Influence of 300MW Fuel Assembly Spacer Grid Improvement on DNBR Analysis

In order to reduce the potential hang up damage risk during refueling operation in Qinshan phase-I NPP, Shanghai Nuclear Engineering Research &Design Institute (SNERDI) carried out improvement work of 300MW fuel assembly space grid (mainly focused on the design of guide vane) to decrease the possibility of snag event between two fuel assemblies. In the whole project, multiple tests and analysis were carried out to demonstrate that the mechanical and thermal-hydraulic performance of new designed grid meets predetermined functional and safety requirement. During above process, DNBR analysis and comparison was performed under the influence of old and new designed spacer grids. For the old design of space grid, W-3 CHF correlation was adopted to calculate the DNBR margin. Thus, the first step was to conform that this CHF correlation can still be used. CFD simulations were employed to calculate the flow and heat transfer phenomenon in 17×17 rod bundles with old and new space grid, to compare the thermal-hydraulic performance of these two kinds of space grids qualitatively. To carry out above DNBR analysis, sub-channel grid loss coefficients are very important parameters. CFD tools and pressure drop tests help us to obtain these parameters. Four special rod bundles were used to calculate ratio of different sub-channels. Based on above work, the typical, thimble, side and corner sub-channel loss coefficient of old and new space grid were obtained, respectively. Finally, based on a safety analysis 14 sub-channel model, DNBR analysis were finished. Results indicated the Minimum DNBR of side and corner cell is a little lower than typical cell model.