Measurement of pressure drops in prototypic BWR and PWR fuel assemblies in the laminar regime - Pressure drop measurement of laminar air flow in prototypic BWR and PWR fuel assemblies

Laminar pressure drops in nuclear fuel assemblies are of interest for evaluating complete loss-of-coolant accident scenarios in spent fuel pools and for performance analyses of dry storage casks. To the knowledge of the authors, this study represents the first attempt to directly quantify pressure losses in prototypic fuel assemblies in the laminar regime. Two commercial fuel assemblies were examined including a 17x17 PWR and a 9x9 BWR. The assemblies were tested in the laminar regime with Reynolds numbers ranging from 10 to 1000, based on the average assembly velocity and hydraulic diameter. Pressure drop measurements were made across individual bundle spans and grid spacers in the mock fuel assemblies using high-sensitivity differential pressure gauges. These gauges are capable of detecting extremely small changes in differential pressure with a resolution of ∼0.02 Pa. This level of sensitivity allows meaningful pressure drop measurements across separate fuel components, even at low Reynolds numbers. The fuel assembly mock-ups were constructed from commercial fuel assembly structural components and stainless steel tubing that is within 0.6 pc of the outer diameter of actual fuel. The outer flow boundary in the BWR assembly bundle was defined by the walls of a prototypic canister. In the PWR assembly, the flow was confined by the walls of different stainless steel storage cells. Two of the PWR storage cell sizes represented dimensions spanning pool and cask cells available in industry. Pressure ports were installed along the length of the assemblies at locations corresponding to the entrance and exit of fuel components. Dry, ambient air was metered into the bottom of each assembly through a flow straightener. The geometries of the tube bundles in 17x17 PWR and 9x9 BWR fuel assemblies are fundamentally different. The PWR bundle has a larger flow area and incorporates more grid spacers compared to the BWR bundle. Additionally, eight of the 74 fuel rods in the 9x9 BWR tube bundle are partial length leaving significantly greater flow area in the top third of the bundle. With fewer grid spacers and expanded flow area in upper bundle, the BWR assembly exhibited less flow resistance at a given Reynolds number compared to the PWR assembly when located in a storage cell analogous to the BWR canister. This 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 or less than that exhibited by the BWR assembly. These measurements should prove useful in independently validating CFD results or constructing numerically equivalent flow elements for use in fuel modeling efforts. (authors)