety design of pool type Liquid Metal Fast Breeder Reactor (LMFBR) and spent fuel pool of the nuclear power plant adopted Light Water Reactors (LWRs). It can affectively mitigate the Loss of Coolant Accidents of LMFBR and spent fuel pool of LWRs. In the present study, the system thermal hydraulic code RELAP5/MOD3 is used to simulate the experimental results of siphon break apparatus of Beijing Tsinghua University.
The simulated results of flow rate in main pipe, pressure drop in downcomer and water level in upper tank over time are compared with the experimental results. The simulated results are also compared with the results of CFD (Computational Fluids Dynamics) analysis. The results have demonstrated that the general trends of the experimental results can be caught by RELAP5 code. Nevertheless, the time duration of siphon break phenomena as predicted by the code is significantly less than the experimental results. A detail analysis of the code simulation results has shown that the two phase multipliers as predicted by the code is about an order of magnitude smaller than the experimental results. It is believed that the difference is due to the assumption of fully developed flow in the code calculation.
Sensitivity studies of the different downcomer pipe diameter and outlet valve opening show that the instability of the flow is stronger when the resistance is larger.

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