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Pressure drop measurement of laminar air flow in a prototypic BWR and PWR fuel assemblies.

For this study, dry air at ambient conditions was metered into the bottom of two unheated prototypic fuel assembly mock-ups. One assembly was a 17×17 PWR and the other was a 9×9 BWR fuel assembly. Pressure drop measurements were made in the laminar regime with Reynolds numbers ranging from 10 to 1000. High sensitivity quartz crystal differential pressure gauges capable of detecting slight changes in differential pressure with a resolution of ~0.02 Pa provided meaningful pressure drop measurements across individual grid spacers and bundle segments. With fewer grid spacers and expanded flow area in the upper bundle, the BWR assembly exhibited less flow resistance at a given Reynolds number compared to the PWR assembly when located in a tight storage cell sized to be analogous to the BWR canister. The tight PWR storage cell was smaller than any used commercially in spent fuel pools or dry storage casks. When the PWR assembly was tested inside of storage cell sizes that spanned pool and cask cells available in industry, the flow resistance at a given Reynolds number was equivalent to or less than that exhibited by the BWR assembly.