Mathematical spacer grid models for single phase flow

The fuel rods in pressurized water reactor (PWR) and boiling water reactor (BWR) cores are supported by spacer grids. Even though spacer grids add to pressure loss in the reactor core, spacer grids have several benefits in light water reactors (LWRs). Some of these benefits are: (i) increasing turbulence at the bottom of the reactor core for better heat transfer in single phase region of LWRs, (ii) improving departure nucleate boiling ratio results for PWRs, and (iii) improving critical power ratio (CPR) values by increasing the thickness of film in annular flow regime in the top section of the reactor core of BWRs. Several mathematical models have been developed for single and two phase pressure loss across the grid spacer. Almost all of them significantly depend on Reynolds Number. Spacer designs have evolved (incorporating mixing vanes, springs, dimples, etc), resulting in complexity of the analysis across the grid. The models have been compared not only theoretically but also quantitatively. For the quantitative comparisons, this work compares results of mathematical spacer models with experimental data of BWR Full Size Fine Mesh Bundle Tests (BFBT). The experimental data of BFBT provides very detailed experimental results for pressure drop by using several different boundary condition and detailed pressure drop measurements. Two bundle types of BFBT, the current 8 × 8 type and the high burn-up 8 × 8 type, were simulated. There are two also types of spacers in the BFBT program - ferrule type and grid type. Therefore, the experimental data of BFBT used a wide range of boundary conditions of BWRs. This paper analyzes mathematical models of spacer grids for a portion of available data. It was observed and discussed that pressure drop values due to spacer models can be significantly different for single phase flow.