流道堵塞模型开发及在QUENCH实验中的应用研究

Development of flow blockage model for core heat transfer and its application in QUENCH experiment

在压水堆冷却剂丧失事故(Loss-of Coolant Accident,LOCA)中,处于高温条件下的燃料棒由于棒内压力过高,可能导致包壳发生鼓胀。包壳形变会造成堆芯局部流道堵塞,进而影响失水事故再淹没阶段的堆芯换热。然而,大多数系统分析程序都是基于假设的流道堵塞率来模拟事故进程,导致模拟结果与实际情况不符合。本文将已开发的燃料棒热-力行为分析模块(Fuel Rod Thermal-Mechanical Behavior,FRTMB)集成在自主开发的严重事故分析程序ISAA(Integrated Severe Accident Analysis Code)中,通过改进已有的流道堵塞模型,使其能够模拟由于燃料棒形变导致的冷却剂流量变化。最后,使用ISAA-FRTMB模拟QUENCH-LOCA-0实验,通过对比包壳峰值温度,验证改进的流道堵塞模型的正确性和有效性,并在此基础上研究包壳形变对堆芯换热以及后续事故进程的影响。

[Background]In a pressurized water reactor(PWR)loss-of-coolant accident(LOCA),high temperature and high internal pressure of the fuel rod can lead to ballooning of fuel rod cladding,which causes a partial blockage of flow area in a subchannel.Such flow blockage would influence the core coolant flow and thus affect the core heat transfer during reflood phase and subsequent severe accidents.However,the commonly used integrated severe accident analysis codes use simple parametric models to simulate these aspects and therefore cannot consider the influence of multiple coupled factors.This results in a lack of accuracy of the simulation results.[Purpose]This study aims to analyze the key phenomena in core degradation,and develop a thermal-mechanical(TM)behavior module for assessing the failure of cladding and analyzing the flow blockage.[Methods]First of all,the fuel rod thermal–mechanical behavior(FRTMB)module developed for analyzing the TM behavior of fuel rods was integrated into the integrated severe accident analysis code(ISAA).Then,on the basis of the FRTMB module,the flow blockage model of the ISAA-FRTMB code was improved to suit for simulating changes in coolant flow rate caused by fuel rod deformation.Finally,the QUENCH-LOCA-0 experiment was simulated by using improved ISAA-FRTMB code to verify the correctness and effectiveness of the model,and the peak cladding temperatures were compared in order to verify the validity of the flow blockage model.[Results]The results including cladding failure time,circumferential strain,flow blockage rate and cladding temperature predicted by the code are in good agreement with the experimental data.The maximum circumferential strain of the simulated cladding,as indicated by the experimental results,is in the range of 25%⁓50%,and the errors of the predicted cladding rupture time and temperature are within 4%.[Conclusion]Under the stress caused by internal pressure,the cladding deforms outward owing to thermal creep with the increase of temperature.Rapid thermal creep and swelling lead to cladding failure.The maximum circumferential strain of the simulated cladding,as indicated by the experimental results,is in the range of 25%⁓50%,and the errors of the predicted cladding rupture time and temperature are within 4%.The correctness and effectiveness of FRTMB module are thus verified.