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

Abstract

BackgroundIn 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.PurposeThis 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.MethodsFirst 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.ResultsThe 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%.ConclusionUnder 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.

Keywords

• flow blockage
• thermal-mechanical behavior
• peak cladding temperature
• quench-loca-0