New design procedure development of future reactor critical power estimation. (2) Sub-channel analysis for scale effect on critical power

In the Practical Design-by-Analysis (PDBA) for BWR core critical power estimation, the effect of the radially non-uniform power distribution on critical power within rod bundle is supposed to be predicted with numerical analysis. At present, the subchannel analysis method based on three-fluid two-phase flow model is only a realistic choice. In this study, the subchannel analysis code was improved by allocating wall friction to each of three fluids, and verified with experimental data of pressure drop, thermal-hydraulic distribution and bundle size effect on critical power. The published critical power experiment was analyzed for two bundle sizes of 6x6 and 12x12 with different peaking rod positions. The measured critical power was a maximum with uniform power distribution and decreased as the critical power rod lattice position went away from the channel wall. However, the calculated critical power under-estimated the measured critical power decrease from the maximum to the minimum. To predict the critical power variation precisely depending on critical power rod lattice positions, it is important to predict properly the variation of radical distribution of two-phase flow in various non-uniform power distributions in the radial direction. The new wall friction model consistent with empirical friction loss coefficient was developed and implemented into the subchannel analysis code. It resulted in better prediction in critical power dependence on critical power rod lattice position. (author)