Abstract
BWRS are operating for many years in our country. The safety in BWR operation is always a big issue due to the aging of internal parts. The shroud of core is connected by welds，in which fracture and material crack would easy happen. So we tried to investigate the detail effects about this point. We used the CFX4.2 simulation system trying to build a model for the cracked welds and downcomer，and ran some simulation tests. First，we ran some isothermal tests and added some parameters for analysis to choose the properly boundary conditions. Then we discussed the temperature effects. Besides， we built two types of crack to more conformable with the real situation and investigated the application of CFX to any boundary condition。For saving the grids and get better computation efficiency ，we set some properties in commend files. we used a better computer software to analyze for the temperature and vector distributions. In the same time，we compared our results with the INER experiments. Our simulation situation was based on the test section of INER experiments. The pressure difference in core and downcomer was the same with NPP#1’s.The inlet downmer flow rate was 0.71 m /s. In final，we used our simulation experience to build the steps and logic for simulation analysis，and hoped that will be the reference for the simulation in safety consideration.