Cross-Shaped Spiral (CSS) fuel rods have been proposed for improving the performance of Light Water Reactors (LWRs). Since these rods touch periodically as they twist axially, they do not require grids for mechanical rigidity. Experimental measurements of frictional pressure drop were performed for 1.5-meter-long model CSS test rods in 4x4 arrays at three twist-pitches (50-cm, 100-cm, and 200-cm), and for reference bare cylindrical rods in order to establish a base case. A single-phase water flow loop was constructed and operated at room temperature and pressure, for Re between 5,000 and 40,000. Results showed a larger pressure drop at the same flow velocity than for bare cylindrical rods. However, this implies a CSS-bundle turbulent friction factor which is only 90% of the expected value given its hydraulic diameter. The effect of twist pitch on pressure drop and friction factor was found to be negligible in the range of twists examined. The elimination of grid-spacers and the reduced friction factor are countered by the reduction in hydraulic diameter of the bundle. Therefore, at the rated power and flow rate, use of CSS fuel instead of the traditional cylindrical rods leads to a reduction in core pressure drop from 22.2 to 20.3 psi (9%), with the use of CSS rods in BWRs, and a reduction from 18.3 to 17.2 psi (6%) for CSS rods in PWRs.
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