Prediction of Regions of Reduced Heat Transfer Downstream of Nuclear Fuel Spacer Grids
- Westinghouse Electric Company LLC, Fuel Engineering, 102 Addison Rd, Windsor, CT 06095 (United States)
A numerical investigation was performed to predict regions of reduced heat transfer occurring downstream of nuclear fuel spacer grids in PWR assemblies. This study was initiated as a result of excessive crud deposits and fuel failures that have occurred on interior and peripheral rods for PWR assemblies at several plants of B and W, CE and Westinghouse design. Computational Fluid Dynamic (CFD) models were prepared for spacer grids with and without mixing vanes to understand how grid design can influence rod heat transfer downstream of the spacer grids. Regions where the forced convection heat transfer coefficients are low may experience Subcooled Nucleate Boiling (SNB), and enhanced crud deposition. It is postulated that variations in heat transfer in combination with available crud sources from Steam Generator tubes can lead to high localized boiling and excessive crud deposits on fuel rods. The increased crud deposition can lead to accelerated corrosion and then fuel failure. Four CFD models were prepared with and without mixing vanes for grids containing small and large thimble geometries. An intermediate flow mixing (IFM) grid was added to one of the models to demonstrate the impact on rod heat transfer. The CFD models contain 3x6 sub-channels for spacer grids with small thimble design and 6x6 sub-channels for spacer grid with large thimble. Peripheral rods and corner rod of the assembly are also included in the CFD domain. Single phase flow with high Reynolds k-{epsilon} turbulent model was used for the simulation by STAR-CD code. The results of the CFD analysis indicate potential SNB regions are located on the outer face of the peripheral rods due to increased hydraulic resistance of the grid perimeter strip and tabs for the grid without mixing vanes. With mixing vanes on the interior region of the spacer grid, more flow is redistributed to the outer region of the peripheral rods so there is improved peripheral rod heat transfer, however SNB regions can still form at the end of the grid span on selected interior rods with higher rod powers. These SNB regions were observed for mixing vane spacer grids for the large and small thimble geometries. For one of the mixing vane grid models an IFM grid was added in the middle of the grid span to observe the impact on rod heat transfer. The addition of the IFM grid significantly reduces the magnitude of the SNB regions. The predicted location of the SNB regions occurring in the CFD models downstream of the spacer grid was similar to the location of excessive crud deposits observed in reactor. The predicted reduction of hotspots with the addition of IFMs is also supported by no crud/corrosion failures occurring on fuel with IFMs. The CFD modeling has been helpful in understanding the formation of these regions of reduced heat transfer downstream of spacer grids and is useful for optimizing grid design and fuel management to mitigate excessive crud deposits. (authors)
- Research Organization:
- American Nuclear Society, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
- OSTI ID:
- 21229334
- Resource Relation:
- Conference: 2007 LWR Fuel Performance Meeting / TopFuel 2007, San Francisco, CA (United States), 30 Sep - 3 Oct 2007; Other Information: Country of input: France; 7 refs; Related Information: In: Proceedings of the 2007 LWR Fuel Performance Meeting / TopFuel 2007 'Zero by 2010', 683 pages.
- Country of Publication:
- United States
- Language:
- English
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