Analysis of the Chimney Effect During the Reflooding Phase of a Large Break LOCA Transient With the 3D Module of the CATHARE2 Code

This study deals with the ability of the three-dimensional module of the CATHARE2 code to simulate the thermalhydraulic behavior of a 900MWe Pressurized Water Reactor (PWR) in Large Break Loss of Coolant Accident (LOCA) conditions. The CATHARE2 code is a “Best-Estimate” system code, developed by the CEA, in collaboration with EDF, AREVA NP and IRSN, used in France in the frame of realistic methodology to evaluate safety margins. Particularly, the realistic simulation of the so-called “chimney effect”, which occurs during the reflooding phase of a Large Break LOCA is of primary importance. Observed during experiments, this effect is indeed characteristic of the hydraulic behavior of a nuclear core presenting a non-uniform radial power profile. Several separate effect tests such as PERICLES 2D reflood and CCTF/SCTF experiments have demonstrated the existence of cross-flows between the hot assembly and the mean assemblies of the core during this reflooding phase. Liquid goes from the mean assemblies toward the hot one beneath the quench front leading to an increase in the heat transfer coefficient in the hot assembly compared to the one in the mean assemblies, and hence to a better cooling of the hot rod. After a literary survey on the “chimney effect”, quantitative information has been found in several publications concerning SCTF and CCTF tests. More precisely, a correlation has been established from the results of these tests providing the increase rate of the heat transfer coefficient in the hot assembly compared to the one in the mean assemblies depending on the power features of the core. The assumptions related to the establishment of this correlation are first validated in case of the PERICLES 2D Reflood test configuration. Then the simulation of the “chimney effect” by the three-dimensional module of the CATHARE2 code is analyzed by comparing simulation and experimental results in the PERICLES 2D Reflood test configuration. Finally, the same kind of study is performed with the simulation of a 900MWe PWR core in reflooding conditions typical of a Large Break LOCA transient. In both cases, the difference between the heat transfer coefficients of the hot assembly and the mean ones obtained during the CATHARE2 simulations is compared to the correlation derived from the SCTF and CCTF experimental results. While the simulation of a Large Break LOCA in a 900MWe PWR has quite well reproduced SCTF/CCTF experimental evidences, the study performed with PERICLES configuration has not given such satisfying results, probably due to the lack of representativeness of the device (only three aligned assemblies).